Just getting the Spirit rover to Mars was an accomplishment for NASA to celebrate.
Two years ago today, the little robot landed on Mars wrapped in a pyramid of air bags. And three weeks later, the rover Opportunity landed the same way, intact and ready to go to work. The two rovers were supposed to last for 90 days each.
At the time, scientists believed, if they were lucky, the rovers would go for 120 Martian days, which are 40 hours longer than Earth days. But then the rovers lasted for 180 days, then 360, then 500. No one quite knows when these days will end.
"Every day is like a brand*new mission to us, because the rovers move," said John Callas, science manager for the project. "And so we are in different locations, there is different terrain, there is different geology, there is something new to explore. We're like Lewis and Clark going upriver and every day there's new phenomena to see."
Spirit and Opportunity still send back valuable information about Mars to Earth. "Some of our most significant discoveries came well after the initial 90-day prime mission," Callas said.
The rovers have discovered there was once water on Mars, as their cameras captured ripple-like marks left by water flowing over rocks. They also found simple salt, which is a residue of water.
The rovers have had their problems; bad wheels, failed motors, worn parts and Martian dust that covered their solar collectors. Spirit wore out its diamond drill bit. Opportunity had a bad shoulder joint and last spring spent a month trapped in sand.
They have also had some good luck. Unexpected Martian winds have repeatedly dusted off the solar collectors, giving both rovers more power and longer life.
These amazing little machines survive in a harsh climate. Temperatures can swing in one day from the melting point of ice to 150 degrees below zero.
Callas said there is no way for scientists to know exactly how long the rovers will last. "Something could just break and that will be it for the rover. There could be one day when the rover has a massive stroke and it will be over."