The DART asteroid impact created a 10,000 kilometer boulder debris field

Enlarge / Dusty debris from the DART collision dominates this image, but there is has boulders present too. NASA, ESA, David Jewitt (UCLA)

NASA's Double Asteroid Redirect Test (DART) mission was a success from a planetary defense perspective, as it successfully relocated an asteroid's orbit. But the mission had a science element, and we're still sifting through debris from the collision to determine what the impact tells us about the asteroid. This is difficult due to the distance to the asteroid and the low amounts of light reflecting off the debris.

Today an article was published by a team that analyzed images of the aftermath using the Hubble Space Telescope. They spotted dozens of boulders that collectively would have originally made up 0.1% of the mass of Dimorphos, the DART target. And as they all drift very slowly away from the collision site, some of them should be able to escape the gravity of the dual asteroid system.

Hitting rocks

Images taken by DART just before its disappearance suggest that Dimorphos was a pile of rubble, a mixture of boulders, small rocks and dust barely held together by their mutual gravitational pull. So what happens when a relatively solid object, like the DART spacecraft, pounds an asteroid at high speed?

For a while the answer was "lots of dust". The first images show a lot of material coming out of the asteroids, spreading out into space and forming a long "tail" chased by the radiation pressure from the Sun. But, over time, the debris cleared up enough that Hubble could get a clear picture of all the larger objects that had been obscured by dust, or rather a number of clear pictures.

The challenge with this is that these larger objects would still be quite small and reflect very little sunlight. As a result, they usually appeared as small dots of light and seemed indistinguishable from cosmic rays hitting the detector or background stars moving across Hubble's field of view during imaging.

Enlarge / The researchers helpfully identified all of the individual boulders, otherwise difficult to see. Jewitt et al.

So the Hubble images had to be long-exposure to capture enough light, and the researchers combined multiple exposures taken by Hubble at different points in its orbit around Earth (which required them to reorient the image so they all showed the equivalent area from the same angle). Light that only appeared in one or a few of the frames has been removed, removing some of the noise.

Once the exposures were combined, the researchers were able to identify about 40 objects that moved with the Didymos/Dimorphos system but distinct from it. Only the brightest of them are visible in the individual images.

Small and slow

Depending on the amount of light they reflect, researchers estimate that the rocks they see are between 4 and 7 meters in diameter. This is based on the average reflectivity of the parent asteroids; obviously any darker or brighter rocks will negate these estimates. The researchers also use a single-density estimate...