NASA has managed to recover rock samples from the Moon by actually landing on it. However, that’s quite a big, slow moving target with relatively flat surfaces on which to land a spacecraft. Recovering samples from fast moving comets, which are much smaller, throw off ice and rock as they travel, and have much more uneven surfaces, is a completely different problem to solve.The solution, according to NASA, is not to even attempt to have a craft come into contact with a comet. Instead, they intend to develop a surgically precise harpoon system that can be fired into the rock multiple times without undue risk for the spacecraft tasked with transporting those samples back to Earth.Testing of a harpoon system capable of collecting a smaple is already underway at the Goddard Space Flight Center. A large metal crossbow has been installed at the facility capable of generating 1,000 pounds of force through the use of a 0.5 inch thick steel cable. The crossbow will be replaced with a cannon for the final firing system, but for now this setup is fine for testing purposes.The crossbow’s target is a 55-gallon drum full of what scientists believe is close to the composition of a comet: a varying mix of sand, salt, and pebbles. Of course, every comet is going to be different, and it needs to be determined how much energy is required for different compositions.These initial tests have already thrown up a few head scratchers. For example, having 1,000 pounds of force means the harpoon should pierce even the hardest of comets, but how does NASA counteract the recoil the spacecraft carrying it will suffer when firing? There’s also work being done on what the best harpoon tip design is, and how much explosive powder will be required for the final cannon system.There doesn’t seem to be a mission planned yet to use this new harpoon collection system. There is a harpoon being used in 2014 as part of the Rosetta mission, but it won’t collect a sample. The Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer (OSIRIS-REx) mission launching in 2016 will collect a sample, but only from the surface of a comet. The next stage is a mission to collect subsurface samples–those untouched by the harsh conditions in space.Read more at NASA
"NASA is developing a cannon harpoon to collect comet samples"