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69

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 ...


59

RTG technology has been applied on Earth, many times, although not for transportation - they don't produce much power for their weight so any RTG powered vehicle would be very slow. Some pacemakers used to have plutonium batteries, and RTGs were used in remote sites to power sensors, lighthouses and the like in remote areas. It isn't used much anymore on ...


52

The Plutonium isotope 238 used in RTGs is highly specialized. It's not produced in large quantities routinely. Not very many radioisotope applications need that much of a highly radioactive isotope, and it's only produced in certain reactors. In fact, there was only one reactor in the USA that produced it. Nuclear stuff is expensive in general and, now that ...


37

Interplanetary communication is mainly dependent on signal strength (for transmission) and antenna size (for reception). The Pioneers use a 9-foot antenna and an 8-watt transmitter. The Voyagers use a 12-foot antenna and a 20-watt transmitter, allowing a substantially stronger signal to be received on Earth.


36

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 ...


30

There is Carnot's theorem for the theoretical maximum efficiency of heat engines. It is valid not only for mechanical engines like steam engines or Stirling engines but also for solid state devices like the thermocouples used in RTGs. The Carnot efficiency depends on the upper and lower working temperature. $$ \eta = 1 - \frac {T_c}{T_h} $$ Tc is the cold ...


28

In addition to a better transmitter, the Voyagers have better power reserves: their RTGs supplied 470 W at launch, while the Pioneer RTGs supplied 160 W at launch. So the Voyager RTGs will take much longer to decay to a point where they can't power the spacecraft. NASA seems to think RTG decay is the primary reason we can't receive Pioneer 10 any more: ...


28

RTGs are expensive to produce, can be politically inconvenient to use, and in the form of a plutonium-bearing device, represent a potential nuclear proliferation hazard (though all RTGs might be used to construct a "dirty bomb"). To compound the issue, their power output simply isn't very big... Perseverance's generator cost about 75 million USD ...


27

The big difference between the two darker RTG fins (Black and Grey) and the white RTG fins, is that the white fins were destined for use in an atmosphere (Mars). The presence of an atmosphere, even as diffuse as Martian air, would allow increased heat transfer from the RTG fins via convection and conduction, vs. the space based versions which would entirely ...


27

Answer: Thermal radiating coating technology has improved, so they are no longer forced to be sub-optimally black in visible light. They can now be white and reflect incident sunlight to improve thermoelectric efficiency by staying cooler. The color has nothing to do with the atmosphere. It has to do with sunshine! Curiosity's MMRTG is producing about 2 kW ...


24

At first glance, the RTG does not pose a risk. It is powered by Pu-238, which is primarily an alpha emitter throughout its decay chain. Alpha particles can be stopped by a sheet of paper. An astronaut is perfectly safe in his suit, even if the RTG were disassembled and the Pu lying around unprotected. The RTG is built to survive a launch failure, i.e. it ...


24

With typical active radiators on spacecraft, heat is transferred away from the sources into the radiators through forced convection - as heated coolant. At that point the only concern remaining is to remove (radiate) it from the radiators (and as little as possible back into the spacecraft or into other radiators). They are big and they face as much into ...


24

Any isotope used as the basis for a radioisotope thermoelectric generator (RTG) has to have a short but not too short half life. A half life of several decades is ideal. Such isotopes effectively do not exist in nature. (Tritium, with a half life of 12.32 years, does exist in nature in trace amounts due to generation by cosmic rays, but the half life is a ...


21

Power and Mass From this paper (emphasis mine): The specific power of an 241Am-fuelled RTG cannot match that of a 238Pu system (except perhaps at small power output levels); however, the design work undertaken provides confidence in potential capability and performance of 241Am systems for future space missions. Medium-sized RTGs in the 10 W to 50 W range ...


20

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 ...


20

Multi-fin radiators are worse per unit mass. But for an RTG, it is absolutely vital to provide a very large thermal gradient between the (very small) core and the outer layers. Adding more fins still improves radiation in sum, you just get less radiation per fin. Since the cooling requirement of an RTG is high and absolute, designers have no other choice ...


19

The probability and consequences of a release of Pu-238 from an RTG in a launch accident are very low, due to the protections in place for such an incident. It's not like they never thought of that. The radioactive material is not "widely dispersed". As for the numbers, the rate of decay is inversely proportional to the half-life. The half-life of U-235 is ...


18

The Dragonfly cruise stage looks rather similar to Curiosity's cruise stage, so I've looked at Curiosity. Curiosity's backshell contains a hatch. This was used to install the RTG at the last possible moment (a few days before launch, long after the rover had been packaged in its backshell plus heat shield. Here is Curiosity's backshell being prepared for ...


18

Partial answer (everything but exactly when in the timeline it was done): The fuel capsule was installed at the launch pad "through a ten-inch access port in the spacecraft structure". The fabulous document ALSEP Flight System Familiarization Manual includes this info and much, much more. The only time given is "after the LM has been fueled". (page 4-5) ...


18

14 years is the design lifetime for the MMRTGs. The thermocouples do degrade over time while exposed to the high temperatures of the hot side and the temperature changes of the cold side. The output power of the RTGs drops over time by degradation, design lifetime ends when there is too few power left. But many RTGs did work better than conservative lifetime ...


17

TLDR: The rover is power limited not daylight limited Lights are not sufficient to enable nighttime driving. The rover is limited by available power. The RTG produced ~114 W at the start of the mission, dropping to 54 W by 2025. It requires 45-70 W during sleep, at least 150 W when awake and 500 W during driving. This means the rover can only drive for a ...


15

Popular Mechanics had a neat article about it last year. Bottom line, nobody knows. We will lose communication with the probes at some point in the next 10-15 years because the fuel supply will run out. The probes are powered by nuclear reactors and scientists expect them to be depleted sometime in the 2020's. They have been proactively shutting down systems ...


15

If we can limit the discussion to modern RTGs used for exploration probes, there really isn't much in the way of precautions. There are precautions to prevent launch failure, and then contamination of Earth's environment. But obviously you can stand next to them as long as the fuel is in solid ceramic form.         &...


15

This is an addendum to @Uwe's answer. RTG lifetime is a topic of much discussion in the planetary science community, and in particular the MMRTG being currently the only available RTG. MMRTGs decay much faster than GPHS-RTGs (see my comment to Uwe's answer), with an output power half-life of a bit over 16 years, the result of both Pu decay and unicouple ...


14

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 ...


13

As you demand more current, the voltage goes down. You eventually brown-out the system. The RTG doesn't care. If you then reduce the load, the voltage goes back up. You cannot drain an RTG like a battery. Its power output depends only on its radioactivity (which goes down over time), the efficiency of the thermocouples (which also goes down a little ...


13

All heat engines, whether mechanical or solid state, produce work based on heat flow across a temperature difference. The maximum efficiency of a heat engine depends on how large that difference is.


13

The answers claiming danger/toxicity are chasing something that's irrelevant. The real issue is that they don't produce all that much power*. If you want a car that can only drive a few hundred meters a day, an RTG will work just fine. Solar panels aren't all that good on Mars. Not only is the sunlight weaker, and reduced even further by sun angle** and ...


12

There are at least two problems with solar photovoltaic cells (not considering concentrators) in the outer solar system: the low power of the sun, and the low temperature of the cells. For the Cassini mission to Saturn (9–10 AU from the Sun), NASA investigated solar as an alternative. They calculated the surface area that would be required, and concluded ...


12

Pioneers Several factors lead to the loss of signal. Radiated power: Pioneer 10's broadcast power is particularly low. 8 W at ~2.2 GHz ① Very narrow beam; the antenna gain is +65dB, which means 1/(10^6.5) the broadcast surface area... which means a pretty narrow cone. (I don't know the math for the actual beam angle.) Input power decreases - the RTGs still ...


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