Free Astronomy Magazine May-June 2023

45 ferred direction rather than ran- domly. “Tracking how the polarisa- tion changes with the orientation of the asteroid relative to us and the Sun reveals the structure and com- position of its surface.” Bagnulo and his colleagues used the FOcal Reducer/low dispersion Spec- trograph 2 (FORS2) instrument at the VLT to monitor the asteroid, and found that the level of polarisation suddenly dropped after the impact. At the same time, the overall bright- ness of the system increased. One possible explanation is that the im- pact exposed more pristine material from the interior of the asteroid. “Maybe the material excavated by the impact was intrinsically brighter and less polarising than the material on the surface, because it was never exposed to solar wind and solar ra- diation,” says Bagnulo. Another possibility is that the im- pact destroyed particles on the sur- face, thus ejecting much smaller ones into the cloud of debris. “ We know that under certain circum- stances, smaller fragments are more efficient at reflecting light and less efficient at polarising it,” explains Zuri Gray, a PhD student also at the Armagh Observatory and Planetar- ium. The studies by the teams led by Bag- nulo and Opitom show the potential of the VLT when its different instru- ments work together. In fact, in ad- dition to MUSE and FORS2, the aftermath of the impact was ob- served with two other VLT instru- ments, and analysis of these data is ongoing. “This research took advan- tage of a unique opportunity when NASA impacted an asteroid,” con- cludes Opitom, “so it cannot be re- peated by any future facility. This makes the data obtained with the VLT around the time of impact ex- tremely precious when it comes to better understanding the nature of asteroids.” T his 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 sev- eral 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 Didy- mos. The white horizontal bar corre- sponds to 500 kilometres, but the asteroids are only 1 kilometre 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. [ESO/Opitom et al.] “Asteroids are not expected to con- tain significant amounts of ice, so de- tecting any trace of water would have been a real surprise,” explains Opitom. They also looked for traces of the propellant of the DART space- craft, 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 Bag- nulo, 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 polarised,” explains Bagnulo. This means that light waves oscillate along a pre- !