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68

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 ...


36

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.


35

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 ...


27

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: ...


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 ...


21

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 ...


20

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 ...


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

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) ...


17

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 ...


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.         &...


13

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

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 ...


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 ...


11

This is propulsion via infrared photons, which is similar to other photon propulsion methods. It's more common to hear about gamma (antimatter drives) and x-ray propulsion because we have mechanisms to drive higher power through it, whereas thermal photons are limited by the Stefan-Boltzmann law. To the question: How much heat would be necessary to ...


11

Can Dragonfly make it to one of Titan's Lakes? tl;dr: Yes! It could be doable in 2-3 years. According to your linked document Dragonfly will use a Multi-mission radioisotope thermoelectric generator or MMRTG: The MMRTG design incorporates PbTe/TAGS thermoelectric couples (from Teledyne Energy Systems), where the TAGS material is a material incorporating ...


10

Not a good assumption. Curiosity would die a very quick thermal death on the surface of Venus. But to answer your question, Curiosity's MMRTG would work on Venus and provide power. The smaller temperature delta reduces the efficiency of conversion, but its not too bad. See this paper. The atmospheric density and wind are not a factor at all for rover ...


8

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 ...


8

The Pioneer Anomaly gives a first-order answer. The Pioneer Anomaly was an acceleration of $(8.74±1.33)×10^{−10} m/s^2$. According to the paper in which the solution for the Anomaly was published, this acceleration was caused by about 50 W of heat output. So you get $10^{-11} m/s^2W$, or 100 GW for 1 $m/s^2$, for a spacecraft that weighs ~250 kg. So 40 MW ...


8

Presumably this is because solar power isn't feasible at large distances from the Sun. There is a possibility to use solar energy as long as the arrays receive a quantity of energy greater than the working level of a photo voltaic cell. This includes the full solar system. The solar cell usability under low intensity is constantly improving. But, right.....


8

The RTGs powered the Apollo Lunar Surface Experiments Package. This is also the first thing that comes up when you google "Apollo RTG".


8

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 few hours a day. Then it has to stop, sleep and ...


7

Some examples: Viking 1 was inadvertently terminated by overwriting the antenna pointing software. Contact was never regained. Viking 2's rechargeable battery died, ending that mission. So neither Viking lander was deliberately terminated. The Galileo orbiter was sent into Jupiter's atmosphere, where it was destroyed. Cassini will be sent into Saturn's ...


7

It's not about reducing radiated power. The shunt is used to keep the bus voltage constant when the power draw changes. A similar system was used on Cassini: Other portions of the power system include the power control boards, with 192 solid-state power switches (SSPSs), a radiator to “shunt” excess, unused power as heat to space, and a shunt regulator ...


6

The equation for aerodynamic drag is: $$ P_d = \mathbf{F}_d \cdot \mathbf{v} = \tfrac12 \rho v^3 A C_d $$ Curiosity has a max. speed of 5 cm/s (0.18 km/h). Air density $\rho$ of Venus is 67 $kg/m^3$, where on Earth it's 1.2 $kg/m^3$. I'll guesstimate frontal area A = 4 $m^2$ and $C_d$ = 1, then at 5 cm/s the drag force due to the rover's own speed ...


6

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 ...


6

There is a good treatment of the subject in the Wikipedia article on RTGs. I've talked with people at the Nuclear & Emerging Technologies for Space (NETS) conferences who have given me other useful tidbits. Some background helps understand why so few nations have undertaken the task of producing RTGs, and for those who have, why they might choose a ...


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