ESO/M. Kornmesser
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.
The evolution of the cloud of debris that was ejected after NASA’s DART spacecraft collided with the asteroid Dimorphos.
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.