Mars rovers Spirit and Opportunity have been given the green light to keep on roving, possibly through to the end of 2009. The rovers' continued good health is the only limit mentioned in NASA's announcement of the mission extension.
The twin rovers landed on Mars in 2004. The original mission called for the pair to spend three months trundling across the Martian surface. They have now been exploring the planet for more than three years.
Alan Stern, associate administrator for NASA's Science Mission Directorate, described the rovers as "amazing machines [that] continue to produce amazing scientific results operating far beyond their design life".
Naturally, after three and a half year on Mars, the rovers are showing their age. After only a year NASA reported that Spirit's drill's teeth had worn away after grinding through five times as much rock as expected. Both rovers also suffered this winter as dust storms cut them off from the Sun, their only source of power, and left their solar panels coated in dust for some time after.
But despite their age, John Callas, rover project manager at NASA's Jet Propulsion Laboratory, says the pair are "in good health and capable of conducting great science".
The rovers have already expanded our understanding of the red planet beyond expectations.
Both rovers have found meteorites, Opportunity has identified rocks that match the composition of chunks of Mars that have been found on Earth, and Spirit has snapped dust devils whirling across the planet's surface.
In addition, Opportunity has sent back compelling evidence of Mars' watery past, including the tantalising possibility that the planet could once have basked in the conditions capable of sustaining microbial life. Spirit, too, has found evidence that water has altered the chemistry of the soil and rocks on the plateau it is exploring.
Most of the interest at the moment is focused on Opportunity as it continues its first steps into the Victoria Crater. At 800 metres wide, it's the largest crater the rover has encountered so far. Mission scientists hope the crater's walls will reveal even more secrets of the planet's past. ®