Rolling On Red

The Mars Rovers are amazing little things. They were designed for a mission of 90 sols (A sol is a Mars day, which is close to an Earth day at 24 hours, 37 minutes), exploring the surface of Mars for evidence of water in two very different locations and sending us back the results. Ninety days. Ninety days in an environment like Mars is no small accomplishment.

But they lasted more than six years. They have discovered conclusive evidence that Mars was once a planet flowing with liquid water. The possibility that Mars does or did contain life is now a thousand times more likely. For that knowledge alone these rovers were worth their modest cost.

The two rovers landed in 2004 on different sides of Mars and from the very start they had different destinies. Everything came easy for Opportunity, as her name coincidentally suggests. She was placed near the equator in a geological wonderland with lots of sunlight giving her power to burn (the rovers are solar powered) and she found evidence for ancient water almost everywhere she looked. Her data came in and was shown off around the world to scientists and the public alike, the world was amazed at the discoveries she was making.

Poor Spirit, on the other hand, needed all the spirit she could muster to survive her series of hardships in her cold location far from the Martian equator. Her landing spot in the Gusev Crater, which seemed like a promising place to look for water from Earth, turned out to be a giant field of monotonous old lava rocks stretching to the horizon in every direction. Then on sol 18 she abruptly stopped communicating with Earth and put herself into "fault mode". She had apparently become stuck in an infinite series of reboots due to corrupted data in her flash memory. Every time her computer would restart, the flash memory corruption would feed it bad data and it would fault again. For a while it seemed young Spirit's career was over. But the engineers at JPL pulled one last trick out of their collective sleeves: they had designed in a last-ditch protocol that would allow the rover to be completely reset via a command sent to and decoded directly by the radio itself, bypassing the computer altogether. In other words, they used the little computer brain in the radio to wipe the main computer's flash memory and reset it. The trick worked and Spirit woke up ready for action. But her troubles were not over. She was continually beset with one harsh situation after another, including further mechanical problems, very nasty terrain, and power-robbing dust on her solar panels in a place with almost no wind to clean them off. Yet she persisted, she worked hard and eventually made important contributions to the mission.

The rovers have both far outlasted their "warranties" and provided us with profound discoveries about our neighbor, the Red Planet. We have exchanged information across the interplanetary divide* with them continually over the last six years. We've sent instructions on where to go next, they've sent back pictures and other data about what they found once they got there. The rovers have become our ambassadors to another world. They have survived five hard Martian winters and an enormous planet-wide dust storm, alone on the ground. These little rovers owe us nothing.

While Opportunity continues to pick her way around in her warmer, sunnier equatorial climate, Spirit's hard life is taking its toll. The telemetry team at JPL noticed one day that her right front wheel was drawing too much current and sure enough, that wheel subsequently locked-up and from that day on Spirit had to roll backwards and drag her lame wheel. And just as another of her wheels began to fail, she became stuck in deep sand and hasn't been able to get out in nine months of trying. Now, although the rovers weigh 400 lbs on Earth, they only weigh 150 lbs on Mars. A ten year old kid could roll Spirit out of this mess in ten seconds. But 300 million miles from home, there is nobody to help. She's on her own, and the last time she tried to escape, she just dug herself in deeper. Here's a picture she took of her front wheels - you can see the track left by her dragging right wheel, and on the left her good wheel has broken through the thin crust and sunk itself deep into the sand. It's a bad situation, and barring some unforeseen new strategy, Spirit will never move again.

There are new explorers on the way. In 2011 NASA will launch the Mars Science Laboratory (MSL), a much bigger and more sophisticated rover that will be loaded with enough scientific equipment and experiments to keep it (and us) busy for a long, long time. In fact, the science experiments on MSL alone weigh ten times what the entire Spirit/Opportunity rovers do. And it won't get stuck in little sand dunes either, the MSL rover is almost the size of a small car, with adaptable six-wheel drive and a new kind of "surface propulsion system"...she's been designed to handle the rough and tumble terrain on Mars with grace and agility (The picture below shows the size difference between the wheels of the three rover generations - the original tiny Pathfinder Sojourner Truth, the mid size Spirit/Opportunity, and the large MSL). We're using all the lessons we've learned from the previous three rovers as well as the Viking missions of the 1970's to build ever bigger and more capable Mars machines. We may eventually have thousands of these things roaming the surfaces of worlds and moons throughout our solar system, sending us back mountains of data. Spirit and Opportunity have shown us that rovers work. They really work.
Someday soon NASA will make the inevitable decision that Spirit has had enough hardship for one little rover, and they will end her mission. But there is no "kill switch" for these rovers. There is no command that can be sent to shut a rover down for good. It's dangerous to even design in such a thing because it might be tripped by accident.

After Spirit's mission ends, she will live on, at least for a little while. Because she cannot move, she's not positioned correctly to gather enough sunlight to keep her warm through the next brutal Martian winter. Electronics cannot handle temperatures of 200 degrees below zero, so her computers will shut down and suffer permanent damage sometime in May. But until then, she'll wake up every morning when the sun hits her solar arrays, just like she has for the past six years. Then she'll point her high-gain microwave antenna at the Earth and call home, as always.

But this time, nobody will answer.

And Opportunity won't be far behind. These rovers have done everything they came to Mars to do and much more. We've driven them harder and farther than they were ever designed for - the equivalent of driving your car more than a million miles. They deserve a little rest.

Rest, maybe, but the Mars rovers will never rust. They will never tarnish. They will doubtless look just as shiny and sexy in a million years as they do today. Maybe someday we'll go looking for them. They won't be hard to find, we know their locations on Mars to within millimeters. The bright white dot in this satellite image is Spirit, in what will probably be her final resting place, next to a big flat rock formation called "Home Plate". Here she will patiently sit out the ages to come.

While it may be sad to see the Spirit and Opportunity missions come to an end, we can marvel at what they have taught us over the last six years and get ready for the next installment of wonder because while these two rovers came to Mars looking for water, the MSL is coming fully-equipped to look for life.


* Mars is between 55 million and 400 million km away from us depending our relative positions, so it takes our messages between 3 and 22 minutes to travel to Mars at light speed.