Donald Wegel, Lead Engineer, NASA Goddard Space Flight Center, Greenbelt, MD
- Created: Thursday, 01 March 2012
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?
Don 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. NTB: So what is the comet harpoon, and how is it designed?
Wegel: The comet harpoon is much like it sounds —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. 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. 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 —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. 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"",NTB: Do you see other applications or uses for a tool like this?
Wegel: We mentioned in some earlier papers that this kind of tool could be used as a rapid retrieval system. On the Moon, if you just wanted to hop into a crater, grab a sample, and hop back out, you could imagine outfitting your system with a sample-collecting harpoon. It’s the same idea on Earth. Let’s say there’s a scenario similar to the nuclear disaster in Japan, where you send some kind of robot in, fire a harpoon, retrieve a sample, and come back out. NTB: The European Space Agency has the Rosetta Mission, which will use a harpoon to grapple a probe to the surface of a comet. Can you talk about the similarities and differences between the two missions?
Wegel: That’s actually one of our early inspirations, and what we look to closely. They spent a lot of time and effort and did a really impressive job on their harpoon grappling system. Using a tether is very challenging, especially in zero g. They behave very strangely and the possibility of getting tangled, especially when retrieving the sample, is a concern of ours. But Rosetta did a great job of designing a reel system that allows you to shoot into different densities, and not knowing what the density is to begin with, you can fire in, and the reel itself will have the clutch mechanism that throws the harpoon down if the comet doesn’t have enough density.
Some of these bodies have densities as low as cotton candy, so you can imagine firing a high-energy shot in, and nothing slowing the harpoon down, except for yanking the tether out of the spacecraft. [The ESA] has a really sophisticated tether design which avoids that, controls that, and allows retrieval of the harpoon. Also, they’ve done a lot of work with what I believe they call a “smart tether.” NTB: What is most exciting to you about sampling a comet?
Wegel: I think by far the most inspiring possibility is finding the primordial ooze or the origin of life on earth. I can’t think of a more incredible thing to be a part of, to shed even any light on that, let alone prove some of that origin. That alone is what inspires me the most.
NTB: What still needs to be done with the harpoon?
Wegel: We’d like to start shooting harpoons from a greater distance. Right now, we’re shooting probably only two feet of free flight. It’s really not designed to fly freely. We can’t be as close to the surface as we wanted to be. We need to make sure that we can have these fly freely for a while, so whether we use rifling and have the harpoon spinning to maintain stability, or whether we use a slight drag with a tether to keep it stabilized, we need to basically fire the harpoon a longer distance. That’s quite a challenge because we need to have this test bed, which right now only shoots two or so feet. We would like to shoot closer to 10 or 20. We’re in the process right now of going to the next level on both longer free flight and a retrieval system, so we’ll play around with and design a tether system that can retrieve the sample cartridge.
NTB: What‘s a typical day for you?
Wegel: Fortunately, there is a lot of variety in this job, which I like a lot. I’m working on several other projects at the same time, but generally there’s one part of the day I spend using Pro/ENGINEER, which is a 3D CAD model program, and literally just design hardware. What I like the most about that is that you start with an idea sort of spinning around in your head, and you spend a little time getting that idea into the computer. You start to see this 3D model of what was in your head and now on the screen. And then you run a little analysis and find that “Well, this part needs to be a little different, this needs to be thicker, this should be thinner, sharper, or whatever,” and it starts to morph into the real part. Then you actually get 2D drawings, you hand somebody a piece of paper and or a file, and you eventually get an actual part that used to be spinning around in your head. You’re holding that piece of hardware in your hand. So that creative process, and actual creation process, I think, is what I like the most.
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