Letter: Worry about plutonium on Earth before you worry about it in space

Anita Roddick has a new target; the Cassini probe to Saturn and its use of a nuclear power source rather than solar panels (11 May). In fact solar panels would be impractical; Saturn is five times as far from the Sun as is Earth and the panels would have to be 25 times the size there. Cassini is already as big as a large bus. A couple of football fields of solar cells would make it impossibly large and costly. Nuclear power is the only possible power source for Cassini.

The Titan IV rocket is not "notoriously unstable" but the safest launch vehicle of its size in Nasa's repertoire. One pound of plutonium, released by a launch failure would, says Ms Roddick, expose everyone on Earth to "deadly radiation". This is mere playing with words. Launch failure might (improbably) release plutonium: true. Everyone would get a dose of the resultant radiation: true or at least unfalsifiable. Radiation is deadly: true - but not in these tiny doses.

Forty Chernobyl-class reactors are still operating in the former USSR. The Russian Northern Fleet's submarines are disinte-grating into radioactive rust. Several hundred kilos of weapons- grade uranium goes missing every year. There are more important things to worry about than a single space probe.

Alexander Campbell

St John's College, Oxford