When the 'White Whale' is a comet

NASA developing a comet harpoon. Paging Captain Ahab. [Link]

Scientists at NASA's Goddard Space Flight Center in Greenbelt, Md. are in the early stages of working out the best design for a sample-collecting comet harpoon. In a lab the size of a large closet stands a metal ballista (large crossbow) nearly six feet tall, with a bow made from a pair of truck leaf springs and a bow string made of steel cable 1/2 inch thick. The ballista is positioned to fire vertically downward into a bucket of target material. For safety, it's pointed at the floor, because it could potentially launch test harpoon tips about a mile if it was angled upwards. An electric winch mechanically pulls the bow string back to generate a precise level of force, up to 1,000 pounds, firing projectiles to velocities upwards of 100 feet per second.

Donald Wegel of NASA Goddard, lead engineer on the project, places a test harpoon in the bolt carrier assembly, steps outside the lab and moves a heavy wooden safety door with a thick plexiglass window over the entrance. After dialing in the desired level of force, he flips a switch and, after a few-second delay, the crossbow fires, launching the projectile into a 55-gallon drum full of cometary simulant -- sand, salt, pebbles or a mixture of each. The ballista produces a uniquely impressive thud upon firing, somewhere between a rifle and a cannon blast.

"We had to bolt it to the floor, because the recoil made the whole testbed jump after every shot," said Wegel. "We're not sure what we'll encounter on the comet -- the surface could be soft and fluffy, mostly made up of dust, or it could be ice mixed with pebbles, or even solid rock. Most likely, there will be areas with different compositions, so we need to design a harpoon that's capable of penetrating a reasonable range of materials. The immediate goal though, is to correlate how much energy is required to penetrate different depths in different materials. What harpoon tip geometries penetrate specific materials best? How does the harpoon mass and cross section affect penetration? The ballista allows us to safely collect this data and use it to size the cannon that will be used on the actual mission."

Comets are frozen chunks of ice and dust left over from our solar system's formation. As such, scientists want a closer look at them for clues to the origin of planets and ultimately, ourselves. "One of the most inspiring reasons to go through the trouble and expense of collecting a comet sample is to get a look at the 'primordial ooze' -- biomolecules in comets that may have assisted the origin of life," says Wegel.

Scientists at the Goddard Astrobiology Analytical Laboratory have found amino acids in samples of comet Wild 2 from NASA's Stardust mission, and in various carbon-rich meteorites. Amino acids are the building blocks of proteins, the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. The research gives support to the theory that a "kit" of ready-made parts created in space and delivered to Earth by meteorite and comet impacts gave a boost to the origin of life.

Although ancient comet impacts could have helped create life, a present-day hit near a populated region would be highly destructive, as a comet's large mass and high velocity would make it explode with many times the force of a typical nuclear bomb. One plan to deal with a comet headed towards Earth is to deflect it with a large -- probably nuclear -- explosion. However, that might turn out to be a really bad idea. Depending on the comet's composition, such an explosion might just fragment it into many smaller pieces, with most still headed our way. It would be like getting hit with a shotgun blast instead of a rifle bullet. So the second major reason to sample comets is to characterize the impact threat, according to Wegel. We need to understand how they're made so we can come up with the best way to deflect them should any have their sights on us.

"Bringing back a comet sample will also let us analyze it with advanced instruments that won't fit on a spacecraft or haven't been invented yet," adds Dr. Joseph Nuth, a comet expert at NASA Goddard and lead scientist on the project.