New study finds that DART changed both orbit and shape of Dimorphos asteroid

by Haygen Warren

On Sept. 26, 2022, NASA’s Double Asteroid Redirection Test (DART) spacecraft slammed into the side of the 170-meter-wide asteroid Dimorphos to test future asteroid deflection tactics in the event an asteroid was threatening Earth. Following impact, DART teams confirmed that the spacecraft’s collision with the asteroid had successfully deflected the asteroid by a small amount — proving the design of the deflector and allowing scientists and engineers to better understand the ways we can protect Earth from asteroids.

However, a new study from a group of scientists shows that DART’s collision with Dimorphos not only changed the shape of its orbit around its host asteroid Didymos but also significantly changed the shape of the asteroid.

Before the collision, Dimorphos was known to be a roughly symmetrical “oblate spheroid” shaped asteroid, similar to a squished ball that is wider than it is tall. Additionally, it took the asteroid around 11 hours and 55 minutes to complete one orbit around Didymos, which, as mentioned, has since been altered following DART’s impact with Dimorphos.

“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,” said navigation engineer and lead author Shantanu Naidu of NASA’s Jet Propulsion Laboratory (JPL) in California.

Illustration showing the shape of Dimorphos both before and after DART’s impact. (Credit: NASA/JPL-Caltech)

To better understand how Dimorphos’ shape and orbit changed following DART’s impact, Naidu et al. employed computer models that utilized three data sources that collected data during DART’s impact. The first source was the images DART’s Didymos Reconnaissance and Asteroid Camera for Optical navigation (DRACO) camera captured of Didymos in the hours, minutes, and seconds before impact. DRACO’s images were transmitted from DART to the Deep Space Network (DSN), and the images allowed the scientists to perform close-up measurements and gauge the dimensions of both Didymos and Dimorphos before impact.

The DSN’s Goldstone Solar System Radar was the second data source, which is located in Barstow, California. During DART’s impact sequence, the Goldstone Radar bounced radio waves off of Dimorphos and Didymos to measure the position and velocity of Dimorphos around Didymos precisely before and after DART’s collision. These measurements are what allowed NASA teams to quickly discern whether or not DART’s impact had indeed deflected Dimorphos in its orbit around Didymos. Teams would eventually find that the effect of DART’s impact on the asteroid greatly exceeded the minimum expectations for the mission.

The final and most significant data source utilized in Naidu et al.’s study was ground telescopes located around Earth that measured the light curve — how sunlight reflects off the surface of the asteroids over time — from both asteroids. Comparing the asteroids’ light curves before and after impact allowed the team to learn more about how DART altered Dimorphos’ position around Didymos.

“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,” Chesley continued, “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,” said co-author Steve Chesley of JPL.

Naidu even confirmed that their models were so sensitive that they were able to detect a slight back-and-forth rocking of Dimorphos as it orbits around Didymos.

What’s more, Naidu et al.’s models were also able to calculate how Dimorphos’ orbital period around Didymos was altered and evolved over time. Immediately following impact, teams found that DART had reduced Dimorphos’ orbital period by 32 minutes and 42 seconds, making its total orbital period around Didymos 11 hours, 22 minutes, and 37 seconds.

However, the asteroid’s orbit would continue to shorten as the asteroid continuously lost more and more surface material from the impact to space. Dimorphos’ orbital period would finally settle to be approximately 11 hours, 22 minutes, and three seconds, meaning that DART’s impact officially altered the orbit of the asteroid by 33 minutes and 15 seconds. The scientists’ calculations are accurate to within 1.5 seconds, and Dimorphos now sits at a mean orbital distance from Didymos of 1,152 meters, which is around 37 meters closer to the asteroid than before DART’s impact.

“The results of this study agree with others that are being published. Seeing separate groups analyze the data and independently come to the same conclusions is a hallmark of a solid scientific result. DART is not only showing us the pathway to an asteroid-deflection technology, it’s revealing new fundamental understanding of what asteroids are and how they behave,” said NASA’s lead scientist for solar system small bodies Tom Statler of NASA HQ in Washington D.C.

The results from Naidu et al.’s study, along with observations of material left around Dimorphos after the impact, indicate that Dimorphos is a loosely-packed “rubble pile” asteroid, similar to asteroid Bennu that was visited and sampled by NASA’s OSIRIS-REx mission. The European Space Agency’s upcoming Hera mission will travel back out to the Didymos-Dimorphos system to further investigate the long-term changes made to Dimorphos by DART’s impact.

Naidu et al.’s results were recently published in the Planetary Science Journal.

(Lead image: Dimorphos imaged by DART’s DRACO cameras just two seconds before impact. Credit: NASA/Johns Hopkins APL)

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