This is the premise: the occupants of a spaceship die hundreds of millions of years in the past orbiting above a Earth-sized planet. It has a supercomputer that allows the ship to still fire its rockets if it detects even slight orbital decay. The ship has enough power (i.e., a megaton neutron reactor, generating 7.50x10^13 joules of energy per kilogram) to sustain its orbit and to power its machines.

However, when the occupants died, the organisms that live on them didn't die. So, now when the bacteria-analogues evolved for hundreds of millions of years into metazoa and all the machinery has rusted except for the rockets, would the ship still stay intact?

I'm asking if the spaceship will stay intact under the radiation of the extreme proximity of a white subgiant star (maybe an F0IV or something like that), stellar wind, interplanetary particles, and cosmic radiation, if that clears it up. I gave the information about the metazoa for background.

The neutron generator works on beta radiation so electrons and protons come out of the rockets. Whatever energy is not used for powering the ship is not drawn from the electrons and so let them fly out of the rocket unhindered. It does not generate neutrons; it generates electricity from beta decay. Once a neutron decays, the electron is launched out of the neutron core. This would mean that when the core decays completely, the products would be a proton core, which would cause the ship to explode because there are too few neutrons. This won't happen anytime in the near future. I found the energy density of neutronium by the mass of a free neutron and the energy that the electron has once it undergoes beta decay.

The atmosphere would be nitrogen dioxide.

Instead of using beta decay, would it be better if I use metallic hydrogen and rely on that to recombine to provide electricity and use something like hydrogen peroxide as propellant?

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    $\begingroup$ Hi Pyrania, welcome to worldbuilding! Can you please focus on what specifically you want to last - the ship as a celestial body in orbit, its hull being fully intact, "megaton neutron reactor" still working etc.? $\endgroup$ – Alexander Aug 29 '18 at 23:25
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    $\begingroup$ One important criteria is missing - the internal atmosphere composition, what regenerates it, and how pressure-tight is the space ship: Even the most ardently-sealed human contraption will eventually leak through the material in a vacuum. The gas molecules, whatever they are, will eventually 'leach' through.almost any known human material. But if your microbes are anaerobic (don't need oxygen) then you can eliminate rusting by eliminating oxygen in the atmosphere. Our moon has been in orbit for a very, very long time without the need for altitude adjustment. $\endgroup$ – Justin Thyme Aug 30 '18 at 1:36
  • $\begingroup$ Survivability from impact objects s is just engineering. $\endgroup$ – Justin Thyme Aug 30 '18 at 1:37
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    $\begingroup$ Throw in some nanobots for repair and you are good to go. $\endgroup$ – Justin Thyme Aug 30 '18 at 1:39
  • $\begingroup$ "The neutron generator works on beta radiation so electrons and protons come out of the rockets." I know this is going to sound stupid, but don't neutron generators generate neutrons? Also, beta radiation are electrons and positrons, not protons. $\endgroup$ – RonJohn Aug 30 '18 at 2:59

Power is necessary but not sufficient, since the purpose of the power (technically the energy) is to propel mass in the "opposite" direction.

Thus, the amount of time your derelict spaceship stays in orbit is based on:

  1. the amount of mass in it's altitude control motors, and
  2. the initial altitude.

The higher the orbit, the longer it will stay in orbit. Specifically, if it's significantly above 26,200 miles, then it could last for a long time.

If you're asking how long before the ship falls apart, that all depends on how durably it's constructed, if it's got "anti-meteor lasers" (hey, it's sci fi!) and whether it's lucky enough to not get hit by something Really Big.

As for whether it could maintain structural integrity for as long as you ask... well, trees are only 370 million years old. In other words, "hundreds of millions of years in the past" is a Really Long Time.

But... it's your story. Just say it's that old, make some comments about how beat up it is and that "they don't make 'em like that any more!", then continue with your story. Readers will accept that.

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    $\begingroup$ All the really sensitive wiring is probably made of gold or another non corrodable. Is silicon non corodable? Either way the electrical systems should last basically as long as the generator. I would worry about a pure oxygen atmosphere in a spacecraft causing corrosion, but an oxidized layer might prevent it from destroying the ship entirely. The biggest threat will definitely be debris from outside the ship. Actually even more than that, the life support that sustains the bacteria might go out. Software errors or components like light bulbs that will burn out eventually. $\endgroup$ – Clay Deitas Aug 30 '18 at 0:13
  • $\begingroup$ @ClayDeitas a "pure oxygen atmosphere" is exceedingly unlikely and aluminum already forms a thin oxide layer of protection. It's the rubber seals that will fail first. But... if you can build an interstellar ship, you can make it out of all varieties of unobtanium. $\endgroup$ – RonJohn Aug 30 '18 at 0:20
  • $\begingroup$ They use rubber seals on spaceships? Also you're right, it wouldn't be pure oxygen. The 20% oxygen would still corrode... well I don't actually know what spaceships are made of, but I know oxygen isn't nice to most elements. $\endgroup$ – Clay Deitas Aug 30 '18 at 0:24
  • $\begingroup$ @ClayDeitas Human spaceships are mostly aluminum, carbon fiber, etc. But this isn't a human ship. Thus, it can be made of unobtanium. $\endgroup$ – RonJohn Aug 30 '18 at 0:39
  • $\begingroup$ @Clay Deitas Sure do use rubber on space ships. research the famous failure of the o-rings on a certain shuttle disaster right off the launch pad. $\endgroup$ – Justin Thyme Aug 30 '18 at 1:27

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