How do you get a core sample from a comet? There’s so little gravity that if you used a scoop or a drill, you’d push yourself right off the surface. To solve this problem, we came up with a harpoon that collects samples. The concept is that the spacecraft flies next to the moving comet and fires from about 30 feet away, we would use a dampening system and propulsion to counteract the recoil. Our prototype harpoon is stainless steel, about one foot long, two inches wide and four pounds. We don’t want it to be too lightweight, because it needs momentum to cut through the comet’s surface. The harpoon hits the comet with its tip open, the material fills an internal cartridge, and a garage-door-like mechanism shuts behind it.

Right now we’re testing the harpoons in a laboratory. We can’t use a cannon indoors—it’s too dangerous—so we use a ballista, a large crossbow. We shoot harpoons at 100 feet per second into 55-gallon drums filled with beach sand, rock salt and pea gravel. Comets can be as soft as cotton candy or as dense as bedrock, so it’s important that our harpoons work on a range of materials.

Comets are thought to contain some of the building blocks for life, so by taking a sample, you not only learn about the solar system’s formation, but you could also find clues to the origin of life on Earth. There are not many jobs that combine comets, harpoons and the origin of life. And even on a bad day, you still get to fire a ballista.

—Don Wegel, a cryomechanical engineer at NASA’s Goddard Space Flight Center