Space & Astronomy

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

In their dying throes, some stars leave behind beautiful planetary nebulae — disk, spiral or even butterfly-shaped clouds of dust and gas (SN: 5/17/18).
How these fantastically shaped clouds arise from round stars is a mystery. New observations of red giant stars suggest that massive planets or other objects orbiting dying stars help stir up stellar winds and shape planetary nebulae, researchers report in the Sept. 18 Science.
“We were wondering how stars can get these beautiful shapes,” says Leen Decin, an astrophysicist at KU Leuven in Belgium. So she and her colleagues examined 14 stars in the red giant phase, before they become planetary nebulae. Data from the Atacama Large Millimeter/submillimeter Array in Chile revealed that stellar winds — fast-moving flows of gas, dust and subatomic particles such as protons — ejected from the red giant stars have different shapes, including spirals, disks and cones.
Mathematical calculations revealed that the mass and orbit of nearby objects, such as planets or another star, could be shaping these stellar winds. The researchers also made 3-D simulations based on the calculations. Stellar wind shapes created in the simulations largely matched those seen in the observations, the team found. The speed of the winds and how quickly a red giant loses mass as it slowly dies also play a role in making those shapes.
Because planetary nebula shapes resemble these winds’ shapes, the researchers conclude that these same forces influence a nebula’s final shape, long before the nebula itself is created.  “The action of the shaping does not happen when the star becomes a planetary nebula,” Decin says, but is already happening hundreds of thousands to millions of years before, during the red giant phase. This means that it might be possible to predict the shapes of planetary nebulae long before they form, she says.
Some aging red giant stars produce stellar winds in a range of shapes, including disks and spirals, as shown in these false-color images. (Red is stellar wind moving away from Earth; blue is stellar wind moving toward Earth).All images: L. Decin et al/Science 2020, ALMA/ESO
Capturing the new images with the same telescope in “great detail and high resolution” gives researchers a way to compare the winds of these dying stars one another, says Quentin Parker, an astrophysicist at the University of Hong Kong. Even when scientists look at different stars, there seem to be some common causes of the various shapes seen in the winds, he says.
Still, there’s sometimes too much time between the red giant phase and the planetary nebula phase to directly connect the two, Parker says. “Although companion objects may indeed play a major role in shaping both red giant winds and planetary nebula,” he says, it doesn’t mean that those stellar winds can always be used for “predicting what the planetary nebula will look like later.” More

For over a decade, astronomers have puzzled over the origins of fast radio bursts, brief blasts of radio waves that come mostly from distant galaxies. During that same period, scientists have also detected high-energy neutrinos, ghostly particles from outside the Milky Way whose origins are also unknown.
A new theory suggests that the two enigmatic signals could come from a single cosmic source: highly active and magnetized neutron stars called magnetars. If true, that could fill in the details of how fast radio bursts, or FRBs, occur. However, finding the “smoking gun” — catching a simultaneous neutrino and radio burst from the same magnetar — will be challenging because such neutrinos would be rare and hard to find, says astrophysicist Brian Metzger of Columbia University. He and his colleagues described the idea in a study posted September 1 at arXiv.org.
Even so, “this paper gives a possible link between what I think are two of the most exciting mysteries in astrophysics,” says astrophysicist Justin Vandenbroucke of the University of Wisconsin–Madison, who hunts for neutrinos but was not involved in the new work.
More than 100 fast radio bursts have been detected, but most are too far away for astronomers to see what drives the blasts of energy. Dozens of possible explanations have been debated, from stellar collisions to supermassive black holes to rotating stellar corpses called pulsars to pulsars orbiting black holes (SN: 1/10/18). Some astronomers have even invoked signals from aliens.

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But in the last few years, magnetars have emerged as a top contender. “We don’t know what the engines are of fast radio bursts, but there’s growing confidence that some fraction of them is coming from flaring magnetars,” Metzger says.
That confidence got a boost in April, when astronomers detected the first radio burst coming from within the Milky Way galaxy (SN: 6/4/20). The burst was close enough — about 30,000 light-years away — that astronomers could trace it back to a young, active magnetar called SGR 1935+2154. “It’s really like a Rosetta stone for understanding FRBs,” Vandenbroucke says.
There are several ways that magnetars could emit the bursts, Metzger says. The blasts of radio waves could come from close to the neutron star’s surface, for example. Or shock waves produced after the magnetar burped out an energetic flare, similar to those emitted by the sun, could create the radio waves.
Only those shock waves would produce neutrinos and fast radio bursts at the same time, Metzger says. Here’s how: Some magnetars emit flares repeatedly, enriching their surroundings with charged particles. Crucially, each flare would excavate some protons from the neutron star’s surface. Other situations could give a magnetar a halo of electrons, but protons would come only from the magnetar itself. If the magnetar has a halo of electrons, adding protons to the mix sets the stage for the double dose of cosmic phenomena.
As the next flare runs into the protons released by the previous flare, it would accelerate protons and electrons in the same direction at the same speeds. This “ordered dance” of electrons could give rise to the fast radio burst by converting the energy of the electrons’ movement into radio waves, Metzger says. And the protons could go through a chain reaction that results in a single high-energy neutrino per proton.
Together with astrophysicists Ke Fang of Stanford University and Ben Margalit of the University of California, Berkeley, Metzger calculated the energies of any neutrinos that would have been produced by the fast radio burst seen in April. The team found those energies matched those that could be detected by the IceCube neutrino observatory in Antarctica.
But IceCube didn’t detect any neutrinos from that magnetar in April, says Vandenbroucke, who has been searching for signs of neutrinos from fast radio bursts in IceCube data since 2016. That’s not surprising, though. Because neutrinos from FRBs are expected to be rare, detecting any will be challenging, and would probably require a particularly bright magnetar flare to be aimed directly at Earth.
Vandenbroucke has made bets with his students on other aspects of their research, but he says he won’t put any money down on whether he’ll see a neutrino from a fast radio burst in his lifetime. “There’s too much uncertainty,” he says.
Still, he’s optimistic. “Even detecting one neutrino from one [fast radio burst] would be a discovery, and it would take only one lucky FRB to produce a detectable neutrino,” he says. More

The NASA Artemis missions aim to send astronauts to the moon by 2024. But to succeed, they’ll need to solve big problems caused by some tiny particles: dust.
Impacts on the moon’s surface have crushed lunar rock into dust over billions of years (SN: 1/17/19). The resulting particles are like “broken shards of glass,” says Mihály Horányi, a physicist at the University of Colorado Boulder. This abrasive material can damage equipment and even harm astronauts’ health if inhaled (SN: 12/3/13). Making matters worse, the sun’s radiation gives moon dust an electric charge, so it sticks to everything.
Horányi and colleagues have discovered a new method for combatting lunar dust’s static cling, using a low-powered electron beam to make dust particles fly off surfaces. It complements existing approaches to the sticky problem, the researchers report online August 8 in Acta Astronautica.

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During the Apollo missions, astronauts relied on a low-tech system to clean lunar dust off their spacesuits: brushes. Such mechanical methods, however, are thwarted by the electrically charged nature of lunar dust, which clings to the nooks and crannies of woven spacesuit fabric.
The newly described method takes advantage of the dust’s electrical properties. An electron beam causes dust to release electrons into the tiny spaces between particles. Some of these negatively charged electrons are absorbed by surrounding dust specks. Because the charged particles repel each other, the resulting electric field “ejects dust off the surface,” says Xu Wang, a physicist also at the University of Colorado Boulder.
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Abrasive, electrically charged lunar dust clings to surfaces and could wreak havoc on equipment and astronaut well-being during missions to the moon. An electron beam may aid future cleaning efforts. As shown here, when a beam hits artificial lunar dust on a glass plate, particles leap off the surface.
“This is a very unique idea,” says mechanical engineer Hiroyuki Kawamoto of Waseda University in Tokyo, who was not involved in the new work. Kawamoto and colleagues have developed their own dust-busting technologies, including a layer of electrodes that can be built into materials. When embedded in a spacesuit or on the surface of equipment, the electrodes generate electrostatic forces and fling away charged dust particles. Such systems are more complex than shooting an electron beam at surfaces, Wang says. But a potential downside to the simpler electric beam idea, Kawamoto says, is that it would require a robot or some other external means to direct it.
Another limitation of the electron beam is that it left behind 15 to 25 percent of dust particles. The researchers aim to improve the cleaning power. The team also envisions the electron beam as one of multiple approaches that future space explorers will take to keep surfaces clean, Horányi says, in addition to suit design, other cleaning technologies and, one day, even lunar habitats with moon dust mudrooms. More