Phase 3 seems a bit wasteful to me. 0.1 c change in velocity at 100,000 light years yields a margin of 100 light years across; that amounts to thousands of stars even in the galactic suburb. Why be so picky about target stars and not simply send more probes instead? If you really need to make course corrections that big, you should do it in stages starting millions of light years out.
In phase 5, can’t we just use the lightsail again? 0.2 c seems very doable. Wolf-Rayet stars are top candidates here since they have a very high photon pressure to gravity ratio. On second thought, this probably won’t work for galaxies on the edge of our cosmic horizon since by the time of our arrival only red dwarfs will be left, but the Virgo Supercluster is fine.
Finally, in phase 6, I don’t see why not to use our fission/fusion/antimatter engine from phase 3 again. Maneuvering a solar system with low thrust ion engines is one of the very few technologies on this intergalactic journey we have already mastered.
Good point on phase 6. For phase 3, smaller changes in velocity further out are fine, but I still think that even with less velocity changes, you’ll still have difficulty finding an engine that gets sufficient delta-V that isn’t fission/fusion/antimatter based. (also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad.) For phase 5, I don’t think a lightsail would provide enough deceleration, because you’ve got inverse-square losses. Maybe you could decelerate with a lightsail in the inner stellar system, but I think you’d just breeze right through since the radius of the “efficiently slow down” sphere is too small relative to how much you slow down, and in the outer stellar system, light pressure is too low to slow you down meaningfully.
Assuming acceleration occurs over a 40 light year distance and uniform acceleration (because why not; we have a variable power source), the ship would experience a constant acceleration of ~0.3m/s^2 (convertalot.com/relativistic_star_ship_calculator.html ).
If we wanted the same peak deceleration using only lightsail and a sun-like star, we’d get a deceleration of 83km/s (back of envelope calculation analogizing photon pressure as a reversed gravitational well), so we’ll need 72 stars in total.
That is quite reasonable considering the star density in the galactic core. The only problem here of course is that your lightsail might be so small that gravitation dominates, in which case you have to look for stars with higher photon-pressure-to-mass ratio, which are less densely populated. It’s a trade-off between peak acceleration, destination constraint and sail size. Our sun for example would be among the worst targets for decelerating an incoming intergalactic spaceship.
also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad
Is it though? Radiation in general tends to attenuate exponentially in matter, so a merely linear increase in shielding should solve the problem completely.
Btw this sequence has been a very enjoyable read; I’m glad I’m not the only speculating about Clarketech-level space travel in free time.
Phase 3 seems a bit wasteful to me. 0.1 c change in velocity at 100,000 light years yields a margin of 100 light years across; that amounts to thousands of stars even in the galactic suburb. Why be so picky about target stars and not simply send more probes instead? If you really need to make course corrections that big, you should do it in stages starting millions of light years out.
In phase 5, can’t we just use the lightsail again? 0.2 c seems very doable. Wolf-Rayet stars are top candidates here since they have a very high photon pressure to gravity ratio. On second thought, this probably won’t work for galaxies on the edge of our cosmic horizon since by the time of our arrival only red dwarfs will be left, but the Virgo Supercluster is fine.
Finally, in phase 6, I don’t see why not to use our fission/fusion/antimatter engine from phase 3 again. Maneuvering a solar system with low thrust ion engines is one of the very few technologies on this intergalactic journey we have already mastered.
Good point on phase 6. For phase 3, smaller changes in velocity further out are fine, but I still think that even with less velocity changes, you’ll still have difficulty finding an engine that gets sufficient delta-V that isn’t fission/fusion/antimatter based. (also in the meantime I realized that neutron damage over those sorts of timescales are going to be *really* bad.) For phase 5, I don’t think a lightsail would provide enough deceleration, because you’ve got inverse-square losses. Maybe you could decelerate with a lightsail in the inner stellar system, but I think you’d just breeze right through since the radius of the “efficiently slow down” sphere is too small relative to how much you slow down, and in the outer stellar system, light pressure is too low to slow you down meaningfully.
Assuming acceleration occurs over a 40 light year distance and uniform acceleration (because why not; we have a variable power source), the ship would experience a constant acceleration of ~0.3m/s^2 (convertalot.com/relativistic_star_ship_calculator.html ).
If we wanted the same peak deceleration using only lightsail and a sun-like star, we’d get a deceleration of 83km/s (back of envelope calculation analogizing photon pressure as a reversed gravitational well), so we’ll need 72 stars in total.
That is quite reasonable considering the star density in the galactic core.
The only problem here of course is that your lightsail might be so small that gravitation dominates, in which case you have to look for stars with higher photon-pressure-to-mass ratio, which are less densely populated. It’s a trade-off between peak acceleration, destination constraint and sail size. Our sun for example would be among the worst targets for decelerating an incoming intergalactic spaceship.Is it though? Radiation in general tends to attenuate exponentially in matter, so a merely linear increase in shielding should solve the problem completely.
Btw this sequence has been a very enjoyable read; I’m glad I’m not the only speculating about Clarketech-level space travel in free time.