Hubble observes cluster of boulders around impacted asteroid

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Last year, NASA deliberately crashed a spacecraft into an asteroid, in a first-of-its-kind test of planetary defense. At the time, telescopes around the world including the Hubble Space Telescope observed the impact between the DART spacecraft and the Dimorphos asteroid, capturing footage of the plumes of dust thrown up. Now, Hubble has observed Dimorphos once again and seen that a number of boulders have been ejected from the asteroid.

The Hubble image shown below was taken on 19 December 2022, around four months after the impact, and shows the bright streak of the asteroid across the sky, surrounded by small boulders which were knocked loose during the impact. This view was only possible after several months as the impact initially sent up large amounts of dust which made it difficult to see the asteroid in detail.

A NASA/ESA Hubble Space Telescope image of the asteroid Dimorphos taken on 19 December 2022.
This NASA/ESA Hubble Space Telescope image of the asteroid Dimorphos was taken on 19 December 2022, nearly four months after the asteroid was impacted by NASA’s DART (Double Asteroid Redirection Test) mission. Hubble’s sensitivity reveals a few dozen boulders knocked off the asteroid by the force of the collision. These are among the faintest objects Hubble has ever photographed inside the Solar System. The ejected boulders range in size from 1 metre to 6.7 metres across, based on Hubble photometry. They are drifting away from the asteroid at around a kilometre per hour. The discovery yields invaluable insights into the behaviour of a small asteroid when it is hit by a projectile for the purpose of altering its trajectory. NASA, ESA, D. Jewitt (UCLA)

Researchers observed 37 boulders in total, ranging in size from 1 to 6.7 meters. The aim of the impact was not to destroy the asteroid but to redirect it — the idea being that, in case an asteroid were threatening Earth, this method could be effective at nudging its orbit so that it would miss the planet. The recent observations also gave further confirmation that the asteroid’s trajectory around its partner asteroid, Didymos. However, to learn more about the exact effects of the impact, we will have to wait for the European Space Agency’s Hera mission, which will visit the asteroid binary to collect more data and which will launch next year.

In total, around 0.1% of the mass of the Dimorphos asteroid was displaced by the impact, with the boulders now drifting away from the asteroid. Hubble scientists explain that these are probably not made from chunks of the asteroid, but rather are boulders that were sitting on the asteroid’s surface when the impact occurred.

“It’s not clear how the boulders were lifted off the asteroid’s surface,” Hubble scientists write. “They could be part of an ejecta plume that was photographed by Hubble and other observatories. Or a seismic wave from the impact may have rattled through the asteroid — like hitting a bell with a hammer — shaking loose the surface rubble.”

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