http://www.oobject.com/category/cars-in-space/

I think you know what to do Yeah - the LRV's were cool - but they did the job they were designed to do. MER has done that, and then some.

MERFTW (and Stooke - none of that Lunar nonsense )

Doug

Duh - what? What did I do?

Phil

I think we're supposed to vote for our favorite rover. It took me a few minutes to figure that out... wasn't immediately clear what the site was about

Ehhhh....some of those rovers appear to be infested with sapient biological entities

Shhh....
The official Party line is to vote for Unmanned cars

Well - we've put the UMSF 'founding sisters' at the top - good work team

There are some rovers missing PROP-M and Minerva

This little one is quite significant but is not there either :

I will disagree slightly with Doug here on his elevation of the MERs because they've outperformed their design criteria.

For one thing, the MERs were *not* designed to complete the 90-day primary mission. They were designed to last far longer, per Steve Squyres in his memoirs. According to the best thinking and best knowledge of Martian conditions (based on some hard data from other landers), the MERs were designed to operate until the dust accumulation on the solar panels drove power levels below a sustainable level, and that period of time was reliably determined to be somewhere between 200 and 400 sols. According to everything we knew about Mars, there should have been no way for the MERs to last longer than that -- surviving a Martian winter, for example, was considered an impossible task, preflight.

We found out about cleaning events when the girls were getting into some pretty power-strapped states, though to their credit they had lasted longer at that point than was predicted. The periodic cleaning of the solar panels was an unforeseen boon. The MERs would never have lasted as long as they have had not Mars provided them with an unpredicted gift of cleaning winds.

So, the extreme longevity of the MERs is more due to serendipity than to their admittedly excellent designs.

And, to be fair, because they have lived far longer than their creators ever intended, they're both showing signs of old age. Instead of dying gracefully under predicted dust accumulation conditions, they are developing bum wheels, arthritic joints, blurred vision and low-energy listlessness. They are more like denizens of a senior citizen's home than noble explorers by now... which makes every byte of data we get back from them that much more sweet.

I'm impressed and pleased by how much information our intrepid rovers have amassed. But I don't credit the engineers who built them, or even the rovers themselves, for their great longevity. I view *that* as a series of gifts from the gods.

-the other Doug

The wind has made it POSSIBLE for them to live this long. However - that's only an enabling factor. The rovers have still been using that power for barely short of 20x what they were asked to. 20x more wheel revolutions. 20x more camera mast movements. 20x radio transmissions. 20x charge cycles. 20x thermal cycles on all the electronics. 20x more arm movements.

Yeah - they're getting arthritis - but to say it's all down to luck that they're still around is grossly unfair and inaccurate. It's down to damn good engineering and a lot of hard work - designing them, building them, and keeping them alive on Mars.

Yep. We've seen all the nasty conditions our beloved machines have had to endure over the years on Mars. I would suggest that their excellent design and construction has permitted the rovers to take advantage of the prevailing winds and other potential advantages they have encountered over the years. You can't plan for all of the specifics, but you can hedge some of your bets...

I'm not trying to belabor a point, here, but it's the "20x" statement that's specious. The MERs have *NOT* performed 20x longer than they were designed to perform. They have performed 20X longer than the primary mission, but they were designed to perform for far longer than their primary mission. Again according to Steve Squyres, when his team figured out how to get the extra solar cell strings onto the rovers, he knew he had a pair of vehicles designed to last not just for 90 sols, but until dust accumulation killed them -- which they thought would be at least 200 sols, and with some luck perhaps as much as 400 sols.

They were *designed* to last far longer than 90 sols. To say that they have outperformed their design lifetimes is valid -- to say that they have outperformed their design lifetimes by a factor of 20 is specious. The factor that they have outperformed the lifetimes *actually* envisioned by their creators is more like 4x to 8x. Still impressive -- but not 20x.

That's all I'm saying...

-the other Doug

EDIT: It also just occurred to me that you also have to ask at what point does reduced functionality cause you to say that a vehicle has stopped performing, at least in a given area? Neither MER is currently operating up to spec -- Spirit is so power-starved she's a barely-active rover, and has had no RATting capability for quite some time now. Neither mini-TES is working at the moment, though my understanding is that Oppy's may be revived at some point. The radioactive sources for the APXS and Mossbauer have faded so far back that integrations that used to take a few hours would now take many, many sols, and the quality of the results goes downhill as time goes on. Has a MER that is at, say, 50% of its design operational capability actually still outperforming its design lifetime? How about 20% Or less? Do we consider a MER that can take a single image a month and can no longer move a still-operational MER? Just wondering... dvd

I guess the question about "operational" is, Is the rover still returning science data? I think there's a parallel debate going on in the Hubble thread right now. Are pretty pictures scientifically useful data? I would say yes, if nothing else than to show change over time.

Some people need to step away from the keyboard from time to time, seriously. I thought I'd post a link to a fun poll we could vote on.

Next time, I wont bother.

Mossbauer, yes. APXS, no. MUCH longer half life for its two sources - 18 years for one source, 6500 for the other.

Stating the APXS is faded significantly is entirely untrue - so I thought it worth correcting.

Um... 18 years? The half-life of cobalt-57 in Mossbauer is 271 days, if it were 18 years the drop in gamma output would be barely noticeable by now.

Cobalt-57 in the Mossbauer - it has dropped - significantly. That bit I agree with. The APXS, however, is a different story.

It's Curium 244 and Plutonium 240 in an APXS. 18.11 years and 6561 years respectively.

(further googling shows that the MER APXS design just included Curium. The Plutonium part was dropped during the derivation from the Sojourner design)

I found a simple half-life calculator that suggests the Curium would be at about 82% of what it was at landing.

The Mossbauer, however, is down to about 1%.

Doug

I stand corrected!



-the other Doug

On the subject of electric power - I remember in the early days of the MER mission there was quite a bit of speculation that the limiting factor for the rovers' survival would be the batteries' ability to hold a charge. I believe some of this talk came from the JPL site and from the press conferences where the most authoritative sources in the program were taking part. I accepted the basic idea readily enough based on personal experience with car batteries in which a "36-month" battery never seems to last much beyond the 36th month. For laptop computers the deterioration is usually pretty noticeable after a similar interval. OK, the JPL engineers are not using the same kind of batteries as those I use day-to-day, but if they failed to anticipate wind gusts cleaning the solar panels, why did they include these overengineered super-batteries? And if they did deliberately engineer the batteries to outlast the solar panels, why was battery death such a frequently-cited possible cause of rover death? I recall discussions about "when the batteries die can the rovers continue to function on direct solar power?"

In the midst of Oppy's recent preparations for the multi-year trek to Endeavour crater there has been little mention of battery life as a limiting factor. Why has this now faded as a source of concern?

I don't ever remember battery life being mentioned as a limiting factor. The JPL tech report server has a paper that looks at the battery life, and whilst it is showing a drop off, it's not massive.

Doug

I think the discution was about number of battery cycles (charge and discharge?). Does it make sense to you?

Yes - I know the sort of issue you're talking about. But I don't think it's ever been discussed as a potential mission-ending scenario for MER.

Hmmmm.. my admittedly imperfect memory does recall something along those lines. Something about 2,000 cycles being what the MER batteries were rated for. As I recall, it was discussed in the same context as the death of Viking 1, where the batteries were actually deteriorating, their shortening charging cycles were creating a low-energy state, and the attempt to rework the charging cycles via a software update resulted in overwriting antenna pointing information in its limited memory and thus broke off its ability to communicate.

Anyone else remember that discussion? I'm positive it had to be here on this forum somewhere...

-the other Doug

Looking at the JPL tech report server - they cite 3000 cycles before they reach 50% of their initial capacity.

Thanks, Doug. I remembered it was in the single thousands.

The battery system for the MERs reminds me of the axiom that you always prepare to fight the last war, not the next one. Everything on Mars Pathfinder was working pretty well, when the battery system started having problems. It died from battery failure before dust accumulation could kill it. I can just imagine the MER designers, with far better battery technology available, said to themselves "these things won't die because their batteries failed!"

-the other Doug

My recollection was that we (well I at any rate) had believed that the batteries had an expected charge\discharge lifetime in the 1000-2000 cycle range but when I went digging on NTRS and at Lithion (who made them) what I found was that the testing of the specific battery type used on the MER's had taken them out to about 2300 full charge\discharge cycles and at that point they were still performing well above the levels that were deemed effective EOL. There is a continuous degradation of total charge storage capacity with Lithium ION (and all other batteries too) over time but it follows a fairly well understood profile and it is directly related to the level of charge stored so if you maintain your battery at some level below 100% fully charged you can trade storage capacity for a longer overall useful cycle lifetime.

One other thing to remember is that the batteries would generally degrade gracefully (in terms over overall Amp hour storage capacity at full charge) and not die as such and while there is also a voltage drop off (and once they fall below a certain voltage level they are effectively dead) that drop off is very slow for these batteries. The other thing to remember is that the MER batteries don't always go through a full charge\discharge cycle every Sol either so you can't just simplistically say that 1000 cycles == 1000 Sols. And to further complicate matters the Rover team know quite a lot about how to keep the batteries healthy in terms of what levels of charge are optimal (in terms of life span) so when they had the option I'm sure they considered keeping the charge levels in the batteries at levels that would improve overall lifespan when they had the option.

This report has some interesting nuggets - http://trs-new.jpl.nasa.gov/dspace/bitstre...9/1/03-1648.pdf . The point that I find most relevant in this is that they say in this that the 28amp hour batteries of the type used on MER were intended to be used on a number of missions and that a 3 year / 900 cycle operational lifespan on Mars was the intended target for this class of battery. It also mentions that even though the testing had not been completed that they intended for this type of battery to be able to handle 30,000 charge\discharge cycles at 40% capacity (for use in LEO Satellites that go through a very rapid "Day") before expiring so I suspect that the MER's batteries really do have a lot of life left in them yet.

I googled "MER battery life" and turned up this thread on the baut forum dating back to 2004.

http://www.bautforum.com/life-space/14277-...ttery-life.html

The essential facts as quoted from a now defunct NASA site were:

MER Battery Characteristics:
16 Li ion cells of 10 Ah
Two parallel strings of 8 cells each.
Energy: 600 Wh
Voltage : 32 -24 V
Capacity: 20 Ah at RT
Operating Temp Range: -20 C to 30 C
Cycle Life: 1000 cycles
Calendar Life: 5 Years

Of course I can't vouch for the accuracy.
Maybe NASA was spinning to lower people's expectations when they put this out.

The "known unknowns" still include:

The expected shape of the decay curve. How much warning to expect when the batteries are getting tired?
The effect of incomplete charge cycles. Sometimes complete cycling helps extend battery life, doesn't it?
The effect of the cruise phase. Long periods of infrequent use - are they better or worse than regular cycling?
The effect of temperature. What is the minimum battery temperature on winter nights? Does deep sleep mode matter?

Most of your four points are covered in the various JPL tech reports that discuss the batteries.