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Musk said at the start of his presentation that the magnitude of earth's gravity is such that we are right on the cusp of rocket flight possibility. If the magn

Musk said at the start of his presentation that the magnitude of earth's gravity is such that we are right on the cusp of rocket flight possibility. If the magnitude were a little more, rocket flight would be impossible. If it were a bit less, it would all be a lot easier. Can anyone expand on that, assuming it's the truth..

avs733 7y ago
I'm gonna go with citation/mathematics needed. Spaceflight is about velocity and energy.
- cbanek 7y ago
  https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation It's also about gravity, since gravity is what you are fighting against.
allannienhuis 7y ago
https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...
- duskwuff 7y ago
  Nice find! Near the end, the author directly addresses this question: > If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.
  - acidburnNSA 7y ago
    Not using chemical rockets. Nuclear propulsion would be able to do it with the ~1000000x energy density advantage of nuclear fuel over chemical fuel.
    - nine_k 7y ago
      The problem is that the super-dense energy of a nuclear blast also evaporates solids and rains gamma rays on everything around. Imagine how hard space exploration would be if the only practical rocket were a Project Orion.
    - Baeocystin 7y ago
      I remember seeing a simple demonstrator some years back of a laser pulsing against the backside of a spinning disc, the geometry of things causing the heat from the laser to provide thrust. I can't find the video I saw, but here's a general link on the topic: https://en.wikipedia.org/wiki/Laser_propulsion#Laser_energized_rocket https://en.wikipedia.org/wiki/Laser_propulsion#Laser_energiz... This comes with many problems, but it does bypass the rocket equation, while not requiring Project Orion levels of trouble.
      - rocqua 7y ago
        I mean you still need to carry propellant: the stuff that is ablated away. It's just that the exhaust velocity is much higher.
        king07828 7y ago
        Air from the atmosphere may be the propellant [1]. If sufficient velocity can be achieved before leaving the atmosphere, then no propellant is needed to be carried up on the way to orbit. [1] https://www.youtube.com/watch?v=XhUasBcoj-Q https://www.youtube.com/watch?v=XhUasBcoj-Q
    - jccooper 7y ago
      Nuclear would be better, but it's not quite as good as it seems. Nuclear engines are rather heavier than chemical.
deleted 7y ago
[deleted]
Already__Taken 7y ago
if gravity were much more we'd be out of chemical reactions that produce the energy to beat the rocket equation.
- api 7y ago
  This is a possible Fermi paradox answer, or at least factor. Maybe the vast majority of habitable worlds are larger than Earth. The inhabitants of those worlds might find space flight too hard or even impossible. They also might have less interest in it since they would make the natural anthropic assumption that worlds capable of supporting life are probably huge like theirs.
  - derekp7 7y ago
    That really makes a lot of sense. Because planets smaller than Earth cool a lot faster, so they lose their molten core and plate tectonics (which keeps land mass rising and out of the water), and also a magnetic shield that keeps the atmosphere from stripping away (like Mars).
    - JohnJamesRambo 7y ago
      I'm going to be hopeful and optimistic and point out that the large planets in our solar system aren't rocky and suitable for life as we know it anyway. Earth is the biggest one and the rest are gas planets. Maybe planets that harbor life (rocky, liquid water, goldilocks zone closer to the sun) usually have low enough gravity for aliens to escape it. This diagram has some exoplanets on it for size comparison- https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/media/File:Diagram_of_different_habitable_zone_regions_by_Chester_Harman.jpg https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/...
    - api 7y ago
      Earth might be right at this threshold of small enough for space flight to be practical (still hard, but practical) but big enough to hang onto water and atmosphere.
uoaei 7y ago
I assume he's relating 1) the force of gravity, composition and distribution of our atmosphere, and incredible velocity required for orbit, to 2) the chemical potential of fuels and the limits of structural strength for materials to carry cargo to orbit. I haven't run the numbers, but he's right: it's pretty hard to get something that can carry itself to orbit. It has to climb out of the atmosphere directly against gravitational force and atmospheric drag, and then as early as possible turn sideways and start accelerating to around 28000kph while making sure the rocket is pointed in the right direction and holding together. You need a lot of propellant to get to that state, which means you have to carry it around until you need it, which means the machine needs to be much bigger than just the cargo, which means that all of your challenges just magnified by many orders of magnitude.
- TheSpiceIsLife 7y ago
  Would it be possible for the numbers to be a bit more difficult to work with so maybe we could go to low earth orbit, but never be able to make it to the moon?
  - JshWright 7y ago
    If you can make it to orbit, you can make it anywhere (it just may take multiple launches to assemble/fuel the spacecraft going elsewhere).
    - fireattack 7y ago
      Wait, really? So what's the deal of first and second cosmic velocity?
      - cyphar 7y ago
        I guess the idea is that if you can make it to orbit (even if you waste all of your fuel), then you could in principle refuel in-orbit (by getting another unmanned rocket with just fuel into orbit) and thus escape the orbit. But yes, you still require more energy (exactly sqrt(2) times as much) to "escape" Earth's gravity entirely than to orbit it with a circular orbit.
happytoexplain 7y ago
I wonder how much this could be mitigated by extra-rocket energy delivered while the rocket is still close to the ground - e.g. a cartoonishly large slingshot.
- drusepth 7y ago
  Or a comically large refueling hose hooked up until it hits orbit. I guess in that case you'd probably be fighting gravity even more trying to push fuel up the hose.
- mey 7y ago
  Ground laser propulsion has been considered. https://en.m.wikipedia.org/wiki/Laser_propulsion https://en.m.wikipedia.org/wiki/Laser_propulsion
- derekp7 7y ago
  There's also using chemical rockets for the first stage, then switching to nuclear for the second stage once clear enough of the ground to minimize fallout. Radiation pollution wouldn't be pleasant, but future advances could potentially minimize that also.
  - marktangotango 7y ago
    NERVA XE was damn near ready to fly during the Apollo era for Mars missions. Would've made a hell of 2nd stage engine. https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
- sdenton4 7y ago
  Unfortunately, drag force is proportional to the square of velocity... So the slingshot (or railgun or whatever) maybe gives you a nice acceleration at the start, but a LOT of that initial force will bleed off as drag early in the flight in the thick thick low atmosphere. (Consider the difference in difficulty of riding a bike at 15mph vs 16mph and 25mph vs 26mph... 25->26 requires a hell of a lot more effort, all due to drag.)
- wahern 7y ago
enjeyw 7y ago
Ok so doing super-approximate maths: A fully fueled and loaded Saturn V weighs about 3,000,000 Kg, and has a payload of about 40,000 Kg to the moon. So that means once you've put all the bits of the rocket together that collectively give you sufficient thrust to get you to the moon, you've only got about 1.3% left for actual payload. S if earth's gravity had been ~1.3% stronger, then the weight of the "getting us there" bits of the rocket would have left no room for payload. Any more gravity, and we wouldn't even be getting to the moon. Equally, if the earth's gravity had been 1.3% weaker, we could have doubled our payload to the moon (or done things way easier). Contrast that with a Boeing 747 which has 50% of total mass as payload, and so is relatively insensitive to changes in gravity.
- avmich 7y ago
  Some (funny) sources list Saturn-V payload as 140 tons on LEO :) so according to this logic to get to orbit one has plenty of margin. And as soon as the payload is in orbit it uses full value of accumulated speed, so it's used to get further and getting to the Moon is no harder than with Earth...
- JohnJamesRambo 7y ago
  I recently realized that if we got in Jupiter's gravity well we could probably never escape it again. Someone correct me if I'm wrong. The thought made me feel really uneasy.
  - hossbeast 7y ago
    If that were true, it would be impossible to leave the solar system, since everything is currently in the Sun's gravity well, and the Sun is larger than Jupiter. But we have sent probes outside the solar system (did they fully leave the Sun's gravity well yet though? Not sure)
    - davidivadavid 7y ago
      But we didn't send the probes from the Sun.
libtard 7y ago
Well, it'd prevent rockets for sure, we'd only have nuclear pulse propulsion then to get into orbit and that's a lot harder to justify to people than a rocket.
- wolfram74 7y ago
  There's a lot of alternatives, the main constraint is that it has to be built from the ground up, so space elevators are out. I think a dynamic structure such as a launch loop https://en.wikipedia.org/wiki/Launch_loop https://en.wikipedia.org/wiki/Launch_loop would work. Also inflatable towers for launch assist https://en.wikipedia.org/wiki/ThothX_Tower https://en.wikipedia.org/wiki/ThothX_Tower For even kookier options we've got lasers! https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion
sidorares 7y ago
He was talking about rocket reusability, specifically reusing orbital stage. Just reaching the orbit is possible in stronger gravity, but returning back whole stage is already very close to the limits of current material science
wmp56 7y ago
I have another wild guess. Not only we have just right amount of fossil fuels to leave the planet, but also the sun has just right amount of energy to reach the stage when people can move to another solar system.
- tdy721 7y ago
  My guess is the Sun probably has more... it’s really big. What if it’s just like fossil fuels and there’s enough energy there for us to smear ourselves out? Oh cool, kinda like fission then!
duchenne 7y ago
The rocket equation states that the amount of fuel that you need (to send a given payload into orbit) increases exponentially with the escape velocity of the planet: mass of the rocket with fuel = mass of the payload * exp(escape velocity / engine exaust velocity) That is because the faster you need to go, the more fuel you need to use, but now, you also need even more fuel to accelerate the fuel that you just added. So, it becomes exponential. So, if the earth was bigger, its escape velocity would increase, and the amount of fuel needed to power a space rocket would increase (exponentially) so much that it would become unpractical. Vice versa, with a smaller planet it would require exponentially less fuel to reach orbit. The only solution would be to have an engine with a higher exaust velocity. With the current technology, ion engines have very high exaust velocity but low thrust are very energy hungry. The thermal nuclear engines have both high exaust velocity and high thrust, working prototypes have been built, but all projects were boxed 60 years ago because of the fear of an accident.
mpweiher 7y ago
> earth's gravity [..] right on the cusp of rocket flight possibility He didn't actually say that. He was talking about "rapidly reusable orbital rockets" and "fully reusable orbital rocket[s]" being the "critical breakthrough" that's necessary, the "holy grail of space" and the "fundamental thing that's required". So fully and rapidly reusable. https://youtu.be/sOpMrVnjYeY?t=1234 https://youtu.be/sOpMrVnjYeY?t=1234
sidcool 7y ago
Gravity is taxing. It's possible that our galactic neighbors live on a larger planet with greater gravity which makes their escape from the planet next to impossible.
- silasdavis 7y ago
  Can we swing by and drop them a ladder?
  - sidcool 7y ago
    If we get sucked by the gravity, we are stuck as well.
    - silasdavis 7y ago
      Presumably we'd need to lower it from geostationary orbit. This is a unique solution though I suppose there is some dependency on the mass of the planet where you might be too deep in the well. But you'd be in orbit so presumably some low thrust low propellant engine powered by sunlight ought to be an option?
kromem 7y ago
Hmmm... That's actually really interesting to think about from the anthropic principle perspective. Very interesting that the only intelligent species we are aware of is on a planet that we can leave. I wonder to what extent higher gravitational planets could sustain love that couldn't leave it. An interesting answer to Fermi's paradox.
- htfu 7y ago
  Nah, again - nuclear. I doubt the safety profile would factor much once it's the only available option.
  - mrfusion 7y ago
    Don’t forget ground based propulsion too.
hoorayimhelping 7y ago
That is pretty much all there is: If earth was more massive, the chemical rockets we've been producing wouldn't have enough thrust-to-weight ratio to reach orbital velocity (about 25k km/h). We would have to load the rockets with so much fuel to break free of gravity that they would be too heavy to lift themselves. This is all assuming yields and fuels we have now. If we lived on a more massive earth and we were trying to escape its gravity, I'm sure we'd be using more exotic and dangerous fuels (like all those fun fluorine and boron fuels Dr. Clark mentions in Ignition![0]) to do the job. We just happened to have the capability in the middle of the century to use a fuel we were already making (refined petroleum) for jet engines. [0] https://www.amazon.com/Ignition-Informal-Propellants-University-Classics/dp/0813595835 https://www.amazon.com/Ignition-Informal-Propellants-Univers...

avs733 7y ago

I'm gonna go with citation/mathematics needed. Spaceflight is about velocity and energy.

cbanek 7y ago
https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation It's also about gravity, since gravity is what you are fighting against.

allannienhuis 7y ago

https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...

duskwuff 7y ago
Nice find! Near the end, the author directly addresses this question: > If the radius of our planet were larger, there could be a point at which an Earth escaping rocket could not be built. Let us assume that building a rocket at 96% propellant (4% rocket), currently the limit for just the Shuttle External Tank, is the practical limit for launch vehicle engineering. Let us also choose hydrogen-oxygen, the most energetic chemical propellant known and currently capable of use in a human rated rocket engine. By plugging these numbers into the rocket equation, we can transform the calculated escape velocity into its equivalent planetary radius. That radius would be about 9680 kilometers (Earth is 6670 km). If our planet was 50% larger in diameter, we would not be able to venture into space, at least using rockets for transport.
- acidburnNSA 7y ago
  Not using chemical rockets. Nuclear propulsion would be able to do it with the ~1000000x energy density advantage of nuclear fuel over chemical fuel.
  - nine_k 7y ago
    The problem is that the super-dense energy of a nuclear blast also evaporates solids and rains gamma rays on everything around. Imagine how hard space exploration would be if the only practical rocket were a Project Orion.
  - Baeocystin 7y ago
    I remember seeing a simple demonstrator some years back of a laser pulsing against the backside of a spinning disc, the geometry of things causing the heat from the laser to provide thrust. I can't find the video I saw, but here's a general link on the topic: https://en.wikipedia.org/wiki/Laser_propulsion#Laser_energized_rocket https://en.wikipedia.org/wiki/Laser_propulsion#Laser_energiz... This comes with many problems, but it does bypass the rocket equation, while not requiring Project Orion levels of trouble.
    - rocqua 7y ago
      I mean you still need to carry propellant: the stuff that is ablated away. It's just that the exhaust velocity is much higher.
      - king07828 7y ago
        Air from the atmosphere may be the propellant [1]. If sufficient velocity can be achieved before leaving the atmosphere, then no propellant is needed to be carried up on the way to orbit. [1] https://www.youtube.com/watch?v=XhUasBcoj-Q https://www.youtube.com/watch?v=XhUasBcoj-Q
  - jccooper 7y ago
    Nuclear would be better, but it's not quite as good as it seems. Nuclear engines are rather heavier than chemical.

deleted 7y ago

[deleted]

Already__Taken 7y ago

if gravity were much more we'd be out of chemical reactions that produce the energy to beat the rocket equation.

api 7y ago
This is a possible Fermi paradox answer, or at least factor. Maybe the vast majority of habitable worlds are larger than Earth. The inhabitants of those worlds might find space flight too hard or even impossible. They also might have less interest in it since they would make the natural anthropic assumption that worlds capable of supporting life are probably huge like theirs.
- derekp7 7y ago
  That really makes a lot of sense. Because planets smaller than Earth cool a lot faster, so they lose their molten core and plate tectonics (which keeps land mass rising and out of the water), and also a magnetic shield that keeps the atmosphere from stripping away (like Mars).
  - JohnJamesRambo 7y ago
    I'm going to be hopeful and optimistic and point out that the large planets in our solar system aren't rocky and suitable for life as we know it anyway. Earth is the biggest one and the rest are gas planets. Maybe planets that harbor life (rocky, liquid water, goldilocks zone closer to the sun) usually have low enough gravity for aliens to escape it. This diagram has some exoplanets on it for size comparison- https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/media/File:Diagram_of_different_habitable_zone_regions_by_Chester_Harman.jpg https://en.wikipedia.org/wiki/Circumstellar_habitable_zone#/...
  - api 7y ago
    Earth might be right at this threshold of small enough for space flight to be practical (still hard, but practical) but big enough to hang onto water and atmosphere.

uoaei 7y ago

I assume he's relating 1) the force of gravity, composition and distribution of our atmosphere, and incredible velocity required for orbit, to 2) the chemical potential of fuels and the limits of structural strength for materials to carry cargo to orbit. I haven't run the numbers, but he's right: it's pretty hard to get something that can carry itself to orbit. It has to climb out of the atmosphere directly against gravitational force and atmospheric drag, and then as early as possible turn sideways and start accelerating to around 28000kph while making sure the rocket is pointed in the right direction and holding together. You need a lot of propellant to get to that state, which means you have to carry it around until you need it, which means the machine needs to be much bigger than just the cargo, which means that all of your challenges just magnified by many orders of magnitude.

TheSpiceIsLife 7y ago
Would it be possible for the numbers to be a bit more difficult to work with so maybe we could go to low earth orbit, but never be able to make it to the moon?
- JshWright 7y ago
  If you can make it to orbit, you can make it anywhere (it just may take multiple launches to assemble/fuel the spacecraft going elsewhere).
  - fireattack 7y ago
    Wait, really? So what's the deal of first and second cosmic velocity?
    - cyphar 7y ago
      I guess the idea is that if you can make it to orbit (even if you waste all of your fuel), then you could in principle refuel in-orbit (by getting another unmanned rocket with just fuel into orbit) and thus escape the orbit. But yes, you still require more energy (exactly sqrt(2) times as much) to "escape" Earth's gravity entirely than to orbit it with a circular orbit.

happytoexplain 7y ago

I wonder how much this could be mitigated by extra-rocket energy delivered while the rocket is still close to the ground - e.g. a cartoonishly large slingshot.

drusepth 7y ago
Or a comically large refueling hose hooked up until it hits orbit. I guess in that case you'd probably be fighting gravity even more trying to push fuel up the hose.
mey 7y ago
Ground laser propulsion has been considered. https://en.m.wikipedia.org/wiki/Laser_propulsion https://en.m.wikipedia.org/wiki/Laser_propulsion
derekp7 7y ago
There's also using chemical rockets for the first stage, then switching to nuclear for the second stage once clear enough of the ground to minimize fallout. Radiation pollution wouldn't be pleasant, but future advances could potentially minimize that also.
- marktangotango 7y ago
  NERVA XE was damn near ready to fly during the Apollo era for Mars missions. Would've made a hell of 2nd stage engine. https://en.wikipedia.org/wiki/NERVA https://en.wikipedia.org/wiki/NERVA
sdenton4 7y ago
Unfortunately, drag force is proportional to the square of velocity... So the slingshot (or railgun or whatever) maybe gives you a nice acceleration at the start, but a LOT of that initial force will bleed off as drag early in the flight in the thick thick low atmosphere. (Consider the difference in difficulty of riding a bike at 15mph vs 16mph and 25mph vs 26mph... 25->26 requires a hell of a lot more effort, all due to drag.)
wahern 7y ago

enjeyw 7y ago

Ok so doing super-approximate maths: A fully fueled and loaded Saturn V weighs about 3,000,000 Kg, and has a payload of about 40,000 Kg to the moon. So that means once you've put all the bits of the rocket together that collectively give you sufficient thrust to get you to the moon, you've only got about 1.3% left for actual payload. S if earth's gravity had been ~1.3% stronger, then the weight of the "getting us there" bits of the rocket would have left no room for payload. Any more gravity, and we wouldn't even be getting to the moon. Equally, if the earth's gravity had been 1.3% weaker, we could have doubled our payload to the moon (or done things way easier). Contrast that with a Boeing 747 which has 50% of total mass as payload, and so is relatively insensitive to changes in gravity.

avmich 7y ago
Some (funny) sources list Saturn-V payload as 140 tons on LEO :) so according to this logic to get to orbit one has plenty of margin. And as soon as the payload is in orbit it uses full value of accumulated speed, so it's used to get further and getting to the Moon is no harder than with Earth...
JohnJamesRambo 7y ago
I recently realized that if we got in Jupiter's gravity well we could probably never escape it again. Someone correct me if I'm wrong. The thought made me feel really uneasy.
- hossbeast 7y ago
  If that were true, it would be impossible to leave the solar system, since everything is currently in the Sun's gravity well, and the Sun is larger than Jupiter. But we have sent probes outside the solar system (did they fully leave the Sun's gravity well yet though? Not sure)
  - davidivadavid 7y ago
    But we didn't send the probes from the Sun.

libtard 7y ago

Well, it'd prevent rockets for sure, we'd only have nuclear pulse propulsion then to get into orbit and that's a lot harder to justify to people than a rocket.

wolfram74 7y ago
There's a lot of alternatives, the main constraint is that it has to be built from the ground up, so space elevators are out. I think a dynamic structure such as a launch loop https://en.wikipedia.org/wiki/Launch_loop https://en.wikipedia.org/wiki/Launch_loop would work. Also inflatable towers for launch assist https://en.wikipedia.org/wiki/ThothX_Tower https://en.wikipedia.org/wiki/ThothX_Tower For even kookier options we've got lasers! https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Laser_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion https://en.wikipedia.org/wiki/Beam-powered_propulsion

sidorares 7y ago

He was talking about rocket reusability, specifically reusing orbital stage. Just reaching the orbit is possible in stronger gravity, but returning back whole stage is already very close to the limits of current material science

wmp56 7y ago

I have another wild guess. Not only we have just right amount of fossil fuels to leave the planet, but also the sun has just right amount of energy to reach the stage when people can move to another solar system.

tdy721 7y ago
My guess is the Sun probably has more... it’s really big. What if it’s just like fossil fuels and there’s enough energy there for us to smear ourselves out? Oh cool, kinda like fission then!

duchenne 7y ago

The rocket equation states that the amount of fuel that you need (to send a given payload into orbit) increases exponentially with the escape velocity of the planet: mass of the rocket with fuel = mass of the payload * exp(escape velocity / engine exaust velocity) That is because the faster you need to go, the more fuel you need to use, but now, you also need even more fuel to accelerate the fuel that you just added. So, it becomes exponential. So, if the earth was bigger, its escape velocity would increase, and the amount of fuel needed to power a space rocket would increase (exponentially) so much that it would become unpractical. Vice versa, with a smaller planet it would require exponentially less fuel to reach orbit. The only solution would be to have an engine with a higher exaust velocity. With the current technology, ion engines have very high exaust velocity but low thrust are very energy hungry. The thermal nuclear engines have both high exaust velocity and high thrust, working prototypes have been built, but all projects were boxed 60 years ago because of the fear of an accident.

mpweiher 7y ago

> earth's gravity [..] right on the cusp of rocket flight possibility He didn't actually say that. He was talking about "rapidly reusable orbital rockets" and "fully reusable orbital rocket[s]" being the "critical breakthrough" that's necessary, the "holy grail of space" and the "fundamental thing that's required". So fully and rapidly reusable. https://youtu.be/sOpMrVnjYeY?t=1234 https://youtu.be/sOpMrVnjYeY?t=1234

sidcool 7y ago

Gravity is taxing. It's possible that our galactic neighbors live on a larger planet with greater gravity which makes their escape from the planet next to impossible.

silasdavis 7y ago
Can we swing by and drop them a ladder?
- sidcool 7y ago
  If we get sucked by the gravity, we are stuck as well.
  - silasdavis 7y ago
    Presumably we'd need to lower it from geostationary orbit. This is a unique solution though I suppose there is some dependency on the mass of the planet where you might be too deep in the well. But you'd be in orbit so presumably some low thrust low propellant engine powered by sunlight ought to be an option?

kromem 7y ago

Hmmm... That's actually really interesting to think about from the anthropic principle perspective. Very interesting that the only intelligent species we are aware of is on a planet that we can leave. I wonder to what extent higher gravitational planets could sustain love that couldn't leave it. An interesting answer to Fermi's paradox.

htfu 7y ago
Nah, again - nuclear. I doubt the safety profile would factor much once it's the only available option.
- mrfusion 7y ago
  Don’t forget ground based propulsion too.

hoorayimhelping 7y ago

That is pretty much all there is: If earth was more massive, the chemical rockets we've been producing wouldn't have enough thrust-to-weight ratio to reach orbital velocity (about 25k km/h). We would have to load the rockets with so much fuel to break free of gravity that they would be too heavy to lift themselves. This is all assuming yields and fuels we have now. If we lived on a more massive earth and we were trying to escape its gravity, I'm sure we'd be using more exotic and dangerous fuels (like all those fun fluorine and boron fuels Dr. Clark mentions in Ignition![0]) to do the job. We just happened to have the capability in the middle of the century to use a fuel we were already making (refined petroleum) for jet engines. [0] https://www.amazon.com/Ignition-Informal-Propellants-University-Classics/dp/0813595835 https://www.amazon.com/Ignition-Informal-Propellants-Univers...