New VLT data reveals more about the effects of a DART-versus-asteroid collision – Ars Technica

Last September, the Double Asteroid Redirection Test, or DART, smashed a spacecraft into a small binary asteroid called Dimorphos, successfully shifting its orbit around a larger companion. We’re now learning more about the ramifications of that collision, thanks to two new papers Data collected by the European Southern Observatory A very large telescope. the first, Posted in The journal Astronomy & Astrophysics examined the debris from the collision to learn more about the asteroid’s formation. the second, Posted in Astrophysical Journal Letters reported how the impact changed the asteroid’s surface.

As mentioned earlier, Dimorphos is less than 200 meters in diameter and cannot be resolved from Earth. Instead, the binary asteroid looks like a single object from here, with most of the light reflected off of the much larger Didymos. What we can see, however, is that Didymus’ system darkens intermittently. Most of the time, the two asteroids are arranged so that the Earth receives the light reflected from both. But Dimorphos’ orbit takes it sporadically behind Didymus from Earth’s perspective, which means we only receive light reflected from one of the two objects – and that causes the dimming. By measuring the time periods of darkness, we can see how long it takes Dimorphos to orbit and thus how far apart the two asteroids are.

Prior to DART, Dimorphos’ orbit took 11 hours, 55 minutes; After impact, it takes 11 hours 23 minutes. For those who hate math, this is 32 minutes (about 4 percent) shorter. NASA estimates that the orbit is now “tens of meters” closer to Didymos. This orbital shift was confirmed by radar imaging. Earlier this month, the journal Nature published five papers that collectively reconstructed the effect and its consequences to explain how the DART collision had such a large effect. These results indicated that colliders such as DART could be an effective way to protect the planet from small asteroids.

The closest cameras (named Luke and Leia) to the collision were aboard the LICIACube, a cube that was flown into space aboard DART and then detached a few weeks before the collision. LICIACube had two cameras on board. Last October, the Italian Space Agency, which managed the LICIACube mission, released several early images, including a distant view of the collision, close-ups shortly after, and animated images showing the sudden brightness after the collision that sent material splashing into space.

The Atlas project and one of the Las Cumbres Observatory telescopes captured images of the Didymus/Dimorphos system as it moved quietly past background stars from Earth’s perspective (with most of the light reflecting off the much larger Didymos). At the moment of impact, the object brightened noticeably, with debris gradually moving to one side of the asteroid.

Why is studying debris important? Asteroids are relics from when our solar system was created, so they can tell astronomers something about the early history of our corner of the universe. But the surfaces of near-Earth asteroids get bumped by small meteorites and the solar wind as they move through the solar system. This causes erosion, or “space weathering,” so looking at an asteroid’s surface doesn’t necessarily tell us how it formed. The DART impact was expected to eject pristine material beneath Dimorphos’ changing crust, giving astronomers a better glimpse into the asteroid’s past.

In Hubble Space Telescope images, the debris material appeared as rays extending from the system’s core, increasing in size and number over the course of the eight hours thereafter. Another Hubble image showed the continued evolution of debris that had been pushed far enough from the asteroids to be free of their gravity and since then been pushed away from the asteroids (which are still moving around the sun) by sunlight. This showed a striking split in the “tail” formed by this wreck. The Webb telescope also imaged the collision, showing distinct plumes of material spewed from the asteroid.

Now scientists armed with VLT data weigh in, too. The authors of the Astronomy and Astrophysics paper tracked how the debris cloud evolved over time with Multi-Unit Spectral Explorer (MUSE), a telescope equipped with a laser-assisted adaptive optical system for creating artificial stars in the night sky. This helps correct for atmospheric turbulence for sharper images.

The team found that before the impact, the debris cloud was bluer than the asteroid, indicating that it was made up of very fine particles. But after the collision, clumps, spirals and a long tail were formed. The spirals and tail are likely made of larger particles because they are now much redder than the initial debris cloud. Although it was a long shot, the team hoped MUSE would also help them detect chemical signatures of oxygen or water coming from the ice in particular. But they came up empty.

“Asteroids are not expected to contain large amounts of ice, so discovering any trace of water would be a real surprise.” said co-author Cyrielle Opitom from the University of Edinburgh. As for finding no traces of fuel, “we knew they were far-reaching, as the amount of gas that would be left in the tanks from the propulsion system wouldn’t be huge. Furthermore, some of it may have traveled too far to be detected with MUSE at the time that We started watching.”

The authors of the Astrophysical Journal Letters paper focused on studying how the DART effect changes the asteroid’s surface, using a spectrophotometer (FORS2) instrument designed to measure the level of polarization of scattered sunlight, that is, when light waves oscillate along a preferred direction rather than randomly.

“When we observe objects in our solar system, we are looking at sunlight being scattered off their surface or by their atmosphere, which becomes partially polarized,” said co-author Stefano Bagnolo, astronomer at the Armagh Observatory and Planetarium in the United Kingdom. Tracking how the polarization changes with the asteroid’s orientation relative to us and the sun reveals its surface structure and composition.“

Bagnolo et al. found that levels of polarization dropped abruptly after the impact, while overall brightness increased. The authors suggest that this may be evidence that the impact caused more pure material to be released from the asteroid’s interior because this material would not have been exposed to the solar wind and radiation. Alternatively, the impact may have smashed large surface particles and sprayed smaller fragments into the debris cloud because smaller fragments would reflect light more efficiently but not polarize the light as much.

DOI: Astronomy and Astrophysics, 2023. 10.1051 / 0004-6361 / 202345960 (about DOIs).

DOI: Astrophysical Journal Letters, 2023. 10.3847/2041-8213/acb261 (about DOIs).