I was wondering: since (especially Spirit) dust accumulation isn't a big concern anymore nowadays, we come to the next vulnerable part of the rovers, being their batteries (which has been discussed some months ago in another thread).
I read that it could take a 1000 load-reload cycles before their capacity declines. Now, suppose that the batteries are history by sol 1000 and they don't have any capacity left: then what ?

Would the rover be dead ? Or would it boot up at sunrise again every morning and be able to do any science or driving solely on the array output ? And could this mean that not even the batteries are a possible fatal malfunction (and thus, the rovers could still continue to stay in touch) ? In that case, this mission could continue well beyond any expectation we all have. A potential fatal breakdown of a cricial (not redundant) part could take many years litterally. Especially thinking about the semiconductors that are so tough, cabling that has been made for the environment up there, everything prooved to be so sustaining, that it gets quite hard to think of anything that ends it right away. They work, they prooved to work for muuuuuuch longer than expected and there's no RTG that can run out of energy.

I want MSL to have arrays. And i want it to land in a windy place.

I'm pretty sure I remember reading somewhere at the start of the mission that it is possible for the rovers to operate on solar power alone, and just turn on when there is enough sunlight to power them. A dead battery most likely will increase the thermal cycle loads on the rover however, since the rover will not be able to operate any heaters at night. Pathfinder was able to last only a few days after its battery died. The rovers might be able to last longer, since they have already been through several cycles of deep sleep without any major ill effects.

A new technology, nanotubes carbon will improve the solar array power. It will be ready very soon. However, the solar array is only good for low altitudes. Now the land requirement for rover with windy places is now no longer necessary since we have learned the importance to have incorporated fans to blow off the dust from the solar array.

The solar array has a big advantage against the RTG: Very cheap, less weight, and not to worry for everything with the radiations.

Rodolfo

BUT....

it required lots of surface area, doesnt work at night, unless you're lucky will trend to lower output over time etc etc

Just a 100W RTG will give you 2400 Whrs per sol - 3 x the average for MER, and it'll give you that power whenever you want it and essentially, indefinitely.

Doug

Essentially "Deep Sleep" is equivalent to operating without batteries since they are disconnected from the bus. The rover automatically switches on when current from the solar panels starts flowing. So, yes operating without batteries is possible, but it definitely is a "degraded mode". No operations at night (or dawn or dusk), i e no overnight integrations and no nighttime comm passes. Also power consumption can never exceed solar panel output even momentarily.

tty

Good to hear your comments. How can a rover drive during the night. It is not worth to drive during the night. There will be lots of missing pictures thru during the night driving? Maybe, the MSL will have ones alike of off-road lights. Just a joke

Does the rover driver thrust the "navigation and hazard-avoidance software" for night driving? I doubt it after the Oppy was saved from the Divine Purgatory Dune.

With 3 times more energy per sol, the limitation factor is of one IDD that does not permit to work in a multi-tasking mode. On the other hand, the MSL rover will not travel faster than MER but about the same. The MSL rover will live about the same as ME with one Mars year and perhaps 1.5 with the new extensions...

But, knowing that MSL will bring up to Mars lots of science instruments:
1) Mars Science Laboratory Mast Camera,
2) Laser Induced Remote Sensing for Chemistry and Micro-Imaging, IDD?
3) Mars Hand Lens Imager,
4) Alpha Particle X-Ray Spectrometer, IDD?
5) X-Ray Diffraction/X-Ray Fluorescence Instrument, IDD?
6) Radiation Assessment Detector,
7) Mars Descent Imager,
8) Gas Chromatograph Mass Spectrometer/Tunable Laser Spectrometer, IDD?
9) Pulsed Neutron Source and Detector, IDD?
10) Meteorological Package with Ultraviolet Sensor

Hope that the design will not put too many science instruments into one IDD.

The nuclear power RTG from New Horizons weight around 56 kilograms and produces 180 Watts/hour. That is much weight in trade for worth science instruments. The MSL case might take around 37 kg of weight and it will feed electrical power to lots of science instruments and so it needs lots of electrical power. That is ok.

Finally, the planned spacecraft which will bring MSL will weigth 3,000 kg (bigger than MRO by 1 TM!!)

Rodolfo

RTGs do not work for infinite time. Their power decreases exponentially. The MERs have this problem with the radioactive sources which power the AlphaXray detector and another instrument, which now need several nights to achieve a detailed integration. Yes RTGs can last many years, but they are not perfect. And once he power lost, there is no hope to recover.

A spin wheel may authorize a RTG to accumulate energy and have a much higher peak power. But this is heavy too.

At last RTGs emit strong radiations, I once saw the radiation map of Cassini, it was dreadful, with neutrons and all, electronic parts have to be hardened. For this reason the RTGs of Cassini were at the end of a long mast, but this cannot be done aboard a rover

This is not to dismiss RTGs, but to say they are not a perfect solution.

Eventually an improvement of RTGs would be to load them with a low-energy isotope which decays in a stronger isotope. I do not know if we can find such a pair of isotopes, but if yes we may have a RTG with a low power at launch, and at maximum power at mission time, in place of wasting power during interplanetary flight.

I know all that - (actualy, it's Mossbauer, not APXS that's suffering) - I didnt state it's infinite -I said it's effectively indefinitely. i.e. you're going to have many years of guarenteed high power.

Doug

When they quote "1000 cycle life" , this would be when the batteries are expected to drop below some pre-determind specification like 80% of new cpacity. They could well last a few more thousand hours before they drop below even 50% of new capacity unless of course they develop a catastrofic internal short.

RTGs emit almost no radiations outside their own casing. The Plutonium-238 that is used is chosen because, among other properties, it is an alpha emmitter. Apart from the heat produced by it, it would be able to be handled with cotton gloves as essentially the only protection

The plutonium is primarily an alpha emitter, but there's spontaneous fission, I'm reasonably sure, with neutron and gamma radiation. After all... that''s why Cassini's RTG's are a radiation hazard.

RTG's also provide a *lot* of heat. You can pipe that heat around to keep electronics boxes nice and warm, even at high latitudes in the winter, if you're otherwise designed for that. Viking 2 kept operating with water frost all around and predawn temperatures close to C02 freezing point... and it wasn't designed for that.

RTG decay is both due to the half life of the isotope and due to radiation damage of the thermocouples that turn heat gradients into voltage.. but it's slow.. Voyagers are going to be getting power starved in about another decade and maybe too low to operate instruments around 2020, though sun tracker problems or attitude control gas depletion is a real problem by then too, if not fatal.

The Mossbauer Cobalt 56 (I think) has a half life around 2/3 of a year... it had already decayed significantly by landing and they're 2 half lives below landed levels now.

The spontaneous fission rate of Pu-238 is very low, I found figures of 2000/gram/s, so neutron and gamma radiation is almost nonexistant. It was considered so low that it was used for some time in mini-RTGs to power human heart pacemakers. The reason that Cassini's RTG's are a radiation hazard is the danger that they may break open and release finely dispersed Pu-238 which couild be inhaled. Alpha emmitter are extreemly dangerous internally because the dead layer of skin that absorbs most external alpha particle is not there for protection and they cause a lot of biological damage.

This is as likely as $20 notes changing into $100s in my wallet

Funny it came to the RTG discussion again. I did not say i want arrays only on MSL. It must have a beefy RTG without doubt (payload needs this kind of power). I was thinking it might give the mission a really long extension after RTG is exhausted (assuming cleaning events are frequent), so it could continue even longer than with RTG only. But it is probably not easy (if not: impossible) from a weight budget/complexity point of view. Not to speak about the timeline (design is quite detailed allready).

Well - look at Voyager. 25 years + with an RTG.

I really really cant see how a solar array could help extend that on Mars.

Doug

I guess you're right about that. Don't know if the "half lightbulb" of energy Voyager still has to consume is in any comparison to what MSL needs. It all depends on the time the RTG takes to get exhausted in relation to the payload thats connected to it and needs a minimum amount of energy to operate nominal. If MSL can still operate after 5 years, then there's no need for arrays and my point should be regarded as not posted. If, however, it can exhaust before that, then solar power might be usefull after all.

Ah, what the heck, there's rumours that it might not be up there for another 8 years. Damn.

See here the properties of Pu238 http://environmentalchemistry.com/yogi/periodic/Pu-pg2.html
it has spontaneous fission, so that it emits neutrons. I remember well, having somewhat involved in Huygens, I was showed this map of Cassini radiations around the RTGs, for purposes of evaluating electronic components. Maybe the neutron flux is not hight, but if you multiply by the mission duration, it becomes a stress to be considered for electronic parts. (and also for the staff in factory, as we cannot protect ourselves from neutrons).


By the way, PU238 has a half-life of 84 years, so my argument falls short.



Also Pu238 decays in U234 with a period of 245,500 years, so it is a long-lived nuclear waste. I know we are not living in Mars for long, but many things may happen in 245500 years...


With my opinion, we could choose shorter lived isotopes, which can wield more power. But they are perhaps more expensive or uncommon.

This is a good article to read, notice the photo of the technician standing next to the RTG without radiation protection

http://en.wikipedia.org/wiki/Radioisotope_...ctric_generator
A quote from this article
"238Pu has become the most widely used fuel for RTGs, in the form of plutonium oxide (PuO2).238Pu has a half-life of 87.7 years, reasonable energy density and exceptionally low gamma and neutron radiation levels."

Spoiler for ABC's 'Lost':














Use of an RTG would solve the mystery of the 16 year long radio broadcast