DART not only worked, it changed the length of an asteroid’s “year”!

Astro Tidbit

A brief synopsis of some interesting astronomy/science news

Yesterday, I wrote about how the DART mission slammed into an asteroid, testing to see if it’s possible to move an asteroid on an Earth-impacting trajectory.

In that article I talked about the mission being a success — it pushed on the asteroid hard enough to measurably move it! — but I didn’t go into any details. Let me rectify that, because this is cool.

The asteroid Dimorphos is shaped like a Tic Tac and is about 170 meters across. It orbits the larger asteroid Didymos, which is slightly elongated and about 800 meters long. It was chosen for the test mission for several reasons. It gets relatively close to Earth, about 11 million kilometers, which cuts down on mission time to get there, and also allows rapid transmission of data (the signal from a spacecraft weakens with distance, so if it’s close by it can send more data). Dimorphos is small enough that an impact by a small spacecraft can do still make measurable change.

But, importantly, it’s a binary asteroid, a big one orbited by a small one. Why is that such a big deal?

We can calculate the orbit of an asteroid around the Sun by taking repeated measurements of its position over time, then use centuries-old equations to get the size and shape of its orbit. But the orbit calculation is usually a bit fuzzy, because there’s an inherent inaccuracy in how we measure its position in our images. We may know the orbit well enough to predict where the asteroid will be in a few weeks or months, even years, but the farther into the future we try to predict the harder it gets to know just where it will be.

So if we slam into a single asteroid orbiting the Sun, we have to wait a long time to see how much we changed its orbit. That’s a pain.

Ah, but there’s way around that. We know the length of time it took Dimorphos to orbit around Didymos with exquisite accuracy, down to a fraction of a second — it was 11 hours, 55 minutes, 18 seconds before impact. After impact, even a small change would be easy to measure.

How do we know that length of time, called the orbital period (think of it as the asteroid’s “year”)? This way: Mutual transits.

We don’t see the orbit exactly edge-on, but Dimorphos is so close in to Didymos, just 1.2 kilometers out, that every time it passes in front of or behind the bigger rock we see one blocking the other a bit. This happens every half-orbit, when first Didymos blocks Dimorphos, then half an orbit later when Dimorphos blocks Didymos. It’s like a partial eclipse, technically called a transit.

When that happens, the light we see from the pair drops a little bit, enough to measure through telescopes. Given the short orbital period it’s possible to measure these transits over and over again, nailing down the orbital period (radar observations using the late great Arecibo telescope also helped determine the orbital period to great accuracy).

Right after DART impacted Dimorphos observations of the pair from the ground were made; nine were made between September 28 and October 6. Given the knowledge of the period before the impact, astronomers could predict when they’d expect to see the transits begin… and what they found is that they happened earlier every single time, giving a new orbital period of 11 hours, 23 minutes (plus or minus a couple of minutes).

In other words, the spacecraft hit Dimorphos hard enough to change its orbital period by over half an hour!

Here’s what that looks like:

Two different transit events are shown; one from September 29 and another from October 4. The combined brightness of the asteroids is plotted versus time, and you can see the dip when one blocked the other. The gray arrow shows where the transit should begin using the old orbital period, and the orange arrow shows where it actually was. In all the observations the period is consistent with a 32 minutes change.

I’ll note the new period isn’t known as well as the old one, because it takes a lot of transits to measure it accurately. Still, a two minute uncertainty is far less than the change actually seen, so we can definitely say the mission was a success. The DART team was hoping for at least a 73-second change! So yeah, they got a much bigger one that that.

Astronomers will be analyzing the data from the DART mission, the close-up images taken by the Italian LICIACube spacecraft that flew alongside DART, and images taken from and above Earth for a long time. As I said in the earlier article there’s a still a lot to learn, but the bottom line is the mission worked. And that’s great news.

Et alia

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