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After DART Smashed Into Dimorphous, What Happened to the Larger Asteroid Didymos?

NASA’s DART mission (Double Asteroid Redirection Test) slammed into asteroid Dimorphos in September 2022, changing its orbital period. Ground and space-based telescopes turned to watch the event unfold, not only to study what happened to the asteroid, but also to help inform planetary defense efforts that might one day be needed to mitigate potential collisions with our planet.

Astronomers have continued to observe and study Dimorphos, well past the impact event. However, Dimorphos is the smaller asteroid in this binary system, and is just a small moon orbiting the larger asteroid Didymos.

The James Webb Space Telescope (JWST) is the only telescope capable of visually distinguishing between the two closely orbiting asteroids. Now, astronomers have made follow-on observations on the system with JWST to see what happened to Didymos after the dust cleared.

In a new preprint paper, a team of scientists, led by Andrew Rivkin, the Investigation Lead for DART, explain how they used two instruments on JWST to measure spectra of Didymos about two months after the DART impact. One of their biggest takeaways is that Didymos and Dimorphos appear to be of the same composition, which is that of an ordinary chondrite. That’s the class of stony meteorites which account for over 80% of total meteorite falls on Earth. This means DART’s test was an extremely good proxy for the type of asteroids that might pose a threat one day.

“One of the benefits of using the Didymos system was definitely that we thought it was representative of most of what’s out there in its properties,” Rivkin told Universe Today via email. “People working on planetary defense often note that the ‘asteroid picks us rather than the other way around,’ but showing that what we did at Dimorphos is broadly applicable is very important.”

For the new observations, the scientists used NIRSpec – the near infrared spectrometer — and MIRI, the mid-infrared instrument, on November 28, 2022.

At the time of the observations, the centers of Didymos and Dimorphos were never separated by more than 0.1” from each other, from JWST’s vantage point. But the team did take advantage of Dimorphos being occulted by Didymos during the MIRI observations.

Didymos MIRI 1
Median averaged slice through the MIRI MRS IFU showing Didymos. Note that Dimorphos was being occulted by Didymos during the entire period of MIRI observations.Credit: Rivkin et al, 2023.

“Didymos is roughly five times larger in diameter than Dimorphos and therefore it has roughly 25 times the cross-sectional area as Dimorphos,” the researchers wrote. “This size difference between the components means that roughly 96% of flux from the system typically comes from Didymos.”

The researchers said several lines of evidence suggest that the asteroid and its moon have similar compositions and the team concludes they “can reasonably estimate the composition of Dimorphos specifically from measurements of the Didymos-dominated flux.”


This image from the DART spacecraft of the light from asteroid Didymos and its orbiting moonlet Dimorphos. Credit: NASA/JPL-Caltech/DART Navigation Team

Given how hard it is to distinguish between Didymos and Dimorphos in long-range observations, I asked Rivkin if they were able to see any noticeable changes in the observations of Didymos after the impact to Dimorphos, given all
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