What DART scientists have learned about asteroid Didymos so far
Smashing a spacecraft into an asteroid isn’t NASA’s usual approach to planetary science, but it was certainly an opportunity.
NASA’s Double Asteroid Redirection Test (DART) spacecraft slammed into a small asteroid called Dimorphos on September 26 to test a potential technique to protect Earth, should we find ourselves on a collision course with a large space rock. But the impact has also given planetary scientists a close, if fleeting, view of the smallest asteroid any spacecraft has visited so far.
“It’s been a thrill to see the data come in,” Carolyn Ernst, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory and instrument scientist on DART’s only instrument, told Space.com. “Everyone has been keeping an eye on them and been busy working on them.”
Related: Asteroid impact: Here’s the last thing NASA’s DART spacecraft saw before it crashed
It is still early to know the asteroid itself; scientists have had the DART data in hand for only a few weeks and will perform countless analyzes before saying anything with too much confidence. “There are a lot of snap observations you can make, but there are a lot of careful things you have to put together before you go too far down any path,” Ernst said.
The 525-foot-wide (160-meter) Dimorphos orbits a larger asteroid called Didymus, which is perhaps 2,560 feet (780 m) wide. Ahead of DART’s launch in November 2021, scientists got an idea of the shapes of both rocks thanks to planetary radar, bouncing a beam of radio waves off the asteroids.
As the DART spacecraft cruised toward its final destination, that’s about all scientists knew about the pair of rocks.
DART’s only instrument, the Didymos Reconnaissance and Asteroid Camera for Optical Navigation (DRACO), was able to change that. For much of DART’s journey, the asteroids appeared as a bright dot, but about 10 minutes before impact, that dot began to transform into two small but unique worlds.
One feature of Dimorphos jumped out as soon as scientists saw the last images from DART before impact: its rocky surface strewn with rocks, dust and everything in between. Spacecraft have seen this type of surface before: Japan’s Hayabusa2 mission Ryugu and NASA’s OSIRIS-REx mission to determine both found themselves exploring clumps of rock, worlds that scientists call “rubble heap” asteroids.
“Because it looks so rubble and from what we know about these other asteroids, I think a lot of people imagine that it’s some kind of rubble pile or some kind of loose collection of rocks.” Ernst said.
That said, DART didn’t reveal Dimorphos’ innards, so the rubble pile look may not hold up. “We don’t have a direct way to measure the interior,” Ernst said. “Could the interior be a bunch of larger objects with smaller things on top? Could it be what it looks like on the surface all the way down? We have no direct idea of that.”
A second feature of Dimorphos that struck Ernst during DART’s approach was its egg-like shape, at least as seen from DART’s approach angle. “It was less patchy than I expected,” he said. “People often call asteroids big potatoes because they have a lot of irregular shapes. So in that sense, I think it was more regularly shaped than I expected.”
As the analysis continues, Ernst added, scientists will look for clues about whether material on the surface of Dimorphos appears to be moving, which could make the asteroid relatively round.
Scientists can also wait for information from the European Space Agency Hera missionwhich will be launched in 2024 on a SpaceX Falcon 9 rocket and they arrive at Dimorphos two years later. Hera will explore the asteroid and the aftermath of the impact in three dimensions and without the rush of DART.
A look at Didymos
DART’s view of Dimorphos’ larger companion, Didymos, was even shallower, as it slipped out of the spacecraft’s field of view near the end of the approach. But planetary scientists are studying Didymos with the data DART sent home.
“We have some interesting and intriguing looks at Didymos that we’ll be able to do some science with,” Ernst said.
Even passing vision showed that Didymos and Dimorphos are two different bodies, despite their proximity. “The surface definitely looks different than Dimorphos,” Ernst said. “You can definitely see some big rocks, especially on the limb. But it doesn’t look like a giant pile of rocks, like Dimorphos does.”
In particular, he noted that Didymos appears to show more variation in terrain between smooth and rough patches compared to the seemingly uniformly rocky surface of Dimorphos.
The differences between Didymos and Dimorphos could influence how scientists try to explain the formation of binary asteroids. an idea suggests that the main body can spin so fast that material flies out of it, eventually coalescing into a moon; a second idea posits that if the asteroid passes too close to a large planet, the planet’s gravity can rip off material that becomes the moon.
Scientists believe about 15% of near-Earth asteroids are actually binary systems, with the occasional three-piece asteroid.
Meaning in the mess
Thanks to the design of the DART mission, the spacecraft’s abrupt demise did not mark the end of the data scientists can use to understand Dimorphos.
First, DART brought along a small companion called the Light Italian Cubesat to image asteroids (LICACube) that was deployed a couple of weeks before the impact. LICIACube was equipped with two cameras and flew past the impact site about three minutes after DART arrived in hopes of catching a glimpse of a crater or perhaps some debris.
“We didn’t know what they were going to show when they set out to do it,” Ernst said. But LICIACube images showed debris flying from Dimorphos in streamers. “These images, they were quite amazing and amazing.”
Such large amounts of debris suggest that Dimorphos consists of relatively weakly bound material; Think of throwing a tennis ball into a sandbox versus bouncing it on a sidewalk. But there is also a downside to the messiness of the impact. LICIACube’s images are so cluttered with debris that scientists can’t decipher much about the asteroid’s natural surface from the images.
But LICIACube was not the only witness to DART. In addition, the mission recruited ground telescopes i in space to see the aftermath of the DART collision.
The planetary defense objective of the mission prompted the primary duty of these observers. If humans discover an asteroid that threatens to collide with Earth, planetary defense experts say, shortening the asteroid’s orbit around the sun could ensure that the two bodies no longer risk trying to be in the same place at the same time time
To this end, the telescopes were focused on the objective of marking how long it now takes Dimorphos to circle Didymos. Dimorphos’ orbit previously lasted 11 hours and 55 minutes; following the impact, this period has decreased by 32 minutes. That was the high end of scientists’ expectations before the launch. And since debris flying from Dimorphos would have contributed to the orbital change, the large decrease underscores the amount of debris that DART created.
But continued observations also tell scientists a lot about asteroids as space rocks, as well as what happens when asteroids collide naturally.
Normally, the Didymos system is a single bright spot for ground-based telescopes. But just two days after impact, the asteroid pair sported a long, bright, comet-like tail it stretched 6,000 miles (10,000 kilometers) into space.
Like the LICIACube images, observations of the tail suggest that DART left a mess. When the pressure of the sun’s radiation pushed the debris into the tail; that debris also reflected sunlight, hence the bright spot.
“It basically looks like a small comet, a temporary comet,” Ernst said.
Scientists have been able to see the tail shift in the weeks following DART’s impact. The Hubble Space Telescope has been particularly important on this front, observing the asteroid 18 times since the collision and catching Dimorphos spawned a second tail, which quotation marks from time to time too.
Dimorphos is not the first asteroid to masquerade as a comet; about one in 10,000 space rock is an “active asteroid” with comet-like features such as a tail. Interestingly, scientists already thought that these confusing sights could occur when a natural impact throws debris from the asteroid’s surface.
But there’s a lot more work to do before scientists are ready to draw big conclusions about asteroids from their sightings of Didymos and Dimorphos. “I think, honestly, it’s going to take a little bit of time for people to piece together what this means,” Ernst said of the queue.
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