Long description

The illustration shows the Earth on the left hand side, and a smaller red planet on the right hand side. An arrow at the top of the illustration goes from the Earth to the planet, showing a spacecraft moving between the planets, and the arrow is labelled ‘forward contamination (terrestrial material). Another arrow at the bottom of the illustration goes from the planet to the Earth, showing a spacecraft moving between the planets, and the arrow is labelled ‘backward contamination (extraterrestrial material).