NASA’s Double Asteroid Redirection Test (DART) mission was successful from a planetary defense perspective, successfully altering the asteroid’s orbit. But the mission had a science component to it, and we’re still sorting through the impact debris to determine what the impact tells us about the asteroid. This is difficult because of the distance to the asteroid and the low amounts of light reflected from the debris.
Today, a paper was released by a team that analyzed images of the impacts with the Hubble Space Telescope. They’ve unearthed dozens of rocks that together would have originally made up 0.1% of the mass of Dimorphos, the DART target. And while they are all moving very slowly from the collision site, some of them should be able to escape the gravity of the double asteroid system.
Images taken by DART just before its demise indicate that Dimorphos was a pile of rubble, a jumble of boulders, small boulders, and dust barely held together by their mutual gravitational pull. So what happens when a relatively rigid object, like the DART spacecraft, hits an asteroid at high speed?
For a while, the answer was “too much dust.” Early images show a lot of material pouring out from the asteroids, dispersing through space and forming a long “tail” driven by the sun’s radiation pressure. But over time, enough debris had been removed that Hubble was able to obtain a clear picture of any larger objects that had been obscured by the dust — or rather, a number of clear images.
The challenge with this is that those large objects will still be very small and reflect very little in the way of sunlight. As a result, they generally appear as tiny points of light and are indistinguishable from cosmic rays hitting the detector or background stars moving across Hubble’s field of view during imaging.
Therefore, the Hubble images had to be long-exposed to capture enough light, and the researchers combined multiple exposures that Hubble had taken at different points in its orbit around the Earth (which required them to reorient the image so that they all showed the equivalent area from the same angle). Light that only appeared in one or a few images was discarded, eliminating some of the noise.
Once the exposures were combined, the researchers could identify nearly 40 objects that were moving along with but diverging from the Didymus/Dimorphus system. Only the brightest ones are shown in the individual images.
Small and slow moving
Based on the amount of light they reflect, the researchers estimate that the rocks they see are within 4-7 metres. This is based on the average reflectance of the parent asteroids; Any darker or brighter rocks will obviously throw off these estimates. Researchers also use a monolithic density estimate based on intact asteroids to figure out the likely masses of the rocks. Collectively, they are estimated to have carried about 0.1 percent of the mass of Dimorphos before impact.
Based on their distance from the impact site, it was possible to estimate their velocities. All very slow. Even the fastest boulders move at less than a meter per second, which translates to nearly four hours to travel one kilometer from the impact site. And the slower speed is only a fraction of that speed.
But due to the very weak gravity of the double asteroid system they came from, the high-velocity objects would be able to escape the force of gravity. In fact, the rock population can be roughly divided in half, with the faster half achieving escape velocity.
The combination of mass and velocity allowed the authors to estimate the total kinetic energy these rocks carried away from the collision. Compared to the energy provided by DART, it is very small, about 0.003 percent of the energy provided by DART.
Since Dimorphos is a pile of rubble, there is no reason to believe that this is a product of DART shattering a larger boulder on impact. Instead, Dimorphos are built from rocks previously shattered by collisions in the distant past. DART only freed a few of them from the gravitational pull of the rubble pile. Based on pre-impact images of Dimorphos, the researchers estimate that the rocks would collectively occupy about 2% of the asteroid’s surface. This corresponds to DART blowing out a crater about 50 meters in diameter.
The crater would likely be smaller if DART transmitted enough seismic energy to loosen material from elsewhere on the asteroid. But since the piles of rubble are expected to be very porous, the seismic energy is unlikely to reach very far into them.
In any case, we’ll have a clearer picture of things once the European Space Agency’s HERA probe reaches the asteroid for a follow-up study. You just have to be patient, because this is not expected to happen for another three years.
Astrophysical Journal Letters, 2023. DOI: 10.3847/2041-8213/ace1ec (about DOIs).
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