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 defenses, but it also gave astronomers a unique opportunity to learn more about the composition of the asteroid from the expelled material.

On September 26, 2022, the DART spacecraft collided with the asteroid Dimorphos in a controlled test of our asteroid deflection capabilities. The impact occurred 7 million miles from Earth, close enough to be observed in detail with many telescopes. All four 8.2-metre telescopes of ESO’s VLT in Chile have 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 remnants from which all the planets and moons in our solar system were created,” 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 how our solar system formed. “Impacts between asteroids happen naturally, but you never know in advance,” 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 a controlled impact, almost like in a lab.”

Opitom and her team followed the evolution of the debris cloud for a month using the Multi Unit Spectroscopic Explorer (MUSE) instrument on ESO’s VLT. They found that the ejected cloud was bluer than the asteroid itself before impact, indicating that the cloud may be made up of very fine particles. In the hours and days following the impact, other structures developed: clumps, spirals, and a long tail that was pushed away by the sun’s radiation. The spirals and tail were redder than the original cloud, so could be made up of larger particles.

Evolution of the debris cloud around Dimorphos and Didymos after the DART impact
This series of images, taken by the MUSE instrument on ESO’s Very Large Telescope, shows the evolution of the debris cloud ejected when NASA’s DART spacecraft collided with the asteroid Dimorphos. The first image was taken on September 26, 2022, just before impact, and the last almost a month later, on October 25. During this period, several structures developed: clumps, spirals and a long tail of dust that was pushed away by the sun’s radiation. The white arrow in each panel indicates 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 are indistinguishable in these images. The background streaks seen here are due to the apparent motion of the background stars during the observations as the telescope tracked the asteroid pair. Courtesy: ESO/Opitom et al.

Using MUSE, the Opitom team was able to split the cloud’s light into a rainbow-like pattern and look for the chemical fingerprints of different gases. Specifically, they looked for oxygen and water from ice exposed by the impact. But they found nothing. “Asteroids are not expected to contain significant amounts of ice, so it would have been a real surprise to detect any trace of water.” explains Opitom. They also looked for traces of the DART spacecraft’s propellant, but found none. “We knew it was a gamble” she says, “Since the amount of gas that would remain in the tanks of the propulsion system would not be huge. In addition, some of it would have traveled too far for MUSE to detect 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 changed the asteroid’s surface.

“When we observe the objects in our solar system, we are looking at the sunlight 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 the surface.”

Bagnulo and his colleagues used the VLT’s FOcal Reducer/low dispersion Spectrograph 2 (FORS2) instrument to track the asteroid and found that the polarization level 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 asteroid’s interior. “Perhaps the material unearthed 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,” Bagnulo says.

Another possibility is that the impact destroyed particles on the surface, sending much smaller ones into the debris cloud. “We know that under certain conditions 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 the different instruments work together. In fact, in addition to MUSE and FORS2, the aftermath of the impact has been observed with two other VLT instruments, and analysis of this data is ongoing. “This research took advantage of a unique opportunity when NASA hit an asteroid,” concludes Opitom, “So it cannot be replicated by any future facility. This makes the data obtained with the VLT around the time of impact extremely valuable when it comes to better understanding the nature of asteroids.”

Magazine references

  1. C. Opitom (Institute of Astronomy, University of Edinburgh, UK [Edinburgh]), B. Murphy (Edinburgh), C. Snodgrass (Edinburgh), S. Bagnulo (Armagh Observatory & Planetarium, UK [Armagh]), SF Green (School of Physical Sciences, The Open University, UK), MM 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). Morphology and spectral properties of the DART impact ejecta with VLT/MUSE. Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202345960
  2. 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). Optical spectropolarimetry of binary asteroid Didymos-Dimorphos before and after the DART impact. Astrophysical Journal Letters DOI: 10.3847/2041-8213/acb261