. Scientific Frontline: First results from ESO telescopes on the aftermath of DART’s asteroid impact

Tuesday, March 21, 2023

First results from ESO telescopes on the aftermath of DART’s asteroid impact

This series of images, taken with the MUSE instrument on ESO’s Very Large Telescope, shows the evolution of the cloud of debris that was ejected when NASA’s DART spacecraft collided with the asteroid Dimorphos.  The first image was taken on 26 September 2022, just before the impact, and the last one was taken almost one month later on 25 October. Over this period several structures developed: clumps, spirals, and a long tail of dust pushed away by the Sun’s radiation. The white arrow in each panel marks the direction of the Sun.  Dimorphos orbits a larger asteroid called Didymos. The white horizontal bar corresponds to 500 kilometers, but the asteroids are only 1 kilometer apart, so they can’t be discerned in these images.  The background streaks seen here are due to the apparent movement of the background stars during the observations while the telescope was tracking the asteroid pair. 
Full Size Image
Image Credit: ESO/Opitom et al.

Using ESO’s Very Large Telescope (VLT), two teams of astronomers have 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 defense, but also gave astronomers a unique opportunity to learn more about the asteroid’s composition from the expelled material.

On 26 September 2022 the DART spacecraft collided with the asteroid Dimorphos in a controlled test of our asteroid deflection capabilities. The impact took place 11 million kilometers away from Earth, close enough to be observed in detail with many telescopes. All four 8.2-metre telescopes of ESO’s VLT in Chile observed the aftermath of the impact, and the first results of these VLT observations have now been published in two papers.

”Asteroids are some of the most basic relics of what all the planets and moons in our Solar System were created from,” 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 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,” continues Cyrielle Opitom, an astronomer also at the University of Edinburgh and lead author of one of the articles. “DART is a really great opportunity to study a controlled impact, almost as in a laboratory.”

This video shows the evolution of the cloud of debris that was ejected after NASA’s DART spacecraft collided with the asteroid Dimorphos. The animation is based on a series of images taken with the MUSE instrument on ESO’s Very Large Telescope (VLT) for one month after the impact.  The first image was taken on 26 September 2022, just before the impact, and the last one was taken almost one month later on 25 October. Over this period several structures developed: clumps, spirals, and a long tail of dust pushed away by the Sun’s radiation.  Dimorphos orbits a larger asteroid called Didymos, but they can’t be discerned in these images.  The background streaks seen here are due to the apparent movement of the background stars during the observations while the telescope was tracking the asteroid pair.  
Video Credit: ESOESO/Opitom et al.

Opitom and her team followed the evolution of the cloud of debris for a month with the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO’s 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 Opitom’s team 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,” explains Opitom. They also looked for traces of the propellant of the DART spacecraft, but found none. ”We knew it was a long shot,” she says, “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 travelled too far to detect it with MUSE by the time we started observing.”

Another team, led by Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in the UK, studied how the DART impact altered the surface of the asteroid. 

This animation shows how the polarization of sunlight reflected by the Dimorphos asteroid changed after the impact of NASA’s DART spacecraft. At the beginning of the video, unpolarized sunlight — represented by wiggly blue lines oscillating in random directions — is reflected off the surface of the asteroid. In so doing it becomes polarized, the reflected waves now oscillating along a preferred direction. The indicator on the lower right shows the degree of polarization of the reflected sunlight.  The DART impact ejected a cloud of debris, and after the collision the amount of polarization dropped, as seen with the FORS2 instrument on ESO’s Very Large Telescope. This drop in polarization could be due to the exposure of more pristine material from the interior of Dimorphos, or the ejection of small particles produced during the impact.  
Video Credit: ESO/M. Kornmesser

“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,” explains Bagnulo. 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.”

Bagnulo and his colleagues used the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument at the 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,” says Bagnulo.

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,” explains Zuri Gray, a PhD student also at the Armagh Observatory and Planetarium. 

The studies by the teams led by Bagnulo and Opitom show the potential of the VLT when its different instruments work together. In fact, in addition to MUSE and FORS2, the aftermath of the impact was observed with two other VLT instruments, and analysis of these data is ongoing. “This research took advantage of a unique opportunity when NASA impacted an asteroid,” concludes Opitom, “so it cannot be repeated by any future facility. This makes the data obtained with the VLT around the time of impact extremely precious when it comes to better understanding the nature of asteroids.”

Published in journal:

Astronomy & Astrophysics: “Morphology and spectral properties of the DART impact ejecta with VLT/MUSE”

Astrophysical Journal Letters: “Optical spectropolarimetry of binary asteroid Didymos-Dimorphos before and after the DART impact”

Additional information: The team who conducted the first study is composed of C. Opitom (Institute for Astronomy, University of Edinburgh, UK [Edinburgh]), B. Murphy (Edinburgh), C. Snodgrass (Edinburgh), S. Bagnulo (Armagh Observatory & Planetarium, UK [Armagh]), S. F. Green (School of Physical Sciences, The Open University, UK), M. M. Knight (United States Naval Academy, USA), J. de Léon (Instituto de Astrofísica de Canarias, Spain), J.-Y. Li (Planetary Science Institute, USA), and D. Gardener (Edinburgh).

The team who conducted the second study is composed of S. Bagnulo (Armagh), Z. Gray (Armagh), M. Granvik (Department of Physics, University of Helsinki, Finland [Helsinki]; Asteroid Engineering Laboratory, Luleå University of Technology, Sweden), A. Cellino (INAF – Osservatorio Astrofisico di Torino, Italy), L. Kolokolova (Department of Astronomy, University of Maryland, USA), K. Muinonen (Helsinki), O. Muñoz (Instituto de Astrofísica de Andalucía, CSIC, Spain), C. Opitom (Edinburgh), A. Penttila (Helsinki), and Colin Snodgrass (Edinburgh).

Source/CreditEuropean Southern Observatory

Reference Number: astr032123_01

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