It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Tuesday, March 28, 2023
Astronomers analyze 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. 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 September 26, 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-meter 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 Ph.D. 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."
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 traveled 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.
"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 Ph.D. 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."
The research highlighted in the first part of this article was presented in the paper "Morphology and spectral properties of the DART impact ejecta with VLT/MUSE," which appears in Astronomy & Astrophysics. The second part of this article refers to the paper "Optical spectropolarimetry of binary asteroid Didymos-Dimorphos before and after the DART impact" in Astrophysical Journal Letters.
More information: C. Opitom et al, Morphology and spectral properties of the DART impact ejecta with VLT/MUSE, Astronomy & Astrophysics (2023). DOI: 10.1051/0004-6361/202345960
Optical spectropolarimetry of binary asteroid Didymos-Dimorphos before and after the DART impact, Astrophysical Journal Letters (2023). DOI: 0.3847/2041-8213/acb261. iopscience.iop.org/article/10. … 847/2041-8213/acb261
Spirals, Tails, And Reflective Dust Were Released In DART Asteroid Collision
Deep observations have revealed insight into the Didymos-Dimorphos system.
DR. ALFREDO CARPINETI Senior Staff Writer & Space Correspondent
Published March 21, 2023
Artist's impression of the DART impact on Dimorphos. Image Credit: ESO/M. Kornmesser
Last September, DART hit asteroid Dimorphos, the small companion of asteroid Didymos. The impact was a test of planetary defense, showing that we can truly shift the orbit of a celestial body. But it was also a chance to study what an impact on an asteroid looks like. And astronomers did not waste time in pointing some of the most powerful telescopes at it.
Using the Very Lage Telescope, part of the European Southern Observatory (ESO), astronomers were able to spot features, composition, and peculiarities of the dust released in the impact. And it gave them a great deal of information about what happens when asteroids collide.
“Impacts between asteroids happen naturally, but you never know it in advance,” the lead author of one of two new studies, Cyrielle Opitom, an astronomer at the University of Edinburg, said in a statement. “DART is a really great opportunity to study a controlled impact, almost as in a laboratory.”
This research team followed the evolution of the dust cloud from mere hours after the impact to a month later. At first, the ejected cloud was bluer in color than the asteroid, suggesting that it was made of finer particles, but as time went by and it expanded, the team saw structures develop such as clumps, spirals, and long tails. And as more time went by, they appeared redder and redder, suggesting large particles were the main components of these.
The team also looked for water ice from the asteroid – there was little hope of finding it as they tend to be very dry, but it was important to check. They also looked for any residual fuels from DART, but it impacted the asteroid almost empty.
“We knew it was a long shot,” Opitom explained, “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.”
The other research team looked at the polarization of light from the cloud of debris following the impact. Polarized light is light with a specific orientation (the electromagnetic fields of it oscillate on a specific plane) and the atmosphere and surface of a celestial body can change and polarize the light of the Sun. Or clouds of particles from a collision.
“Tracking how the polarisation changes with the orientation of the asteroid relative to us and the Sun reveals the structure and composition of its surface,” lead author Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in the UK, explained.
Following the impact, the scientists noticed that the level of polarization decreased but the brightness of the system increased, suggesting that the material ejected might have been more pristine and brighter, coming from the subsurface so not previously exposed to solar radiation. Or it could be a question of size.
”We know that under certain circumstances, smaller fragments are more efficient at reflecting light and less efficient at polarising it,” explained Zuri Gray, a PhD student also at the Armagh Observatory and Planetarium.
This is just the beginning of this data analysis. More work is currently being done to analyze what the ESO observatories have seen in this fantastic event.
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