On September 26, 2022, NASA’s DART spacecraft collided with the asteroid Dimorphos in a controlled test of our asteroid deflection capabilities. The impact took place 11 million km away from Earth, close enough to be observed in detail with many telescopes. All four 8.2-m telescopes of ESO’s Very Large Telescope observed the aftermath of the impact, and the first results of these observations have now been published in two papers in the journal Astronomy & Astrophysics and the Astrophysical Journal Letters.
“Asteroids are some of the most basic relics of what all the planets and moons in our Solar System were created from,” said Brian Murphy, a Ph.D. student at the University of Edinburgh.
Studying the cloud of material ejected after DART’s impact can therefore tell us about how our Solar System formed.
“Impacts between asteroids happen naturally, but you never know it in advance,” said Dr. Cyrielle Opitom, an astronomer at the University of Edinburgh.
“DART is a really great opportunity to study a controlled impact, almost as in a laboratory.”
In their study, Murphy, Dr. Opitom and colleagues followed the evolution of the cloud of debris for a month with the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s Very Large Telescope (VLT).
They found that the ejected cloud was bluer than the asteroid itself was before the impact, indicating that the cloud could be made of very fine particles.
In the hours and days that followed the impact other structures developed: clumps, spirals and a long tail pushed away by the Sun’s radiation.
The spirals and tail were redder than the initial cloud, and so could be made of larger particles.
MUSE allowed the astronomers to break up the light from the cloud into a rainbow-like pattern and look for the chemical fingerprints of different gases.
In particular, they searched for oxygen and water coming from ice exposed by the impact. But they found nothing.
“Asteroids are not expected to contain significant amounts of ice, so detecting any trace of water would have been a real surprise,” Dr. Opitom said.
“We also looked for traces of the propellant of the DART spacecraft, but found none.”
“We knew it was a long shot as the amount of gas that would be left in the tanks from the propulsion system would not be huge.”
“Furthermore, some of it would have traveled too far to detect it with MUSE by the time we started observing.”
This artist’s illustration shows the ejection of a cloud of debris after NASA’s DART spacecraft collided with the asteroid Dimorphos. Image credit: ESO / M. Kornmesser.
In the second study, Dr. Stefano Bagnulo from the Armagh Observatory and Planetarium and colleagues studied how the DART impact altered the surface of the asteroid.
“When we observe the objects in our Solar System, we are looking at the sunlight that is scattered by their surface or by their atmosphere, which becomes partially polarized,” Dr. Bagnulo said.
“This means that light waves oscillate along a preferred direction rather than randomly.”
“Tracking how the polarization changes with the orientation of the asteroid relative to us and the Sun reveals the structure and composition of its surface.”
The study authors used the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument at VLT to monitor the asteroid, and found that the level of polarization suddenly dropped after the impact.
At the same time, the overall brightness of the system increased.
One possible explanation is that the impact exposed more pristine material from the interior of the asteroid.
“Maybe the material excavated by the impact was intrinsically brighter and less polarizing than the material on the surface, because it was never exposed to solar wind and solar radiation,” Dr. Bagnulo said.
“Another possibility is that the impact destroyed particles on the surface, thus ejecting much smaller ones into the cloud of debris.”
“We know that under certain circumstances, smaller fragments are more efficient at reflecting light and less efficient at polarizing it,” said Dr. Zuri Gray, a Ph.D. student at the Armagh Observatory and Planetarium.
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C. Opitom et al. 2023. Morphology and spectral properties of the DART impact ejecta with VLT/MUSE. A&A 671, L11; doi: 10.1051/0004-6361/202345960
Stefano Bagnulo et al. 2023. Optical Spectropolarimetry of Binary Asteroid Didymos-Dimorphos before and after the DART Impact. ApJL 945, L38; doi: 10.3847/2041-8213/acb261
