4 ms·

I first became aware of Tsiolkovsky's equation 55 or so years ago, as an enthusiastic follower of the Apollo program, first venturing into the world of physics

I first became aware of Tsiolkovsky's equation 55 or so years ago, as an enthusiastic follower of the Apollo program, first venturing into the world of physics and the power of equations. At the time, I immediately wondered what it said about the suitability of various exo-planets (non of which were as yet "visible" or identified) for development of space-faring intelligent life. This wonderful article by Astronaut Pettit does the calculation I struggled with then: >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. So, now, to the Fermi paradox: where are they (the intelligent extraterrestrial life we should have met by now)? Stuck, perhaps, in a gravity well?

EvanAnderson 4y ago
> Stuck, perhaps, in a gravity well? Chemical rockets aren't the only means of climbing out of a gravity well, though. Nuclear thermal and nuclear pulsed rockets both provide higher efficiency than chemical rockets.
- tuatoru 4y ago
  There's a second tyranny, though: momentum change rises linearly with exhaust velocity, while energy expended rises quadratically. No engine is 100% efficient, so the spacecraft has to deal with quadratically more waste heat as "efficiency" (exhaust speed) rises.
- effie 4y ago
  You can still imagine a gravity well that even a nuclear powered rocket would be unable to climb out of. Nuclear thermal rocket has greater specific impulse than chemical propellant rocket, but not by that much. But there are other ways to climb out of any gravity well (except black hole) than a usual mass-reaction rocket, not benefiting buoyancy. If the atmosphere is transparent enough, a plane or a ballon could carry a small craft high enough, and then it could be propelled by pressure of a strong laser below. Of course, in alien economics and sufficiently deep gravity well, this could be prohibitively expensive.
bluGill 4y ago
>So, now, to the Fermi paradox: where are they (the intelligent extraterrestrial life we should have met by now)? Stuck, perhaps, in a gravity well? Why should we have met them by now? Even if they have a planet significantly smaller than earth, they still need to escape their star, and then spend years in space - without any breakdowns or hitting anything. There are only a handful of stars within 5 light years -that is 5 years at the speed of life (relativity gets interesting if you can actually get that fast), but realistically they can't get going that fast with rockets, so we are talking thousands of years to make the trip. Assuming they are very advanced they left when they first detected life on earth - a bit over 100 years ago radio signals from us would have reached them. Even if they are only 5 light years away it is impossible that we met them yet. Once you look at some alien more than 10 light years we cannot really establish meaningful science communications as our advances mean anything we are interested in we probably figure out already. (we can still exchange culture) At 50-60 light years out, they can't even get a message back to us now that they know we are here. Once you get to a bit over 100 they don't know we are here yet no matter how sensitive their radios are. In short there is no Fermi paradox: there might or might not be life out there, but they don't even know we are here, and it they couldn't reach us if they did.
- walnutclosefarm 4y ago
  > In short there is no Fermi paradox: there might or might not be life out there, but they don't even know we are here, and it they couldn't reach us if they did. All that assumes two things for which there is no strong argument. First, that an extraterrestial intelligence would only be interested in traveling to another planet because it detected signs of intelligent life there. But why would that be? Why wouldn't a "life friendly" planet be almost as interesting? Suitability for life, and likely even the signal of the presence of life, is something you can get from spectral observation. And second, intelligent life may or may not care a great deal about the time of travel. No reason intelligent "iife" elsewhere should not be self-constructed to be indefinitely patient and repairable.
  - bluGill 4y ago
    There are still a lot of stars, with little reason to suspect any one has a life capable planet. While you can get a lot of information from spectral observation, most of the universe isn't close enough to observe it. The earth is young enough that even if we assume infinite magnification most of the universe cannot observe it as no light from earth has reached them yet.
rz2k 4y ago
If a planet with ten times the surface gravity of Earth had an intelligent species like humans that had evolved to tolerate acceleration equal to a multiple of their surface gravity for a given time, perhaps that would open up additional launch possibilities such as a rail gun. For example humans accelerating 4g for 2 minutes over a 5,000 mile track would only achieve 10,000mph. The extraterrestrials accelerating 40g for 2 minutes over a 50,000 mile track would achieve a speed of 100,000mph. Here the larger circumference of their planet might also help, or the density of energy they dealt with on a daily basis would influence what type of launch facilities were feasible.
martincmartin 4y ago
> 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. Why would that be the practical limit for intelligent extraterrestrial life?
- walnutclosefarm 4y ago
  > Why would that be the practical limit for intelligent extraterrestrial life? Might not be, but it really doesn't matter that much. The problem is that most of the fuel expenditure is going to lift fuel, not rocket or payload. WIth even the most energetic chemical fuels, only 15% of the total takeoff weight is rocket+payload, and improving the weight of the rocket doesn't change that - you just get whatever weight you trim off the rocket materials as potential payload, and the returns diminish quickly.

EvanAnderson 4y ago

> Stuck, perhaps, in a gravity well? Chemical rockets aren't the only means of climbing out of a gravity well, though. Nuclear thermal and nuclear pulsed rockets both provide higher efficiency than chemical rockets.

tuatoru 4y ago
There's a second tyranny, though: momentum change rises linearly with exhaust velocity, while energy expended rises quadratically. No engine is 100% efficient, so the spacecraft has to deal with quadratically more waste heat as "efficiency" (exhaust speed) rises.
effie 4y ago
You can still imagine a gravity well that even a nuclear powered rocket would be unable to climb out of. Nuclear thermal rocket has greater specific impulse than chemical propellant rocket, but not by that much. But there are other ways to climb out of any gravity well (except black hole) than a usual mass-reaction rocket, not benefiting buoyancy. If the atmosphere is transparent enough, a plane or a ballon could carry a small craft high enough, and then it could be propelled by pressure of a strong laser below. Of course, in alien economics and sufficiently deep gravity well, this could be prohibitively expensive.

bluGill 4y ago

>So, now, to the Fermi paradox: where are they (the intelligent extraterrestrial life we should have met by now)? Stuck, perhaps, in a gravity well? Why should we have met them by now? Even if they have a planet significantly smaller than earth, they still need to escape their star, and then spend years in space - without any breakdowns or hitting anything. There are only a handful of stars within 5 light years -that is 5 years at the speed of life (relativity gets interesting if you can actually get that fast), but realistically they can't get going that fast with rockets, so we are talking thousands of years to make the trip. Assuming they are very advanced they left when they first detected life on earth - a bit over 100 years ago radio signals from us would have reached them. Even if they are only 5 light years away it is impossible that we met them yet. Once you look at some alien more than 10 light years we cannot really establish meaningful science communications as our advances mean anything we are interested in we probably figure out already. (we can still exchange culture) At 50-60 light years out, they can't even get a message back to us now that they know we are here. Once you get to a bit over 100 they don't know we are here yet no matter how sensitive their radios are. In short there is no Fermi paradox: there might or might not be life out there, but they don't even know we are here, and it they couldn't reach us if they did.

walnutclosefarm 4y ago
> In short there is no Fermi paradox: there might or might not be life out there, but they don't even know we are here, and it they couldn't reach us if they did. All that assumes two things for which there is no strong argument. First, that an extraterrestial intelligence would only be interested in traveling to another planet because it detected signs of intelligent life there. But why would that be? Why wouldn't a "life friendly" planet be almost as interesting? Suitability for life, and likely even the signal of the presence of life, is something you can get from spectral observation. And second, intelligent life may or may not care a great deal about the time of travel. No reason intelligent "iife" elsewhere should not be self-constructed to be indefinitely patient and repairable.
- bluGill 4y ago
  There are still a lot of stars, with little reason to suspect any one has a life capable planet. While you can get a lot of information from spectral observation, most of the universe isn't close enough to observe it. The earth is young enough that even if we assume infinite magnification most of the universe cannot observe it as no light from earth has reached them yet.

rz2k 4y ago

If a planet with ten times the surface gravity of Earth had an intelligent species like humans that had evolved to tolerate acceleration equal to a multiple of their surface gravity for a given time, perhaps that would open up additional launch possibilities such as a rail gun. For example humans accelerating 4g for 2 minutes over a 5,000 mile track would only achieve 10,000mph. The extraterrestrials accelerating 40g for 2 minutes over a 50,000 mile track would achieve a speed of 100,000mph. Here the larger circumference of their planet might also help, or the density of energy they dealt with on a daily basis would influence what type of launch facilities were feasible.

martincmartin 4y ago

> 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. Why would that be the practical limit for intelligent extraterrestrial life?

walnutclosefarm 4y ago
> Why would that be the practical limit for intelligent extraterrestrial life? Might not be, but it really doesn't matter that much. The problem is that most of the fuel expenditure is going to lift fuel, not rocket or payload. WIth even the most energetic chemical fuels, only 15% of the total takeoff weight is rocket+payload, and improving the weight of the rocket doesn't change that - you just get whatever weight you trim off the rocket materials as potential payload, and the returns diminish quickly.