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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 ...


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

Space is basically a vacuum, so there's no air resistance. A probe that's been launched will travel at the same speed indefinitely. Because New Horizons is moving away from the Sun, it loses some speed to overcome the Sun's gravity. New Horizons was launched on the fastest rocket they could get. Then it used a gravity assist from Jupiter to gain some more ...


27

Yes, the research on shielding from energetic particles of solar wind plasma using dipole magnetic field continues, and perhaps the best indication of that is the filing of the Spacecraft shield patent (1) in 2010, roughly 2 years after the publication of the Plasma Physics and Controlled Fusion journal (2) that was noticed by the author of that Physics ...


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 ...


25

Thanks to @MarkAddler for his search suggestions I've tried to balance length against completeness, and both lost here. However I have included enough material to try to be convincing that RTGs, singe-use storage batteries, and rechargable batteries for higher power events have all been investigated and solutions exist to provide at least the electrical ...


24

From ESA's FAQ page: Why was it necessary to keep Rosetta in hibernation for 31 months? To limit its consumption of power and fuel, and to minimise operating costs. During hibernation it was spinning once per minute and faced the Sun, so that its solar panels could receive as much sunlight as possible. Almost all of the electrical systems were switched off, ...


23

This answer addresses only the US side of the ISS. Like everything else on the ISS, it's complicated. Fuses are not commonly used on the ISS. There are fuses within the battery subassemblies, to protect against internal battery shorts. The common circuit protection device on the ISS is the Remote Power Controller (RPC), a commandable "smart circuit ...


22

Propulsion Until someone solves the N-body problem every spacecraft needs some kind of propulsion to correct its course during the mission. New Horizons uses a Hydrazine based propulsion system including four 4.4 N main thrusters and twelve 0.9 N attitude control thrusters. Its 77 kg fuel tank allows a total post launch delta-v of somewhere over 290 m/s (...


21

It's Monday, so let me rain on this parade a little. Current magnetic shield designs are adequate to protect against ionizing radiation from the sun. They aren't sufficient to protect against galactic cosmic radiation, which has a lot more energy in each particle. To effectively block that would take a shield with energy 100x greater. If Bamford's shield ...


20

Background and Physics Note that there are actually two different but related types of actuators that use conservation of angular momentum1 to control a spacecraft's attitude (both of which may be lumped into "reaction wheel" by KSP): Reaction wheels (RWs, a.k.a momentum wheels) spin along a fixed axis at a variable speed. They change angular momentum by ...


20

There was no single DC voltage useful for the whole electronic system. A lot of different voltages were needed, for very noise sensitive systems dedicated DC sources were used to avoid interference. Using three phase 400 Hz AC current was a well established method used in aircrafts before. Providing 400 Hz AC enabled the use of aircraft instruments within ...


18

Check the DSN Now page when it will show any of its stations communicating with Voyager 1 (code VGR1) or Voyager 2 (VGR2), select that dish and then expand the side column on the right to show all the data. It will show transmit power under up signal section. I'll update this answer as soon as I see that happen (see below for updates), but it would be in the ...


18

The limit isn't due to power, but to engine lifetime and fuel limits. Ion engines produce very little thrust, so in order to reach speeds of 100km/s they must accelerate continuously for months or years. The Dawn spacecraft, for example, was built with three redundant ion thrusters to extend its lifetime, and got nowhere near 100km/s; it carried enough ...


15

All cubesats that I have personal knowledge of (including the Planet Labs fleet) were launched with partially charged batteries, typically at around the 50% level that minimizes degradation in storage. The launch providers do typically impose fairly strict rules requiring redundant physical interlock switches to prevent any electronics from being energized ...


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

The ISS has a number of thermal management systems. The most visible part are the radiators attached to the main truss (they are the two sets of 3 white panels just to the left and right of middle, and yes, there's a tear towards the end of the first one on the right side) Those panels are part of the External Thermal Control System (ETCS), if I'm not ...


13

An RTG certainly can and would work on Venus, since the hot side is about 1200 C. It just wouldn't be as efficient as it would be with a colder cold side. Search for papers by Geoff Landis on this.


13

Martin's answer offers a great overview and a link to a hidden gem - transcribed interview with Michael Foale, covering the incident and its aftermath in a great detail. I'd like to share the most interesting bits here as an answer for reference. The whole transcript is a very very interesting and chilling reading, what follows are some terse excerpts (so ...


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.


12

All the electrical power requirements of the International Space Station are provided by its solar array wings and excess is stored in batteries (1) to be used when the station isn't exposed to direct sunlight, so there isn't any waste per se, the worst it could be said about it is that some of it isn't utilized. And it wouldn't be all that much anyway, for ...


12

Considering fuel consumption or energy expenditure may be misleading, because of the huge change in mass over the flight as fuel is expended. 2/3 of the fuel is expended by the first stage, which only produces 1/3 of the total velocity, for example. Another way to look at the question is through delta-v expenditure; according to Bob Braeunig's simulation ...


12

Here's a brief answer since some people are trying to close the question and prevent answers: As @ikrase points out answers to the Physics SE question Why is the Peltier / Seebeck Effect's efficiency so low in practical devices? are helpful here. Briefly, there are two main parts to an RTG's conversion efficiency Thermodynamics limit The fraction of the ...


11

It would be highly problematic for reaction wheels to serve dual purposes as reaction control devices and energy storage mechanisms. It might be possible, but the implementation would be extremely complicated. Starting with a simpler case of a three-wheel design, a desired spacecraft attitude and/or slew rate uniquely determines the wheel speed. That is, ...


11

The "radio frequency" system draws up to 111.7 W (PDF on Voyager design, table 2 on page 10). This is purely the power amplifier, modulation/demodulation is listed separately at another 11 W. Note that this has nothing to do with the dish. Those 23 W is what comes out of the antenna as radio waves. The dish reflects pretty much 100% of this.


11

High temperature in itself is useless. You need a temperature differential, just like the RTG exploits the difference between the temperature of the Plutonium and that of outer space. Kinetic energy (heat) flows from the high temperature region to the low temperature region and can do work on the way. On Venus, there's no gradient. Everything has the same ...


11

The problem is with the low energy density of cosmic rays in the universe. Individual cosmic rays are indeed very energetic, but there just aren't enough of them to be a significant power source for interplanetary or interstellar vehicles. The Wikipedia article about cosmic rays (https://en.wikipedia.org/wiki/Cosmic_ray) quotes the energy density of cosmic ...


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 ...


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