Safely scooping a sample from a fast asteroid

HostWe have all seen those movies where a crew lands a ship on a big space rock to save the world. It looks easy enough in the films, like parking a car on a very big hill. But in real life, when we want to go out and grab a piece of one of these rocks to bring back home, it's a whole different story. These things are moving at thousands of miles an hour, and they're not always as solid as they look.

HostSince you can't just park on the ground like a normal plane, how do you get close enough to touch it without the whole thing falling apart or throwing you off?

GuestIt's a huge challenge because a space rock isn't like the moon or a planet. The moon is big enough to have a lot of pull, so when you land there, you stay down. These rocks are tiny. They have almost no pull at all. If you tried to land a ship on one like it was a runway, you would just hit it and bounce right back off into the deep dark. It would be like trying to land a fly on a speeding bullet that's also spinning like a top.

HostBut the ship and the rock are usually moving at the same speed when they get close, right? Like two cars driving next to each other on the highway. If you match the speed, it seems like you could just reach out and grab a handful of dirt.

GuestThat sounds right, but the highway is dark and the other car is a jagged mess that's tumbling end over end. Even if you match the speed, you have the trouble of weight. Since there's no gravity to hold you down, the tiniest touch from your ship can shove the whole rock away from you. Or, even worse, the rock can kick you off into space. You're trying to touch something that weighs millions of tons but feels like it has the weight of a feather when you push against it.

HostSo you can't land and you can't just hover there. How do we actually get the sample?

GuestWe use a move called touch and go. It's just what it sounds like. Instead of trying to sit on the rock, the ship stays in flight. It has this long, thin arm that sticks out, kind of like a pogo stick. At the end of that arm is a head that looks like a big coffee can. The ship slowly drifts down, aiming for a flat spot. The arm touches the ground for just a few seconds, and then the ship fires its engines to get out of there as fast as it can.

HostThat seems like a lot of work for such a short window. If you're only touching the ground for five seconds, you can't exactly get out with a shovel and start digging. How do you get the rocks into the ship that fast?

GuestA shovel would be a disaster. If you tried to push a shovel into the ground, you would just push yourself up and away from the rock. There's no weight to keep your feet on the floor. So, the way it works is actually quite smart. They use air. Inside that coffee can on the end of the arm, there's a small tank of gas. The second the arm hits the dirt, it shoots out a quick puff of air. That puff stirs up all the dust and small pebbles on the ground and blows them upward.

HostAnd they just float right into the can?

GuestThey get caught in a trap inside the head of the arm, like a butterfly in a net. Then, while all that dust is still swirling around, the ship zips away. But even that's harder than it sounds. When a ship did this a few years ago at a rock called Bennu, we got a big surprise. We thought the ground would be hard, like a stone road. But when the arm touched down, it didn't hit something solid. It kept going. It was like the ship was sinking into a giant ball pit.

HostWait, if the ground is that soft, why has the whole rock not just fallen apart? If there's no pull, what's keeping all those loose bits from just floating away into space?

GuestThere's just a tiny bit of pull. It's very, very weak, but it's enough to keep the pile of rocks together in a big clump. Think of it like a house of cards. It stays up as long as no one touches it. But the second you poke it with a spaceship, the whole thing wants to move. That ship I talked about actually sank about a foot and a half into the rock in just a few seconds. If the engines hadn't fired right on time, the whole ship might have been swallowed up by the pile of gravel.

HostThat's scary. And I guess you're doing all of this from millions of miles away. You can't exactly use a joystick to steer it in real time.

GuestNo way. The message takes way too long to travel through space. By the time the picture of the ship hitting the dirt reaches the people on Earth, the ship has already finished the job and moved back into a safe spot. The ship has to do the whole thing on its own. It has a map in its brain, and it looks at the rocks and the shadows on the ground to figure out where it is. It has to make its own choices about when to fire the puff of air and when to run. It's like a smart car trying to park in a spot that's moving and spinning in the pitch black.

HostIt's a wonder it works at all. It feels less like a moon landing and more like trying to catch a fly in a room full of dust.

GuestOne of the ships even had a rock get stuck in the door of the trap, which let a trail of space dust leak out behind it like a hole in a bag of flour.

HostThe whole trip shows that catching a piece of a flying rock isn't about finding a place to park, but about knowing exactly when to let go and run.