When NASA’s DART mission crashed into the asteroid Dimorphos, the first stage of the impact saw the spacecraft’s solar panels strike and pulverize two large boulders on the target, debris from which spun off in two directions. That ejection created enough momentum to give Dimorphos an extra kick on top of the direct effects of the kinetic impact, according to a new blockysis of the collision.
DART, the Double Asteroid Redirection Test, slammed into the 558-foot-wide (170-meter-wide) asteroid Dimorphos on Sept. 26, 2022. The force of the impact shortened Dimorphos’ orbit around its larger asteroid companion, Didymos, by about 32 minutes. The point of the mission was to show that we could deflect hazardous asteroids if they’re ever found to be on a collision course with Earth.
However, it now seems that there are more subtleties involved in deflecting asteroids than simple brute force.
Prior to impact, an Italian-built cubesat called LICIACube, which had piggybacked on DART, detached and took images of the immediate aftermath of the impact. As well as general clouds of dust, LICIACube saw two clusters of boulders, ranging in size from 1.3 to 23.6 feet (40 centimeters to 7.2 meters), speeding away from the point of impact. Later observations by the Hubble Space Telescope confirmed the presence of these boulders.
“We saw that the boulders weren’t scattered randomly in space,” Tony Farnham, an astronomer at the University of Maryland and lead author of the new research, said in a statement. “Instead, they were clustered in two pretty distinct groups, with an absence of material elsewhere, which means that something unknown is at work here.”
Furthermore, these two clusters of boulders imparted more than three times the momentum imparted by the DART spacecraft.
“We succeeded in deflecting an asteroid, moving it from its orbit,” said Farnham. “Our research shows that while the direct impact of the DART spacecraft caused this change, the boulders ejected gave an additional kick that was almost as big. That additional factor changes the physics we need to consider when planning these types of missions.”
Twenty years ago, when NASA’s Deep Impact spacecraft was deliberately sent to collide with comet 9P/Tempel, it hit a relatively smooth surface, which was reflected in a smooth and continuous ejecta sheet rather than disparate groups of large pieces of debris.
Dimorphos, in contrast to 9P/Tempel, has a rougher surface that’s littered with large boulders.
Due to this, Jessica Sunshine, who is a professor of astronomy and geology at the University of Maryland and who has worked on both the Deep Impact and DART missions, thinks she knows what happened when DART hit Dimorphos.
“DART’s solar panels likely hit two big boulders, called Atabaque and Bodhran, on the asteroid [in the moment before the main body of the spacecraft hit],” said Sunshine. “Evidence suggests that the southern cluster [of escaping rocks] is probably made up of fragments from Atabaque, a 3.3-meter-radius [10.8 meters] boulder.”
This southern cluster contains 70% of the ejected boulders and was moving at high velocities of up to 32 miles per second (52 meters per second) at shallow angles to the surface. The ejection of the southern cluster alone could have been enough to tilt Dimorphos’ orbital plane around Didymos by up to a degree.
By comparing Deep Impact with DART, it seems evident that objects with different properties to one another can react differently to being impacted. These differences will come into play should we have to deflect a dangerous asteroid or comet for real, because an error brought about by not understanding the effect of a surface on the way debris is ejected could spell disaster.
“If an asteroid was tumbling toward us, and we knew we had to move it a specific amount to prevent it from hitting Earth, then all these subtleties become very, very important,” said Sunshine. “You can think of it as a cosmic pool game. We might miss the pocket if we don’t consider all the variables.”
To learn more, and to confirm these changes predicted by the ejection of the boulders, the European Space Agency’s Hera mission will follow up on DART when it arrives at the Didymos–Dimorphos system in 2026. However, it will have to look out for boulders and other debris produced by the DART impact that could provide their own impact hazard to the spacecraft.
The new results were published on July 7 in the Planetary Science Journal.
