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EU carbon border tax will force others to cut emissions from 2026
Europa Clipper Reveals a New Perspective on Comet 3I/ATLAS
Researchers have been trying to look at interstellar object 3I/ATLAS from every conceivable angle. That includes very unconventional ones. Recently, while 3I/ATLAS passed out of view of the Earth, it moved into a great vantage point for one of our interplanetary probes. Europa Clipper, whose main mission is to explore Jupiter’s active moon, turned its gaze during its six year journey back towards the center of the solar system and observed 3I/ATLAS as it was reaching its perihelion, and out of sight from the Earth.
#776: The Matter – Antimatter Dichotomy
Shortly after the big bang there were almost exactly the same amounts of matter and antimatter in the Universe, but there was just enough of a difference that we live in a matter-dominated Universe. But it didn’t have to be that way! Explaining this mystery has been one of the great mysteries in astronomy, and today we’ll see if there’s been any progress! Why is the Universe the way it is? Specifically, why is it made mostly of matter? This is the question we’ll look at today!
Show Notes- Why Does Anything Exist? Matter, Antimatter, and the Asymmetric Universe
- The Early Universe
- Symmetry, Violations, and Fundamental Physics
- New Ideas and Radical Theories
- Dark Matter Connections
- How Science Moves Forward
- Matter–antimatter asymmetry as a key to understand the existence of universe
- Future experiments
Fraser Cain:
Astronomy Cast, Episode 776 The Matter-Antimatter Dichotomy. Welcome to Astronomy Cast, our weekly, facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, I’m the publisher of Universe Today.
With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the Director of Cosmoquest. Hello, Pamela. How are you doing?
Dr. Pamela Gay:
I am doing well enough.
Fraser Cain:
Enough. Well enough. Yes.
So, I want to talk about something that is very important, and I think a lot of people are very ashamed to admit it to us, and I think this is a safe space. And that is, it’s okay for you to fall asleep to our Delta Tones.
Dr. Pamela Gay:
It’s true. It’s true. We are aware that you do this.
And honestly, it’s a compliment to know that you’re taking us into such a vulnerable spot where we could potentially give you nightmares, and apparently we don’t.
Fraser Cain:
And that somehow the back and forth between the two of us somehow does not wake you up suddenly when I get into some kind of excited rant, and that’s good. And this is one of those great cycle of life kind of things, where I think we use other people’s audio and video to put ourselves to sleep, and I think this is just one of the great discoveries of the modern internet age, is that you can play audio, it puts you to sleep. How wonderful is that?
And I think, you know, what I consider when you’ve got all of these modern options right now where you can go onto YouTube and you can let an AI read you some kind of sleepy time AI slop. Do not do it. Or you can pick the genuine conversations between two human beings who’ve been doing this for 18 years.
We respect and appreciate the fact that you choose to spend your time going to sleep with us.
Dr. Pamela Gay:
I have one correction, however. It’s been 19 years, my friend.
Fraser Cain:
Whatever. 19 years. You’re such a stickler for the amount of time.
I no longer care. Time isn’t weaning. So yeah, use our material, fall asleep.
Don’t even listen to whole episodes. You have no shame. We appreciate your business, and by that I mean you listening to us.
Dr. Pamela Gay:
See, and I’m required to know how many years this has been because we started two weeks after I got married. And if I forget how long I’ve been married, there are consequences. Therefore, I know how long we’ve been doing this episode.
Fraser Cain:
Yeah. See, so my kid, my eldest, is exactly 30 years younger than me.
Dr. Pamela Gay:
Oh, that’s convenient.
Fraser Cain:
Yeah, exactly. So I’m always to just, I just add 30 or subtract 30, and there will be age. Yep.
Shortly after the Big Bang, there were almost exactly the same amounts of matter and antimatter in the Universe, but there was just enough of a difference that we live in a matter-dominated Universe. But it didn’t have to be that way. Explaining this mystery has been one of the great mysteries in astronomy, and today, we’ll see if there’s been any progress.
So how do we even know that there was matter and antimatter in the early Universe shortly after the Big Bang?
Dr. Pamela Gay:
It’s theory. I have to admit. So the assumption is we know that when things like particle accelerators generate vast amounts of localized energy through collisions that you get both matter and antimatter coming out of that energy.
So the story goes that the Universe came into existence as pure energy, and then that energy divided into matter and antimatter in theoretically equal amounts, and thus the Universe evolved.
Fraser Cain:
Right. And so, sorry, let me just put a, like, I’m going to, we’re going to go through this very slowly and carefully. So when we have a particle accelerator, and we are slamming particles together at enormous velocities that turns into energy, that energy freezes out particles, and what we see in our particle accelerators is this distribution between matter and antimatter.
Dr. Pamela Gay:
Mostly. Mostly. So this is where this whole episode is going to have lots of caveats on it, and that’s where the science is.
The science is in the caveats. So there was some recent research looking at the Large Hadron Collider’s results of lambda particles, and they found that they aren’t actually entirely in equal amounts of particles and antiparticles, which was exciting, because to first order, when you look, they appear to be like you do one or two experiments, and you don’t really see any differences. It’s over the accumulation of data over long periods of time, over lots and lots of, this is why we keep slamming things together, particle accelerators, lots and lots of experiments, they’re starting to see at greater than the three sigma level, not quite to the six sigma level, they’re seeing this asymmetry in matter and antimatter with more matter particles being created.
Fraser Cain:
That’s exciting.
Dr. Pamela Gay:
Yes.
Fraser Cain:
We’ll come back around. But before some of these new experiments with the Large Hadron Collider, originally it was like you either accelerate particles, slam them together, generate energy, energy freezes into particles, particles are balanced between matter and antimatter, boom, or early universe, we are simulating essentially the conditions of the early universe, and so we would get this state where you’d be getting, once things had cooled down, you get matter, antimatter.
Okay.
Dr. Pamela Gay:
And a whole lot of physics depends on this being true. So like the idea of how black holes evaporate is predicated on the idea that energy becomes a particle and an antiparticle. All this stuff is like, it’s just what we breathe.
Fraser Cain:
So then we get all of this, the temperature of the universe cools down, these particles freeze out, you get the particles, the antiparticles, but then they annihilate. Right. And what did that cause?
That annihilation of all of this matter, all this antimatter, did that just get you back to the energy again? Like what happened?
Dr. Pamela Gay:
So it got us to the cosmic microwave background being created. All of the light from the cosmic microwave background came out of this annihilation. And for reasons that make no sense.
Fraser Cain:
Well, hold on a second though. I mean, the cosmic microwave background radiation is the result of all of the protons turning into helium infusion, like the core of the star, and then it cools down and it’s like the surface of the star. And then finally you get to this place of last scattering.
Dr. Pamela Gay:
So the majority of the light is actually from the matter, antimatter.
Fraser Cain:
Whoa.
Dr. Pamela Gay:
Yeah. Yeah. So yes, there is light from when the universe was the inside of a star, sort of.
But also the particle antiparticle, there was a whole lot more of that going on earlier in time.
Fraser Cain:
Right. Yeah. But like in the first like millionth of a second after the universe, right?
Dr. Pamela Gay:
Yeah. But those photons are still going to be there getting absorbed and re-emitted and absorbed and re-emitted.
Fraser Cain:
Right. They’re random walking in the way that we see that random walk inside a star. Okay.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Yeah. And it starts out as gamma radiation. You know, matter, antimatter gives you gamma radiation and then it’s got to make its way out of the system.
Okay. Understood. Okay.
But like a lot of people ask this question, like did it help drive the expansion in the universe? Like what role, apart from superheating everything, I guess, did this annihilation play in the evolution of the universe?
Dr. Pamela Gay:
It depends on which papers you read, but for the most part, it was a universe that was so energetic that it’s just like one more part of the soup at that point.
Fraser Cain:
Right.
Dr. Pamela Gay:
Got it. So we have what should have been, according to early theories, equal amounts of matter and antimatter should have annihilated and created a universe of pure energy that would then turn into equal amounts of antimatter and matter that would cure. And you get stuck in a cycle of you don’t get us, but there’s somehow us.
Fraser Cain:
Right. I mean, you could still get the expansion and so you would have the energy and the energy would be starting to get farther apart from each other, but it would always be sort of perfectly balanced. It would be a dramatically different universe than the one we experience.
Dr. Pamela Gay:
So somehow, the matter that we are did not get annihilated.
Fraser Cain:
Okay.
Dr. Pamela Gay:
All right. And this gets us to the initial question.
Fraser Cain:
Right. So before we get into the explanation that we’re sort of moving towards, there’s one additional possibility that I want to just sort of take off the table. And that is that, in fact, there were equal amounts of matter and antimatter, they just weren’t evenly distributed.
Right. Could we look out into the universe and see galaxies and so on where they’re all made of antimatter? And in fact, everything is perfectly balanced.
Dr. Pamela Gay:
So that was one of the early explanations for this. Now, the problem is you have our island of matter universe, you have this other island of antimatter universe, and they could be multiple islands. But between the two of them is going to be the place where matter and antimatter are encountering one another.
And that place should be creating a wall of gamma rays.
Fraser Cain:
Right.
Dr. Pamela Gay:
And we don’t see that. So within our ability to look out across the universe for whatever fraction in the universe we see, everything appears to be matter-dominated, and it is now generally accepted that we live in a matter-dominated universe and not in a universe with galaxies made of both matter and antimatter that are separated.
Fraser Cain:
Right. So if that distribution is over the cosmic horizon, not our problem. Exactly.
But we do see examples of places where antimatter is being generated. There seems to be an excess of gamma radiation coming from the center of the Milky Way. But it’s not very much.
And so even where there’s even just like slight amounts of antimatter being generated by, who knows, tons of pulsars that are acting like particle accelerators and they’re producing antimatter, it’s finding its way to matter. It’s producing gamma radiation. We see this excess.
It is subtle. It’s there. If there was whole galaxies of matter and antimatter – Not subtle.
Not subtle. It would just be the universe would be screaming at us that there’s antimatter and matter colliding over here. Take a look at me.
So no. By every measurement that we see, this universe is made of matter and, you know, and then all of the other stuff. It is definitely not equal amounts of matter, antimatter.
Okay. So hopefully everyone was like, oh, that was the one I was thinking of. Okay.
So we, so now we go back and we say, why, why do we have more matter than antimatter?
Dr. Pamela Gay:
So the next big theory that people chased down was this, this idea that maybe there isn’t the parity we think there is. Maybe there isn’t the charge symmetry we think there is. Maybe time isn’t symmetric the way we think it is.
Fraser Cain:
Yeah. You just hit CPT violation. We’re going to have to break that down a bit.
Dr. Pamela Gay:
Yeah. Yeah. So, so the idea is that when, when you take a particle, it has charge, it has parity, and it moves through time.
And ideally every bit of physics that you see that is happening for, for a matter particle should happen in the opposite time direction for the antimatter particle. And, and so everything should be symmetric for all physics interactions.
Fraser Cain:
Right.
Dr. Pamela Gay:
So beta decay, anti-beta decay, the, this, think of the way neutrons decide, I shall become a proton and an electron now. All these reactions that, that are creating positrons and electrons, these all should have symmetries in them. Right.
But, but it was noticed in, in the, I believe it was in the sixties that there are with, with mesons reactions, these, these are particles that are a quirk and an anti-quirk, there, there are reactions that are actually asymmetric. There are reactions that don’t show the, the CP, it’s CP violations is the way they called it. So charge and parity seem to have moments only involving the weak force.
So electrostatic, fine, strong, fine. Weak force, nope, not going to do it.
Fraser Cain:
Yeah. Which is kind of fascinating, right? That you, you know, you take a bunch of particles, you take a, a dock, you, you, you flip the charge on every single particle in the dock.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
You’ve now made yourself an antimatter dock.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Right. Don’t do this. Don’t do this.
Meet, meet, meet a regular dog, you get annihilation. But, but the point is, is that the antimatter dog is going to behave exactly like the regular matter dog, just that everything is going to be, you know, that, that you’re going to get the same outcome because you’re going to get this, this symmetry. And that, but they found that when you have the charge and the parity, the antimatter dog does not behave like the matter dog in these really subtle ways, but only for the weak force.
Dr. Pamela Gay:
And, and this leads to a small amount of more matter.
Fraser Cain:
Oh, there’s a hint.
Dr. Pamela Gay:
So, so then.
Fraser Cain:
So, so sort of like, sorry, I just got to believe this point. You take dog, you let it decay, I guess you take an anti-dog, you let it decay. Okay.
I shouldn’t be using, I should be using radioactive elements.
Dr. Pamela Gay:
No, we’re not going to decay them.
Fraser Cain:
You take, you take a little bit of radioactive element, you let it decay. You take a little bit of, of anti-radioactive element, you let it decay and, you know, both sort of run by the weak force and they, you don’t get the version and the anti-version. You get a slight difference with this hinting that, okay, maybe there are these forces that are involved in this somehow, something to do with this sort of asymmetry when you, when you, when you flip everything over.
Dr. Pamela Gay:
So, so the question became, does normal baryonic matter that is made of trios of quarks have, have the same CP violation and decades pass, generations pass until finally enough data was, was accumulated by the Large Hadron Collider’s instruments looking at, at baryonic particles that they were able to go, yes, occasionally there is at the five sigma level, which is enough to say it is real. We are seeing baryons, but the problem is they’re seeing them, but the violation isn’t enough to explain the universe we’re in. We have, we have too much matter for this violation.
So, so while yes, CP violation is real, yes, we can start to understand how, how there’s more matter than antimatter. This is not a complete solution.
Fraser Cain:
Okay. So now that does not solve the whole problem.
Dr. Pamela Gay:
No, no. And, and this brings us to a fairly new theory. This is not something I learned in grad school.
It is something that actually came out of the Perimeter Institute fairly recently by, have you interviewed him? Do you know how to actually say his last name? Niall Turok?
Turok?
Fraser Cain:
Niall Turok?
Dr. Pamela Gay:
Niall Turok.
Fraser Cain:
I would say, I would say Turok. Have I interviewed Niall Turok? I have not interviewed Niall Turok.
You should. You should.
Dr. Pamela Gay:
I want you to.
Fraser Cain:
Okay. Yeah. He’s one of those maverick thinkers.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Tons of crazy ideas coming out of, out of him. Yeah. I have not.
Dr. Pamela Gay:
That are science-based and make predictions. So.
Fraser Cain:
Yes. Uh, yes, but also are seen skeptically by large amounts of the particle physics community, which is fine, you know?
Dr. Pamela Gay:
And that is okay.
Fraser Cain:
Like this is the way it’s supposed to be done.
Dr. Pamela Gay:
If it’s disprovable, it’s still science.
Fraser Cain:
Yes. Yeah. As long as you don’t hold onto it after it’s been disproved.
Dr. Pamela Gay:
Yes.
Fraser Cain:
There, there is that part too. Yeah. Let it go when it’s been disproved.
I’m not saying that, that, that Turok isn’t doing this. I’m just saying like, in general, there are people who are making statements that are disprovable and yet when they’re disproved, they’re not releasing the idea. They are continuing to hold onto it and it’s switched from science to marketing, to self-promotion, just saying, just laying that out there, not naming any names, not pointing any fingers.
Dr. Pamela Gay:
And they do not work for the Perimeter Institute.
Fraser Cain:
No, they do not. We are not casting any aspersions.
Dr. Pamela Gay:
No.
Fraser Cain:
We are casting a wide net. Yes. A general how-to, as it, as it were.
Dr. Pamela Gay:
So, so Turok, uh, there’s, there’s this really cool suite of papers that have come out that look at the idea. Well, what if the way to look at CPT violation is…
Fraser Cain:
You’ve got to choose the charge. You’ve got to include the parody and you’ve got to include the time.
Dr. Pamela Gay:
What if the universe at the moment of its creation sent matter in one direction and anti-matter in the other direction through time?
Fraser Cain:
Whoa.
Dr. Pamela Gay:
Yeah. So, so now you have one side that’s matter dominated, another side that’s anti-matter dominated. The way this works out, it actually doesn’t require inflation.
It explains dark matters being, uh, a, a plethora of sterile neutrinos being required. It works out gracefully, except it’s, it’s so novel and weird that there’s a whole lot of people going, no, no.
Fraser Cain:
And hard to detect.
Dr. Pamela Gay:
And hard to detect. It does, it does make predictions about sterile neutrinos. Um, it also makes, uh, there should be a neutrino with no mass.
Um, right.
Fraser Cain:
And so, so this is one example. And so like the point here is, is that, is that if you go all the way to charge parody and time, you flip them all. Yeah.
That you theoretically get that violation. That you get more matter in one direction and more anti-matter in the other other direction. While the sort of expectation is no.
If you, if you take the universe and you flip all the charges and you flip all the parody and you flip all the, the time and you run the universe backwards with that you would not be able to tell.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Right. That you can run a universe backwards, all the charges reverse, all the parodies reverse, you know, up quarks become down quarks, down quarks become up quarks, uh, positive becomes negative. You look at that universe and then you’re like, I can’t tell the difference.
Everything is perfect. And in fact, what you’re saying is maybe there’s like a little difference.
Dr. Pamela Gay:
And it’s super uncomfortable to think that out of the moment of creation, you get two universes that are one universe, but matter anti-matter time for time backwards, parody swaps. And, and because chaos theory is a thing, this doesn’t mean that there’s an anti astronomy cast that is a 13.8 billion years in the before times, um, before Big Bang, so BBB, um, it, it is to say that there were matter and anti-matter created and then chaos theory and everything else allowed perturbations to exist between the two. It’s just cool.
And it’s also not perfect, right? So we have all these different concepts. None of them are complete solutions.
And I, I am of the thinking that someday, just like with dark matter, it’s, it’s a multiple solution. Perhaps with this, it’s going to be a multiple solution of CP, CP violations.
Fraser Cain:
And, and we’re looking for the, and, and you are like, just describing a theory that was presented by one researcher, three researchers. He was, yeah, yeah. Yeah.
I reported about three weeks ago and Dr. Brian Koberlein wrote this up on universe today. Another theory that proposes that in fact, sort of early on in the universe, there were these kind of knots in space-time called a soliton field and that the sort of, while the particles themselves all behave, they were the way they were expected, these soliton fields, uh, sort of were the source of neutrinos, either axions or sterile neutrinos. And that those would, would sort of get in there and mess up the, the matter antimatter sort of side and, and, and they would only be matter.
And so you would get, yeah. And so you would get not only an explanation for dark matter, but you’d also get an explanation for the matter antimatter symmetry problem. And, and that again, like the math, but how do you observe it?
And so I think, you know, like we sat down and really researched, we could probably come up with dozens, if not hundreds of papers that are proposing novel ideas for how to explain this. And, and I think that makes it really exciting. There is this field where everybody acknowledges that this is a problem and everybody sort of is mapping out the parameters of what the solution has got to look like.
And then it’s up to the creativity of the individual researchers, the ingenuity of the people who are making the science experiments to actually perform observations, and this is how science works.
Dr. Pamela Gay:
And, and what I’m really appreciating about this is there’s a chance that they’re going to come up with something that has a single solution to explain dark matter, why it’s a majority matter universe, the expansion of the universe, and potentially inflation or lack thereof, all in one, still not grand unified theory, but one particle physics set of theories.
Fraser Cain:
I mean, that would be delightful, but come on.
Dr. Pamela Gay:
I can’t hope, a girl can’t hope.
Fraser Cain:
Right. You know, it’s going to be this horrible mishmash, inelegant collection of things that slowly, when you do the math, they all add up together to provide the explanation, but it’s a nightmare and, you know, and nobody deeply understands it and that’s just the universe is messier than we expected.
Dr. Pamela Gay:
And, and that is excellent too. And, and this is where, so, so I don’t know if you have one of these, but I have this Patreon Yeti mug that is my favorite thing and it says creativity over everything and like that describes science as well as art and, and this concept that you have to be massively, I’m not creative enough to be a particle physicist. It’s just that simple.
I am designed to be a photometrist. I have a good degree of creativity, not enough to be a particle physicist. And, and this, this is just the universe we live in and this is why we science.
We don’t understand everything. And so we keep exploring on the pursuit of it.
Fraser Cain:
Yeah. Like, you know, that feeling when you’re trying to solve a math equation and like there’s an answer in the back of the book and yet, and yet it’s not just a simple like punch in the numbers. Like it’s literally like prove this mathematical theorem.
And we did this a bit in, in my computer science mathematics class in linear algebra and we did some, some math proofs and it’s next level. Like normally you’re like factored this polynomial and you’re like, no problem. And then you’re like, prove this theorem.
And you’re like, eh, where do I even start?
Dr. Pamela Gay:
I loved that part.
Fraser Cain:
Can I disprove it? Yes. Okay, great.
Done.
Dr. Pamela Gay:
Yeah.
Fraser Cain:
Right. No, I can’t. Okay. So how do I, and so like things like, like there are other people who prove one plus one equals two and that, that there is this whole class of mathematics equations which are, which are literally unproven and it’s just this, I can’t even imagine.
My brain would nope out so hard trying to just blank, clean slate, solve a mathematical problem like this. And you’d, and you have these creative ideas. You’re like, oh, maybe I do this.
And then you spend the next three years of your life filling up one piece of paper with equations. You’re like, nope, that didn’t work. No, that just sounds like hell to me.
But yeah, but there are people out there who love it.
Dr. Pamela Gay:
So I, I particle physics, like kinematics, physics, that kind of like relativity. I’m down with all of that.
Fraser Cain:
Um, but just blue sky, no pure math. No, no, no. But mad respect.
Thanks, Pamela.
Dr. Pamela Gay:
Thank you, Fraser. And thank you so much to all the patrons out there who allow us to put the show together week after week. This show is made possible by our community on patreon.com slash astronomycast. This week, we’d like to thank the following $10 and up patrons, Alan Gross, Andrew Allen, Antisor, Aster Setz, Benjamin Carrier, Bob Krell, Brian Breed, Buzz Parsec, Conrad Howling, Daniel Schechter, David Green, Dr. Whoa, Ed, Fairchild, just as it sounds. Frederick Salvo, Jeff McDonald, Gold, Gregory Singleton, J Alex Anderson, Jason Kwong, Jeremy Kerwin, J O, John Herman, Jordan Turner, Kate Sindretto, Kim Barron, Lab Rat Matt, Les Howard, Mark, Masa Herleu, MHW1961, Super Symmetrical, Michael Regan, Naila, Noah Albertson, Paul Esposito, Peter, Red Bar Is Watching, Robert Plasma, Sachi Takeba, Scone, Sergey Manalov, Steven Rutley, TC Starboy, Thomas Gazzetta, Travis C. Porco, Vitaly, William Andrews.
Thank you all so very much.
Fraser Cain:
Thanks everyone. And we will see you next week.
Dr. Pamela Gay:
Bye bye everyone.
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The Earth Observer: Offering Perspectives from Space through Time
38 min read
The Earth Observer: Offering Perspectives from Space through TimeAn Intertwined History: The Earth Observer and EOS
The Earth Observer, a newsletter issued for nearly 37 years, will release its last online content at the close of 2025. This newsletter evolved in parallel with NASA’s Earth Observing System (EOS). It is almost impossible to speak of this newsletter without mentioning EOS. As The Earth Observer prepares its final publication, NASA also plans to shutter its three EOS flagship satellites (discussed below) possibly as early as the end of 2026.
While EOS was “much more than its satellites,” one cannot deny that the satellite missions and their iconic images provide an entry point to the overarching work conducted by the EOS science teams for almost three decades. These efforts spanned crucial complementary ground- and aircraft-based observations along with focused field campaigns to coordinate observations across multiple levels of Earth system time and spatial scales. The teams worked (and continue to work) closely with the NASA Earth Science Division Earth Observing System Data and Information System (EOSDIS) and related Science Investigator Processing System (SIPS) facilities, as well as developed and enhanced the algorithms that support the satellite products. Readers who wish to learn more about these topics should consult The Earth Observer’s archives page, which contains much of the history of this work.
During this point of inflection, The Earth Observer’s publication team felt it important to pause and reflect on the significance of the work detailed in the newsletter throughout this brief slip of time. The result is the article that follows.
A Flagship of an Idea: Almost Four Decades of Science
As described in the article, A Condensed History of the Earth Observing System (EOS) [June 1989, 1:3. 2–3], what would become known as EOS had its foundation in the recommendations of an ad hoc NASA study group that convened in 1981 to “determine what could and should be done to study integrated Earth science measurement needs.” Initially, the study group envisioned several large platforms in space, each with numerous instruments that would be serviced by the Space Shuttle, similar to servicing of the Hubble Telescope on several occasions. Known as System Z [Sept.–Oct. 2008, 20:5, 4–7], this early vision “laid the groundwork for a Mission to Planet Earth” but was reimagined after the tragic loss of the Space Shuttle Challenger in January 1986. An article written at the end of the Shuttle program included a sidebar that detailed the impracticality of launching shuttle missions into polar orbit to service EOS satellites, see Polar Shuttle Launches: The Path Almost Taken, [Sept.–Oct. 2011, 23:5, 6–7]. Eventually, the large space platform concept morphed into several mid-size flagship satellite missions, known today as Terra, Aqua, and Aura. Smaller satellite missions would supplement and enhance the data gathered by the “big three” satellites – see Figure 1.
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Technological advances further enhanced and refined this vision, allowing satellites to fly in close formation to capture near-simultaneous measurements in much the same way they would if they were on a single platform. The Afternoon Constellation, or A-Train, is a shining example of this international effort and is described in more detail below.
NASA released the first EOS Announcement of Opportunity in 1988, and a panel selected the winning proposals. An EOS Project Science Office was established to manage the projects. During this time of rapid development, NASA leadership was keenly aware of the need to keep the international EOS community abreast of the latest information. Enter The Earth Observer newsletter. First published in March 1989, the newsletter was the natural conduit to bridge this communication gap. To set the stage of how things have changed, an early article, titled Direct Transmissions of EOS Data to Worldwide Users [July–Aug. 1990, 2:6, 2–4], introduced the readership to the World Wide Web, which promoted “a ‘place’ where scientists communicate with each other and with the data they have collected with the help of their professional colleagues from the engineering and operations disciplines.”
In the more than 1000 printed pages published in the past three decades. The Earth Observer has chronicled the story of EOS and NASA’s broader Earth Science program. The publication has captured – often in meticulous detail – the intensive work behind the scenes that has gone into the development of the technologies, algorithms, and data centers that gather data from Earth observing satellites, suborbital observations, and other experiments to inform end users who use the data to address societal issues.
In the years before the first EOS missions launched, the newsletter reported in earnest on Investigator Working Group (IWG) meetings, Payload Panel Reviews (reviewing the instruments planned for the EOS platforms), and Mission and Instrument Science Team Meetings. As EOS matured, the newsletter began reporting on the development and implementation of specific science missions, launches, milestones, and research generated from the data collected. The editorial staff began publishing more feature articles to appear along with the meeting and workshop reports. The newsletter shared news stories developed by NASA’s Earth Science News Team and other bimonthly content (e.g., Education Update, Science in the News). “The Editor’s Corner” column in the newsletter gave the EOS Senior Project Scientist a platform to offer commentary on current events in NASA Earth Science as well as on the content of the current issue of the newsletter. While not formally named for the first few issues, an editorial article has been a cornerstone of the publication since the beginning.
The Earth Observer has produced several articles reflecting on its interwoven history with EOS, such as The Earth Observer: Twenty-Five Years Telling NASA’s Earth Science Story [March–April 2014, 26:2, 4–12] and A Thirtieth Anniversary Reflection from the Executive Editor {March–April 2019, 31:2, 4–6]. These stories expand upon the topics covered in the brief review presented in this article.
Satellite Missions: the Backbone of EOS Science
EOS was originally organized around 24 critical science measurements deemed integral to understand planetary processes and assess variability, long-term trends, and climate change. These science measurements serve as a roadmap for organizing EOS data products and mission objectives. The 24 measurements coalesced into five broad categories that reflect Earth science disciplines:
- Atmosphere: aerosol properties, cloud properties (e.g., fraction and opacity), atmospheric temperature and pressure profiles, water vapor, ozone (O3), trace gases [e.g., carbon dioxide (CO2), sulfur dioxide, and formaldehyde], and total solar irradiance;
- Ocean: ocean color (chlorophyll), sea surface temperature, sea ice cover and motion, ocean surface topography and sea level, and sea surface salinity;
- Land/Cryosphere: land surface temperature, soil moisture, snow and ice cover (extent and elevation), land cover and change (e.g., forest cover), and topography;
- Radiation/Energy Balance: radiant energy balance (incoming and outgoing radiation), and precipitation (e.g., rainfall, snow); and
- Solid Earth: static gravity field and synthetic aperture radar observations.
The Grand Vision of EOS: Three Flagships Leading the Earth Observing Fleet
In the late 1980s and early 1990s, a team of scientists envisaged the concept for two missions – EOS-AM1 and EOS-PM1. The synergy of this system was the ability to make observations in the morning (10:30 AM mean local time, or MLT), a time when cloud cover over the tropical equatorial and other land regions would be at a minimum, and afternoon (1:30 PM MLT), a time when continental convection would peak. The plan was to have two instruments – the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and Earth’s Radiant Energy System (CERES) – overlap on the two platforms along with other instruments unique to each mission.
In parallel, the teams envisioned EOS-CHEM1, a satellite platform identical to EOS-PM1 but carrying a payload focused on atmospheric chemistry. Like EOS-PM1, EOS-CHEM1 would be placed in an afternoon orbit but lag slightly in its equatorial crossing time (1:45 PM MLT) to optimize its position for atmospheric chemistry observations.
Each mission was slated to be the first in a series that would launch at five-year intervals to ensure continuity of critical Earth science measurements. Budgetary realities and technical advances eventually rendered plans for the second and third series of each satellite obsolete; however, all three flagship missions endured far beyond their planned six-year lifetime and have outlasted the originally proposed 15-year timeframe for each series.
Terra
Terra, originally named EOS-AM1, launched in December 1999 – see Figure 2. Terra carries five instruments – MODIS, CERES (two copies), Multiangle Imaging Spectroradiometer (MISR), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and Measurements of Pollution in the Troposphere (MOPITT) – and was designed to capture information about Earth’s atmosphere, carbon cycle and ecosystems, climate variability, water and energy cycle, weather, and the planet’s surface and interior. The Earth Observer captured early Terra data in the article, Terra Spacecraft Open For Business [March–April 2000, 12:2, 24].
After over 26 years in service, Terra remains in orbit and continues to gather data; as of this writing all instruments accept MOPITT remain active. However, since 2020 the spacecraft has been allowed to drift from its carefully maintained 10:30 AM MLT equator crossing time toward earlier MLT crossings. This was done to conserve enough fuel to control Terra’s eventual atmospheric reentry. The Terra team also conducted orbital lowering maneuver on the spacecraft in 2022. A more complete history of Terra is available in the online article, Terra: The End of An Era, published on December 29, 2025.
Figure 2. An artistic rendering of the Terra spacecraft. The image shows the locations of its five instruments. Note that there are two Clouds and Earth’s Radiant Energy System instruments aboard the satellite and one each of the other four instruments. Figure credit: NASA
Aqua
Aqua, originally named EOS-PM1, launched in May 2002 – see Figure 3. An article in The Earth Observer at the time of launch described the mission, Aqua is Launched! [March–April 2002, 14:2, 2]. The second EOS flagship carried six different instruments into orbit – Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit–A (AMSU-A1 and -A2), CERES (two copies), MODIS (both of which also fly on Terra), the Advanced Microwave Scanning Radiometer for EOS (AMSR–E), and Humidity Sounder for Brazil (HSB). Aqua’s mission focused on collecting data on global precipitation, evaporation, and the cycling of water. Aqua paired its data with Terra, offering the scientific community additional insights into the daily cycles for important scientific parameters to understand the global water cycle.
The Earth Observer article, Aqua: 10 Years After Launch [Nov.–Dec. 2012, 24:6, 4–17] provides an overview of the mission’s accomplishments during its first decade in orbit. Due to fuel limitations, Aqua completed the last of its drag makeup maneuvers in December 2021. Like Terra, the satellite is now in a free-drift mode, slowly descending below the A-Train orbit and crossing the equator later and at lower altitudes. A more recent newsletter article, Aqua Turns 20 [May–June 2022, 34:3, 5–12] reflects on Aqua’s accomplishments and legacy after two decades in orbit. As of this writing MODIS, CERES, AMSU, and CERES remain active.
Figure 3. An artistic rendering of NASA’s Aqua satellite. The mission collects data about the Earth’s water cycle, including evaporation from the oceans, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, land ice, and snow cover on the land and the ocean. Figure credit: NASAAura
Originally named EOS-CHEM1, Aura was the third and final flagship mission, and was launched in July 2004 – see Figure 4. The Earth Observer detailed the first post-launch science team meeting, Aura Science Team Meeting [March–April 2004, 17:2, 8–11]. Aura followed a Sun-synchronous, near-polar orbit, crossing the equator 15 minutes after Aqua. Similar to Aqua, Aura completed its final inclination adjustment maneuver in April 2023 to save its remaining fuel to allow for controlled reentry. As a consequence, the satellite has drifted out of the A-Train orbit, slowly continuing to move to a later equatorial crossing time and lower orbit altitude.
Aura’s payload included four instruments: the Microwave Limb Sounder (MLS), High Resolution Dynamics Limb Sounder (HIRDLS), Tropospheric Emission Spectrometer (TES), and Ozone Monitoring Instrument (OMI). These instruments gather information on trace gases and aerosols in the atmosphere. The key mission objectives aimed to monitor recovery of the stratospheric O3 hole, evaluate air quality, and monitor the role of the atmosphere in climate change. The article, Aura Celebrates Ten Years in Orbit [Nov.–Dec. 2014, 26:6, 4–16] detailed Aura’s first decade of accomplishments. The online article, Aura at 20 Years, published Sept. 16, 2024, reported on Aura’s status and achievements as it began its third decade of continuous operations. As of this writing MLS and OMI remain active.
Figure 4. An artistic rendering of the Aura satellite. Aura gathers information on trace gases and aerosols in the atmosphere. Figure credit: NASA
Building and Dismantling the “A-Train”
Between 2002 and 2014, a series of satellites joined the A-Train constellation – see Figure 5. This international effort includes the two EOS flagship satellites with afternoon equatorial crossing times (Aqua and Aura) as well as the Orbiting Carbon Observatory–2 (OCO-2), CloudSat, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). In addition, Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with observations from a Lidar (PARASOL) and Global Change Observation Missions with a focus on the water cycle (GCOM-W) are two international missions that became part of the A-Train constellation.
In the past decade, many of the satellites in the A-Train have either retired or have been allowed to drift out of the constellation. As of this writing, only two satellites – OCO-2 and GCOM-W1 – remain in their positions in the A-Train gathering data.
Three A-Train symposiums have been organized to bring the Earth science community together to discuss the achievements and future synergy of these missions. The outcome from each of these meetings were reported in The Earth Observer. The most recent of these was: The Third A-Train Symposium: Summary and Perspectives on a Decade of Constellation-Based Earth Observations [July–Aug. 2017, 29:4, 4–18].
Figure 5. An artistic depiction of all the satellites that participated in the Afternoon Constellation (A-Train), except for Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with observations from a Lidar (PARASOL). CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lowered their orbits. Called the C-Train, the orbit of these satellites overlapped the A-Train, enabling science observations with other A-Train missions. More details about the A-train is available on the constellation’s website. Figure credit: NASA
Science from the EOS Fleet
The next several sections provide a highlight of science from key missions outside of Terra, Aqua, and Aura. The content has been organized in terms of measurements – with an overarching focus on water (oceans and fresh water), atmosphere, and land. This summary is far from exhaustive. A record of much of the amazing science conducted during these missions is detailed in the archives of The Earth Observer.
Interpreting an Ocean of Data
When viewed from space, Earth has been described as a “blue marble.” The planet’s abundance of liquid water is found in the oceans, and while not potable, the oceans play a critical role in regulating Earth’s climate. Satellites provide an unparalleled way to study the global ocean. With each new mission, the process of data collection has been refined and improved. The scientific community can now measure ocean color as a proxy for surface productivity as well as measure subtle changes in surface ocean salinity. These data have improved weather and climate models to increase the accuracy of storm projection and help the scientific community better understand the movement of energy around the planet.
Aqua was the flagship mission dedicated to studying water on Earth, but other missions have contributed and expanded on this data record. For example, Japan’s GCOM-W1 mission, also known as SHIZUKU (Japanese for droplet), continues to gather information on precipitation, water vapor, wind velocity above the ocean, sea water temperature, water levels on land, and snow depths. These data support weather models to improve forecasts to monitor tropical cyclones. The subsections that follow provide examples of how data from these satellites support different science objectives, as well as examples of the science deciphered by both flagship and ancillary platforms within the A-Train. All of these missions and science have been covered in The Earth Observer over the past several decades.
Discerning the Ocean’s True Colors
Ocean color data are crucial for studying the primary productivity and biogeochemistry of the oceans. The Coastal Zone Color Scanner (CZCS), launched on the Nimbus 7 satellite in 1978 and ceasing operations in 1986 – gave the earliest perspective of the oceans from space. SeaWiFS, which served as a follow-on to CZCS, was launched on the privately owned Seastar spacecraft on Aug. 1, 1997 to produce ocean color data and offered a synoptic look at the global biosphere. This mission was a data-buy, where NASA purchased the data from Orbital Imaging Corporation. An article in The Earth Observer, titled Sea-viewing Wide Field-of-view Sensor [March–April 1998, 10:2, 20–22] detailed how the satellite gathered chlorophyll-a data that was calibrated to field measurements from a Marine Optical Buoy. The research community have used this information to understand primary productivity in the surface ocean and global biogeochemistry. This data offered an early assessment of the role of the ocean in the global carbon cycle. It also produced one of the first global perspectives of the impact of El Niño and La Nina events around the world. Coastal and fishery managers have used this data to improve the health of these important ecosystems. Launched for a five-year mission, SeaWiFs gathered data until December 2010.
More recently, NASA launched the Plankton, Aerosol, Cloud ocean Ecosystem (PACE) satellite in February 2024 to gather data on ocean and terrestrial ecosystem productivity – see Figure 6. While other missions studied ocean color in the interim between SeaWiFS and PACE (e.g., MODIS on Terra and Aqua), PACE offers an exponential leap forward with its three-instrument payload that includes: the Ocean Color Instrument (OCI), Hyper-Angular Rainbow Polarimeter–2 (HARP2), and Spectropolarimeter for Planetary Exploration (SPEXone). The PACE mission aims to clarify how the ocean and atmosphere exchange CO2, a key factor in understanding the evolution of Earth’s climate system. The satellite also examines the role of aerosols in providing micronutrients that fuel phytoplankton growth in the surface ocean. The data gathered extends the aerosol and ocean biological, ecological, and biogeochemical records that were initiated by other satellites. The Dec. 29, 2025 article, Keeping Up with PACE: Summary of the 2025 PAC3 Meeting, reports on three recent meetings related to the mission.
Figure 6. An artistic rendering of the Plankton, Aerosol, Cloud ocean Ecosystem (PACE) observatory and the instrument panels that it carries. PACE focuses on clarifying how the ocean and atmosphere exchange carbon dioxide. Figure credit: NASA
Mapping the Ocean Surface to Reveal the Rising Seas
The Ocean Surface Topography (TOPEX)/Poseidon mission, launched on Aug. 10, 1992, was the first in a series of missions that have measured ocean surface topography, or the variations in sea surface height. This record now extends more than 30 years. TOPEX/Poseidon spent more than 13 years in orbit. The data gathered helped to improve the scientific community’s understanding of ocean circulation and its impact on global climate – including sea level rise. TOPEX/Poseidon produced the first global views of seasonal current changes, which allowed scientists to forecast and better understand El Niño events. These early efforts to distribute data were captured in The Earth Observer article, Jet Propulsion Laboratory DAAC Begins TOPEX Data Distribution [March–April 1993, 6:2, 24].
Jason followed TOPEX/Poseidon to continue the measure of sea level as well as wind speed and wave height for more than 95% of Earth’s ice-free ocean – see Figure 7. Jason consists of a series of satellites, with Jason-1, launched in 2001, remaining in orbit for 11 years. It was followed by Jason-2, also called the Ocean Surface Topography Mission (OSTM), which was launched in 2008. Jason-2 gathered data for 11 years. Jason-3 launched in January 2016 and remains in orbit, continuing the sea level dataset. The Earth Observer has reported on meetings of the Ocean Surface Topography Science Team over the years. The online article, Summary of the 2023 Ocean Surface Topography Team Meeting, was published May 31, 2024 and includes the most recent updates available.
Figure 7. Beginning with TOPEX/Poseidon in 1992, a series of ocean surface topography missions have maintained a continuous record of global sea surface height data with the best possible accuracy along the same exact ground track. Dubbed the “reference” altimetry missions, shown here are TOPEX/Poseidon, Jason-1, and the Ocean Surface Topography Mission/Jason-2 (OSTM/Jason-2) in the tandem orbit pattern. This is used to cross-calibrate each mission to the next. By flying in formation, just one minute apart for a period of several months, scientists can be sure that each successive mission is exactly calibrated to its predecessor. Connecting each record to the next, these reference missions have built a record of sea level that stretches more than 30 years with centimeter level accuracy for every corner of the ocean. The reference mission has now been taken over by, Sentinel 6 Michael Freilich, which will hand the baton to the recently launched Sentinel 6B sometime in 2026. Figure credit: NASA/JPL/CNES
The international partnership between the United States [NASA and the National Oceanic and Atmospheric Administration (NOAA)], the European Space Agency (ESA), and the French Space Agency [Centre National d’Études Spatiales (CNES)] collaborate to create the ESA’s Copernicus Sentinel–6 missions. The Sentinel-6B, launched Nov. 16, 2025, will follow the path of the Sentinel-6 Michael Freilich (originally called Sentinel–6A) satellite, which has been in orbit for five years – see Figure 8. These two Sentinel 6 missions continue the global measurements of sea level, wind speed, wave height, and atmospheric temperature. The data will be used in marine weather forecasts as well as to improve commercial and naval navigation, search and rescue missions, and tracking garbage and pollutants in the ocean. To learn more about Sentinel-6B, see the online article, Sentinel-6B Extends Global Ocean Height Record, published Dec. 22, 2025.
While the Surface Water and Ocean Topography (SWOT) mission is fully described in the next section – with emphasis placed on its novel surface water observation capabilities – it should be noted that SWOT is also an ocean topography mission that obtains data similar to TOPEX/Poseidon, Jason, and Sentinel-6 missions. These data will contribute to the long-term time series of the sea surface height record.
Figure 8. Sentinel-6B, an Earth-observing satellite jointly developed by NASA and U.S. and European partners, will observe the ocean and measure sea level rise to provide insights into our home planet that will improve weather forecasts and flood predictions, increase public safety, and protect coastal infrastructure. The Sentinel missions are part of the European Space Agency’s Copernicus Programme. Figure credit: NASA
Sampling the Salty Seas
Launched June 2011, Aquarius was an international collaboration between NASA and Argentina’s Comisión Nacional de Actividades Espaciales (CONAE). The cooperative effort was detailed in the article, Aquarius: A Brief (Recent) History of an International Effort [July–Aug. 2010, 22:4, 4–5]. The satellite carried a microwave radiometer that was sensitive enough to measure salinity to an accuracy of 0.2 practical salinity units (psu) on a monthly basis. It also carried a scatterometer to measure surface ocean roughness. Pairing data from the two instruments allowed the team to overcome the challenges of measuring salinity from space. This feat is detailed in the article, For Aquarius, Sampling Seas No ‘Grain of Salt’ Task [July–Aug. 2011, 23:4, 42–43]. The more accurate, global measurements of ocean salinity that Aquarius obtained have helped the research community better understand ocean circulation. The mission ended in 2015, after the satellite experienced a power failure.
Focusing on Freshwater
While most water on the planet is housed in the ocean, fresh water is a primary concern for life on the planet. Fresh water accounts for ~3% of the total amount water on the planet. Of that small amount, a significant portion is locked in ice on land and as sea ice. The remaining water flows on Earth’s surface and underground. Maintaining a supply of fresh water is critically important to our survival. The location, status, and purity of this precious resource continues to be an on-going focus for many of the missions.
Monitoring Rain and Snow
The joint NASA/National Space Development Agency of Japan (NASDA – which is now known as the Japan Aerospace Exploration Agency, or JAXA) Tropical Rainfall Measuring Mission (TRMM) carried a Microwave Imager, Visible Infrared Scanner, and Precipitation Radar to gather tropical and subtropical rainfall observations (and two related instruments) – see Figure 9. These data filled a critical knowledge gap – to understand the interactions between the sea, air, and land. Over the years, these data were incorporated into numerous computer models to clarify the role of tropical rainfall on global circulation and formed the basis for experimental quasi-global merged satellite precipitation products. The Earth Observer detailed the early data collection in the article titled TRMMing the Uncertainties: Preliminary Data from the Tropical Rainfall Measuring Mission [May–June 1998, 10:3, 48–50]. The mission was extended twice but eventually the satellite’s maneuvering fuel was exhausted, resulting in a slow decline in the orbital altitude beginning in 2014, with reentry in 2015. Data from TRMM have improved understanding of storm structure of cloud systems, produced reliable global latent heating estimates to improve water transfer estimates within the atmosphere, and continue to be used in calibrating modern precipitation products for the TRMM era.
Figure 9. Artistic rendering of the Tropical Rainfall Measuring Mission (TRMM) in space over a hurricane. TRMM was launched in 1997 and remained in operation until 2015. The satellite was designed to improve our understanding of the distribution and variability of precipitation within the tropics as part of the water cycle in the current climate system. Figure credit: NASA
To continue the efforts that began with TRMM – and extend coverage to most of the globe – NASA and JAXA launched the Global Precipitation Measurement (GPM) mission in 2014. This satellite aims to advance our understanding of water and energy cycles, improve forecasting of extreme weather events, and extend current capabilities to use accurate and timely information of precipitation to directly benefit society. The Earth Observer detailed the accomplishments of this mission in the online article, GPM Celebrates Ten Years of Observing Precipitation for Science and Society, published Oct. 3, 2024.
Surveying Earth’s Surface Water
Introduced briefly in the previous section, the SWOT mission is a joint venture between the United States and France. Launched in December 2022, SWOT is conducting the first global survey of Earth’s surface water – see Photo. The mission was introduced to the EOS community in The Earth Observer article, Summary of the 2022 Ocean Surface Topography Science Team Meeting [May–June 2023, 35:3, 19–23]. SWOT carries the Ka-band Radar Interferometer (KaRIN) – the first spaceborne, wide-swath, altimetry instrument capable of high-resolution measurements of sea surface height in the ocean and freshwater bodies. SWOT covers most of the world’s ocean and freshwater bodies with repeated high-resolution elevation measurements. This data have been applied to monitor rivers across the Amazon basin, simulate land/hydrology processes, and predict streamflow. A more comprehensive overview of SWOT applications is detailed in online article, Summary of the 10th SWOT Applications Workshop, published Sept. 20, 2024.
Photo 1. Workers in a clean room in Cannes, France, load the Surface Water and Ocean Topography (SWOT) satellite into a container in preparation for shipping the spacecraft to the United States. SWOT provides the first global survey of Earth’s surface water. Photo credit: Centre National d’Études Spatiales (CNES), Thales Alenia Space
Gracefully Tracking Water Movement
The twin GRACE satellites were launched on March 17, 2002. The mission, a partnership between NASA and the German GeoForschungsZentrum (GFZ) Helmholtz Centre for Geosciences was developed to measure Earth’s shifting masses – most of which comes from water – and map the planet’s gravitational field using a K-band microwave ranging system and accelerometers. Some early results of the satellites appeared in The Editor’s Corner column [Nov.–Dec. 2002, 14:6, 1–2]. GRACE enabled groundbreaking insights into Earth’s evolving water cycle as the satellites tracked monthly mass variations in ice sheets and glaciers, near-surface and underground water storage, the amount of water in large lakes and rivers, as well as changes in sea level and ocean currents.
GRACE’s mission was extended with the GRACE-Follow On (GRACE-FO) mission launched in 2018 – see Figure 10. GRACE-FO continues comprehensive tracking water movement across the planet, including groundwater measurements that have important applications for everyday life. The most recent developments of the GRACE-FO science meeting was detailed in an online article, Summary of the 2023 GRACE Follow-On Science Team Meeting, published March 30, 2024 – and also published in the final print issue [Jan.–Feb. 2024, 35:7, 19–26]. The data gathered during the GRACE-FO mission details large-scale changes in Earth’s groundwater reservoirs, Greenland and Antarctica’s sensitivity to warming ocean waters, and even subtle shifts deep in Earth’s interior that reveal how large earthquakes can develop.
In 2028, NASA will move into a third-generation of gravity observations with the launch of GRACE-Continuity, or GRACE-C, which will further expand the foundational observations of global mass change and expand the societal and economic applications that have been created from these data.
Figure 10. An artistic rendering of the twin Gravity Recovery and Climate Experiment-Follow-On (GRACE-FO) satellites that, like the original GRACE twins, follow each other in orbit, separated by about 137 miles (220 km). GRACE tracks water movement across the planet’s surface. Figure credit: NASA
Assessing the Atmosphere from Above
Earth has a unique atmospheric makeup that maintains a stable temperature allowing life to thrive. As far as we know, our atmosphere is unique in the universe. Satellites provide an unparalleled perspective to study variability in the column of air extending from Earth’s surface. While Aura has a suite of instruments making a wide range of atmospheric chemistry measurements, other missions also measure the abundance and impact of atmospheric constituents that, while often invisible to the unaided eye, can have profound impacts on Earth’s air quality and climate. These data have also improved climate models and help the scientific community better understand how energy is emitted into space.
Tracking Tiny Particles with Big Impacts
France’s PARASOL mission was an original member of the international A-Train constellation. Launched in 2004. PARASOL sought to capture the radiative and microphysical properties of clouds and tiny atmospheric aerosol particles using a unique multiangle imaging POLDER polarimeter.
NASA’s Glory mission was intended for operation in the A-Train; it carried a multiangle polarimeter as its instrument. Unfortunately, the spacecraft failed to separate from the Taurus rocket due to a fairing separation failure during its launch in 2011. As a result, POLDER on PARASOL was the only atmospheric polarimeter to fly in space until two (SPEXone and HARP2) launched as part of NASA’s PACE mission. Researchers gathered information from POLDER and other A-Train instruments about how aerosols affect the formation of precipitations and clouds, the movement of water around the planet, and the reflection and absorption of radiative energy that impact overall planetary climate. PARASOL was deactivated in 2013 after nine years in service.
Cloud particles form when water vapor nucleates onto aerosols; changes in one can impact the other. After many years and conversations, it was decided to pair two NASA Earth System Science Pathfinder (ESSP) missions – CloudSat and CALIPSO – and fly them in coordination with each other and with other A-Train satellites. By combining the two datasets, it was possible to explore cloud and aerosol processes. This information helped the community drill into the larger climate questions. The two satellites were launched on the same Delta-II rocket from Vandenberg Air Force Base in California on April 28, 2006. CloudSat used a 94 GHz cloud profiling radar that is 1000 times more sensitive than a typical weather radar, capable of distinguishing between cloud particles and precipitation. CALIPSO contained a Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), Wide-Field Camera, and Imaging Infrared Radiometer to detect and distinguish between aerosol particles and cloud particles.
The Earth Observer captured the early data collection of the two satellites in the article, CloudSat and CALIPSO: A Long Journey to Launch…But What a Year It’s Been!! [May–June 2007, 19:3, 7–12]. The later article, A Useful Pursuit of Shadows: CloudSat and CALIPSO Celebrate Ten Years of Observing Clouds and Aerosols [July–Aug. 2016, 28:4, 4–12] provided a review of the accomplishments of the missions after 10 years in orbit. CALIPSO and CloudSat were both deactivated in 2023 after 17 years of service.
An Oracle of High-Altitude Wisdom
The Stratospheric Aerosol and Gas Experiment (SAGE) has experienced several iterations, extending back nearly half a century. The initial SAGE mission launched on Feb. 18, 1979, aboard the Applications Explorer Mission-B (AEM-B) to measure vertical distribution of aerosols and important gases in the upper troposphere and stratosphere (UTS). The satellite failed after three years in orbit. In 1984, SAGE II began collecting data on stratospheric O3, producing a stable record of this important greenhouse gas from 1984–2005. SAGE III was launched on Метеор-3М (SAGE III/M3M). The third-generation satellite produced an accurate measurement of the vertical structure of aerosols, O3, water vapor, and other important trace gases in the upper troposphere and stratosphere. The satellite was terminated on March 6, 2006, following a power supply system failure, resulting in loss of communication with the satellite.
Another version of SAGE III was launched to the International Space Station (ISS) on Feb. 19, 2017, where it was installed on the EXpedite the PRocessing of Experiments to Space Station (ExPRESS) Logistics Carrier [ELC-4] – an unpressurized attached payload platform for ISS. SAGE III/ISS, which is shown mounted on ELC-4 in Figure 11, has completed its prime mission after three years of operation. NASA granted approval to extend the SAGE III/ISS mission through at least 2026 – meaning the instrument will continue to provide the public and science community with world-class vertical profiles of O3, aerosol, water vapor, and other trace gases, e.g., nitrogen dioxide (NO2) and nitrate (NO3), data products for at least another year. An article titled, Summary of the 2024 SAGE III/ISS Meeting, published May 26, 2025, details the latest findings from SAGE.
Figure 11. An artistic rendering of the Stratospheric Aerosol and Gas Experiment-III (SAGE-III), which is externally mounted on the International Space Station’s Japanese Experiment Module–Exposed Facility (JEM-EF) EXPRESS Logistics Carrier (ELC)-4. SAGE III/ISS measures the vertical structure of aerosols, ozone (O3), water vapor, and other important trace gasses in the upper troposphere and stratosphere. Figure credit: NASA
Watching Earth Exhale
The Orbiting Carbon Observatory (OCO) was launched into space in February 2009, but it failed to separate from the Taurus rocket during its ascent, leading to mission failure and loss of the satellite. Undaunted, the EOS community began again and assembled OCO-2, which was successfully launched into orbit, joining the A-Train on July 2, 2014 – see Figure 12. The satellite’s mission focused on making precise, high-resolution measurements of atmospheric CO2. OCO-2 measures reflected sunlight that interacts with the atmosphere. Using diffraction gratings to separate the reflected sunlight into spectra, OCO-2 measures the absorption levels for the different molecular bands to calculate CO2 concentration. This information is invaluable for the quantification of CO2 emissions and can characterize both sources and sinks of this critical greenhouse gas. The mission was detailed in an article, titled Orbiting Carbon Observatory-2: Observing CO2 from Space [July–Aug. 2014, 26:4, 4–12].
On May 4, 2019, NASA launched the third iteration in the OCO group to the ISS. It was subsequently installed on the Japanese Experiment Module–Exposed Facility (JEM-EF). Constructed from parts left over from OCO-2, OCO-3 continues the mission of making CO2 measurements with a focus on daily variability. In particular, the measurements explore the role of plants and trees in the major tropical rain forests of South America, Africa, and Southeast Asia. As of today, both OCO-2 and OCO–3 remain operational and gathering data.
The science team reflected on both these missions in a recent article posted in the online article, A Tapestry of Tales: 10th Anniversary Reflections from NASA’S OCO-2 Mission, published Aug. 12, 2025.
Figure 12. An artistic rendering of OCO-2 in orbit above Earth. OCO-2 measures the concentration of trace gases in the atmosphere. Figure credit: NASA/JPL-Caltech
Tracking the Sun’s Output
In December 1999, NASA launched the Active Cavity Radiometer Irradiance Monitor Satellite (ACRIMSAT) satellite to extend the more than two-decade record of total solar irradiance (TSI). Scientists use this important measurement to quantify the solar energy input to the planet and thereby its interactions with Earth’s oceans, land masses, and atmosphere. It is also a critical component to understand variations of the planet’s climate. The Active Cavity Radiometer Irradiance Monitor 3 (ACRIM3) instrument onboard combined the best features of the ACRIM I (flown on the Solar Maximum Mission), ACRIM II (flown on the Upper Atmosphere Research Satellite), and SpaceLab-1 ACRIM (flown on Space Shuttle Columbia, STS 9). ACRIM3 improved on its predecessors by incorporating a new electronics and package design. The Earth Observer captured the initial information from this mission in the article, The ACRIMSAT/ACRIM3 Experiment — Extending the Precision, Long-Term Total Solar Irradiance Climate Database [May–June 2001, 13:3, 14–17]. ACRIMSAT spent 14 years in orbit and ACRIM3 extended the TSI record to 36 years (i.e., building on measurements from previous ACRIM missions).
NASA continued its quest to observe the incident solar energy budget with the launch of the Solar Radiation and Climate Experiment (SORCE) in January 2003. SORCE focused on measuring solar radiation incident to the top of the Earth’s atmosphere. The Total Irradiance Monitor (TIM) onboard continued the TSI record that the ACRIM series of satellites established. In addition to TIM, the satellite carried a Spectral Irradiance Monitor (SIM), an Extreme Ultraviolet (XUV) Photometer System [XPS], and a stellar observation from the Solar Stellar Irradiance Comparison Experiment (SOLSTICE). The satellite has produced groundbreaking TSI and spectral solar irradiance (SSI) measurements – two key inputs for atmosphere and climate modeling.
Early results from SORCE are detailed in the article, The SORCE (SOlar Radiation and Climate Experiment) Satellite Successfully Launched [Jan.–Feb. 2003, 15:1, 16–19]. The article, The SORCE Mission Celebrates 10 Years [Jan.–Feb. 2013, 25:1, 3–13] details the most significant results from a decade of SORCE observations. Designed for a five-year mission, SORCE gathered data until 2020 – although a degradation of a battery power that began in 2008 increasingly hindered data collection for the remainder of the mission. During its time in orbit, SORCE captured two of the Sun’s 11-year solar cycles and observed the solar cycle minimum in both 2008 and 2019. SORCE’s orbit will decay and re-enter Earth’s atmosphere in 2032.
To continue the crucial long-term TSI and the SSI record that SORCE originated, NASA launched the Total and Spectral Solar Irradiance Sensor (TSIS-1) to the ISS on Dec. 15, 2017, which was installed on JEM-EF ELC-3. The satellite’s mission set out to measure the total amount of sunlight that falls on the planet’s surface – see Visualization 1. This data will clarify the distribution of different wavelengths of light. TSIS-1 was introduced in The Earth Observer article, Summary of the 2018 Sun–Climate Symposium [May–June 2018, 30:3, 21–27]. Similar to SORCE, TSIS-1 carries a TIM and SIM. The instrument extends the multidecadal SSI record and provides highly accurate, stable, and continuous observations that are critical to understanding the present climate conditions and predicting future conditions. The most recent efforts from this mission were detailed in the online article, Summary of the 2023 Sun–Climate Symposium, published July 18, 2024. TSIS-1 has been extended by at least three more years as part of the Earth Sciences Senior Review process. A follow-on mission, TSIS-2, is under development to extend the long-term observational record through continued TSI and SSI measurements.
Visualization 1. NASA’s Total and Spectral solar Irradiance Sensor (TSIS-1) measures the total amount of solar energy input to Earth as well as the distribution of the Sun’s energy across a wide range of wavelengths. The animation illustrates the various wavelengths of light that are partially reflected into space at different places in the column of atmosphere above the ground.Visualization credit: NASA
Chronicling the Changing Land Surface
Along with Terra, other satellites also provide global estimates about the land. Each new mission provides the scientific community more information to refine these measurements. These data have improved climate models as well as improved our understanding of how the planet’s interior is altering the surface of the planet.
Measuring Ice and Vegetation Heights
NASA launched ICESat in 2003 on a three-to-five-year mission to provide information on ice sheet mass balance and cloud properties. It carried the Geoscience Laser Altimeter System (GLAS), which combines a precision surface lidar with a sensitive dual-wavelength cloud and aerosol lidar. ICESat was decommissioned seven years after launch. The science team began efforts for the follow-on mission, ICESat-2, which launched on Sept. 15, 2018 – see Figure 13. Data collected during a series of Operation IceBridge field campaigns to the Arctic and Antarctic helped to fill the data gap between the two satellite missions – allowing for continuity of measurements. ICESat-2 carries a payload of a photon-counting laser altimeter on its three-year mission. The laser is split into six beams capable of measuring the elevation of the cryosphere, including ice sheets, glaciers, and sea ice, down to a fraction of an inch. The laser altimeter also gathers the height of ocean and land surfaces, including forests, snow, lakes, rivers, ocean waves, and urban areas. The mission objective includes quantifying polar ice sheet contribution to sea-level change, estimating sea-ice thickness, and measuring vegetation canopy height. The mission was detailed in The Earth Observer article, ICESat-2: Measuring the Height of Ice from Space [Sept.–Oct.. 2018, 30:5, 4–10]. The research community has been using this information to investigate how the ice sheets of Antarctica and Greenland are changing as the planet warms.
Figure 13. Illustration of the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) spacecraft. ICESat-2 measures the elevation of aspects of the cryosphere, including ice sheets, glaciers, and sea ice. Figure credit: NASA
NASA’s Global Ecosystem Dynamics Investigation (GEDI – pronounced “jedi”) mission was launched to the ISS on Dec. 5, 2018 and was subsequently installed on the JEM–EF ELC-6. From that vantage point GEDI produces high-resolution laser ranging observations of the three-dimensional (3D) structure of Earth that can be used to make precise measurements of forest canopy height and canopy vertical structure – see Visualization 2. These measurements have improved understanding of important atmospheric and water cycling processes, biodiversity, and habitat. Upon completion of its prime mission, which lasted from December 2018 to March 2023, GEDI was moved from the ISS’s EFU-6 to EFU-7 (storage). Since April 2024, the GEDI instrument has been back in its original location on EFU-6 and continues to collect high-resolution observations of Earth’s 3D structure from space. The GEDI research team hopes the mission can continue collecting data until 2030.
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The GEDI mission has been covered in The Earth Observer through summaries of periodic meetings of the GEDI Science Team. The online article, Summary of the 2025 GEDI Science Team Meeting, is the most recent installment of GEDI’s progress, published on Aug. 18, 2025. This article includes discussion of “the return of the GEDI” from hibernation and the science results since then.
Monitoring Earth in Intricate Detail
The Soil Moisture Active Passive (SMAP) mission was designed to measure the amount of water in surface soil across Earth. The satellite was launched from Vandenberg Air Force Base on Jan. 31, 2015. The satellite payload consisted of both an active microwave radar and a passive microwave radiometer to measure a swath of the planet 1000-km (~621-mi) wide. The radar transmitter failed just nine months after launch on July 7, 2015. Although the loss of the radar was unfortunate, the nine months where both instruments functioned provided an invaluable dataset that established the dependence of L-band radar signals on soil moisture, vegetative water content, and freeze–thaw state. Two of these variables (surface soil moisture and freeze–thaw state) are critical variables that influence the planet’s water, energy, and carbon cycles. The three variables influence weather and climate. Furthermore, the SMAP team quickly turned a setback into a success. They repurposed the channels that had been dedicated to the radar to record the reflected signals from the Global Navigation Satellite System (GNSS) constellation in August 2015, making SMAP the first full-polarimetric GNSS reflectometer in space for the investigation of land surface and cryosphere.
The Earth Observer article, SMAP: Mapping Soil Moisture and Freeze/Thaw State from Space [Jan.–Feb. 2015, 27:1, 14–19] offered a preview of SMAP that was published shortly after its launch. A more recent online article, A Decade of Global Water Cycle Monitoring: The Soil Moisture Active Passive Mission, published Aug. 18, 2025, reflects on the achievements of SMAP after a decade of operations.
More specific to vegetation water content, NASA launched the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) to ISS on June 29, 2018. It was subsequently installed on the JEM–EF ELC 10, placing it in close proximity to GEDI (installed on ELC 6) and enabling combined observations. While GEDI focuses on the canopy height and related characteristics, ECOSTRESS monitors the combined evaporation and transpiration of living plants – known as evapotranspiration (ET). ECOSTRESS determines ET indirectly through measurements of the thermal infrared brightness temperatures of plants.
As with GEDI, The Earth Observer has reported on the activities of the ECOSTRESS mission. The most recent coverage was in the article, ECOSTRESS 2019 Workshop Summary: Science, Applications, and Hands-On Training [July–Aug. 2018, 31:4, 15–18.]
Last, but certainly not least, the most recent Earth observing satellite to launch is a joint venture between NASA and the Indian Space Research Organization (ISRO). The NASA-ISRO Synthetic Aperture Radar (NISAR) took to the skies on July 30, 2025, from the Satish Dhawan Space Centre on India’s southeastern coast aboard an ISRO Geosynchronous Satellite Launch Vehicle (GSLV) rocket 5. The mission was designed to observe and measure some of the planet’s most complex processes – see Figure 14. The launch was lauded in the Editor’s Corner published online on Sept. 10, 2025.
NISAR uses two different radar frequencies – L-band and S-band synthetic aperture radar (SAR). The dual system can penetrate clouds and forest canopies to allow researchers to measure changes on the planet’s surface, down to a centimeter (~0.4 in). This level of detail allows the research community to investigate ecosystem disturbances, ice-sheet collapse, natural hazards, sea level rise, and groundwater issues. The satellite will also capture changes in forest and wetland ecosystems. It will expand on our understanding of deformation of the planetary crust that can help predict earthquakes, landslides, and volcanic activity. All of this data will help mitigate damage from a disaster and help communities prepare a disaster response. Some early results from the both NISAR radars are discussed in the Final Editor’s Corner column, published online on Dec. 29, 2025.
Figure 14. The NASA-ISRO Synthetic Aperture Radar (NISAR) Synthetic Aperture Radar can observer Earth’s land and ice with unmatched precision, offering real-time insights into earthquakes, floods, and climate shifts. Figure credit: NASA/Jet Propulsion Laboratory–Caltech
Conclusion
Over the past 36 years, The Earth Observer has borne witness to some of the most monumental scientific achievements of NASA Earth Science and chronicled those stories for the community. While the format of the publication evolved considerably over the years, the satellite missions that have been the focus of this article are one of the primary “lenses” that the newsletter has had to observe and reflect on the story of NASA Earth Science. These continuous global observations have revolutionized society’s knowledge of our home planet and how humans might be altering it.
The staff of The Earth Observer have navigated many different modes of communication over the past three-and-a-half decades, but the commitment to delivering high-quality content has remained constant. It has been the highest honor of every member of our publication team – past and present – to work on this material. While the newsletter is coming to an end, it is hoped that the Archives page continues to be a rich source of historic information about NASA’s EOS and Earth science over the past three and a half decades.
On behalf of the current Editorial Team, we, the authors of this reflection, wish to thank every person who has contributed to the success of this newsletter over the years – and to extend to all in the NASA Earth Science community best wishes for the year ahead and continued success in your remote observation endeavors.
Stacy Kish
NASA’s Goddard Space Flight Center/EarthSpin
stacykishwrites@gmail.com
Alan B. Ward
NASA’s Goddard Space Flight Center/Global Science &Technology Inc.
alan.b.ward@nasa.gov
The Earth Observer: Offering Perspectives from Space through Time
38 min read
The Earth Observer: Offering Perspectives from Space through TimeAn Intertwined History: The Earth Observer and EOS
The Earth Observer, a newsletter issued for nearly 37 years, will release its last online content at the close of 2025. This newsletter evolved in parallel with NASA’s Earth Observing System (EOS). It is almost impossible to speak of this newsletter without mentioning EOS. As The Earth Observer prepares its final publication, NASA also plans to shutter its three EOS flagship satellites (discussed below) possibly as early as the end of 2026.
While EOS was “much more than its satellites,” one cannot deny that the satellite missions and their iconic images provide an entry point to the overarching work conducted by the EOS science teams for almost three decades. These efforts spanned crucial complementary ground- and aircraft-based observations along with focused field campaigns to coordinate observations across multiple levels of Earth system time and spatial scales. The teams worked (and continue to work) closely with the NASA Earth Science Division Earth Observing System Data and Information System (EOSDIS) and related Science Investigator Processing System (SIPS) facilities, as well as developed and enhanced the algorithms that support the satellite products. Readers who wish to learn more about these topics should consult The Earth Observer’s archives page, which contains much of the history of this work.
During this point of inflection, The Earth Observer’s publication team felt it important to pause and reflect on the significance of the work detailed in the newsletter throughout this brief slip of time. The result is the article that follows.
A Flagship of an Idea: Almost Four Decades of Science
As described in the article, A Condensed History of the Earth Observing System (EOS) [June 1989, 1:3. 2–3], what would become known as EOS had its foundation in the recommendations of an ad hoc NASA study group that convened in 1981 to “determine what could and should be done to study integrated Earth science measurement needs.” Initially, the study group envisioned several large platforms in space, each with numerous instruments that would be serviced by the Space Shuttle, similar to servicing of the Hubble Telescope on several occasions. Known as System Z [Sept.–Oct. 2008, 20:5, 4–7], this early vision “laid the groundwork for a Mission to Planet Earth” but was reimagined after the tragic loss of the Space Shuttle Challenger in January 1986. An article written at the end of the Shuttle program included a sidebar that detailed the impracticality of launching shuttle missions into polar orbit to service EOS satellites, see Polar Shuttle Launches: The Path Almost Taken, [Sept.–Oct. 2011, 23:5, 6–7]. Eventually, the large space platform concept morphed into several mid-size flagship satellite missions, known today as Terra, Aqua, and Aura. Smaller satellite missions would supplement and enhance the data gathered by the “big three” satellites – see Figure 1.
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Technological advances further enhanced and refined this vision, allowing satellites to fly in close formation to capture near-simultaneous measurements in much the same way they would if they were on a single platform. The Afternoon Constellation, or A-Train, is a shining example of this international effort and is described in more detail below.
NASA released the first EOS Announcement of Opportunity in 1988, and a panel selected the winning proposals. An EOS Project Science Office was established to manage the projects. During this time of rapid development, NASA leadership was keenly aware of the need to keep the international EOS community abreast of the latest information. Enter The Earth Observer newsletter. First published in March 1989, the newsletter was the natural conduit to bridge this communication gap. To set the stage of how things have changed, an early article, titled Direct Transmissions of EOS Data to Worldwide Users [July–Aug. 1990, 2:6, 2–4], introduced the readership to the World Wide Web, which promoted “a ‘place’ where scientists communicate with each other and with the data they have collected with the help of their professional colleagues from the engineering and operations disciplines.”
In the more than 1000 printed pages published in the past three decades. The Earth Observer has chronicled the story of EOS and NASA’s broader Earth Science program. The publication has captured – often in meticulous detail – the intensive work behind the scenes that has gone into the development of the technologies, algorithms, and data centers that gather data from Earth observing satellites, suborbital observations, and other experiments to inform end users who use the data to address societal issues.
In the years before the first EOS missions launched, the newsletter reported in earnest on Investigator Working Group (IWG) meetings, Payload Panel Reviews (reviewing the instruments planned for the EOS platforms), and Mission and Instrument Science Team Meetings. As EOS matured, the newsletter began reporting on the development and implementation of specific science missions, launches, milestones, and research generated from the data collected. The editorial staff began publishing more feature articles to appear along with the meeting and workshop reports. The newsletter shared news stories developed by NASA’s Earth Science News Team and other bimonthly content (e.g., Education Update, Science in the News). “The Editor’s Corner” column in the newsletter gave the EOS Senior Project Scientist a platform to offer commentary on current events in NASA Earth Science as well as on the content of the current issue of the newsletter. While not formally named for the first few issues, an editorial article has been a cornerstone of the publication since the beginning.
The Earth Observer has produced several articles reflecting on its interwoven history with EOS, such as The Earth Observer: Twenty-Five Years Telling NASA’s Earth Science Story [March–April 2014, 26:2, 4–12] and A Thirtieth Anniversary Reflection from the Executive Editor {March–April 2019, 31:2, 4–6]. These stories expand upon the topics covered in the brief review presented in this article.
Satellite Missions: the Backbone of EOS Science
EOS was originally organized around 24 critical science measurements deemed integral to understand planetary processes and assess variability, long-term trends, and climate change. These science measurements serve as a roadmap for organizing EOS data products and mission objectives. The 24 measurements coalesced into five broad categories that reflect Earth science disciplines:
- Atmosphere: aerosol properties, cloud properties (e.g., fraction and opacity), atmospheric temperature and pressure profiles, water vapor, ozone (O3), trace gases [e.g., carbon dioxide (CO2), sulfur dioxide, and formaldehyde], and total solar irradiance;
- Ocean: ocean color (chlorophyll), sea surface temperature, sea ice cover and motion, ocean surface topography and sea level, and sea surface salinity;
- Land/Cryosphere: land surface temperature, soil moisture, snow and ice cover (extent and elevation), land cover and change (e.g., forest cover), and topography;
- Radiation/Energy Balance: radiant energy balance (incoming and outgoing radiation), and precipitation (e.g., rainfall, snow); and
- Solid Earth: static gravity field and synthetic aperture radar observations.
The Grand Vision of EOS: Three Flagships Leading the Earth Observing Fleet
In the late 1980s and early 1990s, a team of scientists envisaged the concept for two missions – EOS-AM1 and EOS-PM1. The synergy of this system was the ability to make observations in the morning (10:30 AM mean local time, or MLT), a time when cloud cover over the tropical equatorial and other land regions would be at a minimum, and afternoon (1:30 PM MLT), a time when continental convection would peak. The plan was to have two instruments – the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and Earth’s Radiant Energy System (CERES) – overlap on the two platforms along with other instruments unique to each mission.
In parallel, the teams envisioned EOS-CHEM1, a satellite platform identical to EOS-PM1 but carrying a payload focused on atmospheric chemistry. Like EOS-PM1, EOS-CHEM1 would be placed in an afternoon orbit but lag slightly in its equatorial crossing time (1:45 PM MLT) to optimize its position for atmospheric chemistry observations.
Each mission was slated to be the first in a series that would launch at five-year intervals to ensure continuity of critical Earth science measurements. Budgetary realities and technical advances eventually rendered plans for the second and third series of each satellite obsolete; however, all three flagship missions endured far beyond their planned six-year lifetime and have outlasted the originally proposed 15-year timeframe for each series.
Terra
Terra, originally named EOS-AM1, launched in December 1999 – see Figure 2. Terra carries five instruments – MODIS, CERES (two copies), Multiangle Imaging Spectroradiometer (MISR), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), and Measurements of Pollution in the Troposphere (MOPITT) – and was designed to capture information about Earth’s atmosphere, carbon cycle and ecosystems, climate variability, water and energy cycle, weather, and the planet’s surface and interior. The Earth Observer captured early Terra data in the article, Terra Spacecraft Open For Business [March–April 2000, 12:2, 24].
After over 26 years in service, Terra remains in orbit and continues to gather data; as of this writing all instruments accept MOPITT remain active. However, since 2020 the spacecraft has been allowed to drift from its carefully maintained 10:30 AM MLT equator crossing time toward earlier MLT crossings. This was done to conserve enough fuel to control Terra’s eventual atmospheric reentry. The Terra team also conducted orbital lowering maneuver on the spacecraft in 2022. A more complete history of Terra is available in the online article, Terra: The End of An Era, published on December 29, 2025.
Figure 2. An artistic rendering of the Terra spacecraft. The image shows the locations of its five instruments. Note that there are two Clouds and Earth’s Radiant Energy System instruments aboard the satellite and one each of the other four instruments. Figure credit: NASAAqua
Aqua, originally named EOS-PM1, launched in May 2002 – see Figure 3. An article in The Earth Observer at the time of launch described the mission, Aqua is Launched! [March–April 2002, 14:2, 2]. The second EOS flagship carried six different instruments into orbit – Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit–A (AMSU-A1 and -A2), CERES (two copies), MODIS (both of which also fly on Terra), the Advanced Microwave Scanning Radiometer for EOS (AMSR–E), and Humidity Sounder for Brazil (HSB). Aqua’s mission focused on collecting data on global precipitation, evaporation, and the cycling of water. Aqua paired its data with Terra, offering the scientific community additional insights into the daily cycles for important scientific parameters to understand the global water cycle.
The Earth Observer article, Aqua: 10 Years After Launch [Nov.–Dec. 2012, 24:6, 4–17] provides an overview of the mission’s accomplishments during its first decade in orbit. Due to fuel limitations, Aqua completed the last of its drag makeup maneuvers in December 2021. Like Terra, the satellite is now in a free-drift mode, slowly descending below the A-Train orbit and crossing the equator later and at lower altitudes. A more recent newsletter article, Aqua Turns 20 [May–June 2022, 34:3, 5–12] reflects on Aqua’s accomplishments and legacy after two decades in orbit. As of this writing MODIS, CERES, AMSU, and CERES remain active.
Figure 3. An artistic rendering of NASA’s Aqua satellite. The mission collects data about the Earth’s water cycle, including evaporation from the oceans, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, land ice, and snow cover on the land and the ocean. Figure credit: NASAAura
Originally named EOS-CHEM1, Aura was the third and final flagship mission, and was launched in July 2004 – see Figure 4. The Earth Observer detailed the first post-launch science team meeting, Aura Science Team Meeting [March–April 2004, 17:2, 8–11]. Aura followed a Sun-synchronous, near-polar orbit, crossing the equator 15 minutes after Aqua. Similar to Aqua, Aura completed its final inclination adjustment maneuver in April 2023 to save its remaining fuel to allow for controlled reentry. As a consequence, the satellite has drifted out of the A-Train orbit, slowly continuing to move to a later equatorial crossing time and lower orbit altitude.
Aura’s payload included four instruments: the Microwave Limb Sounder (MLS), High Resolution Dynamics Limb Sounder (HIRDLS), Tropospheric Emission Spectrometer (TES), and Ozone Monitoring Instrument (OMI). These instruments gather information on trace gases and aerosols in the atmosphere. The key mission objectives aimed to monitor recovery of the stratospheric O3 hole, evaluate air quality, and monitor the role of the atmosphere in climate change. The article, Aura Celebrates Ten Years in Orbit [Nov.–Dec. 2014, 26:6, 4–16] detailed Aura’s first decade of accomplishments. The online article, Aura at 20 Years, published Sept. 16, 2024, reported on Aura’s status and achievements as it began its third decade of continuous operations. As of this writing MLS and OMI remain active.
Figure 4. An artistic rendering of the Aura satellite. Aura gathers information on trace gases and aerosols in the atmosphere. Figure credit: NASABuilding and Dismantling the “A-Train”
Between 2002 and 2014, a series of satellites joined the A-Train constellation – see Figure 5. This international effort includes the two EOS flagship satellites with afternoon equatorial crossing times (Aqua and Aura) as well as the Orbiting Carbon Observatory–2 (OCO-2), CloudSat, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). In addition, Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with observations from a Lidar (PARASOL) and Global Change Observation Missions with a focus on the water cycle (GCOM-W) are two international missions that became part of the A-Train constellation.
In the past decade, many of the satellites in the A-Train have either retired or have been allowed to drift out of the constellation. As of this writing, only two satellites – OCO-2 and GCOM-W1 – remain in their positions in the A-Train gathering data.
Three A-Train symposiums have been organized to bring the Earth science community together to discuss the achievements and future synergy of these missions. The outcome from each of these meetings were reported in The Earth Observer. The most recent of these was: The Third A-Train Symposium: Summary and Perspectives on a Decade of Constellation-Based Earth Observations [July–Aug. 2017, 29:4, 4–18].
Figure 5. An artistic depiction of all the satellites that participated in the Afternoon Constellation (A-Train), except for Polarization and Anisotropy of Reflectances for Atmospheric Sciences coupled with observations from a Lidar (PARASOL). CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) lowered their orbits. Called the C-Train, the orbit of these satellites overlapped the A-Train, enabling science observations with other A-Train missions. More details about the A-train is available on the constellation’s website. Figure credit: NASAScience from the EOS Fleet
The next several sections provide a highlight of science from key missions outside of Terra, Aqua, and Aura. The content has been organized in terms of measurements – with an overarching focus on water (oceans and fresh water), atmosphere, and land. This summary is far from exhaustive. A record of much of the amazing science conducted during these missions is detailed in the archives of The Earth Observer.
Interpreting an Ocean of Data
When viewed from space, Earth has been described as a “blue marble.” The planet’s abundance of liquid water is found in the oceans, and while not potable, the oceans play a critical role in regulating Earth’s climate. Satellites provide an unparalleled way to study the global ocean. With each new mission, the process of data collection has been refined and improved. The scientific community can now measure ocean color as a proxy for surface productivity as well as measure subtle changes in surface ocean salinity. These data have improved weather and climate models to increase the accuracy of storm projection and help the scientific community better understand the movement of energy around the planet.
Aqua was the flagship mission dedicated to studying water on Earth, but other missions have contributed and expanded on this data record. For example, Japan’s GCOM-W1 mission, also known as SHIZUKU (Japanese for droplet), continues to gather information on precipitation, water vapor, wind velocity above the ocean, sea water temperature, water levels on land, and snow depths. These data support weather models to improve forecasts to monitor tropical cyclones. The subsections that follow provide examples of how data from these satellites support different science objectives, as well as examples of the science deciphered by both flagship and ancillary platforms within the A-Train. All of these missions and science have been covered in The Earth Observer over the past several decades.
Discerning the Ocean’s True Colors
Ocean color data are crucial for studying the primary productivity and biogeochemistry of the oceans. The Coastal Zone Color Scanner (CZCS), launched on the Nimbus 7 satellite in 1978 and ceasing operations in 1986 – gave the earliest perspective of the oceans from space. SeaWiFS, which served as a follow-on to CZCS, was launched on the privately owned Seastar spacecraft on Aug. 1, 1997 to produce ocean color data and offered a synoptic look at the global biosphere. This mission was a data-buy, where NASA purchased the data from Orbital Imaging Corporation. An article in The Earth Observer, titled Sea-viewing Wide Field-of-view Sensor [March–April 1998, 10:2, 20–22] detailed how the satellite gathered chlorophyll-a data that was calibrated to field measurements from a Marine Optical Buoy. The research community have used this information to understand primary productivity in the surface ocean and global biogeochemistry. This data offered an early assessment of the role of the ocean in the global carbon cycle. It also produced one of the first global perspectives of the impact of El Niño and La Nina events around the world. Coastal and fishery managers have used this data to improve the health of these important ecosystems. Launched for a five-year mission, SeaWiFs gathered data until December 2010.
More recently, NASA launched the Plankton, Aerosol, Cloud ocean Ecosystem (PACE) satellite in February 2024 to gather data on ocean and terrestrial ecosystem productivity – see Figure 6. While other missions studied ocean color in the interim between SeaWiFS and PACE (e.g., MODIS on Terra and Aqua), PACE offers an exponential leap forward with its three-instrument payload that includes: the Ocean Color Instrument (OCI), Hyper-Angular Rainbow Polarimeter–2 (HARP2), and Spectropolarimeter for Planetary Exploration (SPEXone). The PACE mission aims to clarify how the ocean and atmosphere exchange CO2, a key factor in understanding the evolution of Earth’s climate system. The satellite also examines the role of aerosols in providing micronutrients that fuel phytoplankton growth in the surface ocean. The data gathered extends the aerosol and ocean biological, ecological, and biogeochemical records that were initiated by other satellites. The Dec. 29, 2025 article, Keeping Up with PACE: Summary of the 2025 PAC3 Meeting, reports on three recent meetings related to the mission.
Figure 6. An artistic rendering of the Plankton, Aerosol, Cloud ocean Ecosystem (PACE) observatory and the instrument panels that it carries. PACE focuses on clarifying how the ocean and atmosphere exchange carbon dioxide. Figure credit: NASAMapping the Ocean Surface to Reveal the Rising Seas
The Ocean Surface Topography (TOPEX)/Poseidon mission, launched on Aug. 10, 1992, was the first in a series of missions that have measured ocean surface topography, or the variations in sea surface height. This record now extends more than 30 years. TOPEX/Poseidon spent more than 13 years in orbit. The data gathered helped to improve the scientific community’s understanding of ocean circulation and its impact on global climate – including sea level rise. TOPEX/Poseidon produced the first global views of seasonal current changes, which allowed scientists to forecast and better understand El Niño events. These early efforts to distribute data were captured in The Earth Observer article, Jet Propulsion Laboratory DAAC Begins TOPEX Data Distribution [March–April 1993, 6:2, 24].
Jason followed TOPEX/Poseidon to continue the measure of sea level as well as wind speed and wave height for more than 95% of Earth’s ice-free ocean – see Figure 7. Jason consists of a series of satellites, with Jason-1, launched in 2001, remaining in orbit for 11 years. It was followed by Jason-2, also called the Ocean Surface Topography Mission (OSTM), which was launched in 2008. Jason-2 gathered data for 11 years. Jason-3 launched in January 2016 and remains in orbit, continuing the sea level dataset. The Earth Observer has reported on meetings of the Ocean Surface Topography Science Team over the years. The online article, Summary of the 2023 Ocean Surface Topography Team Meeting, was published May 31, 2024 and includes the most recent updates available.
Figure 7. Beginning with TOPEX/Poseidon in 1992, a series of ocean surface topography missions have maintained a continuous record of global sea surface height data with the best possible accuracy along the same exact ground track. Dubbed the “reference” altimetry missions, shown here are TOPEX/Poseidon, Jason-1, and the Ocean Surface Topography Mission/Jason-2 (OSTM/Jason-2) in the tandem orbit pattern. This is used to cross-calibrate each mission to the next. By flying in formation, just one minute apart for a period of several months, scientists can be sure that each successive mission is exactly calibrated to its predecessor. Connecting each record to the next, these reference missions have built a record of sea level that stretches more than 30 years with centimeter level accuracy for every corner of the ocean. The reference mission has now been taken over by, Sentinel 6 Michael Freilich, which will hand the baton to the recently launched Sentinel 6B sometime in 2026. Figure credit: NASA/JPL/CNESThe international partnership between the United States [NASA and the National Oceanic and Atmospheric Administration (NOAA)], the European Space Agency (ESA), and the French Space Agency [Centre National d’Études Spatiales (CNES)] collaborate to create the ESA’s Copernicus Sentinel–6 missions. The Sentinel-6B, launched Nov. 16, 2025, will follow the path of the Sentinel-6 Michael Freilich (originally called Sentinel–6A) satellite, which has been in orbit for five years – see Figure 8. These two Sentinel 6 missions continue the global measurements of sea level, wind speed, wave height, and atmospheric temperature. The data will be used in marine weather forecasts as well as to improve commercial and naval navigation, search and rescue missions, and tracking garbage and pollutants in the ocean. To learn more about Sentinel-6B, see the online article, Sentinel-6B Extends Global Ocean Height Record, published Dec. 22, 2025.
While the Surface Water and Ocean Topography (SWOT) mission is fully described in the next section – with emphasis placed on its novel surface water observation capabilities – it should be noted that SWOT is also an ocean topography mission that obtains data similar to TOPEX/Poseidon, Jason, and Sentinel-6 missions. These data will contribute to the long-term time series of the sea surface height record.
Figure 8. Sentinel-6B, an Earth-observing satellite jointly developed by NASA and U.S. and European partners, will observe the ocean and measure sea level rise to provide insights into our home planet that will improve weather forecasts and flood predictions, increase public safety, and protect coastal infrastructure. The Sentinel missions are part of the European Space Agency’s Copernicus Programme. Figure credit: NASASampling the Salty Seas
Launched June 2011, Aquarius was an international collaboration between NASA and Argentina’s Comisión Nacional de Actividades Espaciales (CONAE). The cooperative effort was detailed in the article, Aquarius: A Brief (Recent) History of an International Effort [July–Aug. 2010, 22:4, 4–5]. The satellite carried a microwave radiometer that was sensitive enough to measure salinity to an accuracy of 0.2 practical salinity units (psu) on a monthly basis. It also carried a scatterometer to measure surface ocean roughness. Pairing data from the two instruments allowed the team to overcome the challenges of measuring salinity from space. This feat is detailed in the article, For Aquarius, Sampling Seas No ‘Grain of Salt’ Task [July–Aug. 2011, 23:4, 42–43]. The more accurate, global measurements of ocean salinity that Aquarius obtained have helped the research community better understand ocean circulation. The mission ended in 2015, after the satellite experienced a power failure.
Focusing on Freshwater
While most water on the planet is housed in the ocean, fresh water is a primary concern for life on the planet. Fresh water accounts for ~3% of the total amount water on the planet. Of that small amount, a significant portion is locked in ice on land and as sea ice. The remaining water flows on Earth’s surface and underground. Maintaining a supply of fresh water is critically important to our survival. The location, status, and purity of this precious resource continues to be an on-going focus for many of the missions.
Monitoring Rain and Snow
The joint NASA/National Space Development Agency of Japan (NASDA – which is now known as the Japan Aerospace Exploration Agency, or JAXA) Tropical Rainfall Measuring Mission (TRMM) carried a Microwave Imager, Visible Infrared Scanner, and Precipitation Radar to gather tropical and subtropical rainfall observations (and two related instruments) – see Figure 9. These data filled a critical knowledge gap – to understand the interactions between the sea, air, and land. Over the years, these data were incorporated into numerous computer models to clarify the role of tropical rainfall on global circulation and formed the basis for experimental quasi-global merged satellite precipitation products. The Earth Observer detailed the early data collection in the article titled TRMMing the Uncertainties: Preliminary Data from the Tropical Rainfall Measuring Mission [May–June 1998, 10:3, 48–50]. The mission was extended twice but eventually the satellite’s maneuvering fuel was exhausted, resulting in a slow decline in the orbital altitude beginning in 2014, with reentry in 2015. Data from TRMM have improved understanding of storm structure of cloud systems, produced reliable global latent heating estimates to improve water transfer estimates within the atmosphere, and continue to be used in calibrating modern precipitation products for the TRMM era.
Figure 9. Artistic rendering of the Tropical Rainfall Measuring Mission (TRMM) in space over a hurricane. TRMM was launched in 1997 and remained in operation until 2015. The satellite was designed to improve our understanding of the distribution and variability of precipitation within the tropics as part of the water cycle in the current climate system. Figure credit: NASATo continue the efforts that began with TRMM – and extend coverage to most of the globe – NASA and JAXA launched the Global Precipitation Measurement (GPM) mission in 2014. This satellite aims to advance our understanding of water and energy cycles, improve forecasting of extreme weather events, and extend current capabilities to use accurate and timely information of precipitation to directly benefit society. The Earth Observer detailed the accomplishments of this mission in the online article, GPM Celebrates Ten Years of Observing Precipitation for Science and Society, published Oct. 3, 2024.
Surveying Earth’s Surface Water
Introduced briefly in the previous section, the SWOT mission is a joint venture between the United States and France. Launched in December 2022, SWOT is conducting the first global survey of Earth’s surface water – see Photo. The mission was introduced to the EOS community in The Earth Observer article, Summary of the 2022 Ocean Surface Topography Science Team Meeting [May–June 2023, 35:3, 19–23]. SWOT carries the Ka-band Radar Interferometer (KaRIN) – the first spaceborne, wide-swath, altimetry instrument capable of high-resolution measurements of sea surface height in the ocean and freshwater bodies. SWOT covers most of the world’s ocean and freshwater bodies with repeated high-resolution elevation measurements. This data have been applied to monitor rivers across the Amazon basin, simulate land/hydrology processes, and predict streamflow. A more comprehensive overview of SWOT applications is detailed in online article, Summary of the 10th SWOT Applications Workshop, published Sept. 20, 2024.
Photo 1. Workers in a clean room in Cannes, France, load the Surface Water and Ocean Topography (SWOT) satellite into a container in preparation for shipping the spacecraft to the United States. SWOT provides the first global survey of Earth’s surface water. Photo credit: Centre National d’Études Spatiales (CNES), Thales Alenia SpaceGracefully Tracking Water Movement
The twin GRACE satellites were launched on March 17, 2002. The mission, a partnership between NASA and the German GeoForschungsZentrum (GFZ) Helmholtz Centre for Geosciences was developed to measure Earth’s shifting masses – most of which comes from water – and map the planet’s gravitational field using a K-band microwave ranging system and accelerometers. Some early results of the satellites appeared in The Editor’s Corner column [Nov.–Dec. 2002, 14:6, 1–2]. GRACE enabled groundbreaking insights into Earth’s evolving water cycle as the satellites tracked monthly mass variations in ice sheets and glaciers, near-surface and underground water storage, the amount of water in large lakes and rivers, as well as changes in sea level and ocean currents.
GRACE’s mission was extended with the GRACE-Follow On (GRACE-FO) mission launched in 2018 – see Figure 10. GRACE-FO continues comprehensive tracking water movement across the planet, including groundwater measurements that have important applications for everyday life. The most recent developments of the GRACE-FO science meeting was detailed in an online article, Summary of the 2023 GRACE Follow-On Science Team Meeting, published March 30, 2024 – and also published in the final print issue [Jan.–Feb. 2024, 35:7, 19–26]. The data gathered during the GRACE-FO mission details large-scale changes in Earth’s groundwater reservoirs, Greenland and Antarctica’s sensitivity to warming ocean waters, and even subtle shifts deep in Earth’s interior that reveal how large earthquakes can develop.
In 2028, NASA will move into a third-generation of gravity observations with the launch of GRACE-Continuity, or GRACE-C, which will further expand the foundational observations of global mass change and expand the societal and economic applications that have been created from these data.
Figure 10. An artistic rendering of the twin Gravity Recovery and Climate Experiment-Follow-On (GRACE-FO) satellites that, like the original GRACE twins, follow each other in orbit, separated by about 137 miles (220 km). GRACE tracks water movement across the planet’s surface. Figure credit: NASAAssessing the Atmosphere from Above
Earth has a unique atmospheric makeup that maintains a stable temperature allowing life to thrive. As far as we know, our atmosphere is unique in the universe. Satellites provide an unparalleled perspective to study variability in the column of air extending from Earth’s surface. While Aura has a suite of instruments making a wide range of atmospheric chemistry measurements, other missions also measure the abundance and impact of atmospheric constituents that, while often invisible to the unaided eye, can have profound impacts on Earth’s air quality and climate. These data have also improved climate models and help the scientific community better understand how energy is emitted into space.
Tracking Tiny Particles with Big Impacts
France’s PARASOL mission was an original member of the international A-Train constellation. Launched in 2004. PARASOL sought to capture the radiative and microphysical properties of clouds and tiny atmospheric aerosol particles using a unique multiangle imaging POLDER polarimeter.
NASA’s Glory mission was intended for operation in the A-Train; it carried a multiangle polarimeter as its instrument. Unfortunately, the spacecraft failed to separate from the Taurus rocket due to a fairing separation failure during its launch in 2011. As a result, POLDER on PARASOL was the only atmospheric polarimeter to fly in space until two (SPEXone and HARP2) launched as part of NASA’s PACE mission. Researchers gathered information from POLDER and other A-Train instruments about how aerosols affect the formation of precipitations and clouds, the movement of water around the planet, and the reflection and absorption of radiative energy that impact overall planetary climate. PARASOL was deactivated in 2013 after nine years in service.
Cloud particles form when water vapor nucleates onto aerosols; changes in one can impact the other. After many years and conversations, it was decided to pair two NASA Earth System Science Pathfinder (ESSP) missions – CloudSat and CALIPSO – and fly them in coordination with each other and with other A-Train satellites. By combining the two datasets, it was possible to explore cloud and aerosol processes. This information helped the community drill into the larger climate questions. The two satellites were launched on the same Delta-II rocket from Vandenberg Air Force Base in California on April 28, 2006. CloudSat used a 94 GHz cloud profiling radar that is 1000 times more sensitive than a typical weather radar, capable of distinguishing between cloud particles and precipitation. CALIPSO contained a Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), Wide-Field Camera, and Imaging Infrared Radiometer to detect and distinguish between aerosol particles and cloud particles.
The Earth Observer captured the early data collection of the two satellites in the article, CloudSat and CALIPSO: A Long Journey to Launch…But What a Year It’s Been!! [May–June 2007, 19:3, 7–12]. The later article, A Useful Pursuit of Shadows: CloudSat and CALIPSO Celebrate Ten Years of Observing Clouds and Aerosols [July–Aug. 2016, 28:4, 4–12] provided a review of the accomplishments of the missions after 10 years in orbit. CALIPSO and CloudSat were both deactivated in 2023 after 17 years of service.
An Oracle of High-Altitude Wisdom
The Stratospheric Aerosol and Gas Experiment (SAGE) has experienced several iterations, extending back nearly half a century. The initial SAGE mission launched on Feb. 18, 1979, aboard the Applications Explorer Mission-B (AEM-B) to measure vertical distribution of aerosols and important gases in the upper troposphere and stratosphere (UTS). The satellite failed after three years in orbit. In 1984, SAGE II began collecting data on stratospheric O3, producing a stable record of this important greenhouse gas from 1984–2005. SAGE III was launched on Метеор-3М (SAGE III/M3M). The third-generation satellite produced an accurate measurement of the vertical structure of aerosols, O3, water vapor, and other important trace gases in the upper troposphere and stratosphere. The satellite was terminated on March 6, 2006, following a power supply system failure, resulting in loss of communication with the satellite.
Another version of SAGE III was launched to the International Space Station (ISS) on Feb. 19, 2017, where it was installed on the EXpedite the PRocessing of Experiments to Space Station (ExPRESS) Logistics Carrier [ELC-4] – an unpressurized attached payload platform for ISS. SAGE III/ISS, which is shown mounted on ELC-4 in Figure 11, has completed its prime mission after three years of operation. NASA granted approval to extend the SAGE III/ISS mission through at least 2026 – meaning the instrument will continue to provide the public and science community with world-class vertical profiles of O3, aerosol, water vapor, and other trace gases, e.g., nitrogen dioxide (NO2) and nitrate (NO3), data products for at least another year. An article titled, Summary of the 2024 SAGE III/ISS Meeting, published May 26, 2025, details the latest findings from SAGE.
Figure 11. An artistic rendering of the Stratospheric Aerosol and Gas Experiment-III (SAGE-III), which is externally mounted on the International Space Station’s Japanese Experiment Module–Exposed Facility (JEM-EF) EXPRESS Logistics Carrier (ELC)-4. SAGE III/ISS measures the vertical structure of aerosols, ozone (O3), water vapor, and other important trace gasses in the upper troposphere and stratosphere. Figure credit: NASAWatching Earth Exhale
The Orbiting Carbon Observatory (OCO) was launched into space in February 2009, but it failed to separate from the Taurus rocket during its ascent, leading to mission failure and loss of the satellite. Undaunted, the EOS community began again and assembled OCO-2, which was successfully launched into orbit, joining the A-Train on July 2, 2014 – see Figure 12. The satellite’s mission focused on making precise, high-resolution measurements of atmospheric CO2. OCO-2 measures reflected sunlight that interacts with the atmosphere. Using diffraction gratings to separate the reflected sunlight into spectra, OCO-2 measures the absorption levels for the different molecular bands to calculate CO2 concentration. This information is invaluable for the quantification of CO2 emissions and can characterize both sources and sinks of this critical greenhouse gas. The mission was detailed in an article, titled Orbiting Carbon Observatory-2: Observing CO2 from Space [July–Aug. 2014, 26:4, 4–12].
On May 4, 2019, NASA launched the third iteration in the OCO group to the ISS. It was subsequently installed on the Japanese Experiment Module–Exposed Facility (JEM-EF). Constructed from parts left over from OCO-2, OCO-3 continues the mission of making CO2 measurements with a focus on daily variability. In particular, the measurements explore the role of plants and trees in the major tropical rain forests of South America, Africa, and Southeast Asia. As of today, both OCO-2 and OCO–3 remain operational and gathering data.
The science team reflected on both these missions in a recent article posted in the online article, A Tapestry of Tales: 10th Anniversary Reflections from NASA’S OCO-2 Mission, published Aug. 12, 2025.
Figure 12. An artistic rendering of OCO-2 in orbit above Earth. OCO-2 measures the concentration of trace gases in the atmosphere. Figure credit: NASA/JPL-CaltechTracking the Sun’s Output
In December 1999, NASA launched the Active Cavity Radiometer Irradiance Monitor Satellite (ACRIMSAT) satellite to extend the more than two-decade record of total solar irradiance (TSI). Scientists use this important measurement to quantify the solar energy input to the planet and thereby its interactions with Earth’s oceans, land masses, and atmosphere. It is also a critical component to understand variations of the planet’s climate. The Active Cavity Radiometer Irradiance Monitor 3 (ACRIM3) instrument onboard combined the best features of the ACRIM I (flown on the Solar Maximum Mission), ACRIM II (flown on the Upper Atmosphere Research Satellite), and SpaceLab-1 ACRIM (flown on Space Shuttle Columbia, STS 9). ACRIM3 improved on its predecessors by incorporating a new electronics and package design. The Earth Observer captured the initial information from this mission in the article, The ACRIMSAT/ACRIM3 Experiment — Extending the Precision, Long-Term Total Solar Irradiance Climate Database [May–June 2001, 13:3, 14–17]. ACRIMSAT spent 14 years in orbit and ACRIM3 extended the TSI record to 36 years (i.e., building on measurements from previous ACRIM missions).
NASA continued its quest to observe the incident solar energy budget with the launch of the Solar Radiation and Climate Experiment (SORCE) in January 2003. SORCE focused on measuring solar radiation incident to the top of the Earth’s atmosphere. The Total Irradiance Monitor (TIM) onboard continued the TSI record that the ACRIM series of satellites established. In addition to TIM, the satellite carried a Spectral Irradiance Monitor (SIM), an Extreme Ultraviolet (XUV) Photometer System [XPS], and a stellar observation from the Solar Stellar Irradiance Comparison Experiment (SOLSTICE). The satellite has produced groundbreaking TSI and spectral solar irradiance (SSI) measurements – two key inputs for atmosphere and climate modeling.
Early results from SORCE are detailed in the article, The SORCE (SOlar Radiation and Climate Experiment) Satellite Successfully Launched [Jan.–Feb. 2003, 15:1, 16–19]. The article, The SORCE Mission Celebrates 10 Years [Jan.–Feb. 2013, 25:1, 3–13] details the most significant results from a decade of SORCE observations. Designed for a five-year mission, SORCE gathered data until 2020 – although a degradation of a battery power that began in 2008 increasingly hindered data collection for the remainder of the mission. During its time in orbit, SORCE captured two of the Sun’s 11-year solar cycles and observed the solar cycle minimum in both 2008 and 2019. SORCE’s orbit will decay and re-enter Earth’s atmosphere in 2032.
To continue the crucial long-term TSI and the SSI record that SORCE originated, NASA launched the Total and Spectral Solar Irradiance Sensor (TSIS-1) to the ISS on Dec. 15, 2017, which was installed on JEM-EF ELC-3. The satellite’s mission set out to measure the total amount of sunlight that falls on the planet’s surface – see Visualization 1. This data will clarify the distribution of different wavelengths of light. TSIS-1 was introduced in The Earth Observer article, Summary of the 2018 Sun–Climate Symposium [May–June 2018, 30:3, 21–27]. Similar to SORCE, TSIS-1 carries a TIM and SIM. The instrument extends the multidecadal SSI record and provides highly accurate, stable, and continuous observations that are critical to understanding the present climate conditions and predicting future conditions. The most recent efforts from this mission were detailed in the online article, Summary of the 2023 Sun–Climate Symposium, published July 18, 2024. TSIS-1 has been extended by at least three more years as part of the Earth Sciences Senior Review process. A follow-on mission, TSIS-2, is under development to extend the long-term observational record through continued TSI and SSI measurements.
Visualization 1. NASA’s Total and Spectral solar Irradiance Sensor (TSIS-1) measures the total amount of solar energy input to Earth as well as the distribution of the Sun’s energy across a wide range of wavelengths. The animation illustrates the various wavelengths of light that are partially reflected into space at different places in the column of atmosphere above the ground.Visualization credit: NASA
Chronicling the Changing Land Surface
Along with Terra, other satellites also provide global estimates about the land. Each new mission provides the scientific community more information to refine these measurements. These data have improved climate models as well as improved our understanding of how the planet’s interior is altering the surface of the planet.
Measuring Ice and Vegetation Heights
NASA launched ICESat in 2003 on a three-to-five-year mission to provide information on ice sheet mass balance and cloud properties. It carried the Geoscience Laser Altimeter System (GLAS), which combines a precision surface lidar with a sensitive dual-wavelength cloud and aerosol lidar. ICESat was decommissioned seven years after launch. The science team began efforts for the follow-on mission, ICESat-2, which launched on Sept. 15, 2018 – see Figure 13. Data collected during a series of Operation IceBridge field campaigns to the Arctic and Antarctic helped to fill the data gap between the two satellite missions – allowing for continuity of measurements. ICESat-2 carries a payload of a photon-counting laser altimeter on its three-year mission. The laser is split into six beams capable of measuring the elevation of the cryosphere, including ice sheets, glaciers, and sea ice, down to a fraction of an inch. The laser altimeter also gathers the height of ocean and land surfaces, including forests, snow, lakes, rivers, ocean waves, and urban areas. The mission objective includes quantifying polar ice sheet contribution to sea-level change, estimating sea-ice thickness, and measuring vegetation canopy height. The mission was detailed in The Earth Observer article, ICESat-2: Measuring the Height of Ice from Space [Sept.–Oct.. 2018, 30:5, 4–10]. The research community has been using this information to investigate how the ice sheets of Antarctica and Greenland are changing as the planet warms.
Figure 13. Illustration of the Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) spacecraft. ICESat-2 measures the elevation of aspects of the cryosphere, including ice sheets, glaciers, and sea ice. Figure credit: NASANASA’s Global Ecosystem Dynamics Investigation (GEDI – pronounced “jedi”) mission was launched to the ISS on Dec. 5, 2018 and was subsequently installed on the JEM–EF ELC-6. From that vantage point GEDI produces high-resolution laser ranging observations of the three-dimensional (3D) structure of Earth that can be used to make precise measurements of forest canopy height and canopy vertical structure – see Visualization 2. These measurements have improved understanding of important atmospheric and water cycling processes, biodiversity, and habitat. Upon completion of its prime mission, which lasted from December 2018 to March 2023, GEDI was moved from the ISS’s EFU-6 to EFU-7 (storage). Since April 2024, the GEDI instrument has been back in its original location on EFU-6 and continues to collect high-resolution observations of Earth’s 3D structure from space. The GEDI research team hopes the mission can continue collecting data until 2030.
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The GEDI mission has been covered in The Earth Observer through summaries of periodic meetings of the GEDI Science Team. The online article, Summary of the 2025 GEDI Science Team Meeting, is the most recent installment of GEDI’s progress, published on Aug. 18, 2025. This article includes discussion of “the return of the GEDI” from hibernation and the science results since then.
Monitoring Earth in Intricate Detail
The Soil Moisture Active Passive (SMAP) mission was designed to measure the amount of water in surface soil across Earth. The satellite was launched from Vandenberg Air Force Base on Jan. 31, 2015. The satellite payload consisted of both an active microwave radar and a passive microwave radiometer to measure a swath of the planet 1000-km (~621-mi) wide. The radar transmitter failed just nine months after launch on July 7, 2015. Although the loss of the radar was unfortunate, the nine months where both instruments functioned provided an invaluable dataset that established the dependence of L-band radar signals on soil moisture, vegetative water content, and freeze–thaw state. Two of these variables (surface soil moisture and freeze–thaw state) are critical variables that influence the planet’s water, energy, and carbon cycles. The three variables influence weather and climate. Furthermore, the SMAP team quickly turned a setback into a success. They repurposed the channels that had been dedicated to the radar to record the reflected signals from the Global Navigation Satellite System (GNSS) constellation in August 2015, making SMAP the first full-polarimetric GNSS reflectometer in space for the investigation of land surface and cryosphere.
The Earth Observer article, SMAP: Mapping Soil Moisture and Freeze/Thaw State from Space [Jan.–Feb. 2015, 27:1, 14–19] offered a preview of SMAP that was published shortly after its launch. A more recent online article, A Decade of Global Water Cycle Monitoring: The Soil Moisture Active Passive Mission, published Aug. 18, 2025, reflects on the achievements of SMAP after a decade of operations.
More specific to vegetation water content, NASA launched the ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) to ISS on June 29, 2018. It was subsequently installed on the JEM–EF ELC 10, placing it in close proximity to GEDI (installed on ELC 6) and enabling combined observations. While GEDI focuses on the canopy height and related characteristics, ECOSTRESS monitors the combined evaporation and transpiration of living plants – known as evapotranspiration (ET). ECOSTRESS determines ET indirectly through measurements of the thermal infrared brightness temperatures of plants.
As with GEDI, The Earth Observer has reported on the activities of the ECOSTRESS mission. The most recent coverage was in the article, ECOSTRESS 2019 Workshop Summary: Science, Applications, and Hands-On Training [July–Aug. 2018, 31:4, 15–18.]
Last, but certainly not least, the most recent Earth observing satellite to launch is a joint venture between NASA and the Indian Space Research Organization (ISRO). The NASA-ISRO Synthetic Aperture Radar (NISAR) took to the skies on July 30, 2025, from the Satish Dhawan Space Centre on India’s southeastern coast aboard an ISRO Geosynchronous Satellite Launch Vehicle (GSLV) rocket 5. The mission was designed to observe and measure some of the planet’s most complex processes – see Figure 14. The launch was lauded in the Editor’s Corner published online on Sept. 10, 2025.
NISAR uses two different radar frequencies – L-band and S-band synthetic aperture radar (SAR). The dual system can penetrate clouds and forest canopies to allow researchers to measure changes on the planet’s surface, down to a centimeter (~0.4 in). This level of detail allows the research community to investigate ecosystem disturbances, ice-sheet collapse, natural hazards, sea level rise, and groundwater issues. The satellite will also capture changes in forest and wetland ecosystems. It will expand on our understanding of deformation of the planetary crust that can help predict earthquakes, landslides, and volcanic activity. All of this data will help mitigate damage from a disaster and help communities prepare a disaster response. Some early results from the both NISAR radars are discussed in the Final Editor’s Corner column, published online on Dec. 29, 2025.
Figure 14. The NASA-ISRO Synthetic Aperture Radar (NISAR) Synthetic Aperture Radar can observer Earth’s land and ice with unmatched precision, offering real-time insights into earthquakes, floods, and climate shifts. Figure credit: NASA/Jet Propulsion Laboratory–CaltechConclusion
Over the past 36 years, The Earth Observer has borne witness to some of the most monumental scientific achievements of NASA Earth Science and chronicled those stories for the community. While the format of the publication evolved considerably over the years, the satellite missions that have been the focus of this article are one of the primary “lenses” that the newsletter has had to observe and reflect on the story of NASA Earth Science. These continuous global observations have revolutionized society’s knowledge of our home planet and how humans might be altering it.
The staff of The Earth Observer have navigated many different modes of communication over the past three-and-a-half decades, but the commitment to delivering high-quality content has remained constant. It has been the highest honor of every member of our publication team – past and present – to work on this material. While the newsletter is coming to an end, it is hoped that the Archives page continues to be a rich source of historic information about NASA’s EOS and Earth science over the past three and a half decades.
On behalf of the current Editorial Team, we, the authors of this reflection, wish to thank every person who has contributed to the success of this newsletter over the years – and to extend to all in the NASA Earth Science community best wishes for the year ahead and continued success in your remote observation endeavors.
Stacy Kish
NASA’s Goddard Space Flight Center/EarthSpin
stacykishwrites@gmail.com
Alan B. Ward
NASA’s Goddard Space Flight Center/Global Science &Technology Inc.
alan.b.ward@nasa.gov
A Pioneering Study Assesses the Likelihood of Asteroid Mining
A team led by the Institute of Space Sciences (ICE-CSIC) analyzed samples of C-type asteroids in a recent study. Their findings support the idea that these asteroids can serve as a crucial source of materials if and when asteroid mining is realized.