Donald Wegel, Lead Engineer, NASA Goddard Space Flight Center, Greenbelt, MD

Scientists at NASA’s Goddard Space Flight Center in Greenbelt Maryland are in the early stages of designing a sample-collecting comet harpoon. NASA Goddard’s Donald Wegel, lead engineer on the project, will work with researchers to send a spacecraft to rendezvous with the comet, and then fire a harpoon to acquire samples from specific locations.

NASA Tech Briefs: Why sample comets? What can we learn from comets?

altDon Wegel: Comets are early remnants of the solar system’s formation and might give us clues to the possible origins of life on Earth. They have some of the building blocks of life and could contain the primordial ooze of where we came from. Also, comets and asteroids are potential threats for the Earth, so to understand them better may help us find the best solution to avoid their impact.

NTB: So what is the comet harpoon, and how is it designed?

Wegel: The comet harpoon is much like it sounds — a core-sampling harpoon that is fired into a comet’s surface and designed to take subsurface samples. It will hopefully penetrate anywhere from a meter to several meters deep, depending on the density. There is an inner sample cartridge, which has mechanisms that capture the sample, remove it from the comet, put it into a spacecraft, and send it home for extensive study.

NTB: Can you walk me through how a mission with the comet harpoon would work, from penetrating the surface, to collecting it, and then bringing back the data?

Wegel: A spacecraft will rendezvous with the comet and spend several months studying it. Then we’ll pick an interesting spot, maybe a crevasse or a vent that’s outgassing, and then fire sample-collecting harpoons into the target. As the harpoon penetrates the comet, a sample cartridge, which is located inside the harpoon sheath, will fill with material. Next, the sample-closure system engages and shuts a garage-door-like mechanism that captures the sample. The sample cartridge is then pulled out from the back of the harpoon and into the spacecraft, then transferred to a return capsule. Finally, the return capsule is sent back to Earth, most likely landing in the Utah desert or some large unpopulated area, and then we go pick it up.

NTB: What were the previous ways of collecting samples from a comet?

Wegel: So far it’s never actually been done, but traditional samples are collected on earth and other bodies using a scoop, drill, or shovel. The problem is that on comets and asteroids there is so little gravity that any of these methods would require some other form of grappling to the surface; if you try to employ your drill on the surface, you’ll push yourself back off. And the same with a shovel or scoop. So what’s conceptually done is firing some sort of grappler, or basically a harpoon, to grapple to a surface. Then you use your drill, scoop, or shovel. Joseph Nuth, the scientist on this project, has proposed that we get rid of the drill, scoop, or shovel, and just use the harpoons themselves to not only grapple, but penetrate and collect the sample themselves.

NTB: Because of what this comet harpoon has to do, how has that created design challenges along the way?

Wegel: One of the trickiest parts of the design is packaging all the mechanisms that allow the sample capture into a very small cross-section. The thicker the walls of your harpoon are, the less it will penetrate, and the more energy it will require to break through any surface. But the thinner and smaller you make these mechanisms, the more likely they are to yield and/or break. So you need to have the mechanisms robust enough, but also small enough to fit in the available real estate.

We’ve also decided that it’s too much of a risk, depending on the shape of the comet, to come in close with such enormous solar panels. So we decided to shoot from a little bit further away, up to about ten meters. Maintaining the stability of any projectile in free flight is a challenge, especially when it’s attached to a tether. Also, as it hits the surface, you want to keep it upright, rather than tipping over, to maximize the depth it will collect samples from. Another huge challenge is physically getting the sample cartridge back to the spacecraft. It’s difficult to simulate and test in our terrestrial laboratory, especially when trying to account for such low gravity conditions.