Space & Astronomy

NASA’s OSIRIS-REx spacecraft is a cosmic rock collector. Cheers erupted from mission control at 6:12 p.m. EDT on October 20 as scientists on Earth got word that the spacecraft had gently nudged a near-Earth asteroid called Bennu, and grabbed some of its rocks to return to Earth.
“The spacecraft did everything it was supposed to do,” said mission principal investigator Dante Lauretta of the University of Arizona in Tucson on a NASA TV webcast. “I can’t believe we actually pulled this off.”
OSIRIS-REx arrived at Bennu in December 2018, and spent almost two years making detailed maps of the 500-meter-wide asteroid’s surface features and composition (SN: 10/8/20). Observations from Earth suggested Bennu should be smooth and sandy, but when OSIRIS-REx arrived, it found a treacherous, rocky landscape.
The team selected a relatively smooth patch in a crater named Nightingale. The spot was not without hazards, though — the team was so worried about a particularly large rock nearby that they named it “Mount Doom” (SN: 12/12/19).

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Luckily, the spacecraft did not need to fully land in the crater to complete its mission. As it hovered just above the surface, OSIRIS-REx reached out a robotic arm with an instrument called TAGSAM at the end, for Touch-And-Go Sample Acquisition Mechanism. The instrument tapped the asteroid lightly for several seconds, and released a burst of nitrogen gas to disturb the surface dust and pebbles. Once those small rocks were lofted, some hopefully were blown into the sample collector.
Because signals from Earth took 18½ minutes to reach Bennu, the spacecraft performed the sampling sequence autonomously. When the mission team got the signal that the spacecraft had finished its job and retreated to a safe distance from Bennu, team members pumped their arms in the air, cheered and sent each other socially distant high-fives and hugs.
OSIRIS-REx is not the first spacecraft to grab samples from an asteroid. That distinction goes to Japan’s Hayabusa mission, which brought back grains of asteroid Itokawa in 2010 (SN: 6/14/10). An encore to that mission, Hayabusa2, collected samples of asteroid Ryugu last year, and is on track to land in Australia in December (SN: 2/22/19).
But OSIRIS-REx attempted to collect much more material than Hayabusa2 did. Hayabusa2 hoped to collect 100 milligrams; OSIRIS-REx is aiming for a minimum of 60 grams, or a little more than two ounces.
Hayabusa2’s scientists have no way to know how much material it actually collected until the spacecraft returns to Earth. But OSIRIS-REx’s team plans to find out using the spacecraft itself. On October 24, the spacecraft will extend its arm and spin its whole body. The difference in the way it spins before and after the sample collection will reveal the mass of the sample.
OSIRIS-REx will return to Earth in 2023, where scientists will analyze the rocks in hopes of unlocking details of the history of the solar system and the origins of water and life on Earth (SN: 1/15/19). More

A strange, newly measured clump of stars orbiting the nearby Andromeda galaxy has the lowest level of heavy chemical elements ever seen in one of these mysterious star clusters. Named RBC EXT8, this globular cluster is also surprisingly massive, challenging theories for how such clusters and some galaxies form, astronomers report online October 15 in Science.
“It’s a very unusual object,” says astrophysicist Oleg Gnedin of the University of Michigan in Ann Arbor, who was not involved in the new discovery.
Globular clusters are crowded, spherical collections of stars that orbit a galaxy’s center, though most, including RBC EXT8, live in the galactic outskirts. The clusters are typically billions of years old, so their stars tend to be chemically pristine, meaning they formed before the universe had time to create much of any of the elements heavier than hydrogen or helium, which astronomers lump together as “metals.”
Previous observations of these clusters in the Milky Way and other galaxies had suggested that there’s a limit to how low a globular cluster’s metal content can be. The most metal-poor clusters were about 300 times less rich in heavy elements like iron than the sun, but no less.
But spectra of RBC EXT8, some 2.5 million light-years away, show that the cluster’s metal content is about 800 times less than the sun’s. The globular cluster that held the previous record for lowest “metallicity” has three times that amount.
“It was completely unexpected that we would find a globular cluster that is so metal poor,” says astronomer Søren Larsen of Radboud University in Nijmegen, the Netherlands.
The bigger, fuzzy blob in the inset image at left is RBC EXT8, a globular cluster that orbits about 88,000 light-years from the center of the galaxy Andromeda (shown at right). The cluster has surprisingly few heavy elements for its size, a new study finds.© 2020 ESASky, CFHT
What’s more, given its metal-poor status, this cluster is surprisingly massive, weighing about 1.14 million times the mass of the sun. (A mid-weight globular cluster is about 100,000 solar masses, but some clusters reach 3 million solar masses. RBC EXT8 is heavy, but not the heaviest.)
That mass makes the cluster even harder to explain because across the cosmos, the more massive a galaxy or cluster is, the more heavy elements it normally has.
There are several potential explanations for that trend, but one is simply that more massive galaxies or globular clusters have more stars. A star fuses heavy elements in its core and sprinkles them around its host cluster or galaxy as it ages. Sufficiently massive stars can explode in a supernova, spreading those metals to become part of the next generation of stars (SN: 8/9/19). So more stars means more opportunity for metals to accumulate locally.
More massive objects also have the advantage of gravity, which lets them better hold on to the metals that they do have and remain a cohesive group for billions of years. Less massive globular clusters dissolve into their host galaxies over time.
Those trends together could have explained the apparent “metallicity floor” for globular clusters — all of the less massive, more metal-poor clusters have broken apart over the eons.
RBC EXT8 turns that conventional wisdom on its head. “It’s too big to have as low metallicity as it has,” Gnedin says. “That’s the conundrum.”

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Astronomers aren’t sure how globular clusters form in general, but they probably grow within galaxies, rather than forming outside of them and being pulled in later. And so the clusters reflect the characteristics of their galaxies: small, metal-poor galaxies end up with small metal-poor globular clusters, and vice versa. But based on RBC EXT8’s metal content, it’s galactic birthplace would be less than a million solar masses, so smaller than the globular cluster itself – which is a paradox.
As a result, the cluster challenges some simplified models of galaxy formation. But it doesn’t completely break them, Gnedin says. “It’s one object, it’s not going to overturn things,” he says. “It just makes us people working on these issues have to work harder” and be more open-minded about other ways that galaxies could form.
Open-mindedness and willingness to explore is perhaps responsible for the new finding about RBC EXT8’s metals. Larsen and colleagues spotted the globular cluster at the beginning of a night of observing with the Keck telescope in Hawaii in October 2019. “It was really a serendipitous discovery,” he says. He had a spare hour before the globular clusters in galaxy M33 that his team was planning to look at rose above the horizon, so the observers picked another cluster “more or less at random” to fill the time.
“At first, I couldn’t really believe that what was coming out [in the observations] was right,” Larsen says. “But I kept working on it, and it turned out to hold up.” More

When NASA’s OSIRIS-REx arrived at near-Earth asteroid Bennu, scientists were dismayed to find a surface covered with hazardous-looking boulders.
But new research suggests that those boulders are surprisingly brittle. That’s potentially good news for the spacecraft, which is charged with grabbing a piece of Bennu on October 20 and returning it to Earth in 2023 (SN: 1/15/19). If the rocks are crumbly, that could lower the risk of damaging the spacecraft’s equipment.
That kind of rock also may be too fragile to survive the trip through Earth’s atmosphere without burning up. If so, scientists may be close to getting their hands on a never-before-seen kind of space rock, researchers report in a collection of papers published October 8 in Science and Science Advances.
Data taken from Earth before OSIRIS-REx launched suggested that Bennu’s surface would be sandy. So it was a shock to find a rough landscape strewn with boulders when the spacecraft arrived in 2018 (SN: 12/3/18).
“We had really convinced ourselves that Bennu was a smooth object,” says Daniella DellaGiustina, a planetary scientist at the University of Arizona in Tucson and member of the OSIRIS-REx team. “As everyone saw from the first pictures, that was not the case.”
The team found a relatively clear crater, nicknamed Nightingale, from which to retrieve a sample of the space rock (SN: 12/12/19). Still, the worry remains that the boulders might pose a safety hazard for the sampling system, which was designed to handle pebbles only a few centimeters across.
From late April through early June 2019, planetary scientist Ben Rozitis of the Open University in Milton Keynes, England, and colleagues mapped the way Bennu’s boulders retain heat, a clue to the rocks’ structure. Denser materials hold heat better than finer-grained ones, like how a sandy beach cools quickly after sundown, but single large rocks remain warm.
This map shows where carbon-bearing minerals (represented by redder colors) are located on Bennu’s surface. The opportunity to analyze those minerals could help scientists figure out how carbon got to the early Earth.A. Simon et al/Science 2020
Based on those maps — and maps of other surface properties, described in the series of papers released October 8 — Bennu’s boulders seem to come in two flavors: darker-colored rocks that are weaker and more porous and lighter-colored, denser rocks that are stronger and less porous. Even the denser rocks are much more porous and brittle than meteorites from similar asteroids that have been found on Earth. The least dense meteorites are about 15 percent porous; Bennu’s rocks seem to be between 30 and 50 percent porous, Rozitis and colleagues found.
“This is exciting,” says DellaGiustina, a coauthor of the new papers. The spacecraft and its instruments might “encounter some boulders at the sample site that might otherwise be difficult to ingest,” she says, but “if they’re porous and weak, then they might just break down,” making them easier to collect.
The lighter, denser rocks also appear to be shot through with veins of carbonate, which suggests that they were in the presence of flowing water at some point in their past (SN: 12/10/18). NASA chose Bennu as an asteroid to visit partly because it resembles carbonaceous chondrite meteorites, which scientists think are time capsules of the early solar system. Similar space rocks could have delivered water and organic materials to Earth billions of years ago.
But Bennu’s more porous rocks appear to be unlike anything in scientists’ current assortment of meteorites, Rozitis says. “This is one of the cool things about OSIRIS-REx — it’s quite likely it will pick up new material that isn’t in our meteorite collection,” he says.

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That’s believable, says meteor scientist Bill Cooke of NASA’s Marshall Space Flight Center in Huntsville, Ala. Observations of meteors have shown that low-density space rocks and dust burn up higher in Earth’s atmosphere than higher-density rocks.
“The old conventional wisdom was that the low-density stuff was from comets, and the high-density stuff was from asteroids,” he says. But recent observations show that some of the low-density rocks come from the orbits of asteroids. “So it is very plausible that low-density stuff from Bennu … would ablate higher in the atmosphere and not have a chance to create meteorites at all.”
If Bennu represents a missing piece in our understanding of the solar system’s history, studying that material in labs on Earth “will help us fill in an additional piece of the jigsaw,” Rozitis says. More

When the first-ever image of a black hole was released in April 2019, it marked a powerful confirmation of Albert Einstein’s theory of gravity, or general relativity.
The theory not only describes the way matter warps spacetime, but it also predicts the very existence of black holes, including the size of the shadow cast by a black hole on the bright disk of material that swirls around some of the dense objects. That iconic image, of the supermassive black hole at the center of the galaxy M87 about 55 million light-years away, showed that the shadow closely matched general relativity’s predictions of its size (SN: 4/10/19). In other words, Einstein was right — again.
That result, reported by the Event Horizon Telescope Collaboration, answered one question: Is the size of M87’s black hole consistent with general relativity?
But “it is very difficult to answer the opposite question: How much can I tweak general relativity, and still be consistent with the [black hole] measurement?” says EHT team member Dimitrios Psaltis of the University of Arizona in Tucson. That question is key because it’s still possible that some other theory of gravity could describe the universe, but masquerade as general relativity near a black hole.

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In a study published October 1 in Physical Review Letters, Psaltis and colleagues have used the shadow of M87’s black hole to take a major step toward ruling out those alternative theories.
Specifically, the researchers used the size of the black hole to perform what’s known as a “second-order” test of general relativity geared toward boosting confidence in the result. That “can’t really be done in the solar system” because the gravitational field is too weak, says EHT team member Lia Medeiros of the Institute for Advanced Study in Princeton, N.J.
So far so good for relativity, the researchers found when they performed this second-order test.
The results are on par with those from gravitational wave experiments like the Advanced Laser Interferometer Gravitational-Wave Observatory, which has detected ripples in spacetime from the merger of black holes smaller than M87’s (SN: 9/16/19). But the new study is interesting because “it’s the first attempt at constraining a [second-order] effect through a black hole observation,” says physicist Emanuele Berti of Johns Hopkins University, who was not involved in the new work.
Generally, physicists think of general relativity as a set of corrections or add-ons to Isaac Newton’s theory of gravity. General relativity predicts what those add-ons should be. If measurements of how gravity works in the universe deviate from those predictions, then physicists know general relativity is not the full story. The more add-ons or factors added to a test, the more confidence there is in a result.
In weak gravitational fields, like within the solar system, physicists can test whether “first-order” additions to Newton’s equations are consistent with general relativity or not. These additions are related to things like how light and mass travel in a warped spacetime, or how gravity makes time flow more slowly.
Those aspects of gravity have been tested with the way stars’ light is deflected during a solar eclipse for example, and the way laser light sent to spacecraft flying away from the sun takes longer than expected to return to Earth (SN: 5/29/19). General relativity has passed every time.
But it takes a strong gravitational field, like the one around M87’s black hole, to kick the tests up a notch.
The new result is slightly disappointing for the physicists hoping to find cracks in Einstein’s theory. Finding a deviation from general relativity could point the way to new physics. Or it could help unite general relativity, the physics of the very large, and quantum mechanics, the leading theory that describes the physics of the very small, like subatomic particles and atoms (SN: 3/30/20). The fact that general relativity still refuses to bend is “worrying for those of us who are old enough that we were hoping to get an answer in our lifetime,” Psaltis says.
But there is some hope that general relativity might still fail around black holes. The new study makes the box of possible ways for the theory to break down smaller, “but we haven’t made it infinitesimal,” Medeiros says. The study is “a proof of concept to show that the EHT could do this… But it’s really just step one of many.”  
Future observations from the EHT will make for even more precise tests of general relativity, she says, especially with yet-to-be-released images of Sgr A*, the black hole at the center of the Milky Way. With much more precise measurements of Sgr A*’s mass than any other supermassive black hole, that image may make the possible box around the theory even smaller — or blow it wide open. More

Fresh intel from Mars is sure to stir debate about whether liquid water lurks beneath the planet’s polar ice.
New data from a probe orbiting Mars appear to bolster a claim from 2018 that a lake sits roughly 1.5 kilometers beneath ice near the south pole (SN: 8/18/18). An analysis of the additional data, by some of the same researchers who reported the lake’s discovery, also hint at several more pools encircling the main reservoir, a study released online September 28 in Nature Astronomy claims.
If it exists, the central lake spans roughly 600 square kilometers. To keep from freezing, the water would have to be extremely salty, possibly making it similar to subglacial lakes in Antarctica. “This area is the closest thing to ‘habitable’ on Mars that has been found so far,” says Roberto Orosei, a planetary scientist at the National Institute for Astrophysics in Bologna, Italy, who also led the 2018 report.
Ali Bramson, a planetary scientist at Purdue University in West Lafayette, Ind., agrees “something funky is going on at this location.” But, she says, “there are some limitations to the instrument and the data…. I don’t know if it’s totally a slam dunk yet.”

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Orosei and colleagues probed the ice using radar on board the European Space Agency’s Mars Express orbiter. Short bursts of radio waves reflect off the ice, but some penetrate deeper and bounce off the bottom of the ice, sending back a second echo. The brightness and sharpness of that second reflection can reveal details about the underlying terrain.
The possible lake was originally found using radar data collected from May 2012 to December 2015. Now, in data collected from 2010 to 2019, the team once again found regions beneath the ice that are highly reflective and very flat. They say their findings not only confirm earlier hints of a large buried lake but also unearth a handful of smaller ponds encircling the main body of water and separated by strips of dry land.
“On Earth, there would be no debate” that a bright, flat radar reflection would be liquid water, Orosei says. These same analysis techniques have been used closer to home to map subglacial lakes in Antarctica and Greenland.
While much about these putative ponds remains unknown, one thing is certain: This new report is bound to spark controversy. “The community is very polarized,” says Isaac Smith, a planetary scientist with the Planetary Science Institute who is based in Ontario, Canada. “I’m in the camp that leans towards believing it,” he adds. “They’ve done their homework.”
One question centers on how water could stay liquid. “There’s no way to get liquid water warm enough even with throwing in a bunch of salts,” says planetary scientist Michael Sori, also at Purdue.
In 2019, he and Bramson calculated that the ice temperature — about –70° Celsius — is too cold even for salts to melt. They argue some local source of geothermal heat is needed, such as a magma chamber beneath the surface, to maintain a lake. That in turn has led to other questions about whether contemporary Mars could supply the necessary heat.
Smith — as well as the paper’s authors — thinks this isn’t a problem. As recently as 50,000 years ago, Smith says, the Martian south pole was warmer because the planet’s tilt (and hence its seasons) is constantly changing. Warmer temperatures could have propagated through the ice to create pockets of salty liquid. Alternatively, the ponds may have been there before the ice cap formed. Either way, at very high salt concentrations, once water has melted, it’s hard to get it to freeze again. “The melting temperature is different than the freezing temperature,” he says.
Even so, such liquid may be unlike any that most earthlings are familiar with. “Some supercooled brines at these cold temperatures are still considered liquid but turn into some weird glass,” Bramson says.
Resolving these questions will probably require more than radar. Multiple factors, such as the composition and physical properties of the ice, can alter the fate of the second echo from the bottom of the ice, says Bramson. Seismology, gravity and topography data could go a long way to revealing what lurks beneath the ice.
Whether anything could survive in such water is an open question. “We don’t know exactly what is in this water,” Orosei says.  “We don’t know the concentration of salts, which could be deadly to life.” But if life did evolve on Mars, he speculates, “these lakes could have been providing a Noah’s Ark that could have allowed life to survive even in in present conditions.“ More

A two-month stint on the moon would expose astronauts to roughly the same amount of radiation as they would get living on the International Space Station for five months, according to new measurements from the lunar surface.
Detectors on China’s lunar lander Chang’e-4 measured radiation from galactic cosmic rays at the moon’s surface in 2019, from January 3 to 12 — just after landing on the farside of the moon — and again from January 31 to February 10. An astronaut would be exposed to an average daily dose of 1,369 microsieverts of radiation, researchers report online September 25 in Science Advances.
That’s about 2.6 times as high as the average daily radiation exposure of 523 microsieverts recorded inside the ISS, the scientists say. Being on the moon “for two months would be OK. That is about the same amount of radiation astronauts receive at the ISS [over five months] and wouldn’t be incredibly dangerous,” says coauthor Robert Wimmer-Schweingruber, a physicist at Christian Albrechts University in Kiel, Germany.
The new study is perhaps the first to measure cosmic radiation at the moon’s surface, says Jeffery Chancellor, a physicist at Louisiana State University in Baton Rouge. “This is [a] pretty cool bit of data.” He cautions that radiation levels on other parts of the space station could be higher, so the authors may have overprojected the exposure difference between the moon’s surface and the ISS.
Galactic cosmic rays, high-energy charged particles that zip through space, come from outside the solar system. Earth’s magnetic field protects humans from these rays, but in space, it’s a whole different story.
Long exposure to such radiation can cause cellular and DNA damage resulting in cancers, cataracts, cardiac problems, neurodegenerative diseases and behavioral impairments, animal studies have shown (SN: 7/15/20). So far, it’s unclear exactly what impact such exposure might have on human health. The effects of spending a large amount of time in space may show many years after someone has been exposed, says Marjan Boerma, a radiation biologist at the University of Arkansas for Medical Sciences in Little Rock. 
The findings come at a time when United States and other nations are making plans to land humans on moon for the first time in decades (SN: 12/16/19). NASA has announced its plans to land the first U.S. woman and a man on the moon’s surface by 2024. More