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China's Relay Satellite is in Lunar Orbit
On March 20th, China’s Queqiao-2 (“Magpie Bridge-2”) satellite launched from the Wenchang Space Launch Site LC-2 on the island of Hainan (in southern China) atop a Long March-8 Y3 carrier rocket. This mission is the second in a series of communications relay and radio astronomy satellites designed to support the fourth phase of the Chinese Lunar Exploration Program (Chang’e). On March 24th, after 119 hours in transit, the satellite reached the Moon and began a perilune braking maneuver at a distance of 440 km (~270 mi) from the lunar surface.
The maneuver lasted 19 minutes, after which the satellite entered lunar orbit, where it will soon relay communications from missions on the far side of the Moon around the South Pole region. This includes the Chang’e-4 lander and rover and will extend to the Chang’e-6 sample-return mission, which is scheduled to launch in May. It will also assist Chang’e-7 and -8 (scheduled for 2026 and 2028, respectively), consisting of an orbiter, rover, and lander mission, and a platform that will test technologies necessary for the construction of the International Lunar Research Station (ILRS).
A perilune braking maneuver is vital to establishing a lunar orbit and consists of a thruster firing as the spacecraft approaches the Moon. This reduces the spacecraft’s relative velocity to less than the lunar escape velocity (2.38 km/s; 1.74 mps) so that it can be captured by the Moon’s gravity. Two experimental satellites that will test navigation and communication technology (Tiandu-1 and -2), which accompanied the Queqiao-2 satellite to the Moon, also performed a perilune braking maneuver and entered lunar orbit on Monday.
These two satellites will remain in formation in an elliptical lunar orbit and will conduct communication and navigation tests, including laser ranging with the Moon and microwave ranging between satellites. According to the CNSA, Queqiao-2 will enter a 24-hour elliptical orbit around the Moon at a distance of 200 km (125 mi) at its closest point (perigee) and 100,000 km (62,000 mi) at its farthest point (apogee). Mission controllers will further alter Queqiao-2’s orbit and inclination to bring it into a “200 by 16,000-km, highly-elliptical ‘frozen’ orbit.”
Within this highly stable orbit, Queqiao-2 will have a direct line of sight with ground stations on Earth and the far side of the Moon and will conduct communication tests with Chang’e-4 and Chang’e-6 using its 4.2-m (13.8-ft) parabolic antenna. The mission could also support other countries in their lunar exploration efforts, many of whom are also interested in scouting the Moon’s far side and southern polar region. The satellite also carries scientific instruments, including extreme ultraviolet cameras, array-neutral atom imagers, and lunar orbit Very Long Baseline Interferometry (VLBI) test subsystems.
According to state-owned media company CCTV, the CNSA chose the Queqiao-2 satellite’s present orbit for a multitude of reasons:
“Experts told me that this is an ideal location on the Moon to observe the separation of the Queqiao-2 star arrow, and it also has a deep connection with China’s lunar exploration project. This is the Moon’s rich maria region… Fifteen years ago, on March 1, 2009, it was here that the Chang’e-1 probe of China’s lunar exploration project completed a controlled collision with the Moon… The location of the Sea of Abundance on the moon is also very eye-catching. The next time the moon is full, you look up at the moon and find this dark black patch in the southeast of the moon. This is the Sea of Abundance!”
Visualization of the ILRS from the CNSA Guide to Partnership (June 2021). Credit: CNSAThe satellite will support China’s upcoming Chang’e-6 mission, China’s second attempt to return lunar samples to Earth. Mission controllers will adjust its orbit into a 12-hour period to support the Chang’e-7 and -8 missions. These missions aim to map the terrain and scout resources (particularly water ice) around the South Pole-Aitken Basin. These missions will ultimately support the creation of the ILRS, a joint project between CNSA and Roscomos to create a lunar base that will enable research and development on the Moon.
This program is intended to rival NASA’s Artemis Program, which will send astronauts on a circumlunar flight next year – the Artemis II mission. The program will culminate in 2026 with the first crewed mission to the lunar surface (Artemis III) in over 50 years. NASA also plans to deploy the core elements of the Lunar Gateway next year, an orbital habitat that will facilitate the deployment of the Artemis Base Camp. Along with its international and commercial partners, these elements will support the creation of “a sustained program of lunar exploration and development.”
Further Reading: CGTN
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The Hubble Aims Its Powerful Ultraviolet Eye at Super-Hot Stars
Some stars are so massive and so energetic that they’re a million times brighter than the Sun. This type of star dominated the early Universe, playing a key role in its development and evolution. The first of its kind are all gone now, but the modern Universe still forms stars of this type.
These hot, blue stars emit powerful ultraviolet energy that the Hubble can detect from its perch in Low-Earth Orbit.
In December 2023, astronomers completed a three-year survey of these hot stars. It’s one of the Hubble’s largest and most ambitious surveys. It’s called ULLYSES (Ultraviolet Legacy Library of Young Stars as Essential Standards), and in it, astronomers gathered detailed information on almost 500 stars.
UV emissions from hot young stars provide a window into some of the processes inside these stars. UV can’t be observed from Earth because the ozone layer blocks it. That’s one of the reasons the Hubble was built. From its perch, it can gather high-resolution UV images. That’s the impetus for ULLYSES.
The survey doesn’t contain images of all the stars. Instead, the Hubble gathered spectra from 220 stars and combined them with Hubble archival data on 275 additional stars. Powerful ground-based telescopes also made a contribution, though not in UV. The result is a very rich dataset consisting of detailed spectra from both hot, bright, massive stars and from cool, dim, low-mass stars.
“I believe the ULLYSES project will be transformative, impacting overall astrophysics – from exoplanets to the effects of massive stars on galaxy evolution, to understanding the earliest stages of the evolving universe,” said Julia Roman-Duval, Implementation Team Lead for ULLYSES at the Space Telescope Science Institute (STScI) in Baltimore, Maryland. “Aside from the specific goals of the program, the stellar data can also be used in fields of astrophysics in ways we can’t yet imagine.”
The ULYSSES spectra collected by Hubble can reveal the presence of chemical elements in the stars. Image Credit: Hubble/ STScI/ULYSSESSpectra can tell astronomers more than just the metallicity of the stars. They can also reveal the powerful stellar winds coming from the hot blue stars.
Massive blue stars have powerful winds that shape their surroundings. The Hubble spectra can tell which way the winds travel and how fast they travel. The star represented by the teal line has slower winds than the star shown by the purple line. Image Credit: Hubble/ STScI/ULYSSESSpectra also reveal the metallicity of stars. Stars with lower metallicity are typically older than stars with higher metallicity. A critical part of stellar metallicity concerns the iron content. Astronomers use iron content and its ratio with hydrogen to date stars in relation to our own Sun’s iron and hydrogen ratio.
These spectra show the iron content for two stars. In this image, the star represented by the purple line has less iron, indicating that it’s older than the other star. Iron content affects a star’s lifetime and the strength of its winds. Image Credit: Hubble/ STScI/ULYSSESIn ULYSSES, Hubble targeted hot blue stars in nearby galaxies with low metallicity, the type that would’ve existed in the early Universe. At that point in the Universe’s life, they would’ve contained nothing heavier than hydrogen and helium. This type of galaxy was common in the very early universe. Only once these hot young stars died and spread the elements they created inside themselves would the heavier elements needed for rocky planets, water, and even life be available. “ULLYSES observations are a stepping stone to understanding those first stars and their winds in the Universe and how they impact the evolution of their young host galaxy,” said Roman-Duval.
ULLYSES also observed stellar counterparts to the massive, hot stars: cool, red, low-mass, and dim stars. While the more massive stars form quickly, burn bright, and die soon, these ones are the opposite. They take longer to form, are dimmer, and last much longer. But they still emit winds and energy that shape their surroundings. They’re called T-Tauri stars, stars so young they’re still growing.
As part of the three-year ULYSSES survey, the Hubble also observed cool, dim, low-mass stars like the one in this artist’s illustration, which are still growing by accreting material from their disks. Image Credit: Robert O’Connell (UVA), SOC-WFC3, ESODespite their lower masses, these stars emit powerful radiation. During their formation, they’re known to unleash powerful blasts of both UV and X-ray radiation.
There are outstanding questions about T-Tauri stars and how they behave. Some of their processes are obscured. But the Hubble spectra from ULYSSES can provide some answers. They can reveal how much energy T-Tauri stars release as they grow and how powerful their winds are. Their powerful winds can alter their protoplanetary disks, blowing material away and making it unavailable for planet formation. In some cases, the powerful energy from these stars could eliminate the habitability of any planets forming around them.
The ULYSSES data is not meant to answer any specific question. Rather, it’s a massive database of detailed spectra that researchers can query to serve future research. The overarching goal is to provide an in-depth database of spectra from young stars that are in the first 10 million years of their lives.
“More fully understanding the formation and lives of young stars has connections to many other areas in astronomy, including galaxy formation and evolution, the mechanics and mass loss of supernovas, how stars’ environments impact planet formation, and how their emissions may play a role in the makeup of the interstellar medium, the gas and dust between stars in a galaxy,” the ULYSSES website explains.
ULYSSES is an observing program designed by the research community for the research community. By extension, it also serves those of us who like to follow along as researchers discover new things about the Universe.
“ULLYSES was originally conceived as an observing program utilizing Hubble’s sensitive spectrographs. However, the program was tremendously enhanced by community-led coordinated and ancillary observations with other ground- and space-based observatories,” said Roman-Duval. “Such broad coverage allows astronomers to investigate the lives of stars in unprecedented detail and paint a more comprehensive picture of the properties of these stars and how they impact their environment.”
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