Actually the idea that these rovers were designed to last 3 months is of course complete bullsh...... sorry... nonsense. They were designed and build to operate as long as possible, which could be some years. Early in the design phase it was recognized that dust build up on the solar panels was the most likely thing that would kill them. However, how long this would take was a complete unknown. Many in the program, including Steve Squyres, believed or hoped this would be something like 6 to 12 months. Just to be on the safe side they decided to communicate that 3 month figure as the 'design life time' of the rovers.
As it is, with wind gusts cleaning the panels from time to time, there is nothing magical, brilliant or incredible that both rovers have last this long. Just solid and conservative engineering and a bit of luck.

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It doesn't really work that way. All exploratory spacecraft have a "primary mission" period, the completion of which, along with various accomplishments along the way, is defined as mission success. The spacecraft are specifically designed to have a high probability of completing the primary mission, but obviously if there's a 99% probability of lasting 90 days, there's a nontrivial probability of lasting several times that.

As a result, it's a common occurrence for a mission to last far longer with planned, as with Pioneer, Voyager, Galileo, and many more. What's impressive about the rovers is the endurance despite the unusually hostile environment. As someone who has supported space missions for most of the last 25 years, I do find it most impressive.

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The requirement was for 90 sols. The funding was for 90 sols. They made the arrays big enough so that they would have enough power to operate for at least 90 sols, and tested mechanical and electronics parts for a minimum of 3x the requirement ( 270 sols ).

Go read all the pre-launch technical reports and predictions at the JPL technical reports server. 90 sols isn't nonsense - it's what they designed to be confident of lasting.

Squyres said that he thought they would work 120 - 180 sols (on record on Planetary Radio on the first aniv of landing) - not 6 to 12 months.

Doug

Well, I have seen the designs and specs of the battery, the drive motors, steering actuators and arm joint servos and all I can say is that these things were not designed for 3 months. I think quite some mass could have been saved by really designing them for 3 months. My feeling (I can't proof that) is that the rovers were designed in a very defensive way, with focus on survival rather than on science return. This seems pretty logical, remember the two Mars missions prior to MER? What I'm saying is that this might explain their amazing longevity.

Oh and BTW, if they really were designed for 3 months would you publicly declare that to be a one of your prime mission success criteria?
Suppose they would work for 2 months and 25 days or so. That's what you get with a mission designed for 3 months, but it would have FAILED.
Of course, you simply say that it is designed for 1.5 months and suddenly chances for a 'successful' mission are a lot better.

ps. Doug, I think that 120-180 sol number was for Spirit. They knew from the beginning that Gusev was a much dustier place than Meridiani and that Opportunity would probably last longer.

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You are 100% correct. They were designed and built to last at least 3 x that time as standard JPL life testing requires.

The REQUIREMENT was 90 sols. You design to make damn sure it lasts 90 sols by testing things to last longer so that the gaussian distribution of failure doesn't put any components at significant risk of failure within those 90 sols. And not everything is life limited - you design something strong enough to do a job - and baring something unexpected, it should just carry on doing that job for a long long time. You can't design a camera to fall off after 90 sols, or the battery to explode on the morning of Sol 91. You make them good enough to do a job - and then hopefully it'll keep doing it far longer than you need it to.

We HAVE had failures remember. MERB shoulder joint motor has broken a winding, the cable to the Microscopic Imager is ratty, a thermostat is stuck on and the front right steering actuator is broken. MERA has front right wheel drive motor broken after nearly failing by sol 150.

The warrantee on my camera is 2 years. I don't expect the lens to drop off after 2 years and a day.

And don't for one minute think that JPL knew they'd last this long...if they had - they wouldn't have given the X-Band transmitter on Spirit the same channel as that of MRO - thus causing sequencing issues when both spacecraft are visible from Earth.


Doug

Well, technically, you could, I suppose, in Bizarro world. Sorry, but that just brought a funny image to mind, what with the pyro camera eject and the battery self destruct device. No doubt the activation code would be the ascii text "Zero-Zero-Zero-Destruct-Zero."

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Well OK, if you insist...

Anyway,

The surprise was the wind gusts cleaning the panels. Nobody could have foreseen that. If you would have told the engineers that dust build-up would not be a problem they would have changed Spirit's X-band channel.
Or maybe they wouldn't have believed you...

So, no more complaining about these great machines. Build to last, and last they will!

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The MiniTES on one of the rovers has stopped working, I believe, and the cobalt source on the Mossbauer spectrometers have decayed to nearly half the activity they were when installed.

Jon

Nope - they had issues with the one on Opportunity, but it's working.

The the mossbauers have actually decayed a lot more than half. They use Co57 - 270 days half life. We're at not far short of 5 half lives now - so an 8 hour integration from sol 1 - would now be about 10 day. But this can not be described as a failure - it is simply a resource that's decaying. The spectrometer still works...it just takes a long time to get a good spectrum.


Doug

I am glad they fixed the mini TES.

I thought they were using Cobalt 60, not 57. Five half lives would mean that the radiation intensity is about 3% of the original. It would take 30 times as long as to get the original reading, I would have thought. Agreed it is not a failure as such, but it is an instrument that is not particularly useful any more. Not that Mossabuer was ever that useful!

Jon

Yes - 30x - 8hrs to 10 days. I spoke to Steve Squyres about it last November

Here's a transcript:
http://www.unmannedspaceflight.com/i...ndpost&p=75661

Talking about the Mossbauer on Opportunity


and 12 months before that -
http://www.unmannedspaceflight.com/i...showtopic=1683

I spoke to him about Mossbauer integrations taking so long.

Doug

Interesting, thanks. November 2006 was a a bit over three half lives. That makes sense for a 10 day period of data acquistion. Four half lives (16 days to acquire data) was passed in February, and 5 half lives (32 days) will be passed in the coming November.

Jon

Spirit, today, has become the second longest lasting lander on Mars. The top four list

Viking 1: 2245 sols
Spirit: 1282 sols
Viking 2: 1281 sols
Opportunity: 1255 sols

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Any bets on at least one of the rovers beating out Viking 1?

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Hm, hat's very long. I wonder, what did they do with a stationary lander for over 2000 sols? I mean, was there anything new to do after standing there for 1000 sols?

In the end Viking 1 was killed by human error, not by technical issues. (well, the human error was due to software updates to improve the decaying batteries, but anyway)

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One of the 'level one' requirements was to operate for 90 sols. It was a 'primary objective' to last that long.

A loss of vehicle before that would have been considered a failure by NASA rules.

Doug

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