One year on, the Mars Rovers continue to defy expectations and explore the Martian surface. Here is a summary of the last 12 months of this fantastic human success story.

In space, a year can be a long time. Back at the beginning of 2004, the idea that the Red Planet had once been covered with rivers, lakes and seas was just a theory. Now two quad-bike-sized, roving-laboratories have collected abundant evidence on the ground that turns the idea into an established fact, including stratified sediments, and minerals that probably formed in the presence of water.

A year ago it was scientific heresy even to talk of the possibility of life existing today on Mars and previous missions had been ill-equipped to detect it. But with the proof that water might have lasted for aeons - plus evidence from the European Space Agency's Mars Express orbiter that there was recently methane in the atmosphere - it is now a subject for serious discussion. Water is a pre-requisite for life as we know it, and, on Earth at least, most methane is produced by living organisms.

The story could so easily have been otherwise. After a year of stunning successes by NASA's twin Mars rovers Spirit and Opportunity, it is hard to remember just how much tension surrounded the landings. Following the loss of both the Japanese Mars orbiter Nozomi and the ESA's Beagle 2 probe just weeks earlier, and the Columbia space shuttle disaster in February 2003, much was riding on the missions' success. The roll call of past Mars missions is littered with other failures.

Landing sites were carefully selected to be those that might have flowed with water in warmer periods of the planet's history. NASA's Mars Odyssey orbiter, launched in 2001, had revealed features resembling old lakes, rivers and flood plains, as had its predecessor the Mars Global Surveyor. After a series of delays, rover-laden rockets were launched in June and July 2003.

First to arrive was Spirit on 3 January 2004, at the Gusev Crater - the possible site of a prehistoric lakebed, just south of the Martian equator. Three weeks later, Spirit was followed by its twin, Opportunity, which landed on the other side of the planet at the Meridiani Planum - a site of possible ancient water action.

Both landings were virtually flawless. It has pretty much been that way ever since. Everything has worked better than expected, with just a few glitches, such as problems with a wheel mechanism, computer chips and onboard memory. Future rovers will be able to overcome some problems by self-diagnosis.

Having landed at a site that consisted largely of volcanic rock, Spirit's major success came after 6 months of hard driving to another site called the Columbia Hills. Here it found rocks that had clearly been altered by past water. Spirit also completed other tasks such as profiling the planet's temperature.

On the other side of the planet, Opportunity found good evidence of water much more rapidly. Haematite, a mineral which usually forms in the presence of water, was found to be the main component of tiny spherules termed "blueberries" that littered the plains. While Spirit was trundling its way to the Columbia Hills, Opportunity was set on a course for Endurance Crater - a feature where it later uncovered hints of possible past wet-dry climatic cycles.

Designed to last for 90 days, both rovers have now gone about four times that long and show no signs of stopping any time soon. In fact, with the Martian winter over and dust mysteriously getting cleaned off the solar panels, the rovers are actually gaining power and should continue to do so for a few more months.

Now that we know Mars had water, future missions are aimed at discovering how long it lasted and whether it persists today. More elaborate orbiters, giant rovers and - one day - human landings (plus perhaps even a permanent settlement) are planned to help answer these questions and continue the hunt for life on Mars.

David L Chandler and John Pickrell, 15 January 2005