Astronomy Without A Telescope – Solar Or RTG?

Article written: 9 Oct , 2010
Updated: 24 Dec , 2015


It used to be the case that if you wanted to send a spacecraft mission out past the asteroid belt, you’d need a chunk of plutonium-238 to generate electric power – like for Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, even Ulysses which just did a big loop out and back to get a new angle on the Sun – and now New Horizons on its way to Pluto.

But in 2011, the Juno mission to Jupiter is scheduled for launch – the first outer planet exploration mission to be powered by solar panels. And also scheduled for 2011, in another break with tradition – Curiosity, the Mars Science Laboratory will be the first Mars rover to be powered by a plutonium-238 radioisotope thermoelectric generator – or RTG.

I mean OK, the Viking landers had RTGs, but they weren’t rovers. And the rovers (including Sojourner) had radioisotope heaters, but they weren’t RTGs.

So, solar or RTG – what’s best? Some commentators have suggested that NASA’s decision to power Juno with solar is a pragmatic one – seeking to conserve a dwindling supply of RTGs – which have a bit of a PR problem due to the plutonium.

However, if it works, why not push the limits of solar? Although some of our longest functioning probes (like the 33 year old Voyagers) are RTG powered, their long-term survival is largely a result of them operating far away from the harsh radiation of the inner solar system – where things are more likely to break down before they run out of power. That said, since Juno will lead a perilous life flying close to Jupiter’s own substantial radiation, longevity may not be a key feature of its mission.

Perhaps RTG power has more utility. It should enable Curiosity to go on roving throughout the Martian winter – and perhaps manage a range of analytical, processing and data transmission tasks at night, unlike the previous rovers.

With respect to power output, Juno’s solar panels would allegedly produce a whopping 18 kilowatts in Earth orbit, but will only manage 400 watts in Jupiter orbit. If correct, this is still on par with the output of a standard RTG unit – although a large spacecraft like Cassini can stack several RTG units together to generate up to 1 kilowatt.

So, some pros and cons there. Nonetheless, there is a point – which we might position beyond Jupiter’s orbit now – where solar power just isn’t going to cut it and RTGs still look like the only option.

Left image: a plutonium-238 ceramic pellet glowing red hot, like most concentrated ceramicised radioisotopes will do. Credit: Los Alamos National Laboratory. Right image: the Apollo 14 ALSEP RTG, almost identical to Apollo 13's RTG which re-entered Earth's atmosphere with the demise of the Aquarius lunar module. Credit: NASA

RTGs take advantage of the heat generated by a chunk of radioactive material (generally plutonium 238 in a ceramic form), surrounding it with thermocouples which use the thermal gradient between the heat source and the cooler outer surface of the RTG unit to generate current.

In response to any OMG it’s radioactive concerns, remember that RTGs travelled with the Apollo 12-17 crews to power their lunar surface experiment packages – including the one on Apollo 13 – which was returned unused to Earth with the lunar module Aquarius – the crew’s life boat until just before re-entry. Allegedly, NASA tested the waters where the remains of Aquarius ended up and found no trace of plutonium contamination – much as expected. It’s unlikely that its heat tested container was damaged on re-entry and its integrity was guaranteed for ten plutonium-238 half-lives, that is 900 years.

In any case, the most dangerous thing you can do with plutonium is to concentrate it. In the unlikely event that an RTG disintegrates on Earth re-entry and its plutonium is somehow dispersed across the planet – well, good. The bigger worry would be that it somehow stays together as a pellet and plonks into your beer without you noticing. Cheers.

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14 Responses

  1. Spoodle58 says

    RTGs or even reactors whch have more power, solar is useless in comparsion for deep space at current tech levels or if a mission needs a lot of power.

    Hippies start moaning when they hear the word nuclear which is why an agency like NASA uses solar for a lot of stuff, remember those protests for the Cassini mission, and look at the wonderul science thats doing.

  2. Torbjorn Larsson OM says

    From the posts references:

    “Juno benefits from advances in solar cell design with modern cells that are 50 percent more efficient and radiation tolerant than silicon cells available for space missions 20 years ago. The mission’s power needs are modest, with science instruments requiring full power for only about six hours out of each 11-day orbit (during the period near closest approach to the planet). With a mission design that avoids any eclipses by Jupiter, minimizes damaging radiation exposure and allows all science measurements to be taken with the solar panels facing the sun, solar power is a perfect fit for Juno.”

    Technology progresses, and it fits Juno’s profile. But even if it has pushed the solar panel envelope it may not be a panacea at Jupiter’s solar distance.

    Solar or RTG. What’s the third option, nuclear power plants like the ones tested for Moon colonization?

  3. Emilio says

    Americans, hippies and tea partiers, are in general skeptical of what the government tells them. People in power “do” misuse power.

  4. Lawrence B. Crowell says

    The solar option is certainly possible, but I am surprised this appears to be done with larger solar panels.

    Photons are conserved, so the number of photons passing through a small sphere at a radius r, which might the solar radius, with area A = 4pi r^2 is equal to the number of photons passing through an imaginary sphere with a radius equal to the radius of Jupiter’s orbit A’ = 4pi R^2. So for a solar panel with any given surface area the number of photons reaching it is some small fraction of those which would reach it if it were on the solar surface, given by the ratio (r/R)^2. Now if r is the radius of the Earth’s orbit 150e^6km = 1AUand R = 5.20 UA, the radius of Jupiter’s orbit, this ratio is 0.0369. So is the panels generate 18 Kw at Earth’s orbit they will in principle generate (r/R)^2*P_earth = 0.0369*1.8e^4watts = 664 watts at Jupiter. This is not far off from the article’s expected 400 watts. There are apparently some material issues which reduce this further, probably having to do with the lower temperature.

    Now here is the thing which surprises me. If you want to generate solar power out in this colder and darker region of the solar system, don’t make larger solar panels, but have a fold out reflecting concentrator made of thin material which increases the photon irradiance on the panels. It would be cheaper and lighter.

    The Pu 238 RTG reactor is a good idea for anything out in space. I must confess I do think that leaving an RTG on the surface of Mars is a sort of pollution, which might have some small impact on life, should it exist under the soil where the MSL grinds to its final stop. I don’t know if anyone has thought about how long these objects will remain intact on the Martian surface, but I imagine that weathering will eventually break them down, and the RTG will eventually split open to spill its Pu interior.


  5. Torbjorn Larsson OM says

    Lawrence, the Pu is bonded in the ceramics, and ceramics leach very slowly. The estimated 900 years in warm sea environment may be orders of magnitude more in dry and cold martian environment.

    Pu-238 half life is a mere ~ 90 years, which is presumably why it is used as power source. (And the rest of the decay chain is the standard natural U-234.)
    As soon as it has decayed it isn’t biologically poisonous (but see the decay chain).

    Personally I think Martian life has more dangerous endogenous stuff to worry about (UV radiation, free radicals).

    This is very intriguing:

    There are apparently some material issues which reduce this further, probably having to do with the lower temperature.

    Quantum efficiency, number of free and doped carriers, and bandgap, certainly varies with temperature.

    I haven’t the energy to check, but I believe QE goes up with lower temperature. The other two goes down, unless I’m mistaken. I expect the overall effect to be a lower power output.

    The main lowering effect should come from radiation damage though. Which is why it is surprising to see solar panels on a Jupiter mission. Presumably the orbit is well away from Jupiter’s intense radiation belts.

  6. Member
    Aqua says

    The use of fissionable and/or fusion energy products for space exploration is an obvious path! The use of atomic power to generate electromagnetic shielding and/or power for ion drive engines or even plasma drive engines seems like the most logical next step(s)?

    I like the idea of using a reactor that is built for use in several applications. First, to provide power for rocket propulsion, then upon arriving at say Mars, power for the habitat, transportation and communication systems. And finally, power for water distillation/purification/electrolysis and atmosphere scavenging.

  7. TerryG says

    Torbjorn Larsson OM raises a good point. Go nuclear with U235 as the Russians did with ROSTAT (Radar Ocean Reconnaissance SATellites)

    Unlike RTG fuel, there’s no shortage of U235 and once the spacecraft leaves Earth’s orbit the beer is safe.

  8. Lawrence B. Crowell says

    Most quantum efficiencies are expressed according to wavelength. However, there are dependencies on temperature. As the temperature approaches zero the electrons approach the Fermi surface states. Electrons in states on the band overlap tend to be less occupied. The unique thing about Fermi-Dirac statistics is that the electrons do not collapse into the minimum state, so the QE as T – -> 0 will not itself go to zero. One exception to this is if on the Fermi surface there is BCS bosonization and the onset of superconductivity. However, this is not physics generic to semi-conductors and solar cells. On the flip side if you have a fairly high temperature the Boltzmann distribution of states of electrons becomes spread and this again lowers quantum efficiencies. So there is an optimal condition of rather standard temperatures.

    Radiation in Jupiter’s belts would degrade the performance of a solar cell. The lattice damage done over time would I think cause them to drop in QE over time. It will be interesting to see how this works. There was some outcry, largely misplaced I think, over the use of Pu RTG generators in the Galileo and Cassini spacecrafts. I suppose this is an attempt to get solar power working a bit further out. There is no way this will work out to Saturn and beyond.


  9. Olaf says

    I propose to confuse the hippies and rename RTG to something like “Mr Power”.
    This way they won’t know that something atomic is in it.

  10. Nexus says

    This may be a stupid idea but…

    all these probes require a large radio dish for communications with Earth, and it needs to be pointed approximately sunwards all the time. Is there any reason why you couldn’t make it a big dish-shaped solar panel?

  11. ND says


    I don’t think that will work. Try Mr Coffee or Mr Radar.

  12. Member


    Radio dish surfaces need to reflect (so are generally white or silver) – solar panels need to absorb (so are generally black). The colours don’t matter for reflecting radio, but heat absorption will reduce signal to noise ratio – and can distort the disk geometry. Not sure these issues are vital in the cold of the outer solar system, but are important on Earth.

    Perhaps the key thing is that you don’t really want a big dish on your spacecraft – which is mainly used for data transmission. A big dish will need to draw more power to push out a coherent signal. Better to have small dishes on spacecraft and big ones back on Earth.

    There may also be issues with the electronics I’m not really across 🙂

  13. Jon Hanford says


    “I don’t think that will work. Try Mr Coffee or Mr Radar.”

    LOL. You read my mind: 😀

  14. ND says


    One of my favorites, right up there with 2001 Space Odyssey 🙂

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