On September 26, 2022, the NASA mission DART (Double Asteroid Redirection Test) slammed into the 170-meter wide asteroid Dimorphos at 24,000 kilometers per hour. Dimorphos is a moon of the bigger asteroid Didymos, and the test was done to see how much the impact could shift the moon’s orbit — the idea being that if an asteroid were headed toward Earth impact, we might be able to repeat this experiment to push it out of the way.

The spacecraft’s impact had quite the, um, impact, because it changed the orbital period of Dimorphos by half an hour! That’s a lot. And the bigger idea here was to hit an asteroid moon because we knew the period of Dimorphos (how long it takes the rock to orbit Didymos once) quite well, making any change easier to measure.

But it gets better. New research just published shows that not only was the asteroid moon’s orbit changed, so was the orbit of the pair around the sun! [link to journal paper] Not by much; the system was slowed by about 11.7 (±1.3) microns per second. A micron is a millionth of a meter, or a thousandth of a millimeter — a typical human hair is about 100 microns thick. This tiny amount was measured using very accurate means, like bouncing radar off the asteroid and stellar occultations (when the asteroid passes directly in front of a star as seen from Earth; the timing of this gives a very precise measurement of the asteroid’s position).

Why did this happen? Because the two rocks are connected by gravity. You can say that Dimorphos orbits Didymos, but really the two orbit their mutual center of gravity, called the barycenter. They pull on each other. So when DART hit Dimorphos (and material was ejected into space; more on that below) it changed the moon’s orbital period, but Dimorphos also pulls on Didymos, so the whole system was moved as well.

Not much, but it’s a start. A bigger impact, or a faster one, would move the system more. And if you do this well in advance of an impact, maybe multiple times, it could be enough over time to change the course of an asteroid from an impact to a near miss. When it comes to Earth, I prefer the latter.

There’s other news from DART as well, more straight science than scifi action: Didymos is throwing rocks at Dimorphos! I know, this is weird, but super cool [link to journal paper].

Scientists took images of the smaller asteroid moon from DART and enhanced them, correcting for things like the changing geometry and lighting as DART approached the rock, and found that Dimorphos is covered in odd parallel stripes, fanning out like rays from a single point on its surface.

This looks a lot like what you expect for an impact; we see similar features on Earth’s moon, like the crater Tycho, which is surrounded by radial streaks of lighter material. These are from plumes of matter ejected from the impact that then fall to the surface. Is that what happened on Dimorphos? If so, how did they form? 

The astronomers who did the work think the impacting material came from Didymos, the bigger asteroid. As I’ve written about before, sunlight can make an asteroid spin faster over time. This is called the YORP effect, and I’ve explained it in detail on The Old Blog™. As the asteroid spins faster the centrifugal effect near the equator can get strong enough to overcome surface gravity (which is weak in these small asteroids), so rocks on the surface can actually get flung into space. 

What happens to them next is complicated, because the gravity and orbital motion of the two asteroids make the trajectories of these escaped rocks complex, but they can fall to the surface of Dimorphos on the side facing away from Didymos (which is what’s seen in the images). When they hit at slow speeds, they blow off some dust that then forms a small, light plume heading away from the impact downstream as it were. Because all the impacts happen around the same spot, you get this weird fan of radial lines. 

We’ll get more info about all this soon, too! The ESA mission Hera is on its way to the binary asteroid and will arrive in November 2026, where it will begin extensive mapping of the pair to see what they look like now. The images showing the radial features were taken before the impact, and likely didn’t survive the aftermath, which would have sprayed material all over the place. If so Hera probably won’t see them but there’s still a lot to learn, not only about what the impact did, but also about binary asteroids in general. A decent fraction of near-Earth asteroids have moons, which means a decent fraction of potential impactors have moons. The more we understand them, the better.

But wait! There’s more news!

Via AAS Nova — an American Astronomical Society site that gives highlights of recent research — I read about a study that examined the plume from the DART impact [link to journal paper].

When DART hit, a lot of material was blasted into space. Modeling that observation is key to understanding just what the impact did, how much material was blown out, and what the structure of Dimorphos is like.

In general, scientists like to use simple models at first, because it a) makes the math a lot easier, and 2) once you establish the basics it’s easier to start applying more complexity to better approach what’s really seen.

At first, scientists modeled the plume as a cone, which is roughly the shape of ejected material… assuming the surface is flat, homogeneous, and so on. But when you look at the actual images, that’s not at all what happened.

That is from LICIACube, a small Cubesat that rode along with the larger DART spacecraft, deployed before impact to take images and send them to Earth. The two asteroids are overexposed to see the fainter wisps (even then different contrasts were applied to see the brighter inner material versus the outer, creating the nested rectangles); Didymos is the bright one to the lower right and Dimorphos is to the upper left, though it’s rather buried by the ejecta.

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