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

NTB: This comet harpoon is still in the prototype and testing phase, right?

Wegel: Right now, we’ve basically built a ballista, which allows us to fire a harpoon into different test materials. The reason we chose a ballista, instead of a cannon, is because of safety reasons. You get the same information, you get basically how much energy is required to penetrate different materials, and we can correlate the energy by taking the velocity of the harpoon and its mass, and you get kinetic energy. And that’s the same thing we’ll use to size the cannon for the eventual mission.

NTB: What phase is the comet harpoon in, and what is the team working on now?

Wegel: We have a test laboratory, where we’ve taken hundreds and hundreds of shots with different harpoon tip geometries, into different materials, at different velocities and energies, and correlated that to its penetration. So we’ve come up with a pretty good set of data that shows us what tip geometry works the best. Cross-section is somewhat understood as “the thinner the better.” There’s a certain minimum that you can do with these mechanisms.

The dummy harpoons we’ve used do not contain sample collecting. They just test tip geometry, cross-section, and mass. We’ve come up with three smart harpoons which actually have all of the mechanisms required to not only collect the sample and capture it. You also need to de-couple the cartridge from the sheath when you want it (and you also want to have it coupled to it when you want it). Basically, when you fire the harpoon, you want the sheath and the sample cartridge coupled together. You want them to hang on to each other. You also want them to let go when you’ve collected a sample, so that‘s actually another mechanism that’s tricky — to package both those mechanisms into a functioning sample-collecting harpoon. We’ve shown in several density materials that it does indeed collect a sample by just engaging a simple tug on the tether, which is just one of a hundred ways we may eventually do it. But proving one way is half the battle.

NTB: When will we see these in action, do you think?

Wegel: The Osiris-Rex mission will go to an asteroid and get a surface sample.  The next step is clearly to get the subsurface sample, which is more pristine than the surface, as you might imagine, as the asteroid or comet travels around the sun. As it gets close, the outer layer sees pretty hot temperatures and outgasses and changes quite a bit, but the inside of it could be fairly pristine and almost identical to the early solar system formation, so it’s really important to get the subsurface sample. This Osiris-Rex mission is just funded now, and will retrieve a sample. I think it launches in 2016 and retrieves samples in 2023. So that’s the timeframe for our mission that we’re looking to learn from. We’re pushed off into the future beyond that.

NTB: What kinds of discoveries can be made in the sub-surface as opposed to just the surface?

Wegel: As you might imagine, as the comet or asteroid gets close to the sun, anything that was volatile, which could be building blocks or organics, could burn off and sublime right off the surface. Anything you can capture below the surface may not have either sublimed or changed in some way. The thermal energy of the sun can alter components on the surface; that’s not true on the subsurface.

NTB: In your tests, what kinds of materials will the harpoon penetrate through?

Wegel: So far, our tests have kept things somewhat simple just to make sure the data is well-behaved. We’ve used pretty simple construction sands, so it’s sort of an aggregate of sand sizes, but it’s generally what you’d find at the beach. We’ve also shot at pea gravel, which is another sort of construction material, but they are larger, denser rocks basically. We’ve also shot at sort of a dried corn organic material that’s provided a nice low-density target with consistent particle/particulate size. We’ve also shot at rock salt, the same material that you would put on icy roads, and we’ve been trying to move to mixtures of those.

The challenge of mixing those substances in a test is that you shoot it once, and you get one result, and the next time, it’s difficult to create the exact same layering; if you use the same bucket of layered material, it’s now mixed up a little bit. And we’d also really like to try ices. The same kind of problem is that the ice melts as you’re shooting it, unless your laboratory is very cold, which is another trick to do on its own. And also, each test may break the ice up, and you’re not necessarily shooting at the same target that you were the test before.

NTB: Are there any other modifications that you had to make to the harpoon based on the low-gravity needs of the mission? How is the harpoon designed to accommodate the low gravity?

Wegel: Right now, the harpoon is a square cross-section overall. It’s basically 2" x 2", by about a foot long. It turns out that another challenge of the mechanism inside is that when you try to capture any sample in a square cross-section, it’s much easier than a circular cross-section. I wouldn’t say impossible, but it’s much more challenging to have a closure mechanism that will capture something in our current setup than in a circular cross section. For the real mission, because it would fire out of a cannon and we would probably need to fire some distance, you’d want a round projectile, as you might imagine, like a bullet. We would have the sample cartridge still remain a square, but then nest that into a circular cross-section harpoon sheath, which would provide both the penetrating tip and would take the imparted impact of the explosive charge.