NASA working on a comet harpoon

A rendering of the harpoon embedded in a comet, with its collection cartridge visible inside (Image: NASA/Chris Meaney/Walt Feimer)

Call it Ishmael. Actually, no, call it the Comet Nucleus Sample Return mission spacecraft. Regardless of its name, the NASA vehicle will be wielding a harpoon, not unlike the narrator of Moby Dick.
Instead of hunting a white whale, however, it will be after a comet. Although the spacecraft itself is still a concept, its harpoon is in the works now.

   

In the proposed mission, the spacecraft would pull up alongside a comet, then shoot a tethered harpoon into it. The harpoon would consist of a hollow, open-ended inner cartridge, and an open-ended outer sheath. As the harpoon pierced into the comet, subsurface material would be collected inside the cartridge. Once the harpoon reached its maximum depth, a closure mechanism on the end of the cartridge would be activated, trapping the collected material inside. The cartridge would then be winched back into the spacecraft, leaving the empty sheath behind.
While it might seem like it would be simpler to have the spacecraft land on the comet to take its samples, this apparently wouldn't work - the force of gravity is so low on comets, that any landed vehicle attempting to drill or dig into its surface would end up pushing itself away from the comet, instead.
Considering that comets move at speeds of up to 150,000 mph (241,401 km/h) and are constantly throwing off bits of ice and rock, attempting a landing on one would also be very risky.

Presently, scientists at NASA's Goddard Space Flight Center are working on designs for the harpoon. They are shooting their prototypes using an almost six foot (1.8 meter)-tall metal ballista, which is a type of large crossbow. Its bow is made from a pair of truck leaf springs, and it is loaded using a half-inch steel cable bow string. It generates up to 1,000 pounds (454 kg) of force, delivering harpoons into a drum full of comet-like target material at velocities of over 100 feet (30.5 meters) per second.
Intimidating as that may sound, plans are for the harpoon(s) on the spacecraft to be launched out of a cannon, propelled by explosives. It's a setup that would work well in space, but that isn't practical for indoor lab tests.
The current tests are being conducted to determine how much force is needed to shoot a harpoon down to different depths, in different consistencies of material, as the composition of comets is far from uniform. Harpoon characteristics such as the optimum type of penetrating tip and cross-section are also being studied.
According to lead engineer Donald Wegel, the study of subsurface material from comets could lead to a better understanding of the origin of planets and of ourselves, as comets contain ice and dust left over from the formation of the solar system.