Mars Rover Opportunity Sets Longevity Record

s31523 writes "The Mars rover Opportunity has beaten the original record of six years and 116 days operating on the surface of Mars, originally set by the Viking 1 Lander. While the Spirit rover has been on the surface longer than the Opportunity by three weeks, it has been out of communication since March 22. If Spirit comes back online, it will attain the new Martian surface longevity record. This feat, right on the heels of another longevity feat (Voyager 2 and twin on the verge of entering interstellar space and still kicking) is healing some of NASA's past black eyes. It is quite remarkable given original spec of 90 days for the mission. With the passing of the solstice, warmer temperatures and more sun will likely mean the rover will continue on."

Comments

Go technology go!

[JDSalinger]

Feats like the Mars Rovers show us that our space-engineering prowess is not only continuing to mature, but indeed getting quite robust. From this one mission alone, how much have we learned about vehicle design for dealing with the Martian environment?

And with yesterday's announcement of the creation of synthetic life, we are obviously on the edge of new breath-taking scientific ability. When will we be able to start creating custom bacteria to begin terra-forming mars? I know there is no way to predict the future, but the potential for change in our life-times is mind-blowing. As an anxious futurist, all I can say is "Go technology go!"

Re:Go technology go!

[Anonymous Coward]

I'm sure there is a more sagan-esque way to phrase "go technology go". That said, it certainly shows that a still more glorious dawn awaits!

(For disclosure, it wasn't true synthetic life, but the injection of synthetic DNA into an existing cell, IIRC.)

Re:Go technology go!

[Pharmboy]

Feats like the Mars Rovers show us that our space-engineering prowess is not only continuing to mature, but indeed getting quite robust.

I don't doubt we are getting better at building equipment that can stand the rigors of space or Mars (better alloys, better lubricants, better electronics, simple design) but this also says something about our ability (or lack thereof) to estimate the durability of the things we build. Lasting twice as long as we expect is a great thing, lasting 25x as long means they were afraid to give a real estimate.

Kind of like how Scotty says "It will take 8 hours to get it fixed, Captain!", to which Kirk says "You have 2 hours", and yet it still gets fixed. The engineers are likely underestimating the "average" time to cover their own butts. This actually makes it *harder* to get some science done, as you are scrambling to create new tasks with the extra time you weren't expecting, never knowing when it will give up the ghost.

Re:Go technology go!

[Anonymous Coward]

NASA engineers have learned to underestimate their ability to make themselves look good. The problem now is that future expectations will veer sharply to long lived missions even if engineering estimates are low. I say they've got a good 10-15 years before people will start looking for that next NASA miracle story to justify the budget.

Re:Go technology go!

[Jarik C-Bol]

Ah, just as i suspected, the age old 'they failed, it was over engineered, it should have only lasted 95 days, blah blah blah' shtick.
For the umpteenth time, *it was deliberate* they knew they could not get approval for the budget for a rover designed to last years and years, because of the long standing 'what if it breaks early? then the money is wasted right?' attitude.
NASA knew what it would cost to build a decent rover, so they pitched it at 90 days, that way if it flakes out, it does not look like a huge failure.
They build an excellent rover on (what is thought to be) a 90 day rover budget, send it up, and 95 days later, they can say 'look! this 90 day rover we made is doing great! it well outlasted our expectations! its way cheeper for us to keep driving it around than to build a new one and send it, can we get a little more funding?' I'm confident NASA knew full well what it was doing when it built and sent these rovers. (They probably even had at least rough outlines of things to do with them in the event of an extended project life.)

Re:Go technology go!

[Conchobair]

SMIILE = SM (Space Migration) + II (intelligence increase) + LE (Life extension)

That's the Leary way of saying it.

Re:Go technology go!

[fbartho]

As an Engineer myself I find your comment irritating. Sure any organization that has lived for a respectable amount of time has a certain percentage of people continuously trying to cover their asses. The thing you missed is that NASA wasn't asking the Engineers "how long can it last?" They were telling the Engineers "it must last 90 days, weigh less than X, take up Y space, generate no more than Z interference, cost less than C...". Having a hard stop of lasting 90days and throwing engineers at the problem means they can't give any average age or expectation. Every expected fragile component was replicated in triplicate, time money and other constraints permitting. They had to demonstrate that any realistic scenario was covered from every angle. They literally had to account for unknown unknowns. Voyager just had a single bit flipped on the OS. The last I heard people think this could literally have been a cosmic ray. A particle moving close to the speed of light managed to hit a particular atom that generated enough energy to change the value in a tiny piece of the computer. --> AND they recovered from that!

I've got to give them credit. Just brushing away their work surviving for much longer than their minimum is not acknowledging their accomplishments appropriately. They had effectively one shot at performing the mission they designed for. Normally estimates for life are fuzzy things with ill defined edges. They polished and polished and got the lowerbound edge to be as sharp as possible. Necessarily that pushed out the definition of the *average* expected lifespan and death estimate. I'm glad I don't have the stress their job must entail, but I wish I could be so specific about the likelihood of death of my software.

Re:Go technology go!

[tomhudson]

Plus, it's not like lasting 25x as long takes 25x as much engineering.

Though next time, they should send them in pairs (a sort of "buddy system"). Have them normally maintain a separation of, say, 50 to 500 feet, but be able to pull each other out of a situation where a wheel gets stuck, maybe have one of them equipped with a feather duster to brush off the solar cells for both of them, etc.

Imagine putting 50 of these down and having them just do a march across the desert ... at 1000 feet separation, even at half speed (1 inch per second), they could cover a square mile every 2 days (or with better batteries, the could do experiments at night). That would be 1,100 square miles in 6 years.

Come to think of it, why don't we do this on the moon. It's a lot closer. With the much less time delay, you could even get people to volunteer with helping log the data, point out interesting stuff, and even pay for the privilege of "driving" a rover in real time.

Re:Go technology go!

[Pharmboy]

To be honest, I had not thought of it as you present it: NASA set the spec for 90 days, and the engineers simply over-delivered. And obviously, we had some good luck for a change with the solar panels staying cleaner than anticipated. Of course, that wouldn't have mattered if the rest of the craft had not been built so incredibly well.

My bad. I stand corrected.

Re:Go technology go!

[fbartho]

No worries. TGIF. Go for in-office kegerator.

Re:Go technology go!

[Morty]

In general, NASA builds most spacecraft to considerably higher spec than required to perform the primary mission. This is a basic engineering principle called safety margin. If you calculate that a bridge needs to handle a load of X, then you build it to actually handle 3X.

Spacecraft that complete their primary missions become eligible to do extended mission, usually at a reduced budget. Most spacecraft that survive their primary mission do end up going into extended mission. The primary mission is the set of scientific observations that spacecraft are funded to do. The extended mission is a collection of observations that we do given that we are already "there".

This might seem odd, but actually makes a lot of sense. A lot of the mission cost is up-front cost -- designing instruments, launch vehicle, ground systems, calibration, systems integration, etc. So build the actual spacecraft a whole lot better than spec to make sure all those other costs don't get wasted if something unexpected occurs. Then, after the primary mission, you find yourself with an incredibly expensive asset uniquely placed to do scientific observations, which has just proven itself capable of providing lots of scientific data -- so do you shut it down, or keep pulling value out of it as long as you can?

Disclaimer: I speak for myself, not my employer or work site.

Re:Go technology go!

[hitmark]

it also shows that well maintained tech can last for a long time, contrary to the "common wisdom" of the consumer culture.

As someone once said...

[PmanAce]

Live long and prosper.

Re:As someone once said...

[WrongSizeGlass]

I think we need a 'Mars Rovers: Battle In The Red Sand' fight to the death match. The victor will get the longevity record. The loser? A nice parting gift of new solar panels and a front row seat to the next MArs battle royale.

Mission spec too low?

[Anonymous Coward]

How low are the specs for these missions are set if it's been operating for 25x longer than it was designed to?

Re:Mission spec too low?

[Pharmboy]

How low are the specs for these missions are set if it's been operating for 25x longer than it was designed to?

The problem is that the original specs were for U.S. standard time, and we now use metric time.

Re:Mission spec too low?

[MyLongNickName]

Some assholes are upset because the sun shines too much.

Re:Mission spec too low?

[Jeng]

No one knew if there would be enough wind to wipe the dust off of the solar panels. That was the limiting factor, it was figured it could go for 90 days before its solar panels would be too dusted to power the rover.

The specs were fine, we just under estimated the wind.

At least that is what I have been told.

Re:Mission spec too low?

[Lord Crc]

How low are the specs for these missions are set if it's been operating for 25x longer than it was designed to?

IIRC they expected dust to settle on the solar panels rendering them useless fairly quickly. That the wind clears them as effectively as it does came as a surprise at the time.

Re:Mission spec too low?

[The Grim Reefer2]

No one knew if there would be enough wind to wipe the dust off of the solar panels. That was the limiting factor, it was figured it could go for 90 days before its solar panels would be too dusted to power the rover.

That, and the homeless martians keep squeegeeing the panels.

Re:Mission spec too low?

[obarthelemy]

never had an asshole sunburn ?

Re:Mission spec too low?

[Anonymous Coward]

On behalf of the Martian Squeegee Collective, Dust Determinate Union, and Undersecretary for Homeless Martians, we would like you to cease all postings regarding to past, or future cleaning events as outlined in the Martian Interplanetary Cooperative bi-laws. Your unsanctioned reference, though cataloged thoroughly on Earth(Planet-3:Sol-1), shall not be referenced in any Martian historical informative.

Regards,

Mars(Planet-4) Public Relations

Re:Mission spec too low?

[Brett Buck]

Well, that's part of it. Just ignore the cleaning events and other factors. They probably figured on 3-sigma dust accumulation rates. That means that 99.73% of the time it's going to be less than that. Meaning that they could launch 1000 of them, and you would expect that 3 of them would run out of power in 90 days. So it is not in any way surprising that they didn't get excess dust accumulation.

    Same thing with everything else. All the .9973 from every identified part/subassembly multipled together is total reliability, and so it's not in any way surprising that these things generally last *much much longer* than the specified mission duration.

    This sort of analysis, of course, presupposes that statistically significant measures are applicable. For one-off or very short-run parts, of course there are no statistics so the methodology can tend to the bullshit end of the plausibility scale.

 

Opportunity will outlast Windows 7

[SpaceLifeForm]

Probably outlast Microsoft for that matter.

Re:Opportunity will outlast Windows 7

[Jeng]

define outlast.

Are you talking about the physical object Opportunity, regardless of function?

Or are saying that Opportunity will function longer than an operating system? Cause that makes absolutely zero sense. How in the hell would you even measure the lifetime of an operating system? By first install to last install? By how long Microsoft supports the OS? By how long the physical disk is usable, and if so in which environment? From boot to reboot? You have left my mind in a boggle.

Re:Opportunity will outlast Windows 7

[dgatwood]

I think there's a good possibility that Microsoft will drop support for Windows 7 before NASA drops support for Opportunity. I doubt, however, that Opportunity will still be functioning when Microsoft declares bankruptcy in 2148....

Reduces black eyes, but readies 'em for a beating

[Dynedain]

This feat ... is healing some of NASA's past black eyes. It is quite remarkable given original spec of 90 days for the mission.

Until some congressional asshat takes a look and argues "NASA builds things to last 25 times longer than specified. Ergo they are spending too much and their budget is 25 times higher than it should be."

Late-breaking news from the Council: PROPAGANDA!

[Anonymous Coward]

In response to a recent propaganda barrage from the Blue World, K'Breel, Speaker for the Council, spake thus:

"It has been seventeen years since the first pair of invaders from the blue planet dug the first trenches into our soil. Seventeen years during which we have waged war and succesfully held them at bay. Three years ago since this pair of mobile abominations landed. One has already been frozen in place forever, and the second is still half a year's drive from the defensive troops currently massing at at End-Devaur crater."

"Our Planetary Land Defense Forces are ready, willing, and able to brave any conditions - even working in soils touched by poisonous, corrosive dihydrogen monoxide - in the defense of our world. Yes, the war goes on, but it goes on to victory!"

When a junior reporter suggested that the first pair of stationary invaders were not the vanguard of a planetary invasion force, but were, in fact, merely passive weather observation stations, K'Breel had the reporter's gelsacs scooped out, mounted at right angles to each other atop the trench-digging invader's antenna mast, and used as an anemometer.

Very remarkable indeed

[digitalsushi]

It's remarkable in that M.O. Scotty tells Geordi in that novel turned episode, Relics:

from imdb:

Lt. Commander Geordi La Forge: Look, Mr. Scott, I'd love to explain everything to you. But the captain wants this spectrographic analysis done by 1300 hours.
Scotty: [thinks about it some time] You mind a little advice? Starfleet captains are like children. They want everything right now and they want it their way. But the secret is to give them only what they need, not what they want.
Lt. Commander Geordi La Forge: Yeah. Well, I told the captain I'd have this analysis done in an hour.
Scotty: How long would it really take?
Lt. Commander Geordi La Forge: [annoyed] An hour!
Scotty: [looks unbelieving] Oh. You didn't tell him how long it would REALLY take, did you?
Lt. Commander Geordi La Forge: Of course I did.
Scotty: Oh, laddie. You've got a lot to learn if you want people to think of you as a miracle worker.

NASA Young Guy: This thing should last for 6 years easy!
NASA Old Guy: Er my young peer means it should definitely last for 90 days. Anything past 90 days is amazing.

Re:Very remarkable indeed

[Chris Burke]

Haha, yeah you're so right, except for how you're totally wrong.

90 days never had anything to do with how long the rover's parts would last. Not a single damned thing. No engineer ever said "I can only guarantee this part will last for 90 days" or anything like it.

90 days was how long they thought it would be until the solar panels were too coved in dust for the rover to function. When the Martian wind turned out to be strong enough to clear the panels, NASA issued a press release and said "We were wrong; yay!" and the mission continued.

And not knowing what the environment on Mars was like is exactly why there was never any Scotty-esque sandbagging, and instead the rovers were simply designed as robustly as possible, with a hypothetical lifespan of many years, simply to ensure they worked at all.

Re:Very remarkable indeed

[Hurricane78]

Are you drunk?

No engineer ever said "I can only guarantee this part will last for 90 days" or anything like it.

Yes, they did:

90 days was how long they thought it would be until the solar panels were too coved in dust for the rover to function.

SAME THING. That is “how long it will last”

I even read a quote by some NASA guy, saying exactly that the 90 days was the time they could guarantee the panels to last in the expected dust.

Re:Very remarkable indeed

[Chris Burke]

SAME THING. That is "how long it will last"

Only in the broadest most general sense, and not one relating to how the rover itself was speced.

There's a huge difference between predicting that a part will only last so long before failing, versus predicting that too much sand will build up for a perfectly functioning part to do its job. There's a difference between saying that a solar panel will fail vs that there won't be enough light for solar panels. There's a difference between saying sensors will only last so long before they break versus saying sensors will only last until they don't have power. Or wheels, or processors, or antennae, or whatever.

The difference is made clear by what happened when the environmental prediction didn't come true: suddenly all talk about time limits ended. There was no talk about how some component designed to only last for a 90-day mission might fail and anything beyond that was bonus. Not the wheels, not the electronics, not the radioisotope heaters, not even the solar panels themselves. Because none of them were guaranteed only for 90 days.

90 days never about anything more than the scientists' expectation of dust accumulation, and never had anything to do with the engineering behind the rover except for the decision to go with solar panels to begin with. It's a pleasant surprise the rovers have lasted this long but completely in line with how they were engineered, and this was never presented differently and so there was no Scotty-esque sandbagging. That's the point. Hope it's clear now.

xkcd's take on the matter

[zorro-z]

http://xkcd.com/695/

Team Spirit!

[liquiddark]

I bought donuts for Spirit's overtaking of the record, I guess I did so prematurely.

Obligatory.

[Adys]

http://xkcd.com/695/ (Still not posted? What has slashdot become?

Re:Obligatory.

[Anomalyx]

Doh, you beat me to it. Nobody posted it, but by the time I found which # it was, it had been posted twice!

Re:Obligatory.

[sadness203]

It was... 4 min before you...

Re:Obligatory.

[blair1q]

Next time you open an MER story on /., do this:

ctrl-F
695
Enter
pause
F5
pause
ctrl-F
695
Enter

then go on with your life because it will be there.

Re:Obligatory.

[PPH]

This one's a bit dated. But perhaps it explains why Spirt gave up

http://www.theonion.com/articles/mars-rover-beginning-to-hate-mars,2072/

It beats the record every day

[opus_magnum]

so what's the point reporting it periodically? It's not like it's competing with something else than its own's.

Re:It beats the record every day

[Anonymous Coward]

It's beat the record of Spirit, which started operating earlier but has now been out of communication for a while.

Re:It beats the record every day

[idontgno]

This breaking news just in: Generalissimo Francisco Franco is still dead.

Every frickin' time.

[CraftyJack]

Stop it. It's OK to have a story about the MER mission without a link to xkcd#695.

Re:Every frickin' time.

[blair1q]

Human beings are not connected by a hive mind. Every one of them has to be told something individually. Even in a broadcast situation, you have to put out enough photons and phonons in enough directions to get the message to all the ears. And anyone who isn't in the room when you do it will cause you or someone else (or a webserver) to repeat the message to them personally.

There. All better. Now go play.

Re:Every frickin' time.

[HybridJeff]

I've never seen that one. Thanks for the link.

Re:Every frickin' time.

[Anonymous Coward]

Eat my shit, you fucking shit-eating piece of shit!

Re:Every frickin' time.

[Hurricane78]

You forgot, that not all people are cattle that do whatever they are being told.
What gave you the idea people would care about a dick screaming “stop it” because of his stupid little brain twist?

Re:Reduces black eyes, but readies 'em for a beati

[blair1q]

And then NASA points at the regulations for parts acquisitions and shows that it's Congress that demanded this sort of reliability out of everything because they were tired of paying for stuff that worked during the demonstration but fell apart after one ride.

reliable tech, eh?

[hort_wort]

This thing has an Earth-based identical twin they use to test situations, right? ... Can we send *that* to go fix the frakking oil leak??

Re:reliable tech, eh?

[CraftyJack]

Leela: Depth at 45 hundred feet, 48 hundred, 50 hundred! 5000 feet!
Farnsworth: Dear Lord, that's over 150 atmospheres of pressure.
Fry: How many atmospheres can this ship withstand?
Farnsworth: Well it's a spaceship, so I'd say anywhere between zero and one.

Does it run Linux?

[filesiteguy]

(Don't answer that!)

Actually I remember in '04 ('05?) that they had to update the software and only gave the Spirit five more months of life.

Re:Does it run Linux?

[Anonymous Coward]

VxWorks, actually..

Re:Does it run Linux?

[shiftless]

(Don't answer that!)

*snaps fingers*

dam

Scotty's Rule

[0100010001010011]

Lt. Commander Geordi La Forge: Look, Mr. Scott, I'd love to explain everything to you. But the captain wants this spectrographic analysis done by 1300 hours.
Scotty: [thinks about it some time] You mind a little advice? Starfleet captains are like children. They want everything right now and they want it their way. But the secret is to give them only what they need, not what they want.
Lt. Commander Geordi La Forge: Yeah. Well, I told the captain I'd have this analysis done in an hour.
Scotty: How long would it really take?
Lt. Commander Geordi La Forge: [annoyed] An hour!
Scotty: [looks unbelieving] Oh. You didn't tell him how long it would REALLY take, did you?
Lt. Commander Geordi La Forge: Of course I did.
Scotty: Oh, laddie. You've got a lot to learn if you want people to think of you as a miracle worker.

Re:Scotty's Rule

[Anonymous Coward]

How long does it take for Captain Kirk to realise you're some guy on slashdot?

JPL

[poly_pusher]

http://www.jpl.nasa.gov/



This success is due to Nasa's JPL or Jet Propulsion Laboratory. The successes they have had over the past decade are astounding. I see this as more proof that remote missions are more practical in the short term as opposed to manned missions. Just give JPL some more money and let them do their thing. These are the guys that will discover what we need to know, so as to make manned spaceflight practical.

As a side note, I saw a documentary on spirit and opportunity recently. It was one of the most entertaining and surprisingly dramatic documentaries I have seen.

http://topdocumentaryfilms.com/welcome-mars/

Re:JPL

[ColaMan]

The successes they have had over the past decade are astounding.

Nevermind the fact that Opportunity has only won the longevity race because a software update to Viking I by (cough) JPL essentially bricked it, by accidentally overwriting its antenna pointing code with updated battery conditioning routines.

I notice in all of JPL's press releases about Viking I that they don't mention this little factoid, just that they lost contact and made it a 'memorial base'.

Re:JPL

[FleaPlus]

This success is due to Nasa's JPL or Jet Propulsion Laboratory. The successes they have had over the past decade are astounding. I see this as more proof that remote missions are more practical in the short term as opposed to manned missions. Just give JPL some more money and let them do their thing. These are the guys that will discover what we need to know, so as to make manned spaceflight practical.

It's also worth noting that JPL is NASA's only FederallyFunded Research and Development Center (FFRDC), a type of organization which is quite a bit more flexible and competitive than the typical NASA Center. The Aldridge Commission from 2004 suggested that NASA restructure and turn all of its centers into FFRDCs, but this proposal was quickly killed in Congress as it's much more difficult for pork to be guaranteed for FFRDCs:

http://govinfo.library.unt.edu/moontomars/docs/M2MReportScreenFinal.pdf

(b) NASA Centers. A second cluster of organizational tasks is to ensure that NASA's ten Centers
and their related field facilities are deployed appropriately in supporting the exploration vision.
Properly engaged, these facilities and their workforce provide indispensable resources and talent.
Centers are also powerful economic engines at the state and local level that should help meet mission
objectives and help grow a robust space industry.
As currently organized, NASA's Centers are not optimally configured to carry out the nation's space
exploration vision. They have Apollo-era infrastructure that needs substantial modernization. They
lack institutional incentives that continuously align performance with the vision's need. Personnel
practices have too often ossified, placing insufficient priority on innovation, professional growth,
and managerial mobility. In some instances, they support duplicative capabilities that unnecessarily
raise NASA's cost to the taxpayers. The Centers, as with the rest of NASA, must also contend
with the reality that a large portion of the workforce is now or will soon be eligible for retirement.
In short, the Centers must be renewed, empowered, focused, and more effectively leveraged in support
of future space exploration and scientific discovery.
The Commission proposes a new model for the NASA Centers. We feel that NASA should transition
its Centers through an open, competitive process, to become Federally Funded Research and
Development Centers (FFRDCs).
FFRDCs provide a tested, proven management structure in which many of the federal government's
most successful and innovative research, laboratory, technical support, and engineering institutions
thrive. NASA's Jet Propulsion Lab is currently so configured, as are the Department of Energy's/QT

Designed for worst case [Re:Go technology go!]

[Geoffrey.landis]

Feats like the Mars Rovers show us that our space-engineering prowess is not only continuing to mature, but indeed getting quite robust.

I don't doubt we are getting better at building equipment that can stand the rigors of space or Mars (better alloys, better lubricants, better electronics, simple design) but this also says something about our ability (or lack thereof) to estimate the durability of the things we build. Lasting twice as long as we expect is a great thing, lasting 25x as long means they were afraid to give a real estimate.

The rovers were engineered to survive 90 sols (Martian solar days) under worst-case conditions.

Turns out that the conditions they actually experience were not as bad as worst case.

Most notably, they were designed to operate in the (solar-energy-positive conditions) of Martian summer. They were definitely not designed to survive Martian winter. The fact that they were able to survive Martian winter is a tribute to, yes, the fact that the engineers overdesigned (partly, the fact that they overdesigned to withstand worst-case summer condidtions that didn't actually occur), partly that components used actually do continue to perform despite being well outside the design envelope (nobody had ever subjected the rechangable lithium batteries to these extreme cycles, until we did it on Mars), and partly by great work on the part of the operations crew. And then, after that, it was due to the fact that Mars cooperated by cleaning our solar arrays.

Kind of like how Scotty says "It will take 8 hours to get it fixed, Captain!", to which Kirk says "You have 2 hours", and yet it still gets fixed. The engineers are likely underestimating the "average" time to cover their own butts.

No, it's not a "cover your butt"-- it's the fact that if you are given a spec of, say 900 Martian days, the review board is going to require that the engineers show test results before launch proving that they will meet that spec-- on most of the components, this means testing to three times the design life. For an environment for which a lot of the conditions are not completely known, and so you'll have to test for worst case conditions. This would balloon the cost up unreasonably.

This actually makes it *harder* to get some science done, as you are scrambling to create new tasks with the extra time you weren't expecting, never knowing when it will give up the ghost.

In some ways this is true-- it would be nice to know how long the mission was going to last, if for no other reason than to know for how long I needed to rent an apartment in Pasadena. For MER, however, the team has had no problems coming up with new things to do with the extra time.

its about money.

[crsuperman34]

NASA will pay for a 90 day mission. 1 day of operating expenses costs x dollars. NASA probably would not pay the operating cost of a 6+ year mission. So you they built these things for longevity, knowing very well the mission would be more than 90 days. Once your on Mars making headlines, who's going to agree to pull the plug...? No one.