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In the past two decades, NASA has launched at least three missions that use RTG's:

  • Cassini
  • Mars Science Laboratory
  • New Horizons

Those launches include plutonium, which is a reason for some to oppose the missions in their form, because the launch is involved with considerable risk. For each of them, NASA has published a quite extensive environmental impact statement. This includes an estimate of the number of "latent cancer deaths" in the worst case scenario:

What alternatives are available? NASA considered solar for Cassini in Chapter 2 of the EIS and for the Mars Science Laboratory in Chapter 2 at the EIS. The risk for Cassini was much higher than for MSL (because it had much more plutonium), but the cost of implementing the alternative was also much higher (It might be possible now, but I question if it was possible when Cassini was launched). There was some discussion, e.g. here. Both Juno and Juice, missions to Jupiter, use solar.

For a mission like New Horizons, but also Voyager 1 and 2, solar power really is not feasible. Is there any alternative here?

One esoteric, science-fiction solution that I can think of would be to produce plutonium in space. It's futuristic, but with a nuclear reactor on an asteroid, operated by robots, one could produce fuel that makes exploration of the outer solar system possible. This is, of course, not currently possible, but would get rid of the launch safety issue.

Another, closer-by alternative would not get rid of plutonium in Earth-based sources completely, but to limit it further. New-style ASRG's use only a quarter of the fuel than traditional RTG's do, and payloads get more efficient, so a future beyond Saturn might do with 10–20% of the plutonium. This was proposed for the now-cancelled Titan Mare Explorer.

Are there any alternatives for exploring the outer solar system that do not carry nuclear launch risks? For the sake of this question, I define outer solar system as Saturn or beyond, where solar has never been used.

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    $\begingroup$ @AdamWuerl I have referenced numbers. A different question addresses the question how the risk compares to other risks involved in launching a space probe. Comparing against non-launch-related risks is difficult. $\endgroup$
    – gerrit
    Sep 23, 2013 at 16:10
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    $\begingroup$ At Saturn's distance, the inverse square rule results in only 15 watts of solar flux 'constant' available. Paltry. But what happens if you take a reflector and focus the sunlight onto an ordinary one square-meter photovoltaic panel? A 100:1 concentration would yield 1500 watts at the panel surface, with maybe 500 watts useful power yield. The reflector would have to be large, but solar sail, material already in existence has close to 90% reflectivity. Assuming a parabolic reflector of, say, 113 square meters, the diameter would be something less than 12 meters. $\endgroup$
    – MercuryPlus
    May 5, 2014 at 16:07
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    $\begingroup$ @gerrit: I think that it remains to be indicated that any of the dozens, including the pioneers and the recent Chinese Chang'e 3 Lunar lander (if not hundreds, who knows what the militaries have been doing) actually have caused any damage to the environment during the half a centruy RTGs have been in use. It is the claim of harm to the environment which cannot be verified! There are good reasons to believe that well encapsulated RTGs cannot cause environmental damage. They never have, not even when they rarely have crashed. $\endgroup$
    – LocalFluff
    May 5, 2014 at 17:01
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    $\begingroup$ @LocalFluff None of those EIS worst case scenarios has happened. It is possible but not desirable to verify the prediction on environmental harm. It is dangerous to believe that protection makes environmental harm impossible. There are plenty of historical disasters thought impossible prior to the fact. The fact that RTGs need protection shows there is a risk. This discussion fits better along with this related question on the relative risk of RTGs, that still hasn't received a quantitative answer. $\endgroup$
    – gerrit
    May 6, 2014 at 21:16
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    $\begingroup$ @LocalFluff The risk is not with the RTG itself as a fully contained unit, it is with the RTG potentially rupturing during a launch accident for example, and contaminating a large area with the fallout. I think the figures were conservative. Imagine launch a RTG from Cape Canaveral, having the launch vehicle explode at high altitude, rupturing the RTG casing, and spreading highly radioactive Pu-233 over half the coast of Florida. It's a risk any way you look at it. Why take it when the mission can work on solar? $\endgroup$
    – Drunken Code Monkey
    Jul 7, 2016 at 19:02

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Fission reactors can work just fine for space probes, and that will probably happen. Projects are currently underway at US agencies to develop designs for this. Notably, Demonstration Using Flattop Fissions (DUFF).

Why a fission reactor?

  • It is not highly radioactive at launch
  • It can be compact
  • It can have high power
  • It's not subject to limited supply of fuel

The assumption is that you would use enriched Uranium. Such a reactor would probably use 20% enriched Uranium, because that is the boundary between officially weapons-grade material. Although this material somewhat politically problematic, there are no health concerns until the reactor is turned on. You could hold it in your hands completely safely, although they would never let you.

The next concern people have is "what if it accidentally turns on?" This is why a space reactor will use control drums. We have lots of research on nuclear safety for space reactors, because they've been considered for moon missions and all kinds of things, over many decades. Control rods can be forced in the reactor if a crash happens, but control drums have to rotate, and there's no density difference to cause that. They are locked in place until pretty far from Earth's surface.

If the reactor fell into the ocean it's designed to not go critical. You would race like hell to retrieve it, because you don't want any suspicious group to pick up a free nuclear reactor, and the IAEA tightly watches flows of nuclear materials around the globe. If the thing was burnt up and strewn across a large area, that would be troubling, but no cancers will be caused.

A fission reactor in space can produce extremely high powers. A space probe would use an extremely modest design, with low burnup (fuel efficiency) and passive cooling. Even so, it will give more power than any RTG or solar array would. You'll need a lot of shielding between the reactor and the probe itself, and there will be a good deal of physical separation between the parts.

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  • $\begingroup$ Are there any environmental impact studies done by NASA or other (in principle) independent organisations that you could link to? $\endgroup$
    – gerrit
    Jul 22, 2013 at 14:17
  • $\begingroup$ en.wikipedia.org/wiki/SNAP-10A Apparently they even went as far as intentionally burning up functioning prototypes in the desert. $\endgroup$
    – AlanSE
    Jul 22, 2013 at 14:32
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    $\begingroup$ @AlanSE: the fuels being used (Uranium, Plutonium) are toxic as metals and as radioactives even in non-critical amounts. It's the very same issue as with RTG's. The threat isn't that they'll detonate, but that the fuel will get splattered over a population center. (The concern by the ESA is essentially a gross overstatement of the risks, but it's the risk of radioactive materials being spread by accident they object to.) Even Thorium and Radium are risky. $\endgroup$
    – aramis
    May 5, 2014 at 21:11
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    $\begingroup$ @kimholder Yes, correct, risk of dispersed radioactive material for NTRs is dramatically lower than RTGs. See the comment by aramis mentioning Radon. New fuel has essentially no Radon because it has just been fabricated anew as Uranium-oxide ceramics. This fuel type is ubiquitous in the vast majority of operating nuclear reactors, and with a <20% U-235 enrichment limit, it is still sufficient for deep space probes. Uranium metal is a trivial biological risk compared to either RTGs or reactors that have been operating and producing fission products. $\endgroup$
    – AlanSE
    Jun 7, 2016 at 3:26
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    $\begingroup$ I asked a follow-up question as the difference seems striking and i would like to have a clearer grasp of it quantitatively. space.stackexchange.com/q/16608/4660 $\endgroup$
    – kim holder
    Jun 7, 2016 at 15:50
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Stored Chemical Energy Power Systems (SCEPS) is one of the possible alternatives. From NASA:

Stored Chemical Energy Power Systems (SCEPS) have been used in U.S. Navy torpedoes for decades. This high-energy-density, high-power technology can be reliably stored for years. In Phase I we analyzed the applicability of SCEPS to in situ solar system exploration, looking to see if it could be adapted to power a lander sent to a target with no usable sunlight as an energy source. We developed a candidate mission to the surface of Venus, showing that SCEPS could be used for powering spacecraft and landers. The team compared it to conventional battery and Plutonium powered systems, both of which have deficiencies that are overcome by SCEPS. Our concept holds the promise of a power solution that could far exceed the operational capacity of existing batteries, allowing exciting exploration to continue despite the lack of available Plutonium. We propose to continue the research into applying SCEPS to exploration missions that can't be powered by sunlight. In this study we will mature the Venus mission studied in Phase I. We will also expand our understanding of the usefulness of SCEPS to exploration of moons, comets, asteroids and other targets where sunlight is not sufficient to power the mission. We will engage with the leaders in science planning for small bodies, outer planets, and robotic missions to our own Moon and make a determination of the first, most high-impact use of SCEPS in space. An experiment will be performed to determine SCEPS performance when using CO2 as an oxidizer, approximating the in situ resource utilization of the Venusian atmosphere. Venus science goals will be revisited to prepare the Venus concept for the next level of study. Two key risks stand out. The first is our ability to scale down the power from current SCEPS implementation to levels more in family with spacecraft. Landed systems on Mars, for example, have had power levels on the order of hundreds of watts, far less than the many thousands of kilowatts that SCEPS provides for a U.S. Navy torpedo. The work proposed here would lead to better understanding of SCEPS operations at power levels appropriate to space exploration. The second risk is combustion with in situ resources. In the case of the ALIVE mission, the atmospheric CO2 is proposed as the oxidizer. The analysis performed in Phase I indicates that the reaction would give of the necessary heat to power the lander. The use of in situ resources has its benefits: in the case of the ALIVE mission it reduces the mass of consumables that would otherwise have to be included on launch day by hundreds of kilograms. In Phase II we seek experimental confirmation that this reaction can be initiated and sustained at the power levels required for such a lander. We see an opportunity to expand our understanding of the impact that SCEPS could have on solar system exploration. The sunless environment of Venus may indeed be explored through the use of SCEPS, but many cold, sunless regions may also benefit. Sending a SCEPS system to power a lander on the surface of Europa or the lakes or dunes of Titan may return substantial science that would be otherwise left unknown, or at least greatly delayed as the community works to solve the Plutonium-availability problem. We will develop a multi-variable model for SCEPS function and performance using advanced trade space visualization and exploration tools and techniques. The trade space will include the information gleaned from the stakeholders. The trade space tools will allow us to see the intersection of SCEPS capability and mission utility. The collective results of the study will be used to create a roadmap for further maturation of SCEPS for use in space. In Phase II we seek to expand the understanding of how best to target this technology and plan a path for development by developing a roadmap for TRL advancement of SCEPS in space that mirrors NASA’s solar system science goals in this decade.

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    $\begingroup$ This concept uses gasoline or similar to drive an internal combustion engine. It's a good fit for torpedoes, which need large amounts of power for a few minutes. For long-term missions you'd need unfeasibly large amounts of fuel, making this not an ideal replacement for an RTG. $\endgroup$
    – Hobbes
    May 31, 2016 at 13:57
  • $\begingroup$ @Hobbes Nothing is ideal. It can replenish resources, use CO2 from Venus atmosphere for example, as stated in the text. $\endgroup$
    – mark.g
    May 31, 2016 at 14:12
  • $\begingroup$ Sure, that means you don't have to carry the oxidizer, but you still have to carry the fuel. $\endgroup$
    – Hobbes
    May 31, 2016 at 14:29
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    $\begingroup$ Yes, fuel is a limiting factor, but shorter mission is better than no mission at all. $\endgroup$
    – mark.g
    May 31, 2016 at 17:31
  • $\begingroup$ @Hobbes but an RTG is not ideal in the first place, because you can't control it's power output at all. So you always need to make a tradeoff between sufficient power to operate the instruments and longievity; ²³⁸Pu has quite a decent tradeoff for a mission like Cassini, but e.g. for Voyager it was actually rather silly – full power (as much as the science-intensive flybys would need) for all that eventless time in interplanetary space. And no way to save energy for ever-seldomer measurements in interstellar space; it won't be too long until it'll just be dead. $\endgroup$
    – leftaroundabout
    Dec 13, 2017 at 1:04
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Depending on the usage and also what you mean by the "outer solar system", solar panels are getting to the point where they can be used. For example, Juno, currently en route to in orbit around Jupiter, uses solar panels. As solar panels become more efficient, they may be more useful for the planets which are more remote. On the other hand, and as you noted, the Mars Science Lab's Curiosity rover uses RTG's.

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    $\begingroup$ And lunar missions, with 14 days of dark will not work well with Solar. $\endgroup$
    – geoffc
    Jul 16, 2013 at 22:15
  • $\begingroup$ Lunar Missions will work just fine... half the time. Unlike Mars, where the atmosphere puts dust over the panels if they don't move, on Luna, dust only flies from impacts, and 14 days of dust there is unlikely to matter much. (and if something hits close and hard enough to matter, it's probably also going to do more than just block the panels. $\endgroup$
    – aramis
    Jul 23, 2013 at 9:41
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    $\begingroup$ One of the Lunokhods died because dust got onto the radiators. They were driving inside a crater and accidentally scooped up some dust from the crater rim with the 'lid' (thermal cover). When the cover closed for the lunar night, it dumped the dust onto the radiator, which caused the rover to overheat the following day. $\endgroup$
    – Hobbes
    Mar 6, 2014 at 21:03
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Space travel has utilized three different power methods over the last fifty years.

  1. Solar panels: Ideal, require no fuel, but as distance from the sun increases efficiency drops considerably. Solar output drops in half from Earth to Mars, past the Asteroid belt it's fairly useless.
  2. Fuel Cells: Powered Apollo, Space Shuttle. Provide power for days/weeks. but Malfunction and explosion risk, though they have a decent track record for reliability. But no where near the endurance desired for long distance transit.
  3. RTG's: Reliable, largely maintenance free, run for years, Downside is power output is very poor, averaging 150-300 watts. No US built RTG has had a failure resulting in contamination of the environment. Russia's RTG's it built to power remote lighthouses and beacons, after decades of neglect have since fallen into disrepair and prone to either vandalism or metal theft.

Nuclear reactors offer the only real potential power supply for long term, Long Distance exploration of the outer planets. But it's not the reactor that's the problem it's the power conversion method. A turbine or stirling engine could be such a generator; However both have moving parts which if they fail renders the entire system dead, Thermoelectric conversion is doable but the process isn't very efficient, however fission operates at a higher temperature, the efficiency of the thermoelectric conversion process is slightly improved but the wear and tear of higher temperatures takes it's tole on the thermocouples. An extremely reliable and simple engine must be designed, tested and built. Rather than reliability of miles like a truck, but measured in reliability of hours. Fortunately the reactor doesn't have to run at full speed/output during transit, when the mission proceeds the reactor ramps up (along with the engine) when mission begins. Thermoelectric conversion with no moving parts seems safer, but using engines produces more power for more, robust and sophisticated instruments. With a reactor power supply providing nearly 1000 times the power output and entire array of advanced sensors and high resolution cameras and transmitters can be used. And Thermoelectric generators in tandem with engines; would seem more safe for utilization.

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  • $\begingroup$ thermoelectric conversion does not have moving parts! This sentence needs correction: " A turbine or stirling engine or thermoelectric conversion, However both have moving parts which if they fail renders the entire system dead." $\endgroup$
    – Uwe
    May 29, 2020 at 9:12
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    $\begingroup$ This is not an answer - the question is after safe reactor designs for space use $\endgroup$
    – GremlinWranger
    May 29, 2020 at 9:54
  • $\begingroup$ For point 2. "But no where near the endurance desired for long distance transit." I think that should be reconsidered because modern fuel cells are showing very high endurance. This is especially evident when considering the decision of major auto manufacturers to invest so much in fuel cell vehicle development (Toyota, Hyundai, etc.) $\endgroup$
    – aranedain
    May 30, 2020 at 22:54

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