Four years after they were supposed to have turned into dust-clogged doorstops on the Martian surface, the robotic rovers Spirit and Opportunity are still rolling along.

They have covered and photographed 17,000 meters of the landscape between them, beaming 200,000 images to Earth -- more than enough to keep the Fotomat at NASA busy for years to come.

Credit goes to two well-planned missions, superior design, dependable but relatively inexpensive technology and the surprisingly accommodating environment of Earth's nearest planetary neighbor.

If Spirit and Opportunity had merely accomplished what they were programmed to do during the original three-month mission, NASA would have been ecstatic. But they have held on long enough to receive an infusion of new intelligence to help them navigate the planet's surface -- mapping software that didn't exist when the rovers took off from Earth.

The new software, which up-linked to the rovers recently, will help Spirit and Opportunity maneuver around more challenging obstacles on the Martian surface by using a mapping system that helps the machines "remember" difficult terrain.

Developed at Carnegie Mellon University's Robotics Institute by research professor Tony Stentz and former Carnegie Mellon student David Ferguson, the software is still in the testing phase. When it is fully activated, Spirit and Opportunity won't need humans as often to help them out of a jam.

The Martian rovers have already earned enormous acclaim for the NASA team that launched and still maintains the mission. It's a point of local pride that Carnegie Mellon has a piece of that mission's glory.