Unexpected Results From NASA Mission To Deflect Asteroid

In 2022, NASA directed a spacecraft to impact an asteroid approximately the size of the Great Pyramid of Giza to alter its trajectory.

The mission demonstrated that redirecting objects in space is feasible. In addition to changing the course of the asteroid Dimorphos, the impact caused large boulders to detach from the asteroid, with sizes ranging from 1 to 7 meters (3-23 feet) in diameter. Recent research tracking the resulting boulders found that their movement occurred in unexpected directions, which may influence future asteroid deflection strategies.

"There has yet to be a full accounting of the total momentum in all directions, but the ejecta cone spreads out sideways, as well as in the direction of the incoming spacecraft," the team explains in their paper. "Thus, a significant component of the momentum, possibly several times that contributing to the β factor, was carried out perpendicular to Dimorphos’s velocity vector. To fully understand the impact’s effect on Dimorphos’s orbit, it is necessary to explore the spatial distribution of the debris field and use it to ascertain the net momentum of all its components."

The team examined the distribution of 104 boulders imaged by the Light Italian Cubesat for Imaging of Asteroids (LICIACube), which accompanied the Double Asteroid Redirection Test (DART) mission. These boulders, with radii between 0.2 and 3.6 meters (0.7 to 11.8 feet), moved away from Dimorphos at speeds up to 52 meters per second (116 miles per hour) following the impact. While the redirection of the asteroid was anticipated, the paths taken by these boulders were less expected.

"We saw that the boulders weren't scattered randomly in space," Tony Farnham, lead author of the paper and a research scientist at UMD's Department of Astronomy, 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."

The largest debris cluster contained approximately 70 percent of the ejected objects and moved south at high velocities at a shallow angle to the asteroid's surface. The team suggests these may be fragments of large boulders that were broken apart by the impact with DART's solar panels.

"DART's solar panels likely hit two big boulders, called Atabaque and Bodhran, on the asteroid," second author Jessica Sunshine, a professor of astronomy and geology at UMD, added. "Evidence suggests that the southern cluster of ejected material is probably made up of fragments from Atabaque, a 3.3-meter-radius boulder."

According to the research, the boulders released by the impact carried more than three times the momentum of the DART spacecraft itself. Additionally, since the ejected boulders generally moved perpendicular to DART's direction, the mission may have altered the asteroid's orbital plane by up to one degree, potentially causing it to rotate or tumble in space.

"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," Sunshine added. "You can think of it as a cosmic pool game. We might miss the pocket if we don't consider all the variables."

Although Dimorphos does not pose a threat to Earth, future asteroid deflection missions will need to consider the complex dynamics observed in this case.

"Deep Impact hit a surface that was essentially very small, uniform particles, so its ejecta was relatively smooth and continuous," Sunshine, who was deputy principal investigator on NASA's Deep Impact mission, continued. "And here, we see that DART hit a surface that was rocky and full of large boulders, resulting in chaotic and filamentary structures in its ejecta patterns."

"Comparing these two missions side-by-side gives us this insight into how different types of celestial bodies respond to impacts, which is crucial to ensuring that a planetary defense mission is successful."

The team indicates that a comprehensive analysis of the momentum of surface boulders is necessary to determine how future impacts will affect similar objects. The European Space Agency's Hera mission is scheduled to arrive at Dimorphos in 2026 to further study the impact.

"We succeeded in deflecting an asteroid, moving it from its orbit," Farnham added. "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."

The study is published in The Planetary Science Journal.