Highly speculative thoughts off the top of my head (only with what little I can remember from my high school physics):
The main factor that determines escape velocity is the mass of the planet (there’s also atmospheric drag, but it’s generally manageable unless the world is a perpetual hurricane hell, in which case I doubt it has any civilization). After a certain mass threshold, the planet is likelier to be gaseous than rocky. I don’t think Neptune-like or Jupiter-like worlds are suitable for life (but their moons are another story). In general, I’d say if the world is too big to jump out of, it’s too gaseous for anything to have walked on it anyway. Edited to add: Inhabited moons of Jupiter-like worlds would also need to take into account the planet’s escape velocity, even if it’s lower where they are.
If the planet is a big Earth (that is, quite massive but still mostly rocky), the greater gravity will result in a thicker and denser atmosphere, but I don’t know enough aerodynamics to tell how much, if any, this detail will add to the problem of escape velocity. But this difference may change the rules as to which fuels will be solid, liquid or gaseous under that planet’s normal conditions.
Another, related problem is payload. For example, if the planet’s intelligent species is aquatic, the spaceship will need to be filled with water instead of air; this will increase the total mass horribly and require a much more potent fuel (but all this is assuming that an aquatic species has had the opportunity to discover fire in the first place).
In worlds too big to escape by propulsion, people may come up with the idea of the space elevator, but the extra gravity will require taking into account the structure’s weight. The counterweight at the upper end will need to be heavier and/or farther. Issues related to which material is best suited for this building scenario and whether there’s a limit to how big a space elevator you can build are beyond my knowledge. According to Wikipedia, nanotubes appear to be a workable choice on Earth.
Some world out there may have a ridiculously tall mountain that extends into the upper atmosphere. Gravity at the top will be lower, and if a launch platform can be built there, takeoff will be easier. Of course, this is an “if” bigger than said mountain.
India has a huge coastline, but for mythical/cultural reasons, Hinduism used to have a taboo against sea travel. In the worst scenario, our heavy aliens may stay on ground, not because they can’t, but because they won’t; maybe their atmosphere looks too scary or their planet attracts too many meteorites or it has several ominous-looking moons or something.
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.
In practice even for a planet with as thin an atmosphere as Earth, getting past the atmosphere is more difficult than actually reaching escape velocity. One of the most common times for a rocket to break up is near Max Q which is where maximum aerodynamic stress occurs. This is generally in the range of about 10 km to 20 km up.
In worlds too big to escape by propulsion, people may come up with the idea of the space elevator, but the extra gravity will require taking into account the structure’s weight.
Getting enough mass up there to build a space elevator is itself a very tough problem.
Some world out there may have a ridiculously tall mountain that extends into the upper atmosphere. Gravity at the top will be lower, and if a launch platform can be built there, takeoff will be easier. Of course, this is an “if” bigger than said mountain.
Whether gravity is stronger or weaker on top of a mountain is surprisingly complicated and depends a lot on the individual planet’s makeup. However, at least on Earth-like planets it is weaker. See here. Note though that if a planet is really massive it is less likely to have large mountains. You can more easily get large mountains when a planet is small. (e.g. Olympus Mons on Mars).
India has a huge coastline, but for mythical/cultural reasons, Hinduism used to have a taboo against sea travel. In the worst scenario, our heavy aliens may stay on ground, not because they can’t, but because they won’t; maybe their atmosphere looks too scary or their planet attracts too many meteorites or it has several ominous-looking moons or something.
This would require everyone on the planet to take this same attitude. This seems unlikely to be common.
You got me curious, and I read a bit more, and found this on Wikipedia:
A rocket moving out of a gravity well does not actually need to attain escape velocity to escape, but could achieve the same result (escape) at any speed with a suitable mode of propulsion and sufficient propellant to provide the accelerating force on the object to escape. Escape velocity is only required to send a ballistic object on a trajectory that will allow the object to escape the gravity well of the mass M.
In lay terms, I guess this means that, unlike a cannon ball, which only gets one initial “push”, a rocket is being “pushed” continually and thus doesn’t need to worry about escape velocity.
Because of the atmosphere it is not useful and hardly possible to give an object near the surface of the Earth a speed of 11.2 km/s (40,320 km/h), as these speeds are too far in the hypersonic regime for most practical propulsion systems and would cause most objects to burn up due to aerodynamic heating or be torn apart by atmospheric drag. For an actual escape orbit a spacecraft is first placed in low Earth orbit (160–2,000 km) and then accelerated to the escape velocity at that altitude, which is a little less — about 10.9 km/s. The required change in speed, however, is far less because from a low Earth orbit the spacecraft already has a speed of approximately 8 km/s (28,800 km/h).
So first they get the rocket high enough to be safe from the air, and then they speed it up.
Highly speculative thoughts off the top of my head (only with what little I can remember from my high school physics):
The main factor that determines escape velocity is the mass of the planet (there’s also atmospheric drag, but it’s generally manageable unless the world is a perpetual hurricane hell, in which case I doubt it has any civilization). After a certain mass threshold, the planet is likelier to be gaseous than rocky. I don’t think Neptune-like or Jupiter-like worlds are suitable for life (but their moons are another story). In general, I’d say if the world is too big to jump out of, it’s too gaseous for anything to have walked on it anyway. Edited to add: Inhabited moons of Jupiter-like worlds would also need to take into account the planet’s escape velocity, even if it’s lower where they are.
If the planet is a big Earth (that is, quite massive but still mostly rocky), the greater gravity will result in a thicker and denser atmosphere, but I don’t know enough aerodynamics to tell how much, if any, this detail will add to the problem of escape velocity. But this difference may change the rules as to which fuels will be solid, liquid or gaseous under that planet’s normal conditions.
Another, related problem is payload. For example, if the planet’s intelligent species is aquatic, the spaceship will need to be filled with water instead of air; this will increase the total mass horribly and require a much more potent fuel (but all this is assuming that an aquatic species has had the opportunity to discover fire in the first place).
In worlds too big to escape by propulsion, people may come up with the idea of the space elevator, but the extra gravity will require taking into account the structure’s weight. The counterweight at the upper end will need to be heavier and/or farther. Issues related to which material is best suited for this building scenario and whether there’s a limit to how big a space elevator you can build are beyond my knowledge. According to Wikipedia, nanotubes appear to be a workable choice on Earth.
Some world out there may have a ridiculously tall mountain that extends into the upper atmosphere. Gravity at the top will be lower, and if a launch platform can be built there, takeoff will be easier. Of course, this is an “if” bigger than said mountain.
India has a huge coastline, but for mythical/cultural reasons, Hinduism used to have a taboo against sea travel. In the worst scenario, our heavy aliens may stay on ground, not because they can’t, but because they won’t; maybe their atmosphere looks too scary or their planet attracts too many meteorites or it has several ominous-looking moons or something.
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?
Good analysis! A few remarks:
In practice even for a planet with as thin an atmosphere as Earth, getting past the atmosphere is more difficult than actually reaching escape velocity. One of the most common times for a rocket to break up is near Max Q which is where maximum aerodynamic stress occurs. This is generally in the range of about 10 km to 20 km up.
Getting enough mass up there to build a space elevator is itself a very tough problem.
Whether gravity is stronger or weaker on top of a mountain is surprisingly complicated and depends a lot on the individual planet’s makeup. However, at least on Earth-like planets it is weaker. See here. Note though that if a planet is really massive it is less likely to have large mountains. You can more easily get large mountains when a planet is small. (e.g. Olympus Mons on Mars).
This would require everyone on the planet to take this same attitude. This seems unlikely to be common.
You got me curious, and I read a bit more, and found this on Wikipedia:
In lay terms, I guess this means that, unlike a cannon ball, which only gets one initial “push”, a rocket is being “pushed” continually and thus doesn’t need to worry about escape velocity.
So first they get the rocket high enough to be safe from the air, and then they speed it up.