Mars Sample Return Options

[SteveM]

AFAIK MSL is unlikely to be still around in the 2020 timeframe - RTGs can't be cleaned by lucky gusts of wind.

AFAIK, dust will have little effect on the operation of an RTG (it may hinder the cooling of the RTG but that shouldn't cause major problems) and Pu238 has an 87.7 year half-life.

Steve M

[Stu]

I clicked on this link because I was naturally excited to hear a local firm is involved in a Mars mission. Iassumed, before reading, it was related to MSL... turns out it's an ESA sample return mission...?

http://www.cumberland-news.co.uk/workingto...rPath=business/

[ustrax]

It looks like they're talking about ExoMars there Stu...

[Stu]

I guessed that. I was just surprised by the details about the "sample return" plans.

[Hungry4info]

AFAIK, dust will have little effect on the operation of an RTG (it may hinder the cooling of the RTG but that shouldn't cause major problems) and Pu238 has an 87.7 year half-life.

Steve M

While we're on the subject, assuming MSL lands safely, and some point through the mission doesn't get obliterated by an asteroid or fall down a cliff or whatever else could unexpectedly end the mission, what is the limiting factor on the lifespan of MSL? What about useful lifespan? I sometimes imagine after a 5 or 10 year mission, a crippled rover staying stuck in one spot, but alive, for 20 more years.

What will limit the lifespan of MSL? RTG energy output?

[helvick]

What will limit the lifespan of MSL? RTG energy output?

At this stage I'd be willing to bet that the most likely limiting factor will be budgetary constraints or simple disinterest on the ground because there will be more profitable things to spend the money on in terms of supporting active assets on Mars at that point in time.

No matter what amount of power the RTG is actually able to produce I think that it would be highly improbable for MSL to ever "go dark" because it ran out of juice. Mechanical or electrical failure might kill it, dust build up on sensors\cameras\antennae might render it useless and it might even be driven off a cliff (!) but once it lands the most likely end of mission scenario for it is going to be some human on the ground deciding to turn it off, or at least turn off the things on the ground that listen to it.

The Pu-238 in the RTG has a half life of 87.7 years. However the RTG hardware itself degrades over time aswell so looking at the Voyager RTG's as an example of a mission tested over a sufficiently long period of time the evidence is that an operational RTG of this general type has an effective half life of around 71.5 years.

The MER's initially had an expected power output of 700-900 watt hours / Sol and we initially believed that they would rapidly fail\die once power levels dropped below about 280 watt hours / sol. As it happens while the 280 watt hours per sol number was the minimum value needed to operate the rovers as they were initially intended to be operated, the clever engineers\drivers\designers\PI's and the rest have proven that they can be kept alive (with human intervention) more or less indefinitely provided power levels can be kept above 180 watt hours / sol. It seems reasonable to me to conservatively estimate that they improved on the efficiency of the design by about 35% just by being smart about how they operated the rovers when you just look at it in terms of survivability relative to available power.

Since MSL is known to have a dependable power source the design will certainly not include the sort of 3x power margins that the MER's had to have built in (in case of dust storms in the first 90 days, worse dust deposition than was actually seen etc) but I'd still expect that during the mission the MSL team will learn to operate her more efficiently and it would surprise me if they failed to find at least 50% of the sort of savings that the MER's have found. So let's say they can only realistically expect to "find" 17% savings in terms of survivability vs available power over the lifetime of the mission.

Also lets assume that they build in zero power margin for the end of primary mission - ie MSL is intended to end its first Martian year generating precisely as much power as the design says is the minimum survivable level (equivalent to what we all thought was 280 Watt hours for the MER's way back when).

Assuming the RTG's have not improved at all since the Voyager days and that operational brilliance can only eek out another 17% margin over the designed minimum power demands of MSL then it might live for an additional 19 years or so before its power levels fell below 17% of the minimum designed for the primary mission (assuming the above mentioned 71.5 year effective half life for the RTG).

And I think I'm being conservative there. So back to my original point, EOM for MSL will be somebody down here turning it off not MSL running out of juice. Assuming she lands safely and all that, that is.

[mcaplinger]

No matter what amount of power the RTG is actually able to produce I think that it would be highly improbable for MSL to ever "go dark" because it ran out of juice.

I think battery life is still a significant limiter on the mission lifetime. That's what ultimately killed VL2 and would have killed VL1 had it not been miscommanded first.

Also, there are many factors in RTG degradation, not just the Pu decay. See "DEGRA : a computer model for predicting long term thermoelectric generator performance" at
http://trs-new.jpl.nasa.gov/dspace/handle/2014/38760

[dvandorn]

Exactly -- recall that NASA tried to turn the ALSEPs back on in the early 1990's and none responded. The best theory as to why none responded was that the voltage being generated by the RTGs, less than 25 years after emplacement, had fallen below minimum levels for operating the transmitters.

-the other Doug

[helvick]

Battery lifetime wasn't something that I'd considered. The LiIon batteries for MSL and the power management\battery control subsystem will be pretty similar to those used on the MERs so the best data we have on that is the MER experience, I'm pretty sure they are actually running longer than the longest tests of those batteries so the best we can say is that MSL's batteries should at least be able to match the MER's. The detailed engineering performance data on the MER batteries should give a much better idea of precisely how long they can be expected to last but the only reports that I can find online for free say that the MER batteries had degraded by ~10% in terms of charge capacity after 30 months of operation. With an RTG power source the management of the battery lifetime on MSL should be easier than on the MER's but it's pretty much guaranteed that the mission planners will be driving the batteries hard during the primary mission, at least I certainly hope they will. So 5 years with the batteries still capable of supporting mobility is pretty much guaranteed.

Thanks for the Degra link Mike - I'd just (stupidly) pulled some data from a source that said the Voyager RTG's were delivering 80% of their initial power after 23 years. I should have thought that was a bit off. The report you link puts the actual power drop off at ~30% after 30 years which is more or less equivalent to an effective 40 year half life. Using that as a better model MSL's RTG should degrade to 17% below the planned EOM power levels after 11 years.

In any case she will have more than enough power to keep running in some fashion for a very long time after the end of the 1 year primary mission.

[Stephen]

I clicked on this link because I was naturally excited to hear a local firm is involved in a Mars mission. Iassumed, before reading, it was related to MSL... turns out it's an ESA sample return mission...?

http://www.cumberland-news.co.uk/workingto...rPath=business/

"Workington engineering firm Gravatom will help design a lab to hold rock samples from the red planet, following a planned trip in 2016."

Would I be interpreting that correctly as somebody contemplating an MSR mission for 2016? Or has somebody simply made a typo (for 2018; or 2026)?

======
Stephen

[Mariner9]

Quoting the article:

A rocket will be launched that will send a rover to the surface of Mars to drill into the surface and test for any evidence of organic material.

A subsequent ‘Mars sample return mission’, involving all of the major world space agencies, will then be launched.



How I read that is the 2016 mission would be only a rover, and that that rover would have a sample cache on it.
The subsequent mission would be the MSR, launched at a later date.

Now, given that MSR has been studied in different forms for well over 20 years (including joint missions with ESA and or the French), I don't think this is anything close to a sure bet.

[vjkane]

Now, given that MSR has been studied in different forms for well over 20 years (including joint missions with ESA and or the French), I don't think this is anything close to a sure bet.

see http://futureplanets.blogspot.com/2009/02/...server-and.html

[Fran Ontanaya]

Wouldn't it be more interesting to thoroughly analyze and understand the martian dust background and ice background to remove that noise and provide a 'blank' profile for the hundreds of in situ analysis? I.e. if the clays, carbonates or perchlorates that Phoenix detected ever go airborne, knowing about them beforehand would have been useful to design the TEGA and MECA instruments.

What could be the cost of landing on top of Olympus Mons, sit idle collecting atmospheric dust and ice, and send it to LMO --including lower biohazard costs, a smaller sample, and simpler sampling hardware--, as opposed to a fully fledged MSR mission?

[stevesliva]

What could be the cost of landing on top of Olympus Mons, sit idle collecting atmospheric dust and ice, and send it to LMO --including lower biohazard costs, a smaller sample, and simpler sampling hardware--, as opposed to a fully fledged MSR mission?

Landing with that little atmosphere is tough. Volcanic rock is also probably way down the list of rock types they want.

[Stu]

This week's Carnival of Space (the 101st) is being hosted at the "Robot Explorers" blog of writer David S.F. Portree. http://robotexplorers.blogspot.com What's that got to do with MSR? Well, David's site has lots - and I mean LOTS - of very detailed info about MSR missions that have been proposed and dsigned over the years. If MSR is an interest of yours, go take a look.