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What happens to defunct lunar and Mars rovers?

snow covered rover

What technological barriers prevent humanity from creating a perpetual rover with sun batteries or nuclear portable reactors for use on Mars, the Moon, etc., with daily visual transmissions and scientific data transmissions forever?

If radiation was the main issue, does humanity now have the technology to overcome this?

For example, this device named Zephyr can, in theory, fly forever in the Earth's atmosphere.

                                                                

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    $\begingroup$ Simple answer would be entropy: Eventually, something's going to wear out. I don't think this is a full enough answer, but it is one of the basic reasons. Good question, though. $\endgroup$ – PixelArtDragon Aug 2 '13 at 20:22
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    $\begingroup$ Well, Opportunity is still going strong, so maybe we have ... $\endgroup$ – Mark Adler Nov 27 '13 at 8:36
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    $\begingroup$ Spirit and Opportunity were originally expected to work for only 90 mars-days. It's close to a miracle they survived for so long. Tough little robots, indeed. $\endgroup$ – Philipp Nov 27 '13 at 9:06
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    $\begingroup$ @Philipp - Meh. That was a JPL budget ploy. Those rovers were originally expected to work for a lot more than 90 Mars days. The budget only had ops money for 90 days. What was NASA going to do if they lived longer than that? Tell JPL to turn them off? Rumor has it that that is exactly what NASA threatened JPL with with regard to plans for Curiosity. Curiosity's ops budget is for two years rather than 90 days, and it has a lot less redundancy (many single points of failure) than did the elaborately redundant Spirit and Opportunity. $\endgroup$ – David Hammen Mar 16 '14 at 19:20
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    $\begingroup$ Many years later, a castaway astronaut finds a Mars rover, and uses its remaining battery to make a last radio call. Then, he even crawls into the capsule for samples, and uses it to fly back to Earth ! $\endgroup$ – Nicolas Barbulesco Jun 8 '14 at 14:10
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Temperature shifts

Earth's temperature swings from extreme to extreme is quite moderate. That is, coldest is generally around -40C and warmest is around 40-45C.

The Moon has two weeks in blazing hot sunlight, much hotter (depending on your reflectance) but well over 100C, and two weeks of much much colder. Not quite absolute zero (-273C), but pretty cold to around -173C.

That is a 273 degree Celsius difference. Versus, worst case of 80C on Earth. Things expand and contract a lot more over 273 vs 80. So designing circuitry that can handle the extremes is tough. Most commercial circuit boards will crack due to thermal expansion alone. Welds/solder are very tough to maintain across these temperature variations.

Most rovers and other probes that have an RTG, use it for heat, as much as for power as, to keep them warm overnight.

Mars is not as bad since it actually has an atmosphere to moderate it a bit. The temperature swings are not as bad, but still pretty bad. From a high of 20C to -153C (173 degree vs 273 for the moon).

Clearly it can be done as the current rovers have shown, but it is tricky and hard. It requires hibernating and using battery power to keep the heat going for some of it.

Radiation

The moon, lacking an atmosphere does not have much to protect from solar radiation. Mars has a bit better having a thin atmosphere.

Distance

Stuff breaks. Nothing lasts forever. Grease needs to be replaced eventually. With no way to fix it, more redundancy is required. However, because launches are so expensive, you are limited by mass.

Dust

Both the moon and Mars have surprisingly fine dirt that seems to get into and onto everything. The Apollo astronauts had trouble getting it off the space suits after the moon walks. Mars also has a lot of dust, as the various rovers have shown.

This stuff destroys moving parts as it manages to get in there somehow.

If you use Solar arrays for power on Mars, it also covers the arrays reducing power, as the wind deposits it. The Opportunity/Spirit missions found that the wind both deposited, and occasionally cleaned off the dust.

Now you need a solar array cleaning mechanism to maintain power, and yet another mechanical component to break, far away from anyone to fix it.

Battery lifespan

Rechargeable batteries have finite lifespans before they stop holding charge, or before it degrades to a level too low to survive.

Bearings

Moving parts can be affected by the dust, but the bearings and some kind of grease will eventually wear out.

Funding

While not a technological limit per se, when funding runs out to run the program, even if the vehicle/orbiter/probe is still working, it may have its life cut short by program management.

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    $\begingroup$ Re: Grease - I'd say bearings in general. $\endgroup$ – Deer Hunter Aug 2 '13 at 20:41
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    $\begingroup$ Bearings are all well and good, until the dust kicked up by the rover moving around gets lodged in a bearing. Now the rover will intermittently jam up, and when there's a 15-minute round-trip delay from here to Mars, the jiggles the operator will take to get the rover unjammed can eat up an entire day or more. Any bushing or bearing is only going to last as long as its seal against dirt will hold, and that seal is going to be relatively short-lived in an environment with almost 200*C difference between sunlight and shadow. $\endgroup$ – KeithS Aug 2 '13 at 21:37
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    $\begingroup$ Add batteries to the mix. Most accumulators have up to ~3000 charge-discharge cycles; either you kill them eventually or hibernate without any power through the night. There are designs that are resistant to most of these problems (think hamster ball) but then their usefulness is limited. $\endgroup$ – SF. Aug 4 '13 at 22:17
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    $\begingroup$ Money. I think we can already make rovers that outlast our attention span, as measured in funds for continued operations. It is mind boggling that NASA is considering the possibility of shutting down either Cassini or Curiosity due to lack of funds. $\endgroup$ – Mark Adler Nov 27 '13 at 8:41
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    $\begingroup$ @KeithS: The jamming problem could be partially mitigated by having on-board software detect the jam and jiggle the rover automatically. Good point otherwise, though. $\endgroup$ – Keith Thompson Nov 27 '13 at 16:31
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In addition to Geoff's answer, the comment by Garan contains a major reason why things stop working eventually. Any machine with moving parts is subject to wear and tear. You can keep a machine going for very long if you maintain it; look at the steam engines in various industrial heritage museums for instance. These machines are/were monitored constantly by a man with an oil can, and parts were swapped out or repaired when needed.

A planetary rover is a whole different ballgame: we can't maintain or repair it. Once a part malfunctions, it's gone for good. With careful design, you can add some redundancy. Every rover we've built has at least two main computers: if one fails, the other can take over.

For mechanical parts, this is more difficult. Wheels for instance, are a major point of failure: the wheel itself gets damaged, and there are lots of sensitive moving components (see Geoff's comments on bearings). If you want to be able to jettison a damaged wheel assembly, you're adding more moving parts and more chances of failure. Adding the mechanics to replace damaged parts makes things even more complicated, making the rover larger and heavier, which quickly makes the project more expensive.

The power supply has a similar problem. Solar panels, radiothermal generators, fission plants all decay eventually. Solar panels are down to ~70% of their initial output in 25 years. In an RTG, the thermocouples become less efficient over time. Even on Earth, we have to decommission fission plants eventually due to corrosion and decay of critical components. RTGs and fission plants also run out of fuel eventually.

In addition to the technological barriers, there are two major reasons not to build rovers for a very long lifespan:

  1. Cost. Adding redundancy adds weight and complexity, and increases the launch cost. A Rolls-Royce can be counted on to remain operational for far longer than a Ford, but the Rolls costs enough that you can afford to buy ten Fords vs. one Rolls.

  2. New science. We've only been doing this for a few decades. Every planetary mission has new and unexpected findings. It makes sense to run a rover for a few years, and then sit down and examine what it has found that warrants further study with new, improved instruments. One long-lived rover limits you to one set of instruments. Science advances quickly enough that each rover we've built is vastly more capable than the previous one.

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  • $\begingroup$ Not sure about the Ford vs Rolls Royce comparison - what's the cost of a Rolls-Royce rover compared to the launch/spacecraft required to put it on mars? I doubt you could launch 10 Ford rovers for the same $$ $\endgroup$ – user2813274 Oct 12 '15 at 3:29
  • $\begingroup$ Your #2 point is something I never thought of. Probes become obsolete, and a new one will do more for less. $\endgroup$ – dex3703 Jun 13 '17 at 22:05
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Yet another reason is how much data you can squeeze. A rover is a very limited lab, and after some time, it has gathered all data it can from a specific area. They are slow, so travelling to far away lands is very impractical.

Curiosity, the most advanced rover, has a mean speed of 1 cm/s on flat land. It takes more than a full day to travel one kilometre on ideal conditions. On the actual surface, you have to avoid rocks and climb up and down hills; that will probably reduce the speed by a rough factor of 2. My hometown is about 200 km away from the next geological unit, so it would take Curiosity more than a year to just get there, without stopping to do science. So far, including scientific experiments, it has travelled about 5 km in almost two years, so the actual mean speed is less than a millimetre per second..

Of course in the same area there are different kinds of rocks, and it takes a long time to fully sample an area, and you never know when you are going to find a new thing. But even then, the investment necessary to get the next big new thing is huge (because it is far away). And, by the time the rover has gotten there, we could have sent a new one, with better more sensitive equipment, to another part of Mars looking for exciting more new things.

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  • $\begingroup$ I am not sure how this addresses the question(s). I gives opinion on why would not want to try, but does not address the "technological barriers" to doing it. $\endgroup$ – James Jenkins Jun 11 '14 at 13:22
  • $\begingroup$ A journey around a world, begins with the first move. $\endgroup$ – James Jenkins Jun 11 '14 at 13:23
  • $\begingroup$ @JamesJenkins almost all engineering problems can be solved if we put enough time and resources on solving them (for example ceramic bearings can reduce tear and avoid grease). My answer is a reason why we don't try to do it in the first place. We need much better rovers for it to be worth. $\endgroup$ – Davidmh Jun 11 '14 at 13:33
  • $\begingroup$ Our conversation here inspired this kind of related question space.stackexchange.com/questions/4778 $\endgroup$ – James Jenkins Jun 11 '14 at 19:07

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