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The Mars Spirit and Opportunity rovers exceeded expectations by working for many years past the planned 90 day mission. Why was a Nuclear Battery System not used for power generation, instead of solar panels?

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    $\begingroup$ related but not duplicate space.stackexchange.com/questions/1270/… and discussion of risks space.stackexchange.com/questions/17/… $\endgroup$ Oct 1, 2020 at 10:47
  • $\begingroup$ This is a good question. There can be several possible, plausible and defensible reasons, but it will be harder to state and support the actual reason. $\endgroup$
    – uhoh
    Oct 1, 2020 at 10:48
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    $\begingroup$ Primarily weight. It's really REALLY hard to soft-land things on Mars. By and large RTGs are used only for missions headed so far from Sol that irradiance levels are far too low for reasonably-sized solar panels to collect enough energy. $\endgroup$ Oct 1, 2020 at 11:43
  • $\begingroup$ @CarlWitthoft RTG's scale with the spacecraft size, and I'm not sure that a solar panel plus battery and battery management technology is lighter than an RTG that can deliver uninterrupted power (though a smaller battery might come in handy). It also provides a direct source of heat at night. Are few kilogram RTG's possible with similar mass-specific power to current designs? $\endgroup$
    – uhoh
    Oct 1, 2020 at 15:45

2 Answers 2

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Spacecraft design is based on conservative, pessimistic estimates, "stuff that's proven to work". When those estimates are (usually) exceeded, the next generation "faster horse" can be designed with the updated proven estimates as guidelines. This is the very careful way of doing things.

As such, this starts out with the first rover on Mars, Sojourner.

  • Solar powered
  • Planned 7 sols mission
  • Lasted 83 sols.

Solar power had then proved itself. If you have got 83 sols worth of time, you can design the next generation of rover to fill all that time with objectives, instead of just 7 sols.

The next pessimistic estimate was for these next generation rovers where a low 90 sols. Good enough for the extended lifetime Sojourner had demonstrated.

Spirit and Opportunity where thus designed to be as likely as possible to achieve their goals, even in the face of very pessimistic estimates.

When Sprit and Opportunity eventually managed to run for years and years, the goalpost for the next generation rover was moved proportionally. When operational lifetime becomes this long, one begins to look out for more stable power sources like RTGs.


In hindsight, one may question the performance of solar power, now that years of operational life time is a fact, not wishful thinking. But the rovers where not designed based on wishful thinking, they were designed to last for a ~90 sol operation. For that goal, solar power was a working, proven, technology.

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    $\begingroup$ Wasn't cost a factor? And the availability of the proper isotopes? $\endgroup$ Oct 1, 2020 at 18:25
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    $\begingroup$ Certainly. It's a solution with extra constraints. Nobody wants extra constraints unless they can't avoid it. $\endgroup$ Oct 1, 2020 at 18:30
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    $\begingroup$ Spirit and oppy were in fact, on a budget. And they weren't even supposed to last that long, so why waste $65M -$90M on something that shouldn't last more than 90 sols. (of course, they did last 6 and 14 years, so NASA has nothing to complain about) but solar panels are cheaper to make. $\endgroup$ Aug 11, 2022 at 16:22
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    $\begingroup$ @DekoRevinio That 90 days was the bare minimum amount of time the rovers had to operate without the mission being deemed an abject failure. It was essentially a guarantee to Congress. There would have been hearings galore had they not lasted that long. To ensure that the guarantee would hold the missions were designed to last a lot longer. $\endgroup$ Aug 11, 2022 at 17:50
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Mars Spirit and Opportunity rovers exceeded expectations by working for many years than the planned 90 day mission.

Ninety days was not the planned mission. It was instead the bare minimum amount of time the rovers had to work for the mission to be deemed a "success".

Mechanical and electrical equipment tends to have a high failure rate early on, a lowish failure rate after that "infant mortality" period has passed, and much later, a high failure rate as components inevitably wear out. If the Mars rovers did get past the initial high "infant mortality" period, they had a good chance of continuing for years -- which they did.

(Why) was Nuclear battery system not used for Mars Spirit and Opportunity rovers?

At the time the mission was planned, neither NASA nor the Jet Propulsion Laboratory had yet to prove themselves worthy of the large expense and extreme rarity of an RTG. NASA, along with other space exploration agencies, had instead experienced a long history of mostly failures with regard to exploring Mars. That string of mostly failures started to change with the Mars Pathfinder / Sojourner mission. There was no knowing at the time that that was the inflection point.

The Sojourner rover had just proven itself at the time work was started on the Mars Exploration Rovers. The Sojourner rover was a very low cost (in NASA terms) technology demonstrator add-on to the low cost Mars Pathfinder mission. The Mars Exploration Rovers greatly extended the limited capabilities of the Sojourner rover. (Sojourner cost a mere 25 million US dollars (1997 dollars). It did not have many capabilities.)

It would have been madness to outfit the Mars Exploration Rovers with RTGs.

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  • $\begingroup$ That's true. Even in the early 2000's NASA wasn't considered a very important branch of the government. At the time NASA received about half a cent of every federal tax dollar. now they recive about 2 cents. (my sources may be wrong.) $\endgroup$ Aug 11, 2022 at 16:25
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    $\begingroup$ @DekoRevinio Your sources are wrong. NASA's budget remains at less than half a cent for every federal dollar. $\endgroup$ Aug 11, 2022 at 17:42
  • $\begingroup$ They did not know if they had the ability to survive the winter months on mars. They hd zero experience with nursing a rover on solar power trough the martian winter. So they fully expected the rovers to fail during that time. Once they knew how to get trough the winter they would have been able to adapt the estimates... you can't plan for the unknown. $\endgroup$
    – TrySCE2AUX
    Aug 15, 2022 at 8:48
  • $\begingroup$ @TrySCE2AUX JPL didn't expect the rovers to fail over the winter, and it's rather obvious that they designed the rovers robustly so they would survive multiple winters, and even more severe, multiple Mars dust storm seasons. Whether they would have guaranteed that to NASA, Congress, and the president -- that's a different question. They obviously did not do so. The guarantee was 90 days, and the landings were timed to make that guarantee likely. $\endgroup$ Aug 15, 2022 at 9:55
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    $\begingroup$ @TrySCE2AUX Re And you don't promise congress stuff you're not 100% sure you can pull it off... Exactly. People will (and have) lost their jobs by overpromising/underdelivering, (especially to Congress) or they have been promoted to head a one-person department with an "office" that used to be a broom closet. $\endgroup$ Aug 15, 2022 at 13:45

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