DART successfully deflected the orbit of an asteroid, but by how much?

Using data from NASA’s successful double asteroid redirection test in September 2022, a team of scientists including Siegfried Eggl, an aerospace engineer at the University of Illinois Urbana-Champaign, calculated the momentum transferred to the target asteroid on impact to be greater than they expected it would be. They found the momentum was significantly enhanced by the recoil created from streams of particles produced by the impact.

“The smaller asteroid of the binary Didymos, Dimorphos, that was hit by the DART spacecraft is basically a rubble pile—an agglomeration of lots of tiny and not so tiny pebbles—so I wasn’t surprised at the larger quantity of ejecta the impact created,” said Eggl. He is one of the authors of a paper published in the journal Nature.

ASI’s LICIACube satellite acquired this image just after its closest approach to the Dimorphos asteroid, after the Double Asteroid Redirect Test, or DART mission, made impact on Sep. 26, 2022. In this image, it is possible to observe the Didymos and Dimorphos from a different perspective, which can be useful to determine the shapes of the asteroids. Credit: ASI/NASA

The team of scientists calculated the momentum transferred to Dimorphos as a result of the impact, the Beta factor, and learn its displacement with respect to its primary asteroid Didymos.

He said, going into it, they could infer the shape of Dimorphos from images recorded by DART’s DRACO camera, but without knowing the mass it was difficult to estimate the momentum transfer onto Dimophos,. After the impact, they had new information that made a big difference in their ability to make accurate calculations—they knew that the impact changed the period of Dimorphos’ orbit by 33 minutes.

“I think the most important outcome is what we've learned in terms of how we anchor our simulations,” Eggl said. “We’ve tried to derive all the deflection predictions based on first principles, but we didn't have a single, actual data point. Now we have that and can compare which results match and which ones give us a better understanding, so that we have better predictions in the future.”

The team ran Monte Carlo simulations using random combinations of variables such as whether Dimorphos was more or less massive than Didymos. Eggl’s role was to ensure that the simulations were statistically correct and accurately reflect reality.

He said, because near Earth asteroids are very diverse in their makeup, more tests like DART are needed.

The last complete image of asteroid moonlet Dimorphos, taken by the DRACO imager on NASA's DART mission from ~7 miles (12 kilometers) from the asteroid and 2 seconds before impact. The image shows a patch of the asteroid that is 100 feet (31 meters) across. Dimorphos' north is toward the top of the image. Credit: NASA/Johns Hopkins APL

“If we had pictures of the primary asteroid beforehand, our pre-impact predictions may have been more in line with the results coming out of DART,” Eggl said. “The amount of momentum transported by ejecta was not completely out of the possible range that we calculated, but it was on the high end. We need to do more tests on other kinds of asteroids, to better understand which uncertainties we are looking at when we try to deflect an actual impactor.”

Eggl noted that thanks to the joint work of his Ph.D. student Rahil Makadia and researchers at the NASA Jet Propulsion Laboratory, California Institute of Technology, we know that neither Dimorphos nor Didymos pose any hazard to Earth before or after DART’s controlled collision with Dimorphos. According to Eggl another mission conducted by the European Space Agency, Hera, will accurately measure the mass of Dimorphos by sometime in 2027. That will allow the DART team to further refine the results of world’s first dedicated asteroid deflection mission.

About the DART project, which he has been working on since it began in 2013, Eggl said, “A lot of the work I do is predictive in nature and yet to be validated. In that respect DART was unusual. The mission had a very quick turnaround. Because we had done all of the preparation and made predictions using the software and models, once we got the actual data from the mission we were able to immediately run the simulations and learn the results quickly.”

“I think what we also learned is that if people work together, we can achieve amazing things, like to actually deflect an asteroid.”

The NASA/Johns Hopkins University Applied Physics Laboratory Double Asteroid Redirection Test team which includes Eggl and two of his Ph.D. students Rahil Makadia, and Bhaskar Mondal, is receiving the 2023  AIAA Award for Aerospace Excellence. The award states it is “In recognition of humanity’s first time purposely changing the motion of a celestial object by a team of protectors of our home planet.”

The study, Momentum Transfer from the DART Mission Kinetic Impact on Asteroid Dimorphos,” by Andrew F. Cheng, et al, is published in the journal Nature. DOI: 10.1038/s41586-023-05878-z

Johns Hopkins Applied Physics Lab built and operated the DART spacecraft and manages the DART mission for NASA’s Planetary Defense Coordination Office as a project of the agency’s Planetary Missions Program Office. LICIACube is a project of the Italian Space Agency, carried out by Argotec. For more information about the DART mission, visit https://www.nasa.gov/dart or https://dart.jhuapl.edu.