Planetary scientists attending the first major conference since NASA's Double Asteroid Redirection Test (DART) permanently moved a space rock offered early findings about the asteroid and the impact.
DART slammed into a small asteroid called Dimorphos on Sept. 26, 2021, in humanity's first test of a planetary defense technique that might be used to protect Earth, should we discover a large asteroid threatening a collision. DART was a stunning success: Just weeks after the impact, team members announced that Dimorphos' orbital period around a larger asteroid called Didymos had sped up from 11 hours and 55 minutes to 11 hours and 23 minutes, shaving off 32 minutes — on the high end of predictions.
At the annual meeting of the American Geophysical Union held last month in Chicago and virtually, DART scientists tweaked that finding just a little, updating the calculation to 33 minutes. That value could still change by a minute in either direction, Cristina Thomas, a planetary scientist at Northern Arizona University who leads the DART observations working group, said during her presentation.
While only a tiny difference, pinpointing the amount Dimorphos' orbit changed is crucial; planetary defenders will need that information to be as accurate as possible to calibrate an effective asteroid intervention.
Carolyn Ernst, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Maryland and instrument scientist for DART's sole instrument, offered a second intriguing number during her presentation, noting that the impact represented 11 gigajoules of energy. (For comparison, detonating one ton of TNT releases a little more than 4 gigajoules; the average U.S. household uses about 40 gigajoules of energy each year.)
Other work Ernst and her colleagues are doing recreates the impact itself. They calculated that the spacecraft flew in on a trajectory about 73 degrees above the local surface. "So not quite vertical, but pretty close," she said. That's another valuable insight for planetary defense, since the impact angle influences how much a collision changes an asteroid's path.
The team also worked to reconstruct what happened — a crime scene analysis where everyone knows the perpetrator.
That's a little tricky, since scientists won't see the wreckage up close until late 2026, when the Hera mission from the European Space Agency arrives at the Didymos system. Where DART's mission was fleeting, Hera is designed to stay awhile, reconnoitering the two asteroids in detail, including with two tiny hitchhiker spacecraft.
But even without a detailed view of the crashed spacecraft, Ernst and her colleagues have mapped how it hit the asteroid's rocky surface. To do so, they used DART's final trajectory information and its last photo.
That photo shows more than 950 boulders, she said. Two particularly big rocks — the larger about 21 feet (6.5 meters) across — are located at the center of DART's final photo, and they're likely where the spacecraft and its two long solar array wings met their end, Ernst said.
"The first point of contact was likely that one solar array with boulder number two; that's the first thing that hit," she said. "And then it does look like the second wing probably hit boulder number one just before the bus hit in the middle." (Bus is a term for the main body of a spacecraft; DART's was about 4.3 feet, or 1.3 m, across.)
But scientists' work hasn't all been focused on the impact itself; researchers are also studying data from the DART mission to understand Dimorphos simply as an asteroid. After all, close observations of a space rock are precious.
That work includes developing a sense of the space rock's shape from the spacecraft's final few images.
Helpfully, the DART images show not only the side of Dimorphos that the spacecraft flew into, but also a more distant portion of the asteroid's edge. That's because the larger Didymos is reflecting light off its sun-facing side; that "Didy-shine" then illuminates part of Dimorphos, allowing scientists to sketch both edges of the space rock's full disk, Terik Daly, a planetary scientist at JHUAPL, said during his presentation.
From DART's glimpse, scientists have realized their pre-impact model of the shape of Dimorphos doesn't quite fit. Before DART's arrival, they had only some fragmentary views of parts of the space rock's edges, which offered hints that the distance between the rock's poles would be its longest dimension.
The team's analysis of DART's data suggests otherwise. "The shape of Dimorphos is not elongated like a bean; it is, in fact, more like a candy-covered chocolate," Daly said. "It looks like an M&M a little bit."
Of course, scientists have only had a couple months to dig into DART's observations so far, with lots of work left to do. And there's more data to come as well.
Scientists plan to check up on Didymos for a couple more months to better understand the aftermath of the epic crash. After that, they'll need to wait until Hera's arrival in December 2026 for fresh observations. Hera and its two tiny cubesats will give scientists a more detailed view of the crash site and help fine-tune analyses of how the crash unfolded. With its exploration of Didymos and Dimorphos, Hera will also offer scientists their first-ever detailed view of a binary asteroid.
"In the months and years ahead, there'll be interesting work along that line," Daly said of attempts to explain Dimorphos' shape.