NASA's two Mars rovers, Spirit and Opportunity, were designed to last 90 days. But Tuesday night marks the two-year anniversary of Spirit's landing, and both rovers are going strong. The robotic geologists have provided invaluable data about whether Mars could once harbor life.
MICHELE NORRIS, host: From NPR News, this is ALL THINGS CONSIDERED.  I'm Michele Norris.</s>ROBERT SIEGEL, host: And I'm Robert Siegel.</s>ROBERT SIEGEL, host: As we speak, two remarkable robots are hard at work more than 30 million      miles away on the surface of Mars.  The six-wheeled explorers known as      Spirit and Opportunity were supposed to last 90 days.  Well, tonight is      the two-year anniversary of Spirit's landing, and both rovers are going      strong.  They've provided scientists with data to back up their theories      about whether Mars was once a wetter, warmer planet, one that could have      harbored life.  NPR's Joe Palca reports.</s>ROBERT SIEGEL, host: JOE PALCA reporting:</s>ROBERT SIEGEL, host: Getting a spacecraft to Mars is an amazing technological feat.  Getting      it there and landing it safely is even harder.  But a team from NASA's      Jet Propulsion Laboratory in Pasadena didn't just do that once in January      2004; they did it twice, first with a rover called Spirit that landed in      the giant Gusev Crater and then three weeks later with Opportunity on the      other side of Mars in a vast plain called Meridiani.</s>ROBERT SIEGEL, host: Think about it.  In about six and a half minutes the spacecraft had to go      from more than 10,000 miles an hour to a dead stop.  This was      accomplished by a heat shield, a parachute and some retro-rockets and,      finally, air bags that inflated just before touchdown.  The air bags      cushioned the precious rovers that didn't so much land as bounce down to      the surface.</s>ROBERT SIEGEL, host: On the night of January 3rd, 2004, the control room in building 264 at      JPL was packed.  As the spacecraft descended, it sent back signals when      critical events occurred:  heat shield separation, parachute deployment,      retro-rocket firing.  Finally, the signal everyone was waiting for.</s>ROBERT SIEGEL, host: Unidentified Man #1:  We have signs of bouncing on the surface.</s>PALCA: But then the spacecraft went silent.</s>PALCA: Unidentified Man #1:  Hang on, everybody.  Please be quiet.  We don't see      a signal at the moment.  No signal at the moment.</s>PALCA: Unidentified Man #2:  We saw an intermittent signal that indicated we      were bouncing.  However, we currently do not have signal from the      spacecraft.</s>PALCA: Unidentified Man #1:  Please stand by.</s>PALCA: For 16 excruciating minutes engineers stared at their consoles      and waited.</s>PALCA: Unidentified Man #3:  All I see is...</s>PALCA: Unidentified Man #4:  Do you see it?  Do you see it?  Do you see it?</s>PALCA: Unidentified Man #5:  What do we see?</s>PALCA: Unidentified Man #1:  There it is.</s>PALCA: NASA's deep space network picked up the rover's signal, and all      was well.  The next few days went better than anyone on the rover team      dared to imagine.  Less than a day after touchdown, chief scientist Steve      Squyres reported good news.</s>Dr. STEVE SQUYRES (Chief Scientist): We've gotten the first images from      the descent camera back.  They're beautiful.  They show distinctive      patterns of impact craters.  We landed in the sweet spot.</s>PALCA: For Squyres, the success was particularly satisfying.  He teaches      at Cornell University, and like a lot of planetary astronomers, he      dreamed of sending scientific instruments to Mars.  It took years to      persuade NASA officials and his fellow scientists that his instrument      package was the one most worth sending.</s>PALCA: One of the most important instruments was the Pancam, a high-resolution      camera that would give the best views ever of the martian surface.  In      his book "Roving Mars," Squyres recalls the first time he saw Pancam      pictures on Mars. He was planning to get some sleep after staying up all      night after the landing when his assistant told him that the pictures had      arrived.</s>Dr. STEVE SQUYRES (Chief Scientist): (Reading) `I run back to building 264 and up the three      flights of stairs, not waiting for the elevator.  Trying to catch my      breath, I walk down the hallway to the Pancam room expecting it to be      mobbed.  Instead the room is dark.  I step inside, and there is Jim Bell,      the lead scientist for Pancam, all alone slumped in a chair and staring      at a monitor.  There are tears in his eyes.  On the screen is Mars with      colors so perfect and detailed so sharp it's like being there yourself.      He looks up at me.  "It works, man. It works."</s>Dr. STEVE SQUYRES (Chief Scientist): This is so good.  I can't believe how good this feels, but I'm not going      to get all misty-eyed.  I'm cool.  I go back outside, do a couple more      interviews and then head home and microwave up some frozen spaghetti.      I'm sitting on the couch in a darkened apartment in my underwears eating      my spaghetti when suddenly the events and the emotions of the day come      tumbling down on me. Pancam is really on Mars after all these years.  The      whole damn thing is on Mars.  I dissolve into tears.'  I remember that      real well.</s>PALCA: Now NASA sent the rovers to Mars to search for signs that liquid      water once flowed there because where there's water, there could be life.      Scientists carefully chose both landing sites because they looked like      places water had once flowed.  But at first Spirit's landing site at      Gusev Crater was a bit of a dud.  It looked like a dry lake bed from      space but none of the nearby rocks showed any sign of a watery past.</s>PALCA: On the other side of the planet, Opportunity was having more success.      That rover plunked down near some outcrops that showed patterns like what      you'd see in rocks sitting in slowly moving water.  But looks can be      deceiving.  Rover scientists waited for data from rover instruments that      measure the chemical composition of rocks.  By April of 2004, Squyres and      his colleagues had amassed enough data from Opportunity to make a bold      claim.</s>Dr. STEVE SQUYRES (Chief Scientist): We found an enormous quantity of sulfur in this rock, too      much to explain by any other mechanism, we believe, than this rock being      full of sulfate salts.  That's a telltale sign, we believe, of liquid      water.</s>PALCA: The water is long gone.  It's evaporated, leaving the salts      behind. But where Squyres sees a watery past, others see a drier one.      Tom McCollom is a geochemist at the University of Colorado in Boulder.      He doesn't dispute the fact that Opportunity found sulfate salts.  He      just disagrees with Squyres about how they got there.</s>PALCA: Mr. TOM McCOLLOM (Geochemist, University of Colorado):  The sulfate salts      that are in there come from reaction of the rock with sulfuric acid      rather than from evaporation of a fluid.</s>PALCA: McCollom's work appears in the latest issue of the journal      Nature. Squyres says more recent rover data confirms a watery history at      Opportunity's landing site.  But the question of when and whether this      part of Mars was once wet is likely to be kicked around at scientific      meetings for years.  Tom McCollom says scientists have the rovers to      thank for that.</s>PALCA: Mr. McCOLLOM:  We now have this tremendous data set that's being returned      by the Mars rover that--I mean, we wouldn't have any debates if that data      wasn't there.  There wouldn't be anything to debate about.</s>PALCA: Both rovers are showing some signs of age.  Opportunity has a      sore shoulder in its robotic arm, and one of its wheels is stuck at a      slight angle so the rover handles more like a wobbly shopping cart.  Over      in Gusev Crater, Spirit had been running low on energy as the martian      dust began to cover the rover's solar panels.  But then, miraculously,      there was what engineers call a cleaning event.  They don't know for      sure, but apparently a mini tornado passed over the rover blowing all the      dust away.</s>PALCA: And Gusev is no longer the scientific dud it appeared to be.  As Spirit      began ascending some hills about two miles from its landing site, it      found signs that water was once on this part of Mars, too.  John Graham      is a rover scientist at the Smithsonian's Air and Space Museum.  He says      Spirit has just come down one hill and is heading for another.</s>Mr. JOHN GRAHAM (Smithsonian Air and Space Museum): And we're on a very      tight time line, and the reason for that is we need to get to the base of      the next hill to the south, which has got north-facing slopes, so that we      can get up and basically repeat what we did last winter on Husband Hill:      turn the solar panels towards the sun and survive.  So we are definitely      in a strategic mode here where we're planning on making it though the      winter and going on.</s>PALCA: Perhaps for another year, perhaps longer--not bad for a mission      that was supposed to last 90 days.  Joe Palca, NPR News.</s>MICHELE NORRIS, host: This is NPR, National Public Radio.