Last September, the Double Asteroid Redirect Test, or DART, smashed a spacecraft into a small binary asteroid called Dimorphos, successfully altering its orbit around a larger companion. We're now learning more about the aftermath of that collision, thanks to two new papers reporting on data collected by the European Southern Observatory's Very Large Telescope. The first, published in the journal Astronomy and Astrophysics, examined the debris from the collision to learn more about the asteroid's composition. The second, published in the Astrophysical Journal Letters, reported on how the impact changed the asteroid's surface.
As we've reported previously, Dimorphos is less than 200 meters across and cannot be resolved from Earth. Instead, the binary asteroid looks like a single object from here, with most of the light reflecting off the far larger Didymos. What we can see, however, is that the Didymos system sporadically darkens. Most of the time, the two asteroids are arranged so that Earth receives light reflected off both. But Dimorphos' orbit sporadically takes it behind Didymos from Earth's perspective, meaning that we only receive light reflected off one of the two bodies—this causes the darkening. By measuring the darkening's time periods, we can work out how long it takes Dimorphos to orbit and thus how far apart the two asteroids are.
Before DART, Dimorphos' orbit took 11 hours and 55 minutes; post-impact, it's down to 11 hours and 23 minutes. For those averse to math, that's 32 minutes shorter (about 4 percent). NASA estimates that the orbit is now "tens of meters" closer to Didymos. This orbital shift was confirmed by radar imaging. Earlier this month, Nature published five papers that collectively reconstructed the impact and its aftermath to explain how DART's collision had an outsize effect. Those results indicated that impactors like DART could be a viable means of protecting the planet from small asteroids.
The closest cameras (named Luke and Leia) to the collision were on board LICIACube, a cubesat that was carried to space on board DART and then separated a few weeks before impact. LICIACube had two onboard cameras. Last October, the Italian Space Agency, which ran the LICIACube mission, released several early images, including a distant view of the collision, close-ups taken shortly after, and an animation showing the sudden brightening after the collision scattered material into space.
The ATLAS project and one of the Las Cumbres observatory's telescopes captured images of the Didymos/Dimorphos system moving peacefully past background stars from Earth's perspective (with most of the light reflected off the far larger Didymos). At the moment of collision, the object brightened significantly, with the debris gradually moving off to one side of the asteroids.
Why does studying the debris matter? 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 slammed by tiny 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 expel pristine material below Dimorphos' weathered crust, giving astronomers a better glimpse into the asteroid’s past.
In Hubble Space Telescope images, the debris material showed up as rays that extended from the core of the system, and they grew in size and number over the course of eight hours afterward. Another Hubble image showed the continuing evolution of the debris that got pushed far enough from the asteroids to be free from their gravity and has since been pushed away from the asteroids (which are still moving around the Sun) by the Sun's light. That showed a striking split to the "tail" formed by this debris. The Webb Telescope also imaged the collision, showing distinct plumes of material coming off the asteroid.
Now scientists armed with VLT data are weighing in as well. The authors of the Astronomy and Astrophysics paper tracked how the debris cloud evolved over time with the Multi Unit Spectroscopic Explorer (MUSE) instrument, a telescope equipped with a laser-assisted adaptive optic system to create artificial stars in the night sky. This helps correct for atmospheric turbulence to get sharper images.
The team found that before the impact, the debris cloud was bluer than the asteroid, suggesting it was composed of very fine particles. But after the collision, clumps, spirals, and that long tail formed. The spirals and tail are likely made up of larger particles since they are now redder than the initial debris cloud. Even though it was a long shot, the team hoped MUSE would also help them detect the chemical fingerprints of oxygen or water coming from ice in particular. But they came up empty.
"Asteroids are not expected to contain significant amounts of ice, so detecting any trace of water would have been a real surprise," said co-author Cyrielle Opitom of the University of Edinburgh. As for not finding any traces of propellant, "We knew it was a long shot, 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 authors of the Astrophysical Journal Letters paper focused on studying how the impact of DART changed the surface of the asteroid, using a spectrographic instrument (FORS2) designed to measure the level of polarization of scattered sunlight—i.e., when light waves oscillate along a preferred direction rather than randomly.
“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,” said co-author Stefano Bagnulo, an astronomer at the Armagh Observatory and Planetarium in the UK. “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 et al. found that the polarization levels dropped suddenly after the impact, while the overall brightness increased. The authors suggest this could be evidence that the impact did kick up more pristine matter from inside the asteroid since that material would not have been exposed to solar wind and radiation. Alternatively, the impact may have smashed large surface particles and sprayed smaller fragments into the debris cloud since smaller fragments would reflect light more efficiently but wouldn't 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).
