HOTTEST
It worked! Humanity has, for the first time, purposely moved a celestial object.
As a test of a potential asteroid-deflection scheme, NASA’s DART spacecraft shortened the orbit of asteroid Dimorphos by 32 minutes — a far greater change than astronomers expected.
The Double Asteroid Redirection Test, or DART, rammed into the tiny asteroid at about 22,500 kilometers per hour on September 26 (SN: 9/26/22). The goal was to move Dimorphos slightly closer to the larger asteroid it orbits, Didymos.
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Neither Dimorphos nor Didymos pose any threat to Earth. DART’s mission was to help scientists figure out if a similar impact could nudge a potentially hazardous asteroid out of harm’s way before it hits our planet.
The experiment was a smashing success. Before the impact, Dimorphos orbited Didymos every 11 hours and 55 minutes. After, the orbit was 11 hours and 23 minutes, NASA announced October 11 in a news briefing.
A small spacecraft called LICIACube, short for Light Italian CubeSat for Imaging of Asteroids, detached from DART just before impact, then buzzed the two asteroids to get a closeup view of the cosmic smashup. Starting from about 700 kilometers away, this series of images captures a bright plume of debris erupting from Dimorphos (right in the first half of this gif), evidence of the impact that shortened its orbit around Didymos (left). At closest approach, LICIACube was about 59 kilometers from the asteroids.ASI, NASA
“For the first time ever, humanity has changed the orbit of a planetary body,” said NASA planetary science division director Lori Glaze.
Four telescopes in Chile and South Africa observed the asteroids every night after the impact. The telescopes can’t see the asteroids separately, but they can detect periodic changes in brightness as the asteroids eclipse each other. All four telescopes saw eclipses consistent with an 11-hour, 23-minute orbit. The result was confirmed by two planetary radar facilities, which bounced radio waves off the asteroids to measure their orbits directly, said Nancy Chabot, a planetary scientist at Johns Hopkins Applied Physics Laboratory in Laurel, Md.
The minimum change for the DART team to declare success was 73 seconds — a hurdle the mission overshot by more than 30 minutes. The team thinks the spectacular plume of debris that the impactor kicked up gave the mission extra oomph. The impact itself gave some momentum to the asteroid, but the debris flying off in the other direction pushed it even more — like a temporary rocket engine.
“This is a very exciting and promising result for planetary defense,” Chabot said. But the change in orbital period was just 4 percent. “It just gave it a small nudge,” she said. So knowing an asteroid is coming is crucial to future success. For something similar to work on an asteroid headed for Earth, “you’d want to do it years in advance,” Chabot said. An upcoming space telescope called Near Earth Asteroid Surveyor is one of many projects intended to give that early warning. More
A new study casts a haze over a hint of the universe’s first glimmers of starlight.
In 2018, researchers claimed that a subtle signature in radio waves from early in the universe’s history had revealed the era when the first stars switched on, known as the cosmic dawn. But the first experiment to test that study’s conclusions found no sign of those early stars, scientists report February 28 in Nature Astronomy.
Just after the Big Bang about 13.8 billion years ago, the universe was a hot stew of matter. Stars probably didn’t flicker on until at least 100 million years later — a poorly understood era of the cosmos. Finding signs of the first beams of starlight would flesh out the cosmic origin story. So the 2018 claim of pinpointing those earliest gleams, from the EDGES experiment in the Australian outback, caused an astronomical hubbub (SN: 2/28/18).
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“It definitely completely excited our whole community with this fascinating result,” says radio astronomer Saurabh Singh of the Raman Research Institute in Bangalore, India.
The researchers reported detecting a dip across particular wavelengths of radio waves, a sign of light from the first stars interacting with surrounding hydrogen gas. But the result quickly raised skepticism, because the dip was deeper than expected. To know whether the hint of the first starlight was real, scientists would need to make more measurements.
Singh and colleagues did just that with the Shaped Antenna Measurement of the Background Radio Spectrum 3, or SARAS 3. Similar to EDGES, the experiment uses an antenna to pick up radio waves. But SARAS 3 has a different design from EDGES, with a differently shaped antenna. And SARAS 3 is designed to float atop a lake. “That gives us a very distinctive advantage,” Singh says.
On Earth, radio waves come from a variety of sources, which must be carefully accounted for to reveal the subtler signal from the cosmic dawn. Misunderstanding those other sources of radio waves could lead to an unaccounted-for experimental error that might give incorrect results.
In particular, experiments on land must contend with radio waves emitted from the ground, which are difficult to estimate due to the complex, layered nature of soil. When the antenna is atop a lake, it’s easier to estimate what kinds of radio waves come from the uniform water below. Data taken from two lakes in India revealed no sign of the dip.
The new study “highlights just how difficult this measurement is,” says physicist H. Cynthia Chiang of McGill University in Montreal. It’s uncomfortable that the two studies disagree, she says, but notes that the disagreement “isn’t quite enough to make any definitive conclusions at this point.”
And some of the same types of experimental issues that may affect EDGES could also affect SARAS 3, says experimental cosmologist Judd Bowman of Arizona State University in Tempe, a member of the EDGES team. “We still have more work ahead to reach the final outcome.”
An improved version of EDGES will be deployed later this year, and the SARAS 3 team has additional deployments planned. Other experiments are also working on similar measurements. Those tests may finally illuminate the universe’s transition from darkness to light. More
The Milky Way might have once looked more like a legume than a starry whirlpool.
Over their unfathomably long lifetimes, spiral galaxies like the Milky Way are generally thought to morph into lentil-shaped “lenticular” galaxies and then into elliptical blobs (SN: 4/23/18). But an analysis of nearby galaxies suggests that our galaxy, and others like it, was once lenticular, astronomer Alister Graham reports in the July Monthly Notices of the Royal Astronomical Society. If correct, Graham’s proposed update to the evolutionary sequence of galaxies would rewrite the history of the Milky Way.
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}“Lenticulars have always been sort of the abandoned stepchild of [galaxy] morphology,” says astronomer Christopher Conselice of the University of Manchester in England, who wasn’t involved in the study. But this paper puts them into focus, he says, as being a major aspect of how galaxies change.
Lenticulars get their name from the way their entire halo of stars, when viewed edge on, bulges in the middle and thins out toward the sides, much like a lentil. These galaxies exhibit a confusing mix of properties that’s made their presumed place in the middle of galaxy evolution sequences rather suspect.
“We’ve known for a while that that’s almost certainly not correct,” Conselice says. Particularly puzzling is that lenticulars, despite their spiral-like disks, don’t have lots of gas, which hinders them from producing new stars. Spiral galaxies do have lots of star-forming gas, and scientists aren’t sure why lenticular galaxies don’t.
Graham, of Swinburne University of Technology in Hawthorn, Australia, found new clues to this mystery of galaxy evolution by considering black holes.
Most galaxies harbor a supermassive black hole in their center, and when galaxies merge, so do those black holes. This makes the mass of a galaxy’s black hole a kind of record of its past collisions. If a galaxy got big by gobbling up its neighbors rather than by sucking up surrounding gas, its black hole should be hefty relative to the swarm of stars that surrounds it.
Using images from the Hubble and Spitzer space telescopes, Graham compared the black hole and stellar masses of about 100 nearby galaxies. For galaxies of the same shape, he saw that black hole mass and stellar mass tend to be linked in a predictable way — except for the lenticular galaxies.
When Graham took a closer look at the lenticulars, he realized they are actually two distinct groups that had been lumped together: those that have lots of interstellar dust and those that do not. This division, which he previously reported in the May Monthly Notices of the Royal Astronomical Society, could have been a superficial aesthetic difference. But the galaxies’ black hole masses suggest otherwise.
Dust-poor and dust-rich lenticulars have entirely different relationships between their black hole masses and stellar masses, suggesting different histories and explaining the apparently scattered behavior of lenticular galaxies. The dusty galaxies tend to have a heftier supermassive black hole than the ones found in both spirals and dust-poor lenticulars. Dust-poor lenticulars are usually on the small side in terms of both black hole mass and stellar mass.
This led Graham to conclude that spiral galaxies are actually in between the two types of lenticulars, evolutionarily speaking. His new analysis suggests that dust-poor lenticulars become spirals after capturing small “satellite galaxies” and other minor mergers — bumping up their black hole masses — and scooping up nearby gas.
When spirals collide with other substantial galaxies, he proposes, they become dust-rich lenticulars — and indeed, he adds, every dust-rich lenticular in his dataset was previously recognized as the remnant of a spiral galaxy merger. Collisions between these dust-rich lenticulars are then enough to finally erode the galaxies’ discs of stars and destroy their dust, producing blobby elliptical galaxies.
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Black holes are a good tracer of galaxy evolution, Conselice says, but the new sequence could be controversial. One issue, he says, is that lenticular galaxies in the nearby universe are usually such lightweights that they would need to merge tens or even hundreds of times — far more than the expected average of around three over 10 billion years — to form a large spiral galaxy.
But things might have been different in the early universe, he adds. Long ago, there could have been more massive lenticulars. Figuring that out might be possible with the James Webb Space Telescope, which can see incredibly faint infrared light, so is allowing scientists to peer farther away — and further back in time — than ever before (SN: 12/16/22).
“If you could look in the more distant universe, you could potentially see some of these galaxies when they’re first forming, or when they’re evolving,” Conselice says. “We could potentially really test this idea.” More
A spacecraft plunging into the sun’s atmosphere has revealed the likely source of powerful blasts of plasma in fast solar winds.
Far from the sun, the solar wind is a nebulous, turbulent plasma. But when NASA’s Parker Solar Probe dipped within about 8 million kilometers of the sun’s surface, it detected narrow plasma streams (SN: 12/15/21). The streams appear to be guided by magnetic fields tracing back to two relatively cool regions of the sun’s atmosphere known as coronal holes, researchers report June 7 in Nature.
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}The solar wind — the steady stream of charged particles flowing from the sun — has two distinct speeds, dubbed as “slow” and “fast.” Until now, it wasn’t clear what was speeding the fast solar wind along. The plasma in most of the solar wind consists of protons, electrons and the nuclei of atoms blowing at hundreds of kilometers a second. But plasma from the area above the coronal holes can travel more than 10 times as fast.
The fast plasma streams were associated with events known as switchbacks, in which portions of the magnetic fields near the sun reverse direction (SN: 1/15/21). Switchbacks can result when magnetic field loops close to the sun’s surface connect to long field lines extending away from the sun. The reconnection leads to a sharp switchback kink that kicks the plasma on its way into space as the kink straightens out.
Higher energy streams, the researchers propose, are the result of newly reconnected field lines with sharply kinked switchbacks, while lower energy streams come from field lines with older kinks that had become more smoothed out.
Although the evidence that the fast solar wind is due to magnetic reconnections is compelling, it’s still circumstantial, says astrophysicist Stuart Bale of the University of California, Berkeley. “Our results suggest that the solar wind can be accelerated to high speeds very close to the sun,” Bale says. “This is somewhat different [than the] standard scenario of wind energization,” where the acceleration takes place farther from the sun.
Getting a handle on processes like the origin of the fast solar wind is important for practical reasons, Bale says. “We also believe that magnetic reconnection is responsible for solar flares and is involved in the release of coronal mass ejections, which have a major impact in space weather.” Solar flares and the space weather they create have led to electrical grid disruptions on Earth and interrupted radio communications, and they may threaten astronauts (SN: 7/30/2020).
The findings are also an important clue in the enduring mystery of why the sun’s atmosphere is millions of degrees Celsius hotter than its surface, says astrophysicist Gary Zank, who was not involved in the study (SN: 8/20/17). “But they do not identify the heating mechanism.”
The observations identify the importance of magnetic reconnection close to the sun, says Zank, of the University of Alabama in Huntsville. “The next step is to link that to the dissipation of the magnetic energy in determining the actual heating process.”
Another interesting aspect of the study is how the structure of the solar surface seems to be imprinted on the solar wind near the sun despite the presence of turbulence, says Amitava Bhattacharjee, a Princeton University astrophysicist who was not involved with the study. Other researchers have proposed that it can happen, he says, but in the new study, “the experimental and simulation evidence is more convincing.”
Parker’s ever closer encounters with the sun could confirm the source of the fast solar wind more definitively. And future passes will come as the sun heads into a phase with increasing numbers of sunspots and more solar wind, on the way to peak solar activity around July 2025. “As we move into solar maximum,” Bale says, “we expect some serious surprises.” More
If you’re a small galaxy and want to mint new stars, come to the Milky Way — but don’t get too close if you want a long-lasting star-making career. New observations with the Gaia space telescope show that our galaxy is both friend and foe to the lesser galaxies that revolve around it.
Some 60 known galaxies orbit the Milky Way. About a dozen of these satellite galaxies are dim dwarf spheroidals, which each emit just 0.0005 to 0.1 percent as much light as the Milky Way (SN: 12/22/14). Their few stars are spread out from one another, giving the galaxies such a ghostly appearance that the first one found was initially suspected to be only a fingerprint on a photographic plate.
But these ghostly galaxies once sparkled with young stars. A new study finds that most of these galaxies lit up when they first crossed into our galaxy’s gravitational domain as fresh stars arose. But then, in most cases, the little galaxies stopped making stars soon afterward, because the Milky Way stripped the dwarf galaxies of gas, the raw material for star formation.
Astronomer Masashi Chiba of Tohoku University in Sendai, Japan, and his then-graduate student Takahiro Miyoshi studied seven of the dwarf spheroidal galaxies orbiting the Milky Way. The researchers used the European Space Agency’s Gaia spacecraft, which had measured the galaxies’ motions, to compute their orbits around the Milky Way’s center. The orbits are elliptical, so the galaxies approach and then recede from our galaxy’s center. The astronomers then compared those paths to the times when the galaxies formed their stars.Sign Up For the Latest from Science News
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“We found that there’s a very nice coincidence between the timing of the first infall of the satellite [toward the Milky Way] and the peak in the star formation history,” Chiba says. In work posted online at arXiv.org on October 23, the astronomers attribute the burst of star formation in the small galaxies to the Milky Way. Encountering the giant galaxy squeezes the dwarf galaxies’ gas, causing that gas to collapse and spawn lots of new stars.
As an example, the Draco dwarf galaxy first crossed into the Milky Way’s domain 11 billion years ago and formed numerous stars then — but never again. More recently, the Leo I dwarf galaxy entered our galaxy’s realm just 2 billion years ago, a time that coincided with its last burst of star birth. But today Leo I creates no new stars and, like Draco, has no gas to do so.
Dwarf galaxies that kept their distance also kept their gas longer, the researchers found. The galaxies that came closest to the Milky Way’s center, such as Draco and Leo I, ceased all star formation soon after crossing the Milky Way’s frontier. However, the galaxies that entered our galaxy’s domain but remained farther out, such as Fornax and Carina, fared better.
“Those two galaxies kept their interstellar gas inside them, so that the star formation still continued,” Chiba says. Both galaxies managed to eke out new stars for many billions of years after crossing into the Milky Way’s realm. Today, however, neither galaxy has any gas left.
“I think it all makes sense,” says Vasily Belokurov, an astronomer at the University of Cambridge, who notes how essential the Gaia spacecraft was to the discovery. “It’s a beautiful demonstration of what we’ve never been able to do before Gaia, and suddenly we can do these magical things.” More
