The smallest planet you can probably maintain an atmosphere on for gigayears of time is probably half to a third of an earth mass (barring the effects of geology). That gives you an escape velocity between 70 and 80 % that of here given similar composition and no thousand km thick hot ice layers or anything.
EDIT: If you assume an escape velocity of Earth’s and a specific impulse similar to a Merlin engine and ignore all gravity drag and atmosphere, using the rocket equation an SSTO to LEO requires a fuel to payload+structure mass ratio of at least 12.0. If you assume an escape velocity of 75% that of Earth, it requires a mass ratio of at least 6.5. Probably doubles your mass to orbit per unit fuel. If you have an escape velocity of 1.25x that of Earth, your SSTO requires a mass ratio of 22.4. Mars, by comparison, reads as a mass ratio of 3.1 under these optimistic assumptions.
Of course staging improves all of these numbers and squishes them together some, as does using better fuel than kerosine, while dealing with an atmosphere and gravity drag and propellants worse than kerosine makes things much worse. For a reality check, existing real multistage Earthly launch systems I just quickly looked up have mass ratios between ~35 and ~15 (though the 15 includes the total mass of the space shuttle not just the payload, while the upper stage is not included in other higher numbers for other systems).
Assuming an advanced civilization, the main limiting factor for the viable commercial use of nuclear energy would be the abundance of radioactive elements in the planet. During the formation of the planet, its mass will have an effect on which elements get captured. Unfortunately, Wikipedia isn’t helpful on the specifics of planet mass vs. planet composition, but we know it depends on the composition of the protoplanetary nebula, which depends on the type of star. Too many factors.
Nitpick: It wouldn’t have to be commercial use of nuclear energy. Even if we’re limited to human institutions, it could be governmental use, and I have a notion that religion might be the best sort of institution for getting people off the planet. Religions have a potential for big, long term projects that don’t make practical sense.
Thanks for looking into the question of planetary mass and getting off the planet—once the question occurred to me, it exploded into a lot of additional questions, and we haven’t even gotten to whether planetary mass might have an effect on the evolution of life.
One additional factor: the amount of radioactive elements still usable (that is, not completely decayed) vs. how many billion years it took to evolve from alien amoeba to alien tool-users.
Thank you. I’m also interested in planets with less mass/lower escape velocity and non-chemical fuel methods. Atomic or nuclear fuel? Laser launch?
The smallest planet you can probably maintain an atmosphere on for gigayears of time is probably half to a third of an earth mass (barring the effects of geology). That gives you an escape velocity between 70 and 80 % that of here given similar composition and no thousand km thick hot ice layers or anything.
EDIT: If you assume an escape velocity of Earth’s and a specific impulse similar to a Merlin engine and ignore all gravity drag and atmosphere, using the rocket equation an SSTO to LEO requires a fuel to payload+structure mass ratio of at least 12.0. If you assume an escape velocity of 75% that of Earth, it requires a mass ratio of at least 6.5. Probably doubles your mass to orbit per unit fuel. If you have an escape velocity of 1.25x that of Earth, your SSTO requires a mass ratio of 22.4. Mars, by comparison, reads as a mass ratio of 3.1 under these optimistic assumptions.
Of course staging improves all of these numbers and squishes them together some, as does using better fuel than kerosine, while dealing with an atmosphere and gravity drag and propellants worse than kerosine makes things much worse. For a reality check, existing real multistage Earthly launch systems I just quickly looked up have mass ratios between ~35 and ~15 (though the 15 includes the total mass of the space shuttle not just the payload, while the upper stage is not included in other higher numbers for other systems).
Assuming an advanced civilization, the main limiting factor for the viable commercial use of nuclear energy would be the abundance of radioactive elements in the planet. During the formation of the planet, its mass will have an effect on which elements get captured. Unfortunately, Wikipedia isn’t helpful on the specifics of planet mass vs. planet composition, but we know it depends on the composition of the protoplanetary nebula, which depends on the type of star. Too many factors.
Nitpick: It wouldn’t have to be commercial use of nuclear energy. Even if we’re limited to human institutions, it could be governmental use, and I have a notion that religion might be the best sort of institution for getting people off the planet. Religions have a potential for big, long term projects that don’t make practical sense.
Thanks for looking into the question of planetary mass and getting off the planet—once the question occurred to me, it exploded into a lot of additional questions, and we haven’t even gotten to whether planetary mass might have an effect on the evolution of life.
One additional factor: the amount of radioactive elements still usable (that is, not completely decayed) vs. how many billion years it took to evolve from alien amoeba to alien tool-users.
Giant capacitor plates and you suddenly remove the insulation?