DART Changed the Shape of Asteroid Dimorphos, not Just its Orbit

On September 26th, 2022, NASA’s Double Asteroid Redirection Test (DART) collided with the asteroid Dimorphos, a moonlet that orbits the larger asteroid Didymos. The purpose of this test was to evaluate a potential strategy for planetary defense. The demonstration showed that a kinetic impactor could alter the orbit of an asteroid that could potentially impact Earth someday – aka. Potentially Hazardous Asteroid (PHA). According to a new NASA-led study, the DART mission’s impact not only altered the orbit of the asteroid but also its shape!

The study was led by Shantanu P. Naidu, a navigation engineer with NASA’s Jet Propulsion Laboratory (JPL) at Caltech. He was joined by researchers from the Lowell Observatory, Northern Arizona University (NAU), the University of Colorado Boulder (UCB), the Astronomical Institute of the Academy of Sciences of the Czech Republic, and Johns Hopkins University (JHU). Their paper, “Orbital and Physical Characterization of Asteroid Dimorphos Following the DART Impact,” appeared on March 19th in the Planetary Science Journal.

The Didymos double asteroid system consists of an 851-meter-wide (2792 ft) primary orbited by the comparatively small Dimorphos. The latter was selected as the target for DART because any changes in its orbit caused by the impact would be comparatively easy to measure using ground-based telescopes. Before DART impacted with the moonlet, it was an oblate spheroid measuring 170 meters (560 feet) in diameter with virtually no craters. Before impact, the moonlet orbited Didymos with a period of 11 hours and 55 minutes.

Before the encounter, NASA indicated that a 73-second change in Dimorphos’ orbital period was the minimum requirement for success. Early data showed DART surpassed this minimum benchmark by more than 25 times. As Naidu said in a NASA press release, the impact also altered the moonlet’s shape:

“When DART made impact, things got very interesting. Dimorphos’ orbit is no longer circular: Its orbital period is now 33 minutes and 15 seconds shorter. And the entire shape of the asteroid has changed, from a relatively symmetrical object to a ‘triaxial ellipsoid’ – something more like an oblong watermelon.”

Naidu and his team combined three data sources with their computer models to determine what happened to the asteroid after impact. The first was the images DART took of Dimorphos right before impact, which were sent back to Earth via NASA’s Deep Space Network (DSN). These images allowed the team to gauge the dimensions of Didymos and Dimorphos and measure the distance between them. The second source was the Goldstone Solar System Radar (GSSR), part of the DNS network located in California responsible for investigating Solar System objects.

The GSSR was one of several ground-based instruments that precisely measured the position and velocity of Dimorphos relative to Didymos after impact – which indicated how the mission greatly exceeded expectations. The third source was provided by ground-based telescopes worldwide that measured changes in the amount of life reflected (aka. light curves) of both asteroids. Much like how astronomers monitor stars for periodic dips (which could indicate a transiting planet), dips in Didymos’ luminosity are attributable to Dimorphos passing in front of it.

By comparing these light curves from before and after impact, the team learned how DART altered Dimorphos’ motion. Based on these data sources and their models, the team calculated how its orbital period evolved and found that it was now slightly eccentric. Said Steve Chesley, a senior research scientist at JPL and a co-author on the study:

“We used the timing of this precise series of light-curve dips to deduce the shape of the orbit, and because our models were so sensitive, we could also figure out the shape of the asteroid. Before impact, the times of the events occurred regularly, showing a circular orbit. After impact, there were very slight timing differences, showing something was askew. We never expected to get this kind of accuracy.”

According to their results, DART’s impact reduced the average distance between the two asteroids to roughly 1,152 meters (3,780 feet) – closer by about 37 meters (120 feet). It also shortened Dimorphos’ orbital period to 11 hours, 22 minutes, and 3 seconds – a change of 33 minutes and 15 seconds. These results are consistent with other independent studies based on the same data. They will be further tested by the ESA’s Hera mission, scheduled to launch in October 2024, when it makes a flyby of the double-asteroid and conducts a detailed survey.

Further Reading: NASA