By Paul Rincon
BBC News Online science staff
Negotiations are under way to further extend the missions of the US space agency's Mars Exploration Rovers.
The rovers were originally given a 90-day Martian mission
The rover team wants the robots to keep on with their science operations for longer than the 250 Martian days (or sols) they are expected to work.
Representatives from the team are making the science case to Nasa officials in order to secure funding for the mission beyond September.
The robotic explorers were originally due to work for just 90 Martian days.
In April, scientists and engineers based at Nasa's Jet Propulsion Laboratory (JPL) announced that this period would be extended to 250 sols due to the robots' better than expected performance in Mars' harsh environment.
BBC News Online understands that rover project manager, Jim Erickson, and JPL's director of Solar System exploration, Firouz Naderi, are in talks with Nasa's associate administrator, Ed Weiler, to extend the mission still further.
Dr Ray Arvidson, deputy principal investigator for the rover science payload, explained there might be significant discoveries waiting to be made on the planet.
"We've literally and figuratively just scratched the surface of the Columbia Hills and we're about halfway through the Endurance mission," he said.
Dr Arvidson said July was the last full month the rover team would be based at JPL. After that, mission scientists will use teleconferencing and distributed processing of data to carry the mission forward.
This would also help to make the mission more cost-effective, said Dr Arvidson: "[Money] is a huge factor. We have to minimise the costs as much as possible."
Erickson and Naderi will also have to prove that the rovers are healthy enough to carry on past September.
Dr Arvidson said both Opportunity and Spirit were in excellent shape, although the time available for rover operations had been gradually diminishing.
This is largely due to the northern movement of the Sun as viewed from Mars in the lead-up to winter on the planet.
The rovers found plenty of evidence for past liquid water on Mars
This means days are shorter for the rovers and the Sun less intense, limiting the energy available through their solar panels to charge batteries.
The steady accumulation of dust on the rovers' solar panels will eventually cut the power that can be drawn from the Sun. But this was not currently proving a significant hindrance to the mission, Dr Arvidson said.
In about 30 sols' time, scientists will position the rovers' solar panels in order to soak up as much of the Sun's energy as possible to help push the vehicles through the Martian winter.
Spirit will be placed in a "deep-sleep" mode during the night, which involves shutting down all electronics and relying on solar power to wake the rover up. Opportunity has already been in this mode due to a faulty heater that was sapping power.
"We're not going to do a lot but stay alive and occasionally do some remote-sensing of the sky and surface. We'll become a long-term weather station," said Dr Arvidson.
The rovers might begin driving again by early 2005, when conditions improve on the planet.
If Nasa headquarters agrees to an extension, it is possible the rovers could still be roving well past their first anniversary on the Red Planet.