Using ESO’s Very Large Telescope (VLT), two teams of astronomers observed the aftermath of the collision between NASA’s Double Asteroid Redirection Test (DART) spacecraft and the asteroid Dimorphos. The controlled impact was a test of planetary defenses, but also gave astronomers a unique opportunity to learn more about the asteroid’s composition from the fragments ejected.
On September 26, 2022, the DART spacecraft collided with the asteroid Dimorphos during a controlled test of our asteroid deflection capabilities. The impact occurred 11 million kilometers from planet Earth, close enough to be observed in detail by many telescopes. The four 8.2-metre telescopes of ESO’s VLT in Chile observed the aftermath of the impact, and the first results of these observations have been published in two articles.
“Asteroids are some of the most fundamental remnants of the elements that make up all the planets and moons in our solar system,” says Brian Murphy, a PhD student at the University of Edinburgh in the UK and co-author of one of the studies. Studying the cloud of matter ejected after the DART impact can therefore tell us how our solar system formed. “Impacts between asteroids happen naturally, but you can never predict them,” continues Cyrielle Opitom, an astronomer also at the University of Edinburgh and lead author of one of the papers. “DART is a really great opportunity to study controlled impact, almost like in a laboratory.”
Cyrielle Opitom and her team followed the evolution of the debris cloud for a month using ESO’s VLT instrument MUSE (Multi Unit Spectroscopic Explorer). They found that before impact, the ejected cloud was bluer than the asteroid itself, indicating the cloud could be made up of very fine particles. In the hours and days after impact, other structures developed: clusters, spirals, and a long tail pushed back by solar radiation. The spirals and tail were redder than the initial plume and may therefore be composed of larger particles.
MUSE allowed Cyrielle Opitom’s team to split the light from the cloud into a spectrum and look for the chemical fingerprints of different gases. In particular, they searched for oxygen and water from the ice exposed by the impact. But they found nothing. “We don’t expect to find the slightest amount of ice in asteroids, so finding the slightest trace of water would have been a real surprise,” explains Cyrielle Opitom. They also looked for traces of propellant from the DART spacecraft, but found none. “We knew it wasn’t won,” she says, “because the amount of fuel that would be left in the propulsion system tanks wasn’t going to be huge. Also, some of it would have gone too far for MUSE to recognize at the time the observations began.”
Another team, led by Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in the UK, studied how the DART impact altered the asteroid’s surface.
“When we observe the objects in our solar system, we look at the sunlight reflected from their surface or atmosphere and partially polarized,” explains Stefano Bagnulo. This means that the light waves do not oscillate randomly, but in a preferred direction. “The observation of polarization changes as a function of the asteroid’s orientation relative to us and the Sun reveals its surface structure and composition.”
Stefano Bagnulo and his colleagues used the FORS2 (FOcal Reducer/low dispersion Spectrograph 2) instrument on the VLT to monitor the asteroid and found that the degree of polarization dropped suddenly after the impact. At the same time, the overall brightness of the system increased. One possible explanation is that the impact uncovered more virgin material from inside the asteroid. “Perhaps the material ejected by the impact was inherently lighter and less polarizing than the material on the surface because it was never exposed to the solar wind and solar radiation,” says Stefano Bagnulo.
Another possibility is that the impact destroyed particles on the surface, thereby ejecting many smaller particles into the debris cloud. “We know that smaller fragments may reflect light more efficiently and polarize light less efficiently,” explains Zuri Gray, also a PhD student at the Armagh Observatory and Planetarium.
The studies carried out by the teams around Stefano Bagnulo and Cyrielle Opitom show the potential of the VLT when its different instruments are combined. In addition to MUSE and FORS2, the aftermath of the impact was actually observed by two other VLT instruments, and analysis of this data is ongoing. “This research benefited from an extraordinary situation when NASA hit an asteroid,” concludes Cyrielle Opitom, “so it cannot be repeated with any other device. This makes the data obtained with the VLT at the moment of impact extremely valuable for a better understanding of the nature of asteroids.”