NASA is planning to use a giant crossbow and harpoon to capture samples from a speeding comet.
The plan is to have a spacecraft hover above the comet to fire the device, allowing samples to be collected from areas that are too hazardous for a spacecraft to land.
The latest prototype consists of a metal ballista, or large crossbow, nearly six feet tall. Its bow is made from a pair of truck leaf springs, while the string is a half-inch-thick steel cable.
An electric winch mechanically pulls the bow string back to generate a precise level of force, up to 1,000 pounds, firing projectiles as fast as 100 feet per second.
"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," says lead engineer Donald Wegel.
"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."
Right now, the team is working out the best tip design, cross-section and explosive powder charge for the harpoon, using the crossbow to fire tips at various speeds into different materials like sand, ice, and rock salt.
They are also developing a sample collection chamber to fit inside the hollow tip.
The spacecraft will probably have several sample collection harpoons with a variety of powder charges, so it can handle different surface compositions.