The researchers want to send a spacecraft to rendezvous with a comet and then fire a harpoon to rapidly acquire samples from specific locations with ‘surgical precision’ while hovering above the target.
A metal ballista (large crossbow) measuring almost 6ft (1.8m) in height, with a bow made from a pair of truck leaf springs and another bow string made of steel cable, has been set up in a lab to give the researchers an idea of how the comet harpoon would work in reality.
The ballista is set up in the lab to fire vertically downwards into a bucket of target material. For safety, it is pointed at the floor, because it could potentially launch test harpoon tips about a mile if it was angled upwards.
An electric winch mechanically pulls the bow string back to generate a precise level of force, up to 1,000lb (450kg), firing projectiles to velocities upwards of 100ft/sec.
‘We had to bolt it to the floor, because the recoil made the whole test bed jump after every shot,’ said Donald Wegel, the project’s lead engineer.
‘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.’
Wegel believes that there will be areas with different compositions on the comet and is therefore designing the catapult so that it can penetrate a reasonable range of materials.
The team will be looking at which harpoon tip geometries penetrate specific materials best and how 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,’ said Wegel. ‘The immediate goal, though, is to correlate how much energy is required to penetrate different depths in different materials.’
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