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Spirit Marks One Martian Year
hrayr writes to tell us NASA is reporting that Spirit, their proclaimed "wonder child", sent to explore Mars has just wrapped up its first Martian year, equivalent to two Earth years. Originally designed to only last 90 days the small six wheeled machine has lasted far beyond the original scope to bring us immense amounts of data and some 70,000 images. There is still great hope that this data, and more to come will bring us one step closer to Mars habitation.
Because its made of several parts intended for space grade usage and cost millions of dollars. Nobody wants to face owing the space research center their money back because their part failed first, so they set a number as a minimum. Clearly they aimed too low, although I've heard unfounded assertions that the 90 day target was because it was easier to budget the manpower that way.
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It's just the warranty, or mission success criterium. If you look at the specs, there isn't a single part of the rovers with such a limited lifetime. For example, the batteries have a 1000 cycle lifetime. The big problem is that there were many unknowns. Landing on mars is a tricky business, two out of three missions failing. They sent two rovers to maximize the chance of at least one of them reaching the surface intact. Further bad luck (a global dust storm, or landing in the middle of a nasty dune field of for example), could further have limited the mission lifetime. And they really needed succes, so they set the goals to "realistic" levels. (Or put another way, they over-designed the robots).
But once they had the two rovers safely on the ground, you could already tell they were going to outlast the warranty. Why even try to reach the Columbia Hills if the rover is near the end of it's lifespan?
I still remember a comment by Steve Squyres not that far in the mission stating that the rovers could still be be going "next summer". He wasn't specific (on purpose, I guess), but I think he didn't mean the next earth summer (a few months), but the next martian summer (it was still autumn on mars back then, so that's more than an earth-year).
"It's too bad that stupidity isn't painful." - Anton LaVey
- the temperature is extremely cold, and thermal stresses could crack electronic solder joints and/or ruin components
- the batteries find it tough going at low temperatures
- the silicon solar cells degrade over time, losing efficiency
- an unknown amount of dust collects on the cells, how much and how long can you drive it? One of the surprises is that there are "dust devils" on Mars; some of these have actualy blown over the rovers and cleaned much dust off the solar cells! See the link for an amazing time-lapse movie of such winds caught by the rover cameras!
http://www.nasa.gov/vision/universe/solarsystem/me r_main.html
Bottom line: when you have thousands of parts in a harsh environment, you just don't know. They built them tough but light, and thought they had a good chance of exceeding 90 days. Thankfully, luck has been on JPL's side, and they're still going! What a success story.
At almost the same time the rovers landed, a European probe was also to land. It was never heard from again, and presumably crashed.
In addition to what other people have said: because the Sojourner rover lasted about 90 days before it collected too much dust and couldn't operate. It was only expected to last something like 2 weeks, at which time the batteries ran out and it had to trickle charge from it's undersized solar panel. I'm sure experience from the Viking missions and the Pathfinder lander was also used in setting the mission-success criteria.
Actually, NASA is hardly in a slump. They've shown a pretty good unmanned mission success over the last 4-5 years. The last major failure was the Mars Polar Lander, which was lost in 1999. Then of course there was the Genesis mission that failed to deploy it's parachute after successfully capturing solar wind particles. The sample was tainted, but not considered completely lost. In the meantime, they've launched the Spitzer, Cassini has arrived on station, Deep Impact smacked a comet, Voyager reached the termination shock, Galileo finished an excellent tour of Jupiter, Stardust is on it's way back to earth with a comet sample, Odyssey is orbiting Mars as planned, and the Reconnaissance Orbiter is halfway there. If you consider the past record, or even the trials of other nations (ESA losing the Beagle amd part of the data from Hguyens, Japan having trouble with Hayabusa and losing their Martian probe, Russia in the doldrums of a limited budget), the success rate lately has been outstanding. I hope this is a sign that we're finally getting the hang of it.