NEXT July, a trio of spacecraft will begin their journeys to Mars. The US, Europe and China are all taking advantage of a favourable planetary alignment to launch rovers to the Red Planet and search for signs of current and past life. But what happens if a stray Earth microbe hitches a ride?

That is where the principle of planetary protection comes in. The idea is to make sure that microorganisms from Earth don’t end up on other worlds – and vice versa . This is both …