DART followup: Whacking a space rock

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Astronomy News

It’s a big Universe. Here’s a thing about it.

DART worked. And it worked really, really well.

DART is the Double Asteroid Redirection Test, a small 500-kilogram spacecraft that slammed into the asteroid Dimorphos on September 26, 2022. Dimorphos is the 170-meter wide moon of the much larger asteroid Didymos, and the point of the mission was the hit the asteroid moon hard enough to change the way it orbits the bigger primary.

The impact, at 24,000 kilometers per hour, created a big flash of energy and ejected a lot of rocky dust into space. I covered a lot of this in BAN Issue #467 if you want details. I have images showing the plume of material blasted away within minutes of the hypervelocity impact.

The length of time it took Dimorphos to orbit Didymos, called the orbital period, was known down to a fraction of a second, so any change would hopefully be obvious. Scientists had hoped to show a period change of at least 73 seconds. The actual change? 32 minutes.

So yeah. Success! This is great news. It means that this technology to change the velocity of an asteroid actually works. If we see an asteroid heading towards Earth on an impact trajectory, we might very well be able to do something about it. There are a ton of variables; the size of the asteroid, its composition, the actual orbit it has around the Sun, and most importantly how much time we have before a predicted impact. We can only change the velocity of an asteroid a little bit, so the more time we have to change it the better.

Think of it this way: If you’re standing in the street and a car is one second away from hitting you, it has one second to move a meter to the left or right to avoid the impact. That’s a huge change, one meter per second. But if it’s a hundred kilometers away, it has an hour to move the same one meter left or right, so it only needs to move about a third of a millimeter per second to avoid hitting you, a much smaller change. So the longer the lead time, the less you have to affect an asteroid to make it miss.

But the point here is, we hit an asteroid. And the spacecraft did so autonomously, making needed course corrections on its own as it approached. It hit practically dead center. An outstanding achievement, and a hopeful one.

In Issue 467 I also talked a bit about how the material ejected by the impact formed a long plume, and how that actually helps move the asteroid even more, since it acts like a rocket exhaust, pushing on the rock. I also showed some cool images of it the plume.

A few days later though a different image was released: It was taken by the Southern Astrophysical Research (SOAR) telescope two days after impact. You might think the dust would have all dissipated by then, but that’s not what happened:

I recognized this right away, because I’ve looked at countless images of comets orbiting the Sun: The dust plume forming a long trail away from the asteroid. However, the circumstances here a little different than from a comet.

Comets are made of ice and rock and dust. When they get near the Sun the sunlight warms the surface, turning the ice into gas. This also slowly releases dust, and you get two tails: the dust tail and the ion tail (the latter is gas that gets hit by ultraviolet sunlight and loses an electron; it’s technically called a plasma, not gas, and is ionized). Sunlight exerts pressure on this dust, pushing it away from the comet. It tends to trail behind the comet as the material is pushed into a higher orbit from the Sun, where it moves more slowly. This creates along, curved dust trail.

The Dimorphos trail is different because the dust wasn’t ejected slowly. It expanded away from the asteroid rapidly. The impact was on the side of the asteroid facing away from the direction of its travel, so most of that dust was blasted out backwards relative to its orbital motion (think of it like throwing a baseball backward out of a moving car). That takes away orbital energy of the dust, so it falls toward the Sun. But that means it’s in a lower orbit and so it moves faster than the asteroid around the Sun, stretching it out into a trail. Moreover, sunlight pressure pushes on it, which also helps sculpt it into a long line.

In this case, the SOAR images show the trail is about 10,000 km long, nearly the width of the Earth. Images like this help us understand how the ejected impact material moves away and at what velocity, which could be important for a near-Earth asteroid that we may have to hit. It can also show us the composition of the tail; is it most rocky pebbles, or very fine dust, or something in between?

About that: a Hubble image taken on October 8, 2022, less than two weeks after impact, showed something peculiar. Two tails!

That was unexpected. I’m not sure why there are two tails, but I have a guess. A lot of pulverized material was ejected, rocks crushed to fine dust by the force of the impact. There may have also been bigger grains of material ejected as well, say like the size of grains of sand. The dust would be pushed harder by sunlight, forming one tail, and the larger grains would be affected less, forming a second tail.

Again, I’m speculating. I’d expect to see more of a fan-shaped tail if this were the case, because the grain sizes would fall along a distribution from teeny to bigger, so you’d get a more continuous distribution of them around the asteroid. So maybe I’m wrong, or maybe there’s more to this. I’ll be interested to see any papers analyzing this Hubble image.

Also, the Virtual Telescope Project took an image of the asteroid on November 1, 2022, over a month after impact, and the dust plume is still visible! It’s about 10,000 kilometers long in that shot. That’s very cool.

As usual, images like these are quite lovely, and there’s still so much to learn from them. We can figure out what the asteroid is made of, what its composition is like, what happens when you whack it with a washing-machine-sized spacecraft at six times faster than a rifle bullet, and, maybe maybe maybe, how we might be able to one day save the world.

The images are art and science and engineering and heroic. Taken all together, I’d say that makes them quite… impactful.

There’s more DART news, too, and I’ll have that for you in tomorrow’s article!

Et alia

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