Damage report: DART impact changed orbit and deformed astreoid Dimorphos

When NASA’s DART (Double Asteroid Redirection Test) mission deliberately crashed into a 170-meter-wide asteroid on September 26, 2022, it left its mark in more ways than one.

The DART mission demonstration showed that a kinetic impact could deflect an asteroid dangerous if it were ever on a collision course with Earth.

Now, a new study, published in the journal The Planetary Science Journalshows that the impact changed not only the asteroid’s movement, but also its shape.

DART’s target, the asteroid Dimorphosis a moon of Didymos, a larger asteroid that orbits relatively close to Earth.

Before impact, Dimorphos was a “oblate spheroid” roughly symmetrical – like a squashed ball that is wider than it is tall.

With a well-defined circular orbit and at a distance of about 1189 meters from Didymos, Dimorphos it took 11 hours and 55 minutes completing a lap around his “big brother”.

“When DART made the impact, things got very interesting“, explains Shantanu Naidua navigation engineer at NASA’s Jet Propulsion Laboratory in southern California, who led the study.

“The orbit of Dimorphos stopped being circular: its orbital period – the time it takes to complete an orbit – is now 33 minutes and 15 seconds shorter. And the entire shape of the asteroid changed, from a relatively symmetrical object to a ‘triaxial ellipsoid’ – something more like an oblong watermelon.”

Dimorphos damage report

Naidu’s team used three data sources in their computer models to deduce what had happened to the asteroid after the impact.

The first source was on board DART: the spacecraft captured images as it approached the asteroid and sent them to Earth via NASA’s DSN (Deep Space Network).

These images provided measurements of the distance between Didymos and Dimorphos, while also assessing the dimensions of both asteroids immediately before impact.

A second data source it was DSN’s GSSR (Goldstone Solar System Radar), located near Barstow, California, which reflected radio waves from both asteroids to accurately measure the position and speed of Dimorphos relative to Didymos after the impact.

Radar observations quickly helped NASA conclude that DART’s effect on the asteroid far exceeded minimum expectations.

The third and most significant data source: Ground-based telescopes around the world that measured the “light curve” of both asteroids, that is, the way sunlight reflected from the asteroids’ surfaces changed over time.

By comparing the light curves before and after the impact, researchers were able to learn how DART changed movement from Dimorphos.

As Dimorphos orbits, it periodically passes in front of and then behind Didymos. In these so-called “mutual events“, one asteroid can cast a shadow on another or block our view from Earth.

In both cases, the telescopes register a temporary decrease in brightness – a drop in the light curve.

“We used the times from this precise series of dips in the light curve to deduce the shape of the orbit, and because our models were very sensitive, we were also able to figure out the shape of the asteroid,” he said. Steve Chesley, principal investigator at JPL and co-author of the study. The team discovered that Dimorphos’ orbit is now slightly elongated, or eccentric.

“Before the impact,” Chesley continued, “events occurred regularly, showing a circular orbit. After the impact, there were very slight time differences, showing that something was wrong. We never expected to get this kind of precision.”

The models are so accurate that they even show that Dimorphos rocks back and forth as it orbits Didymos, Naidu said.

Orbital Evolution

The team’s models also calculated the evolution of Dimorphos’ orbital period. Immediately after impact, DART reduced the average distance between the two asteroids, shortening the orbital period of Dimorphos in 32 minutes and 42 seconds, to 11 hours, 22 minutes and 37 seconds.

Over the next few weeks, the asteroid’s orbital period continued to decrease as Dimorphos lost more rocky material to space, eventually settling at 11 hours, 22 minutes and 3 seconds per orbit – 33 minutes and 15 seconds less than before impact.

This calculation has an accuracy of 1.5 seconds, said Naidu. Dimorphos now has an average orbital distance to Didymos of approximately 1152 meters – about 37 meters closer than before the impact.

“The results of this study agree with others that are being publisheds,” he said Tom Statlerlead scientist for small Solar System bodies at NASA Headquarters in Washington.

“Seeing separate groups analyzing the data and reaching the same conclusions independently is a mark of a solid scientific result. Not only is DART showing us the way to asteroid-deflecting technology, it is also revealing fundamental new knowledge about what asteroids are and how they behave.”

These results and observations of the debris left after the impact indicate that Dimorphos It is a “pile of rubble” type objectsimilar to the asteroid Bennu.

ESA’s Hera mission, scheduled to launch in October 2024, will travel to this pair of asteroids to carry out a detailed study and confirm how DART altered Dimorphos.