What DART scientists have learned so far about the asteroid Didymos
Crashing a spacecraft into an asteroid isn’t NASA’s usual approach to planetary science, but it was certainly an opportunity nonetheless.
NASA Asteroid Double Redirection Test (DART) spacecraft slammed into a small asteroid called Dimorphos on September 26 to test a potential technique to protect Earth, should we end up on a collision course with a large space rock. But the impact also gave planetary scientists a close, albeit fleeting, view of the smaller asteroid any spacecraft has visited to date.
“It’s been a pleasure to see the data coming in,” Carolyn Ernst, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory and instrument scientist for DART’s sole instrument, told Space.com. “Everyone eagerly leaned over it and got busy working on it.”
It is still early to know more about the asteroid itself; scientists have had the DART data in hand for only a few weeks and will perform countless analyzes before saying anything too confidently. “There’s a lot of instant observation you can make, but there’s a lot of careful stuff you have to put together before you go too far on any road,” Ernst said.
The 525-foot-wide (160-meter) Dimorphos orbits a larger asteroid called Didymos, which is possibly 2,560 feet (780 m) in diameter. Before DART launched in November 2021, scientists had an idea of the shape of the two rocks thanks to planetary radar, bouncing a beam of radio waves off the asteroids.
As the DART spacecraft sailed to its final destination, that’s about all scientists knew about the pair of rocks.
DART’s only instrument, 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 single bright spot, but about 10 minutes before impact, that spot began to morph into two small but unique worlds.
One feature of Dimorphos became apparent as soon as scientists saw the last DART images before impact: its rocky surface strewn with boulders, dust and everything in between. Spacecraft have seen this type of surface before: the Japanese Hayabusa2 mission to Ryugu and NASA’s OSIRIS-REx mission to Bennu both found themselves exploring clumps of rock, worlds scientists call “rubbish piles” asteroids.
“Because it looks so scattered and because of what we know about these other asteroids, I think a lot of people imagine it to be some sort of rubble pile or some sort collection of rocks loosely held together,” Ernst said.
That said, DART hasn’t revealed Dimorphos’ innards, so the look of the pile of rubble may not hold up. “We don’t have a direct way to measure the interior,” Ernst said. “Could the inside be a bunch of bigger stuff with smaller stuff on it? Could that be what it looks like on the surface all the way through? We don’t have a direct idea about it.”
A second feature of Dimorphos that struck Ernst when approaching DART was its egg-like shape, at least seen from DART’s approach angle. “It was less erratic than I expected,” she said. “People often call asteroids big potatoes because they have a lot of irregular shapes. So in that sense, I think it had a more regular shape than I expected.”
As the analysis continues, Ernst added, scientists will look for clues as to whether material on Dimorphos’ surface appears to be moving, which could make the asteroid relatively round.
Scientists can also look forward to information from the European Space Agency Mission Herascheduled for launch in 2024 on a SpaceX Falcon 9 rocket and arriving at Dimorphos two years later. Hera will explore the asteroid and the aftermath of the impact in three dimensions and without the haste of DART.
An overview of Didymos
The DART view of Dimorphos’ larger companion, Didymos, was even more cursory, as near the end of the approach it slipped out of the spacecraft’s line of sight. But planetary scientists are studying Didymos with the data DART sent home.
“We have some interesting and intriguing looks at Didymos that we can absolutely do science with,” Ernst said.
Even the passing view showed that Didymos and Dimorphos are two separate bodies, despite their proximity. “The surface is definitely different from Dimorphos,” Ernst said. “You can definitely see big boulders, especially on the limbus. But it doesn’t look like a giant pile of boulders, like Dimorphos does.”
In particular, she noted that Didymos appears to show more terrain variation between smooth and rough areas 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 matter flies away, eventually merging into a moon; a second idea posits that if the asteroid strays too close to a large planet, the planet’s gravity can tear off the material that becomes the moon.
Scientists believe about 15% near-Earth asteroids are actually binary systems, sometimes with a three-part asteroid.
Meaning out of order
Thanks to the design of the DART mission, the starship’s abrupt disappearance didn’t mark the end of the data scientists can use to understand Dimorphos.
First, DART carried with it a small companion called Light Italian Cubesat for Imaging of Asteroids (LICIACube) that it deployed a few weeks before impact. LICIACube was equipped with two cameras and flew over the impact site about three minutes after the DART arrived in hopes of spotting a crater or possibly some debris.
“We didn’t know what they would show when they set out to do this,” Ernst said. But LICIACube footage showed rubble flying off Dimorphos in streamers. “These images, they were quite striking and amazing.”
Such large amounts of debris suggest that Dimorphos is made of material relatively loosely held together; consider throwing a tennis ball into a sandbox instead of bouncing it off a sidewalk. But there is also a downside to the mess of impact. The LICIACube images are so full of debris that scientists can’t decipher much about the asteroid’s natural surface from the images.
The mission’s planetary defense objective drove the primary duty of these observers. Should humans discover an asteroid that threatens to collide with Earth, planetary defense experts say, shortening the asteroid’s orbit around the sun could ensure the two bodies are no longer at risk of trying to collide with Earth. be in the same place at the same time.
To that end, telescopes have focused on the goal of timing how long it now takes Dimorphos to circumnavigate Didymos. Dimorphos’ orbit previously lasted 11 hours and 55 minutes; following the impact, this period has decreased by 32 minutes. This was the high end of scientists’ expectations before launch. And because rubble flying off Dimorphos would have contributed to the orbital shift, the sharp decrease underscores the amount of debris created by DART.
But the ongoing observations also tell scientists a lot about asteroids as space rocks, as well as what happens when asteroids collide naturally.
Usually the Didymos system is a single bright spot for ground-based telescopes. But just two days after impact, the pair of asteroids sported a long, glowing comet-like tail that stretched 6,000 miles (10,000 kilometers) into space.
Like the LICIACube images, the tail sightings suggest that DART left quite a mess. As radiation pressure from the sun pushed the debris into the tail; that the debris also reflected sunlight, hence the bright spot.
“It basically looks like a small comet, a temporary comet,” Ernst said.
Scientists were able to observe the tail change over the weeks following the DART impact. The The Hubble Space Telescope has been particularly prominent on this front, observing the asteroid 18 times since the collision and catching Dimorphos sprouting a second tail, which comets sometimes do too.
Dimorphos is not the first asteroid to disguise itself as a comet; on one in 10,000 space rocks is an “active asteroid” with comet-like tail features. Curiously, scientists already thought these puzzling sights could occur when a natural impact throws rubble onto the asteroid’s surface.
But there’s still a lot of work to do before scientists are ready to draw big conclusions about asteroids from their glimpses of Didymos and Dimorphos. “Honestly, I think it’s going to take a long time for people to rebuild everything on what that means,” Ernst said of the tail.