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When I first heard about the Juno spacecraft, I was surprised that it would be using solar panels to generate power throughout the mission. Why wasn't a radioisotope thermoelectric generator (RTG) used instead of solar panels?

Is it because RTGs are more expensive than solar panels are now? All past missions to Jupiter used RTGs for power.

Peter Mortensen
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Hyperdrive Enthusiast
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4 Answers4

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That is precisely it. Plutonium-238, which is used in the creation of radioisotope thermoelectric generators (RTGs) is very difficult to come by. There are plenty of news articles on this, from Popular Science to Space News.

Basically, it comes down to the fact that plutonium-238 is in short supply now, and it is difficult to make more because of nuclear non-proliferation. Of the stock that NASA has left, they can make three RTGs. One of them is definitely going to be used for Mars 2020, because it's using the same design as Curiosity. The other two have not been allocated yet. The United States Department of Energy may start making more plutonium-238, but how much and when is yet to be seen.

Phiteros
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    Dr. Emmett Brown: "I'm sure that in 1985, plutonium is available in every corner drugstore, but in 1955, it's a little hard to come by!" Turns out it's still a little hard to come by. – Mark Adler Jul 06 '16 at 17:36
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    @MarkAdler I guess we're a little behind schedule on the Mr Fusion. – Anthony X Jul 07 '16 at 01:09
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    I heard this presentation of the plutonium RTG production process. It is as if Rube Goldberg is in charge of it. By doctor Makenot: http://spirit.as.utexas.edu/~fiso/telecon/McNutt_4-16-14/ I don't say it could be done in any better way, it just does not look like the big future for energy supply in space flight. – LocalFluff Jul 07 '16 at 01:16
  • @LocalFluff thanks for the link to that presentation, that was fascinating. – Organic Marble Jul 07 '16 at 01:24
  • Another factor is the huge advances that have been made in solar power generation in the last thirty years. That said, I suspect that if the designers of Juno had their druthers, it would be powered by RTGs rather than solar arrays. – David Hammen Jul 07 '16 at 12:34
  • Add to that solar panels are much better now. – GdD Jul 07 '16 at 12:54
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    Besides the cost of the battery itself being a hefty part of the $700mio strict budget, the amount of insurance you have to pay for those is easily in the tens of millions, being at least in the range, if not higher, than the Pu238 cost itself (4.8kg of Pu238 for Curiosity are estimated to cost $15mio, for future generated ones like from LANL it will likely be more) – PlasmaHH Jul 07 '16 at 13:59
  • What is $… mio ? I thought we use USD, or the $ alone? – JDługosz Jul 07 '16 at 19:19
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    @JDługosz I think that they mean 700 million USD. – Phiteros Jul 07 '16 at 20:06
  • @AnthonyX I blame the invention of transistors. Without them we'd have intelligent robots with jet thrusters by 2077 (and a nuclear war shortly after that). Instead we have ... what? Tiny RC helicopters with multiple propellers? – John Dvorak Jul 08 '16 at 04:00
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    Oak Ridge National Laboratory has already started producing Plutonium. I think last year they produced a couple of ounces. I don't know how long it will take them to scale up, but the goal is to produce around 5 lbs per year. – ventsyv Jul 08 '16 at 21:26
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    @PlasmaHH New Horizons' budget is $700 mil and it has RTG. Also, Juno exceeded it's $700 mil budget by quite a bit, I think the new budget is around $1 billion. – ventsyv Jul 08 '16 at 21:32
  • @ventsyv Juno can have solar panels and run efficiently at Jupiter's orbital distance. But out by Pluto, the sunlight is way too weak to get by with solar panels. An RTG is the only option. – Phiteros Jul 08 '16 at 22:09
  • An interesting point @PlasmaHH makes. 4.8kg of PU-238 strapped to (how many tonnes?) of rocket fuel and accelerated to (what speed?) in out atmosphere? How does one assess the insurance cover for this? (or happily take the risk in the first place?) – PCARR Jul 09 '16 at 12:19
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Another interesting note is that this mission more than any other mission to the outer solar system can use solar power. Why? Juno is in a polar orbit, and will continually be in the sun. Solar panels are also becoming more powerful than they have previously. Between the two of these, solar was a more attractive option than it has been in the past. If it was absolutely required, it could use an RTG, but in fact solar panels were possible, and thus they were used.

PearsonArtPhoto
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    This is a really important point - if you come across an illustration of the orbit, that might be a great addition. Something that shows why the orbit never passes through the shadow, since some polar orbits certainly could, considering Juno is coming from the inner solar system. – uhoh Jul 07 '16 at 01:25
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    All I really know for sure is the only eclipse in it's entire mission was the flyby of Earth, where it went into safe mode. – PearsonArtPhoto Jul 07 '16 at 01:26
  • Well, the NASA Juno video in this question is no help - to me at least. However, this other NASA Juno video says "The view here is toward Jupiter, as seen from Earth over the course of the science mission. Jupiter's north pole is up." which roughly gives an idea of the sun's direction. While the shadowing may be wrong or confusing in the video in the question, it may be "correct" in the other video. – uhoh Jul 07 '16 at 01:54
  • Hmm, will it never even briefly be eclipsed by one of the Jovian moons? Or is that short and harmless? – gerrit Jul 07 '16 at 10:31
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    Remember, it is there to explore Jupiter, not it's moons. It won't even closely approach one of the moons. And even if there was a slight eclipse, it does have batteries that would allow it to withstand a small eclipse. – PearsonArtPhoto Jul 07 '16 at 11:51
  • It's also important to remember that the orbit is highly elliptical. The science passes are short, ~8 hours out a ~14 day orbit, and of that 8 hour period near perijove, it will only be in shadow for a short period of time. Even if it's not in shadow, proper pointing of the spacecraft for science trumps pointing for solar power. The vehicle has two fairly hefty batteries for this purpose. – David Hammen Jul 07 '16 at 12:31
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    @uhoh: About polar orbits and eclipsing; I think the way to look at it is that any non-eclipsing orbit must be polar. Any equitorial or inclined orbit has to go into shadow so the only way to avoid the shadow is to be polar. However, as you suspect, many polar orbits can also go into eclipse. – Oscar Bravo Jul 07 '16 at 12:51
  • @PearsonArtPhoto It has had to operate on battery power other times, though. During the Jupiter Orbital Insertion, it was facing away from the sun. – Phiteros Jul 07 '16 at 14:00
  • @OwenBoyle I think Juno will not experience an eclipse by Jupiter for the next year or so. Correct? – uhoh Jul 07 '16 at 14:50
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I had the opportunity to tour JPL a few months ago and asked this exact question to our tour guide. The solar panels on it are enormous and typically, spacecraft going beyond the asteroid belt are equipped with RTGs, so why doesn't Juno have one? He told us that the US was on very short supply of Plutonium-238 at the time and that they would have had to ask Russia for it. So instead of being dependent on another country, NASA decided to tackle a much more challenging design feat. I'm sure that there was a lot more to it than that. I think the other answers did a good job addressing those other factors, but I just found this very interesting and wanted to share.

C. Spencer
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Phiteros' answer states the fact that plutonium is in scarce supply and PearsonArtPhoto's answer points out that Juno's mission profile allows it to use solar panels. These issues are legitimate and (the scarcity one anyway) somewhat overriding of other issues. Nonetheless, I wish to bring up something else: Public relations and politics.

In the opinion of some people, using radioactive materials in a device that could explode is too risky. Thus, every time NASA uses RTG's on a mission, there are protests about the risk of exposing the public to radiation should there be a launch accident. This brings about negative news reports about the agency. It also likely brings some pressure on NASA from some politicians to find another way to power its spacecraft.

Note: I'm not agreeing with the position that RTG's are too dangerous to launch into space, just pointing it out.

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GreenMatt
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