On Tuesday, NASA announced that its first test of a potential planetary defense system was a notable success. The Double Asteroid Redirect Test (DART) successfully smashed a spacecraft into an asteroid in late September, hoping to alter its orbit around a larger companion. Any changes in the orbit, however, would be difficult to pick up and potentially require months of follow-up observations. But the magnitude of the orbital shift was large enough that ground-based observatories picked it up already.
Meanwhile, a lot of hardware is also picking up the debris that shot out from the impact, giving scientists a lot of information about the collision and the asteroid.
New orbit, who dis?
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.
DART’s impact was designed to be head-on and slow Dimorphos. This would result in it dropping into a lower orbit that takes less time to complete. So, even though we’ve slowed the asteroid down, we expect its orbit to be completed more quickly. How quickly? In modeling done before the impact, NASA concluded that, at a minimum, it would be over a minute shorter but was likely to be more substantial. “The team had been looking at a broad range of parameters for the potential physical properties of Dimorphos and from those models, had estimated we would make a change of between a few minutes and several tens of minutes,” said NASA’s Lori Glaze.
Over time, the difference between expectations of when you’d see a dimming, given Dimphos’ prior orbit and when the dimming takes place, should grow. And a variety of telescopes have grabbed observations that have a wide enough time window to capture both the expected orbital dimming and the full range of potential timings based on NASA modeling. The results clearly show that the orbit has been shortened.
By how much? 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, which can resolve the two asteroids (albeit barely, as Dimorphos occupies a single pixel in these images).
