Space crashes on the ground
In Flipside 32, Sean Blair considers the threat from orbit junk left by half a century of flights into space. Designers now have to factor in the problem of orbital debris when working on new spacecraft, which means replicating high-velocity space collisions on the ground…
 It is the speed, not the size, of space junk that throws up the real danger to spacecraft. The average impact at low-Earth orbit occurs with a relative closing speed of 50,000 km per hour (31,000 mph).
At such velocities a 1-mm metal (0.04 inch) particles strikes with the strength of a 22-caliber rifle bullet, while a pea-sized screw collides like a 180 kg (400-lb) safe falling at 100 km/h (60 mph). While objects larger than 10 cm (4 inches) can be tracked and manoeuvred around if needed, smaller items cannot be reliably detected – instead they have to be engineered for.
In an isolated hangar in NASA’s White Sands centre in New Mexico, gas guns fire 0.25 to 22 mm (0.01 to to 0.9 inch)diameter aluminium projectiles to speeds approaching 7 km per second (16,000 miles per hour). Officially known as the Hypervelocity Impact Test Facility (HIT-F), its purpose is to simulate orbital collisions, and test spacecraft materials to destruction.
The destructive potential of space debris is spelled out in kinetic energy equation E=1/2 mv^2, where ‘E’ stands for energy, ‘m’ for mass and ‘v’ for velocity. What the equation shows is that energy increases with the square of the speed – so an object moving three times faster than another object of equal size will possess nine times more kinetic energy.
Like rockets, the gas guns used at White Sands work on a multi-stage basis. The first stage is a cylinder filled with hydrogen, which is compressed by exploding gunpowder. The hydrogen pressure reaches hundreds of square kilograms (pounds) until a disc at the other end of the cylinder gives way, injecting pressurised hydrogen into a vacuum-pumped second stage. This propels the projectile towards its target at the far end of the barrel. High-speed cameras image the impact at a rate of two million frames per second.
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