Date: April 13, 2012

Title: Harpooning A Comet

Podcaster: Bob Hirshon

Organization: American Association for the Advancement of Science (AAAS)

Link: www.aaas.org

Description: Comets are too small to land on, so if you want a sample from one, you have to fly alongside and grab it somehow. AAAS Science Update host Bob Hirshon spoke with cryomechanical engineer Donald Wegel at NASA’s Goddard Space Flight Center about his efforts to build and test a powerful crossbow for firing a sample-collecting harpoon into a comet.

Bio: Bob Hirshon is Senior Project Director at the American Association for the Advancement of Science (AAAS) and host of the daily radio show and podcast Science Update. Now in its 24th year, Science Update is heard on over 300 commercial stations nationwide. Hirshon also heads up Kinetic City, including the Peabody Award winning children’s radio drama, McGraw-Hill book series and Codie Award winning website and education program. He oversees the Science NetLinks project for K-12 science teachers, part of the Verizon Foundation Thinkfinity partnership. Hirshon is a Computerworld/ Smithsonian Hero for a New Millennium laureate.

Sponsor: This episode of “365 days of Astronomy” is sponsored by the Education and Outreach team for the MESSENGER mission to planet Mercury. Follow the mission as the spacecraft helps to unlock the secrets of the inner solar system at www.messenger-education.org.

Transcript:

365 Days of Astronomy Podcast
04/13/12
Harpooning a Comet

Hirshon:

Welcome to the 365 Days of Astronomy Podcast. I’m Bob Hirshon, host of the AAAS radio show and podcast Science Update.

In an unfinished work area in a concrete office building at NASA’s Goddard S pace Flight Center in Greenbelt, Maryland, cryomechanical engineer Donald Wegel enters a makeshift office containing a desk, a computer, and a six-foot high crossbow pointing down toward a 55-gallon trash can full of sand.

Wegel:

This is a ballista, which is really just a large crossbow, and made out of two truck leaf springs. It pulls about 1100 pounds of force at full pull and it shoots these projectiles at about 100 feet per second, or 30 meters per second, depending on the mass and obviously how hard you pull it back.

Hirshon:

And how do you pull that string? Roll up your sleeves and climb up there?

Wegel:

We have a mounted a four-wheeler winch at the top and it is connected to a trigger block and that acts like your hand would on the bow, so the trigger block has sort of a set of fingers that grab the bow string which is half-inch stainless steel cable. And then there is a linear actuator that is remotely operated and that allows you to basically release your fingers outside the room from a safe distance and fire your harpoon.

Hirshon:

The harpoon is a 12-inch projectile with a square cross section and pointed end. It’s hollow like a square gun barrel.

Wegel:

I load the projectile in. Slide a shear pin in. So the shear pin you want to be just strong enough to hold the weight of it, but not strong enough to disturb its flight.

Hirshon:

So that’s a little piece of wood?

Wegel:

It’s a piece of birch dowel.

Now we’re locked and loaded and we’ll leave the room and pull the safety door shut.

Hirshon:

And then he gives me the honor of hitting the button to fire the bow.

[Ballista fires]

Hirshon:

So why is NASA interested in firing harpoons? Well, Wegel explains the goal is to collect a sample from the subsurface of a comet and bring it home.

Wegel:

So traditionally when you get a sample of something you might use a shovel or coring drill or scoop, in this case we decided to use a core-sampling harpoon because on a comet there is so little gravity that a drill, or a shovel or a scoop, you’d push yourself right off the surface. So in concept people use harpoons to hold on, and then they use their scoop or shovel. And the scientist I’m working with, Joseph Nouth thought, “why not get rid of the harpoon and shovel and combine it, so you have a sample-collecting harpoon.”

Hirshon:

And the goal is to get a sample and bring it back to Earth?

Wegel:

Yes, so the biggest challenge and the key is to sample/return: it’s not an analysis when you get there and stay there and send the data back to earth; we’re actually collecting a sample and bringing it back to Earth. The advantages of that is when you bring a sample back to earth, you can study it for future generations, you can have terrestrial laboratories with much more sophisticated equipment work on this. When you’re limited, if you do what they call in situ analysis at the comet, you’re limited with what you can bring, and you’re also limited in the technology. You may bring a sample back and in 50 years, something is invented to analyze that sample that was never even dreamed of. So it’s a huge advantage to bring it back although an enormous challenge.

Hirshon:

Why sample a comet?

Wegel:

Comets are early remnants of the solar system’s formation. And if you were to get a sample you may find a lot of interesting ideas of how the solar system formed, and most importantly you may find the origins of life on earth. They’re thought to have the building blocks that may have deposited life. So if you study them, you may find what I like to say, “the primordial ooze of the solar system.”

Hirshon:

But why would comets have that material as opposed to any other celestial body objects?

Wegel:

Good question. Comets haven’t gone through the same processes that planets and stars have gone through so they haven’t experienced the intense heat and intense pressure and chemical change that have gone on on the planets. So they all—all the ingredients were there at the beginning in the solar system’s formation but due to, again, this formation intense heat and pressure, they—all those components change in terms of planets and the sun, but comets, as I said, are still just floating around, early remnants, somewhat pristine, especially inside the comet. So that’s our goal is to shoot a harpoon into the subsurface, into the nucleus of the comet, get a sample, and you’ll get the pristine material that perhaps has the origin of life.

Hirshon:

So what goes into developing a harpoon to go into something you’ve never been to before?

Hirshon:

It’s funny, when I first started as an engineer, I’d want to know, you know, “what am I shooting at?” And we really don’t have an incredibly clear picture of exactly what we’re shooting at. So the range of densities is really large. Basically, from cotton candy—these are like loosely coalesced bodies, so just granulars of ice and rock barely held together, almost no gravity. So although it’s made of rock and ice, which are dense, it’s, those, it’s not held together very tightly. So the harpoon may, if you hit, fly right through it. You know, you may, you have to design something, you may hit bedrock and it may hit something with almost no resistance. So, it’s a huge challenge.

Hirshon:

It sounds almost like Wiley Coyote, where you’re going to shoot the harpoon, it’s going to go straight through the other side and then pull the spacecraft against a rock and just smash it.

Wegel:

And that’s a huge concern, we have to design it so that the harpoon will be connected to the spacecraft with a tether, and so you need to worry, if there’s no resistance then you need either to cut the harpoon loose or prepare for a pretty serious snag. So that’s our goal to avoid that. We would spend a lot of time studying the comet when we get there, so a couple months trying to figure out what is the density, what are the more interesting areas—you don’t want to shoot straight into bedrock. That’s, your harpoon is not going to go in very deep, and you don’t want to shoot into cotton candy. It’s going to go too deep. So you study it for a while and decide your specific interesting location. And then you fire your harpoon, take a sample, make this over simplified, but you put it in your return pod, and send it back to earth.

Hirshon:

Wegel says the actual mission will use one or more cannons instead of a ballista.

Wegel:

The main reason we use a ballista right now is because a cannon’s not safe to fire in the lab and we would take longer to fill out the paperwork to run the cannon than to do any experiments. So we designed the ballista for safety reasons. But in terms of mass savings, the ballista itself is far too massive. We have thought about tradeoffs of using a spring based cannon or launcher but there are tradeoffs with all these things and pros and cons.

Hirshon:
A key part of the apparatus is, of course, the harpoon itself which must penetrate the comet, collect a sample, and then deliver the sample back to the spacecraft.

Wegel:

You’ve got a harpoon sheath, which basically takes the brunt of the explosive force of the cannon and impact force. So it’s got a sharp tip and a kind of robust back end. And then the sample cartridge itself basically a cartridge that slides inside the harpoon sheath that has a garage door like mechanism that kind of pulls a foil around the sample and captures it. So as the harpoon goes in, this fills up with material, the door shuts and this g ets pulled back into the spacecraft by a tether. It gets pulled back specifically into a return capsule—sort of looks like a clam shell. After you’ve collected all your samples, you shut the shell and send it back home.

Hirshon:

And this part stays in the comet?

Wegel:

Correct. So one of the nice things is the sheath stays in the comet and so you guarantee that the hole you dug is a little bigger than the thing you’re pulling out. Cause one of the concerns is let’s say you shoot a couple meters in, if that material collapses back around the hole, you’re gonna have a tough time getting it back out.

Hirshon:

That would suck if it just went in and got stuck…

Wegel:

Yeah, and so a lot of the design, now we’re working on the tether dynamics, it’s really a challenging problem to know, if it gets stuck and you yank on it, you don’t want the harpoon flying back at the spacecraft, you don’t want the sample cartridge kind of uncontrolled flight back towards the craft, or any tangling.
Hirshon:

You don’t want to take the whole comet back with you by mistake…

Wegel:

A lot of people think that’s what we’re doing. When you hear “harpooning a comet,” you’re going to drag her back home.

Hirshon:

Wegel says the comet mission is probably a decade away, so they’ve got lots of time to work out any kinks. And he says since the work has begun, other missions have been interested in the harpoon system, too.

Wegel:

We propose even using these harpoons not only for low gravity but as a rapid sample retrieval. These things fire quickly, collect the sample, and get out of there, so you could think about even on our moon, where you have long periods of night, you would have no power, depending on what your power source is, if it’s solar, you need to get into your area, say a crater, get a sample and get back out quickly. Same idea on Titan. We’ve looked at one of Saturn’s moons, using this to take samples from perhaps a balloon-borne carrier, so you’d have a balloon floating around, shooting harpoons down below, and rapidly retrieving samples.

Hirshon:

In the next few months, Wegel says his ballista will be leaving the lab to fire the harpoons in a more realistic setting, from an outdoor tower.

Wegel:

It used to be a laser ranging tower. It’s 20 feet tall with an empty platform mount, that we’re going to lift the ballista up onto and we’re going to fire down the center of it so we can simulate free flight of the harpoons. Right now they’re only flying a couple feet. We want to shoot closer to 20 or 30 feet to simulate tether dynamics and how to keep the harpoon stable over that long distance.

Hirshon:

The tests this summer reflect increased interest not only in the harpoon system itself, but also in sample return missions overall.

Wegel:

We started this conceptually probably five or six years ago. And there wasn’t a huge push for sample return. And now it’s really exciting to see that people are lining up, and trying to support us.

Hirshon:

Well, that’s all for today’s podcast. Don’t forget to add your comments and questions for future podcasts to this page. Thanks for listening. For the 365 Days of Astronomy podcast, I’m Bob Hirshon.

End of podcast:

365 Days of Astronomy
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