How would we power probes or spaceships over hundered or thousands of years?

Question

There are different optimal power sources for probes and spaceships for different kinds of missions.

• Solar
  • Well suited for many missions in the inner solar system. Only limited degradation over time but have to be adjusted towards to sun for optimal performance
• Batteries
  • relatively cheap but don't like cold very much and deplete quite quickly af not recharged in some way
• RTG
  • can't be recharged but provide a steady (but declining) source of heat and power over decades
• Fission rectors
  • Don't know anything about that

Now if we want to send probes on interstellar cruises we need to power them for centuries or even millenia. They are too far away from the sun or any other star to get any useful power from solar panels, batteries would be depeted before the probe reached the outer solar system and even an RTG with todays technology would only deliver miniscule amounts of power after a century in space.

So what could be the solution? Are there power sources that provide useful amounts of power over such timeframes? Could we use materials with a longer halflife for an RTG and thus increase the time it works? Could we use solar and just put the probe to deep sleep during transfer and it would wake up as soon as it gets close enough to the target star (I see some risks there)?

Answer 1

Americium-241 is a RTG candidate for longer interstellar missions. It has a much lower power density (a quarter of Pu-238) but it’s half-life is 432 years (vs 88yr for Pu-238). If you want it to provide power for over a millennium, the mass of Am vs. Pu will be significant.

Part of your spacecraft’s power problems will be power storage. The longer the mission, the less often you will be transmitting, but the stronger your signal will need to be (distance squared). If your power source has a fixed output, you will need to store power for transmission bursts. Chemical batteries and capacitors function poorly at low temperature. Perhaps you could use waste RTG heat to maintain the temperature of chemical fuel synthesis unit. The fuel could be stored at ambient temperature, and re-warmed for power generation at the time of transmission.

Thermal Nuclear Rocket propulsion has advantages over chemical and ion thrusters for interstellar propulsion. Once the nuclear fuel is spent, residual heat from isotope decay could heat an RTG. However, compared with Am-241, the heat of isotope decay of spent reactor fuel decreases rapidly (3 orders of magnitude over a millennia.) https://en.wikipedia.org/wiki/Spent_nuclear_fuel#/media/File:Activitytotal1.svg

A totally speculative electrical power source in interstellar space is Galactic Cosmic Rays GCR. These are mostly high energy protons produced by energetic events such as supernova. A sub-relativistic spacecraft is struck by about 2.0 x 10^5 particles/sec/cm3. The energy flux is about the same as starlight in the visible spectrum. https://iopscience.iop.org/article/10.3847/1538-4357/ac6a50/pdf

It is obviously impractical to build PV panels to collect starlight. But protons are charged, so could theoretically be collected by a magnetic field, as proposed in a Bussard Ram Jet. It is doubtful a Bussard Ram Jet could concentrate protons adequately for nuclear fusion, but could a magnetically compressed stream of protons be used for electrical generation? The magnetic field could double as a radiation shield since GCR protons are very high energy.