Why did engineers not expect spirit and opportunity to last longer than 90 days?

Question

Please help.

Thank you

x x x

Answer 1

I don't think they really were serious when they made that estimate.  There was always the chance that they would stir up too much dust moving around that would block the solar panels and cause them to lose power, but usually we make very conservative estimates for how long spacecraft will function - to make ourselves look really good when it lasts much longer.

Answer 2

because its a safegard, if it broke quickly they could be "we warned you" and if they last longer they can be like "look how good we are at our jobs!"

basically, im sure one compenent was rated to last 180 days in the martian atmosphere (probably a solar pannel or maybe a capacitor or something) so they cut it in half so they couldnt be blaimed!

Answer 3

I think it would be more accurate to say they expected them to last a minimum of 90 days. That was the minimum length of time needed to complete the basic mission, so if they came up with anything less it would have required redesigning to meet the requirements. The estimates would have assumed the worst case for temperature variation, dust, hard landings, component reliability, etc.

Answer 4

The Mars environment is so harsh that the landers can not be built tough enough with what they need to last longer then that.  

Between the winds, the sand, the temperature difference (between night and day is over 100 degrees not to mention that the temp from one's head to toes changes like thirty degrees (just what I remember from watch Discovery)), its very tough there.

Mars is a harsh world.

Answer 5

They calculated this as estimate, how long the mission could last with some certainty (ignoring failures like getting eaten by Martians) for the initial mission planning.

For this, they looked at the possible mean time between failures of the parts, calculated how fast dust should accumulate on the solar arrays or inside the wheel bearings or the batteries aging (as the batteries have only a limited number of recharge cycles).

With this known, they came to the conclusion, that they can only guarantee a mission duration of 90 days and that it might be possible to extend the mission, when the state of the rover is known at the end of the 90 days. These calculations had been done already in Phase 0 - before even the first part was really chosen. All that information was needed to know for defining which scientific payloads are possible and if the mission can be flown in a useful way at all. If they came to the conclusion, that the 90 days are not enough for answering scientific questions, it would not have flown.

When the rovers got finally designed, it was sure that it was to be build for the 90 days in mind.

What they did not know at that time was for example, that the solar arrays got blown free from dust by the Martian land sprouts. Or that the batteries did not age as fast as theoretically possible.

As nobody is there on Mars to repair the rovers and clean the bearings, 90 days was a pretty good estimate - and that they lasted longer also a sign of good engineering.

It is a far worse scenario for the engineers, when they say 90 days and the rover fails already after 45 - that would be a mission failure. Anything past these 90 days is a success and thus, engineers prefer understatement, if this means a higher chance of success.

The next rover would very likely be designed to last more than 90 days and the engineers will do their best to ensure it based on the experience gained from these two rovers. But it will still be the same process. If they come to the conclusion, that a mission would be feasible to last one year, and the mission gets approved, they will build a rover to last this year at least... and maybe a few months longer.

Answer 6

It's not that they didn't expect them to last longer, it's just that from an engineering standpoint they couldn't give any guarantees that they would last longer. Trying to keep a remote control vehicle operational in a very dusty, very cold environment hundreds of millions of kilometers from the nearest repair shop is not a very easy task, and NASA wanted to make sure it had a reasonable timespan in place for its projected mission. The fact that the mission has been going on for over 15 times that long and both rovers are still more or less functional just goes to show that the people at NASA are one, good at building machines, and two, lucky.

Answer 7

That was just the lifespan that was guaranteed by the builders. The fact that they have far surpassed that design lifetime surprises no-one, and pleases everyone, especially the builders, who don't have to eat the cost of their construction.