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.
Why did Mars lose its air?
The distance of Maven's orbit around the planet will vary and it will do five "deep dips", swooping into the Martian atmosphere to collect samples.
It will measure how quickly air molecules are being lost from the planet's atmosphere, to try to work out how and why the atmosphere's changed in the billions of years since the planet was warmer and wetter than it is today.
Mars's atmosphere is now extremely thin and is mostly made up of carbon dioxide. Scientists think that Mars probably lost most of its atmosphere from being battered by the Solar wind - the powerful blast of energetic particles from the Sun.
They think that because Mars doesn't have a protective magnetic field as the Earth does, it couldn't keep hold of its atmosphere.
Once Maven has completed its original mission, it will have enough fuel on board to keep it running for years, relaying information back to Earth from the Mars rovers like Curiosity.