A new generation of robotic explorers are going to search for water on Mars.
The rover is a mobile geology lab
Mark Adler, deputy manager on the US space agency (Nasa) mission, talks to BBC News Online's Helen Briggs about what they might encounter.
HB: How does each rover work?
MA: It's a solar powered robot. At about 9 am it will wake up and by 2-3 pm in the afternoon it will send back what it has done. It then goes to sleep and our job begins here on Earth.
We have about 19 hours to figure out the data and send a series of instructions to the rover next morning.
HB: How similar are they to Pathfinder's Sojourner, the first Martian rover?
MA: Pathfinder was quite similar in terms of its landing system for entry and descent to the surface of Mars. From that point on it looks a lot different to Pathfinder.
The purpose of that mission overall was a technology demonstrator, but it did do some good science.
Mars Exploration Rovers (Mer) is a science mission. Once the landers have delivered the rovers, they are discarded. Now the rovers are completely independent - they can go off to do work and have no reason to go back to the landers.
HB: Will the Mer mission be looking directly for evidence of life?
MA: Our rovers' main science objective is to understand the water environment of Mars. They're not looking for life, they're looking for ancient environments that could have harboured life.
What we learnt from Viking is that it is very difficult to come up with specific experiments to look for something you don't really know what to look for.
This is taking a more measurable approach - trying to understand Mars as a global environment and using the rovers to look at the local environment.
To answer those (life) questions definitively we need to bring samples back (to Earth).
What we want to do is find the right places to go to Mars using our orbiters and landers, then go to these places and bring samples back.
Even if the rovers don't strike gold, sending a set of instruments to Mars has always taught us new things.
HB: Do you think this latest mission will capture the public imagination?
MA: Part of the mission is to inspire the next generation. The mission is an independent rover that can go off over the hill to explore the other side. The public will participate through the website.
We're going to two new places where we haven't been before. Meridiani has chemical evidence of a mineral - haematite - we believe could have formed in water. The evidence comes from Mars Global Surveyor.
Gusev is a morphological site, a crater that could have been a large lake in the ancient history of Mars. The instruments will tell us if it was a lake.
Within weeks, there will be some preliminary answers. There may well be argument about the data for years afterwards.