... Just one: a very stringent requirement on the relative positions of Earth and Mars at the time of launch. ... Suppose the spacecraft which has landed on Mars is a robot , which collects a sample and immediately takes off again for its return trip. ... Since Earth and Mars constantly change their relative position, it stands to reason that if we sufficiently delay the return trip, Earth will move from position (2) relative to Mars to position (3), at which time the return trip can begin. ...