Small rocks and debris fly near Earth, many just passing by. Some, however, come too close to Earth, with a potential threat of collision. Defending Earth from these unwanted objects is a growing concern globally. Planetary defense explores threat characterization, risk mitigation, and policy to defend Earth. One mitigation approach is sending an impactor to collide with the target object to deflect its trajectory from the original course toward Earth. This approach, known as kinetic deflection, is practical for intruders with a diameter up to a few hundred meters.

NASA’s Double Asteroid Redirection Test (DART), led by Johns Hopkins University’s Applied Physics Laboratory, was the first full-scale kinetic deflection mission to test how kinetic deflection could effectively push an asteroid measuring 150 meters in diameter. The 580-kg spacecraft (impactor) collided with the target asteroid, Dimorphos, at a speed of 6.1 km/second on September 26, 2022, making the target’s speed 2.7 mm/s. This speed change could gradually make the course deviate from the original one. The more time that elapses after impact, the further it moves away from the Earth. Even though Dimorphos was not a threat before the impact, it was chosen as a test target for DART’s kinetic deflection test.

Georgia Tech Professor Masatoshi Hirabayashi’s critical contribution to DART was recently published in Nature Communications. The study, “Elliptical ejecta of asteroid Dimorphos is due to its surface curvature” analyzed the behavior of fragments coming out by the high-speed DART impact and their push of the asteroid. This work was in collaboration with Professor Fabio Ferrari from Politecnico di Milano, who jointly published the study, “Morphology of ejecta features from the impact on asteroid Dimorphos.”  

Imagine a cannonball flying through the air and hitting a concrete wall. The wall shutters and fragmented pieces disperse at high speeds. Those smaller fragments, called ejecta, are known to be a key factor in controlling the asteroid push.

The study found that the ejecta from the impact site on Dimorphos highly depends on the asteroid’s shape. As a rule of thumb, a cannonball hitting a flat concrete wall creates ejecta departing from the wall at an angle of about 45 degrees from the wall’s surface. The cloud of ejecta thus looks like a waffle cone. However, if the concrete wall’s surface is tilted against the impact direction, the fragment ejection changes, making the ejecta structure differ even if the impactor has the same mass and speed. 

“This changes the asteroid push dramatically. Dimorphos has a squashed round shape, like an M&M,” Hirabayashi explained, “If the impact is large, more ejecta fly out of the surface but are more affected by surface tilts. This process makes the ejecta deviate from the ideal direction, reducing the asteroid push.” 

For the DART impact on Dimorphos, the study identified the impact scale and the asteroid’s rounded surface lowered the asteroid push by 56% compared to when Dimorphos was tested as an entirely flat wall. Thus, sending a large impactor does not mean a big push, and considering how to send impactors strategically is necessary. One way to keep the asteroid push effective is to send multiple small impactors rather than a single large impactor. This way, each small impactor may avoid the target’s rounded shape, and the net asteroid push by multiple impacts can be more efficient than the single impactor.