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Are there any alternative energy sources to power inter-stellar probes such as Voyager I and II developed or in development by any space agency?

Plutonium (238-Pu) has a half-life of about 87,7 years, which is a big limitation to the lifespan of such missions, even if we have a lot of spare energy at the start. Full nuclear reactor is too heavy.

Is there any concept of powering such probes with an energy source, that could work for hundreds of years? Otherwise, the only possibility to reach further is to accelerate probes to higher velocities.

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    $\begingroup$ Can you clarify why a "Full nuclear reactor is too heavy.", seems like most of the weight/mass is in shielding the world from the radiation, and cooling. Both of which get much less expensive (mass wise) in space. $\endgroup$ – James Jenkins Jul 21 '13 at 19:31
  • $\begingroup$ See also: Are there any safe-te-launch alternatives to RTGs.... $\endgroup$ – gerrit Jul 22 '13 at 10:25
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Actually, a full nuclear reactor is not too heavy, and in fact, has been accomplished. There have been quite a few, Wikipedia has an article on the subject. The United States has launched 1 nuclear reactor into space, the Soviet Union has launched around 40. These would have a lifetime of much longer than RTGs, as the half-life of Uranium 235 is in the millions of years, in other words, it won't go away unless it is being used. One would simply have to be careful enough not to use the entire supply of fuel before reaching the destination, but that shouldn't be too difficult of a problem.

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  • $\begingroup$ To be precise: these were not "normal" nuclear reactors with turbines but rather thermionic/thermoelectric reactors. Neither can be relied upon to work for hundreds of years (due to embrittlement and fuel burnup). $\endgroup$ – Deer Hunter Jul 22 '13 at 19:40
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    $\begingroup$ Nuclear reactors don't operate because the fuel decays (half-life), but because it's fissile and can sustain a chain reaction. Depending on reactor design, refueling cycles can be decades-long (Nimitz-class carriers last about 20 years) or just a few years (power plants refuel about every 18-24 months). $\endgroup$ – Nick T Oct 11 '13 at 23:34
  • $\begingroup$ ...never mind they can be shut down and then act as very weak thermoelectric generators (e.g. sufficient to provide enough power for minimal systems to perform a wake-up to start them again). So it's 20 years of actual "high-power operation" but centuries in "sleep mode". $\endgroup$ – SF. Nov 25 '15 at 10:31
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When (if) we get cold fusion under control, the probe could be scooping free hydrogen out of the interstellar space. While there's not much of it, if the probe moves fast enough it will sweep huge volumes of space per each second of flight. That would be fully sustainable, since the hydrogen is pretty much everywhere, but the probe would eventually reach a break-even speed where drag of captured particles would outweigh energy gain from fusion of "burning them up" for accelerating.

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    $\begingroup$ Use the Source, Luke: this is called Bussard ramjet $\endgroup$ – Deer Hunter Jul 22 '13 at 19:41
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    $\begingroup$ The Bussard ramjet would not need cold fusion. Hot fusion would suffice. But the problem is making a fusion process with a net positive energy output. Short of a star, I don't think anyone has done that yet. $\endgroup$ – Osteoboon Jul 23 '13 at 21:06
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    $\begingroup$ @CopyrightX: Yes, but an army of scientists is trying to develop that, and the question was about techniques "in developed". $\endgroup$ – SF. Jul 23 '13 at 21:13
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    $\begingroup$ There is no cold fusion. $\endgroup$ – Mark Adler Oct 11 '13 at 23:32
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If General Fusion achieves their goal, an electric propulsion probe powered by nuclear fusion could not be that far-off as we thought:

Piston-driven fusion prototype

The idea seems to be worthy of consideration: a vortex of melted lead-lithium mixture, some spheromaks of D-T plasma are injected in the vortex, and then the pistons create a spherically convergent shock wave. The numbers say that fusion conditions could be reached, but in this case, the big if are the hydrodynamic instabilities, instead of the plasma

http://nextbigfuture.com/2013/10/general-fusion-targeting.html

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