While the Phoenix Mars lander is the new kid on the block (of ice, salt, or whatever it is), don’t forget those two long-lived rovers: NASA’s Spirit and Opportunity.

I asked Steve Squyres, the principal investigator for the Mars Exploration Rover mission, to provide an update on the overall health of the dynamic duo.

“Both are doing well,” Squyres responded. “Opportunity has plenty of power and is very busy driving and doing science. Spirit is very low on power (the winter solstice is just a few weeks away), but is hanging in there just fine,” he added.

Regarding Spirit, rover scientists and engineers feel very confident about the robot’s health through the winter. There’s even a decent chance that Spirit can make it through without having to disable a survival heater on the rover’s Mini-Thermal Emission Spectrometer - known in Mars jargon as Mini-TES.

“That’d be good news if it happens, because we want Mini-TES to be healthy when spring comes. We’ve been fortunate this winter, in that skies at the Spirit site have been clearer than we anticipated.”

Around the planet, at the Opportunity rover site, that Mars machinery has a new problem with a shoulder joint that swings the robot’s arm left and right.

“We have managed to get it to move, however, and we used it to put the arm out in front of the vehicle. What we’ve then done is a lot of analysis and testing on the ground to convince ourselves that we can safely drive the vehicle with the arm unstowed,” Squyres told me.

The rover wasn’t designed for that, but by keeping the arm unstowed, rover operators protect themselves against a possible future failure of that particular shoulder joint.

“If the joint were to fail permanently with the arm stowed we would never be able to deploy the arm again. Keep it unstowed…and it’s always ready to go,” Squyres said.

So now Opportunity is driving with the arm out. What happens when it comes time to make use of the robot’s arm? If that particular shoulder joint works again, then that’s good news. But if it doesn’t, it’s still okay, Squyres noted.

“We’ve still got four good joints, and those allow us to contact the surface with all the instruments and do the things we normally do. If we do need any side-to-side motion and that joint doesn’t cooperate…all we have to do is turn the rover,” Squyres observed.

BTW: A trip down Mars memory lane - Those two robots have been at work on Mars since January 2004.