Could Mars once have supported life? Nasa's Maven mission aims to find out whether the dusty Red Planet may have once looked a lot more like Earth.
The Maven orbiter will study the planet's high atmosphere, to try to find out why most of its air has disappeared over the millennia.
The probe will blast off from Florida on Monday evening and is set to arrive at Mars in September 2014.
It will spend at least a year circling the planet and gathering samples.