PASADENA, Calif. - Whatever the outcome of NASA's current restructuring of future Mars missions, rovers -- ambulatory robots that are equal parts geologist and public relations envoy -- will in all likelihood continue to play as high a profile role in exploration as the plucky, six-wheeled Sojourner did in 1997’s successful Pathfinder mission.

Indeed, the next time a NASA spacecraft lands on Mars, probably in 2004, it may very well dispatch a tiny rover to roam the surface of the Red Planet.

And although it will not happen before 2005 if not ’07, rovers may also help gather the first martian samples to be returned to Earth.

As NASA struggles to reshuffle its entire Mars program, even contemplating the cancellation of entire missions, it has reserved a berth for a rover on nearly every spacecraft it does intend to land on the fourth planet from the sun over the next decade.

"The bottom line is, mobility is still a big player," said Barry Goldstein, a rover expert at NASA’s JPL.

Although the failed Mars Polar Lander did not carry a rover – it instead would have used a robotic arm to examine the martian soil – NASA tentatively plans for its 2001, 2003, 2005 and 2007 lander missions to carry wheeled robots.

However, the 2001 mission has been all but been definitively pushed back until 2003, forcing a roughly 26-month delay to each subsequent shot at sending a spacecraft to land on Mars.

NASA is considering a wide range of rovers to send to Mars, with two standouts: Marie Curie and Athena. However, a third, intermediate class of rover, nicknamed "Fetch," could appear on the scene in the 2005 time frame.

"The important thing isn’t size, it’s functionality," said Steve Squyres, a professor of astronomy at Cornell University and the principal investigator on rover-carried instruments on the 2001 and 2003 shots.

Marie Curie, named for the Polish chemist and physicist, is essentially a carbon copy of Sojourner. The rover was a late addition to the 2001 mission, which originally was have kicked off NASA’s efforts to return samples from Mars using the far larger Athena rover.

A mockup of the Marie Curie rover and the 2001 lander that would have carried it to Mars.

The lander would use a robotic arm to pluck Marie Curie from its carriage and place it on the martian surface. There it would conduct a number of tasks that would prepare for the future use of Athena to actually select, collect and assemble samples of Martian rock and soil on a later mission.

Athena would travel to Mars no sooner than 2005. The rover will be the largest and most capable mobile robot NASA has ever sent to Mars, in that it will be able to independently select and collect samples over a vast area of terrain like a true field geologist.

Athena prototype, FIDO, on the prowl

An Athena prototype, FIDO, is currently being tested at JPL. Field trials will continue in May, with a team remotely controlling the rover from JPL as it roams over an undisclosed, Mars-like location somewhere in the United States.

Although less likely than Marie Curie or Athena to ever make it to Mars, Fetch is being studied as a Goldilocks-like "just right" alternative. The intermediate-class rover, neither too big nor too small, could fly to Mars as early as 2005 to collect the first samples for return to Earth for study.

While Athena would have the capability to navigate and conduct in situ scientific work on its own, Fetch – as its name implies – would be a dumbed-down version. The rover would venture forth to simply collect samples and spirit them back to the lander, where a small rocket would blast them into martian orbit for pick up.

For sample return, NASA may use two landers in the 2005 time frame to kick off its collection efforts, with just one carrying a rover – either Athena or Fetch – and the other employing a robotic arm to do the sample taking.

"There’s a feeling some mobility can be sacrificed in the sample return mission," said Frank Jordan, the program architect for the space and Earth sciences directorate at JPL.

NASA also recognizes the public relations benefit of sending a rover to Mars.

Sojourner was the clear star of the Pathfinder mission, and was later honored with its own $3 United States postage stamp and Mattel toy.

"Public engagement is part of the whole architecting process," McCleese said of the ongoing NASA efforts to restructure its campaign to explore Mars. "And rovers are one way of conveying to the public the excitement of exploration."