The Iridium network of satellites could self-destruct if just one of the satellites is hit by space debris according to computer simulations by scientists at the CNUCE Institute in Pisa, Italy (Nature 399 743). Most space debris gradually drifts into the atmosphere and burns up on re-entry. However the large number of satellites planned for the Iridium network - 66 are needed to ensure global mobile phone coverage - increases the chances of one of them being hit by space debris. And once one satellite is destroyed, the debris from this explosion increases the probability that another satellite in the constellation will be hit and destroyed, and so on.
Space agencies have become increasingly concerned about space debris after both the Russian Mir Space Station and the US Space Shuttle were damaged by flecks of paint travelling at over 13 kilometres per second. And last year a French spy satellite became the first satellite to be completely knocked out of operation by space debris. There are already 10 million pieces of debris larger than 1 millimetre surrounding the Earth. The danger of collisions is particular acute for satellites orbiting at an altitude between 800 and 1400 kilometres. Mobile phone companies are expected to launch hundreds of small satellites into this orbit in the next few years.
According to the CNUCE researchers, there is a 10% chance that one of the Iridium satellites will be destroyed by debris within a decade, but the probability will increase to 10% within five years if one of the satellites is destroyed. If such a chain reaction starts it could make the entire low-Earth orbit unsuitable for satellites within one hundred years -five times faster than current estimates.
