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70 votes
Accepted

Why wasn't an RTG used on the Juno spacecraft?

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 ...
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37 votes

Why wasn't an RTG used on the Juno spacecraft?

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 ...
  • 119k
30 votes

Why exactly does Curiosity's RTG make more electricity on cold days?

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 ...
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28 votes
Accepted

What powers New Horizons?

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 ...
  • 122k
27 votes

Why are RTGs different colors?

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 ...
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27 votes
Accepted

Why are RTGs different colors?

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 ...
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25 votes
Accepted

Potential high temperature power sources for a Venus lander

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, ...
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24 votes
Accepted

How powerful are reaction control wheels?

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 ...
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24 votes
Accepted

WHY did Rosetta have to go into hibernation for 2.5 years?

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 ...
  • 122k
23 votes
Accepted

How does electrical safety system work on ISS?

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 ...
22 votes

What powers New Horizons?

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 ...
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21 votes

How much power would a spacecraft's magnetic shield require?

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 ...
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20 votes
Accepted

How much RF transmit power does DSN need to send commands to Voyager?

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 ...
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20 votes

Why did Apollo spacecraft use both AC and DC equipment? Still used in present and future designs?

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. ...
  • 47.8k
19 votes

What is the maximum speed an ion engine can propel a spacecraft at?

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 ...
15 votes
Accepted

Are cubesats deployed with fully discharged batteries? Even those on Sherpa?

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....
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14 votes

Why wasn't an RTG used on the Juno spacecraft?

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 ...
13 votes

Potential high temperature power sources for a Venus lander

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.
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13 votes

How did the Mir crew recover from the power outage following collision with Progress 34?

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 ...
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13 votes

Why exactly does Curiosity's RTG make more electricity on cold days?

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.
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13 votes
Accepted

Why are RTGs so inefficient?

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 ...
  • 148k
12 votes
Accepted

How much power does the Voyager radio dish draw?

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 ...
  • 122k
12 votes
Accepted

What is the distribution of ∆V amongst different stages of rocket flight?

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 ...
11 votes
Accepted

Would it be possible to power a Venus lander using a temperature differential?

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 (...
  • 122k
11 votes

Cosmic rays can be new energy source

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 ...
10 votes

Total Mass of the ISS Solar Array

I am going to answer your specific question, to wit: So is EACH IEA a cube 5 meters on a side weighing 7.7 metric tons, or are ALL FOUR of these IEAs, in total, these dimensions? This question was ...
10 votes

How could a fast powerful rover on Mars be powered electrically?

An option not mentioned so far are solar power satellites. A large antenna in orbit could focus a tight, high-power microwave beam on the rover. From a low Mars orbit (300 km), a 1 km antenna could ...
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10 votes

WHY did Rosetta have to go into hibernation for 2.5 years?

Solar arrays are pretty much worthless beyond 4 AU or so from the Sun (depending on the size of the arrays of course). The Juno spacecraft will be the first mission to Jupiter (about 5 AU from the Sun)...
10 votes
Accepted

Solar cells in series

No, it doesn't make sense, for two key reasons. First of all, it is a loss of redundancy. If one of the cells fails, then the entire string fails. You really don't want that to happen... If you build ...
  • 119k
10 votes

How does a solar eclipse affect satellites?

Orbiting satellites have to deal with darkness all the time when Earth is between the satellite and the sun, and these periods last for much longer than an eclipse does for the satellite. So although ...

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