DART modified the form of the asteroid Dimorphos, not simply its orbit

On September 26, 2022, NASA's Double Asteroid Redirection Test (DART) collided with the asteroid Dimorphos, a moon orbiting the larger asteroid Didymos. The purpose of this test was to evaluate a possible planetary defense strategy. The demonstration showed that a kinetic impactor could change the orbit of an asteroid that could potentially one day impact Earth – so. Potentially Hazardous Asteroid (PHA). According to a new study conducted by NASA, the DART mission impact not only changed the asteroid's orbit, but also its shape!

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

The Didymos double asteroid system consists of an 851 meter wide primary asteroid orbited by the comparatively small Dimorphos. The latter was chosen 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 collided with the moon, it was an oblate spheroid 170 meters (560 feet) in diameter with virtually no craters. Before the impact, the moonlet orbited Didymos for a period of 11 hours and 55 minutes.

Artist's impression of the DART mission hitting the moon Dimorphos. Photo credit: ESA

Before the encounter, NASA stated that a 73-second change in Dimorphos' orbital period was the minimum requirement for success. Initial data showed that DART exceeded this minimum benchmark by more than 25 times. As Naidu said in a NASA press release, the impact also changed the shape of the moon:

“Once DART took effect, things got very interesting. Dimorphos' orbit is no longer circular: its orbital period is now 33 minutes and 15 seconds shorter. And the asteroid's entire shape has changed, from a relatively symmetrical object to a “triaxial ellipsoid” – something more like an elongated watermelon.”

Naidu and his team combined three data sources with their computer models to determine what happened to the asteroid after the impact. The first were the images taken by DART just before Dimorphos hit, which were sent back to Earth via NASA's Deep Space Network (DSN). Using these images, the team was able to estimate 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 in California, which is responsible for studying solar system objects.

The GSSR was one of several ground-based instruments that precisely measured the position and speed of Dimorphos relative to Didymos after impact – showing that the mission far exceeded expectations. The third source came from ground-based telescopes around the world that measured changes in the reflected amount of life (also known as light curves) from both asteroids. Similar to how astronomers monitor stars for periodic dips (which could indicate a passing planet), dips in Didymos' luminosity are due to Dimorphos passing in front of it.

Artist's impression of ESA's Hera mission meeting Dimorphos. Photo credit: NASA

By comparing these light curves before and after impact, the team learned how DART changed 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. Steve Chesley, senior research scientist at JPL and co-author of the study, said:

“We used the timing of this precise series of light curve dips to infer the shape of the orbit, and because our models were so sensitive, we were also able to determine the shape of the asteroid.” Before the impact, events were regular and showed a circular orbit. After the impact, there were very slight differences in time, showing that something was wrong. We never expected to achieve such accuracy.”

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

Further reading: NASA

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