An explosive-powered harpoon designed to skewer a comet would be the easiest way to grab a chunk of the icy body for study back on Earth, according to NASA researchers who are currently testing the device.
Since comets have very low gravity—maybe a millionth that of Earth's—landing a probe on one can be a challenge.
But getting close to a comet, even within 15 feet (5 meters) or so, is no problem, said project leader Joe Nuth, of NASA's Goddard Space Center in Maryland.
"Let's say you're 10 meters [33 feet] away from a comet [with all engines and control systems] turned off. It would still take you half an hour to fall to the surface of a large comet," Nuth said. "There's plenty of margin for error."
Hovering over the comet, "you can control [the craft] with just a small puff of gas—really just a sneeze."
Hovering Craft to Fire Harpoon?
The hypothetical harpooning mission would approach a comet when the object is 288 million miles (463 million kilometers) away from the sun.
At this distance, the comet would still be in a region where a spacecraft's solar arrays could get enough sunlight to generate propulsion.
But solar radiation wouldn't yet be causing large amounts of the comet's ices to vaporize, creating a thick halo of debris—the coma—that could interfere with the spacecraft's solar arrays and navigation system.
Once the spacecraft is hovering near the comet, the harpoon would be fired with an explosive charge deep into the comet's nucleus.
The harpoon would have an internal sample-collection chamber that would close around a chunk of comet and detach from the harpoon's tip, sailing back to the spacecraft, which would eventually return to Earth.
Harpoon Tests Use Giant Crossbow
Right now Goddard scientists are testing a proof-of-concept harpoon by firing it in the lab from a six-foot-tall (almost two-meter-tall) crossbow.
The team is trying to find the best design for the harpoon's tip and collection chamber as well as the best explosive powder charge for the harpoon.
The crossbow is helping the researchers fire tips at various speeds into different materials—such as sand, ice, pebbles, and salt—thought to be similar in texture to comets.
"I think it'd be kind of neat to carry a crossbow" on the craft, Nuth said, but ultimately explosives would be more efficient, considering the complex machinery needed to spring-load a crossbow in space.
Besides, an explosion would help propel the spacecraft away from the comet's surface, which would assist in sample return.
"If you pull [the harpoon] out in microgravity, it's just going to flop around," Nuth said. "But if you're accelerating backward, it holds the weight on the end fairly stiff."
Comets and Asteroids Part of Cosmic Continuum
The comet harpoon hasn't yet been selected as a NASA mission, but if it ever launches, the harpoon could provide a wealth of information on the relationship between comets and asteroids.
"When I was in grad school, comets and asteroids were completely different things. But over the last decade or more, there's been more evidence that these things are [part of] one continuum."
Nuth said he would especially like to study 4015 Wilson-Harrington, a comet that was "lost" in the mid-20th century.
"When they recovered it, they recovered it as an asteroid," Nuth said. In other words, the comet had mysteriously stopped emitting dust and gas.
This could be because it developed a layer of dust on its surface thick enough to protect subsurface ices from vaporizing, making the object a comet in asteroid's clothing.
"So the relationship between comets and asteroids is very interesting," Nuth said.