NASA鈥檚 decision this August to send up a robotic vehicle to steer the Hubble Space Telescope back to Earth is looking like a very bright idea indeed. The space agency鈥檚 calculations now show that uncontrolled re-entry would carry a 1 in 250 risk of death or serious injury to people on the ground, far greater than the 1 in 10,000 risk that NASA deems acceptable.
Without repairs, Hubble鈥檚 batteries and gyroscopes are expected to fail by 2007. The spacecraft has no propulsion system, so its orbit will gradually decay until it re-enters the atmosphere at an altitude of 122 kilometres in about 2020. Being relatively fragile, Hubble would break up about 78 km above the ground, says Nicholas Johnson, chief scientist for orbital debris at the Johnson Space Center in Houston, Texas.
鈥淯ncontrolled re-entry would carry a 1 in 250 risk of death or serious injury鈥�
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NASA鈥檚 computer model predicts that about one-fifth of the 10.7-tonne Hubble will reach the ground, strewing debris along a 1220-km path. Most pieces will be heavy enough to cause casualties if they hit anyone (The Orbital Debris Quarterly News, vol 8, p 3).