NASA has developed a harpoon/crossbow/ballista for penetrating the surface of comets. The bow in this test version are made from truck suspension springs, the string is a 1/2″ steel cable. The test is launched into a barrel of dirt (which apparently is a good approximation of a comet’s surface), because the crossbow could launch the harpoon a mile into the air.
A close-up of the harpoon with an internal sample collection chamber.
Via (and high-res images): NASA