In 1996, a briefcase-sized piece of debris from the French rocket Ariane I collided with the military reconnaissance satellite Cerise in Earth’s orbit. It was travelling at nearly 50,000 kilometres (31,000 miles) per hour – fast enough to vaporise part of Cerise’s stabilisation boom and send the satellite spiralling into an erratic trajectory. Fortunately, ground control was able to set Cerise back on track so it could continue its mission. However, this could have been disastrous for a manned craft or space station like the ISS, which had its own near-miss with a fragment of the Russian satellite Cosmos 2251 that was travelling at 26,000 kilometres (16,155 miles) per hour, earlier in 2012.
NASA is well aware of the dangers of the space junk that our half-century of spaceflight has left in orbit. The Space Surveillance Network in Colorado Springs, USA, monitors thousands of orbital objects travelling at an average 33,000 kilometres (21,000 miles) per hour. Large chunks can be avoided so the biggest threat to any mission is from the smaller pieces of debris – those between two and ten centimetres (one to four inches) in diameter that zoom around the Earth like super-accelerated bullets and number in the tens of thousands, yet are too small to track.
For every kilogram of shielding placed on a shuttle, it costs NASA tens of thousands of dollars extra, so the precise amount required for any mission is calculated at the White Sands Test Facility in New Mexico. This cutting-edge shooting gallery is home to four enormous guns with 36-metre (118-foot) barrels that can propel aluminium ball bearings at similar speeds to those which can be reached in orbit. NASA engineers will fire many of these at shuttle casings until they find one whose armoured outer layers prevent the pressure wall from being punctured.