'Thar She Blows!' NASA Designing a Comet Harpoon
Comets are a source of endless fascination for astronomers. Those frozen balls of ice and dust could hold key information about the origin of the planets in our solar system, and of life itself. That might explain why NASA scientists are developing a giant harpoon, intended to be deployed from a hovering spacecraft and take a sample from a moving comet.
Why does NASA need such thing? It’s tough to land on an icy body that is both rotating and hurtling through space at 150,000 miles per hour. “A spacecraft wouldn’t actually land on a comet; it would have to attach itself somehow, probably with some kind of harpoon,” according to NASA comet scientist Joseph Nuth. “So we figured if you have to use a harpoon anyway, you might as well get it to collect your sample.”
Indeed, the European Space Agency’s Rosetta mission is equipped with a harpoon to attach a probe to a comet’s surface. But it’s not designed to collect a sample. Instead, it will tether a small probe to the comet, allowing on-board instruments to conduct in situ analysis.
And NASA’s OSIRIS-REx mission can collect a sample from an asteroid’s surface, but not the subsurface. This latest project will build on these previous designs to sample the interior of a comet.
The latest NASA prototype (shown below) relies in part on a six-foot crossbow, positioned to fire vertically downward into a 55-gallon drum of “cometary simulant” — basically a mix of sand, salt, and pebbles intended to simulate the surface of a comet. A crossbow is simply a bow with a stock and trigger mechanism, so you can “cock” the weapon like a rifle before launching your chosen projectile at a target.
Why a crossbow? It’s an excellent means of launching a projectile, that’s why! I’ll let the nice folks at How Stuff Works explain:
If you press down on a spring, it expands to its original shape when you let go. The same thing happens if you pull its ends in opposite directions. This is because of the spring’s elastic potential energy — the energy it stores because of a change in its shape. When you pull one end of a spring, it stores elastic potential energy until you let go. Its potential energy then becomes kinetic energy, the energy of movement, allowing the spring to resume its normal shape and sometimes to bounce around.
The NASA prototype uses an electric winch to pull back the bowstring. The winch can be precisely adjusted via a dial to generate just the right amount of force to penetrate the target material. The projectile can reach speeds as high as 100 feet per second.
And apparently it packs a killer recoil. “We had to bolt it to the floor, because the recoil made the whole testbed jump after every shot,” Donald Wegel, NASA’s lead engineer on the project, said in the press release.
This is where the harpoon part of the apparatus comes into play. Fans of Moby Dick will recall that a harpoon is a long spear with a barbed detachable head on some kind of fishing line.
In the case of whales, it’s usually a thick rope or chain, given the size of the prey. The barbed head is launched from the harpoon (much like a crossbow) and penetrates the skin and thick layers of whale blubber, embedding itself in the flesh. This lets the whalers drag their prey back to their ship.
NASA is going for something a bit more sophisticated, of course. They don’t want to rope a comet; they’re trying to design a harpoon head that will pull out a sample of the comet’s interior. That means designing a special hollowed-out tip, as well as a sample collection chamber that fits inside that tip.
The chamber must open at the point of impact, as the tip penetrates the comet’s surface, in order to collect the sample, and then close tightly and detach from the tip — which will remain embedded in the comet — so it can be pulled back into the spacecraft.
Because there really is no existing data for this kind of thing, the team is currently experimenting in the lab with lots of different kinds of materials likely to be found on a comet’s surface — a list that does not include whale blubber.
“The surface could be soft and fluffy, mostly made up of dust, or it could be ice mixed with pebbles, or even solid rock,” Wegel explained. “Most likely, there will be areas with different compositions, so we need to design a harpoon that’s capable of penetrating a reasonable range of materials.”
They also need to determine the optimal amount of powder charge to ensure the harpoon doesn’t bounce off the comet’s surface or — yowza! — go all the way through. The lab prototype is designed to do just that. Said Nuth, “Bringing back a comet sample will also let us analyze it with advanced instruments that won’t fit on a spacecraft or haven’t been invented yet.”
Image credit: NASA