On Tuesday, NASA announced that its first test of a potential planetary defense system had been a notable success. The Double Asteroid Redirect Test (DART) Passed crashed a spaceship into an asteroid late September, hoping to alter its orbit around a larger companion. However, any change in orbit would be difficult to detect and potentially require months of follow-up observations. But the magnitude of the orbital shift was large enough that ground-based observatories were already detecting it.
Meanwhile, a lot of hardware is also picking up debris that sprung up from the impact, giving scientists a lot of information about the collision and the asteroid.
New orbit, who is it?
Dimorphos is less than 200 meters in diameter and cannot be resolved from Earth. Instead, the binary asteroid looks like a single object when viewed from here, with most of the light reflecting off the much larger Didymos. What we can see, however, is that the Didymos system darkens sporadically. Most of the time, the two asteroids are arranged so that Earth receives light reflected from both. But Dimorphos’ orbit places it sporadically behind Didymos from Earth’s perspective, which means we only receive light reflected from one of the two bodies, causing the dimming.
By measuring the periods of obscuration, we can determine how long it takes Dimorphos to orbit and therefore how far apart the two asteroids are.
DART’s impact was designed to be direct and slow Dimorphos. This would result in a drop into a lower orbit that takes less time to complete. So even though we’ve slowed the asteroid down, we expect its orbit to end faster. How fast? In pre-impact modeling, NASA concluded that at a minimum it would be a minute shorter, but likely more substantial. “The team had looked at a wide range of parameters for the potential physical properties of Dimorphos and from these models estimated that we would make a change between minutes and tens of minutes,” said NASA’s Lori Glaze.
Over time, the difference between expectations of when you would see dimming, given Dimphos’ previous orbit, and when dimming occurs, should increase. And a variety of telescopes have captured observations that have a time window wide enough to capture both the expected orbital gradation and the full range of potential timings based on NASA modeling. The results clearly show that the orbit has been shortened.
Of how much? Before DART, Dimorphos’ orbit took 11 hours and 55 minutes; after impact it fell to 11 hours and 23 minutes. For those averse to math, it’s 32 minutes less (about 4%). NASA estimates that the orbit is now “tens of meters” closer to Didymos. This orbital shift has been confirmed by radar imagery, which can resolve both asteroids (albeit barely, as Dimorphos occupies a single pixel in these images).