Hazards of Space Debris

By Derek Allen

Space debris in orbit around the Earth continues to multiply - more is being created than is being burned up by re-entry into the atmosphere. The oldest known large chunk is Vanguard 1, launched in 1958. Now, forty six years later, the number of known orbiting objects at least 10 cm across is nearly 11,000, of which only some hundreds are actual operational satellites. Objects in the lowest orbits, just above the atmosphere, whizz around at 7-8 km/sec Bits only a few millimetres across at those speeds can be very dangerous. In 1996 the explosion of a Pegasus rocket's upper stage created around 300,000 such bits larger than four millimetres across. The largest bit of debris up there is a Cosmos 382 Soviet lunar program test vehicle with a mass of 10,000 kg. The total mass of space debris in orbit is estimated to be four million kilograms.

To combat this hazard, in 1996 NASA had the idea of using a ground-based laser to deflect debris away from the path of any shuttle, spacecraft or the International Space Station (ISS). The laser would vaporise part of the debris's surface to create a small thrust, sufficient to move it out of the way. Another possibility was to mount a laser on the ISS itself and shoot at debris, reminiscent of an old Asteroids video game! However, a ground-based laser would have to be extremely large, while an ISS-based one would need more power than the Station could currently generate. Both would have been very costly and of questionable reliability, so nothing came of this.

A passive shielding system is now being used instead on the ISS. It has hundreds of tailor-made shields, each consisting of a layer of aluminium shielding with a "stuff-shield" of bullet-proof Nextel or Kevlar between the aluminium and the ISS module being protected. The shielding is 10 cm thick, sufficient to stop an object one centimetre across travelling at a relative speed of 10 km /sec.

Space Command in Colorado tracks orbiting objects of ten or more centimetres across and is in the process of improving its radar for this to track objects down to five centimetres across. If they identify an object which will make a close approach to the ISS within 72 hours (which is deemed to be a significant risk), the ISS's orbit can be changed by a couple of kilometres, sufficient to reduce the probability of an impact to a negligible level of risk. On average, this manoeuvre has to be done about once a year.

Smaller objects also pose a hazard. Millimetre particles can do dangerous damage to a space shuttle and could kill an astronaut on a space walk outside a shuttle or the ISS. Twice in the last year the ISS crew have reported unexplained loud bangs which could be attributed to the impact of debris on the shielding, but this has not been verified.

As long as spacecraft are launched and satellites go into orbit, it is difficult to see how the generation of debris can be prevented, so it is going to remain a hazard, even an increasing one. Is it just luck that it has not yet caused a major incident up there?

Reprinted with permission from 'Mercury' Vol. 19 No. 4, October 2004, the Newsletter of the Stirling Astronomical Society.