Fascinating. This may weigh down the Drake equation, particularly in reducing the average time civilizations survive on planets with high gravity because their ability to become multiplanetary and survive great filters is limited.
Fascinating. This may weigh down the Drake equation, particularly in reducing the average time civilizations survive on planets with high gravity because their ability to become multiplanetary and survive great filters is limited.
The converse of this was kind of an open problem in the early days of rocketry. Given the theoretical rocket concept, was there a propellant combination with sufficient exhaust velocity to make an orbital rocket practical? The answer was not immediately obvious, and there's a Goddard paper where he talks about just how big the rocket has to grow as you lower the propellant velocity to get equivalent performance. Eventually you're burning entire mountains of gunpowder just to get a few dozen miles up.
It was a nice surprise (and a relief) to the early rocket pioneers to realize that we lived on a planet where gravity and chemistry would make orbital rockets possible. The rest was just engineering.
Wouldn’t it be fascinating if there were an advanced civilization on a planet with gravity that was much higher than earth that couldn’t build chemical rockets and was therefore forced to build nuclear rockets?
What if that actually made the exploration of their solar system easier, since once they left the gravity well of their planet getting to other planets with nuclear rockets was comparatively trivial?
These things are fun to imagine, but the real fun gets to be when you start talking about all the downstream effects. For example, if you can't build rockets you can't build GPS. Building a global communication system is much harder, which means things like shipping and flying are much more difficult. Not to mention that the gravity is much higher in the first place so flying is going to require way more fuel so how long does it take for them to get to that stage of civilization and how does their technological path differ? It gets even trickier once you start thinking about how the atmospheric composition will be different as gases follow similar escape velocities (e.g. Earth loses 3kg H/s but only 50g He/s) and it also determines what can even stay aloft. In general much of the technological paths are fairly straight forward, always iterating off of the current state (leaps and bounds are not common as they're more often a lack of domain expertise or not properly contextualized around the historical knowledge). But I think people forget how connected a lot of these things are. Then again, people often question why it is important that we build rockets, while asking those questions on their handheld computer connected to a global communication network. It's quite incredible how complex these interaction chains actually are and I think make you only admire the beautify of it all that much more.
Interestingly, development of rockets has only made a bunch of the things you mention cheaper (to the multiple orders of magnitude), not impossible.
Eg. determining location through radio signal triangulation can tell you a location pretty well, but would require placing a lot of signal stations throughout the world. Eg. remember the time-synchronisation mechanisms for watches through AM signals (including in hand watches).
Similarly, we did build a global communications network by placing expensive undersea cables across the world, but systems like StarLink are much cheaper (once you get to economies of scale for launching satellites).
So, like many things, rockets have accellerated discovery and progress, but are ultimately not the be-all solution: they work in tandem with the rest of science and engineering (including cultural development).
GPS done via land radio systems would be so flakey and expensive it would still likely not be implemented. Easy to jam too. And subject to control by terrestrial authorities.
Putting a dozen satellites in orbit - and out of reach of local authorities - is so much cheaper and more reliable, it’s not just a matter of cost - it’s an entirely different product.
Same with starlink. A big part of its advantage is someone can’t just walk over and cut a cable. And no one needs planning approval to put a cable in.
Line of sight to low orbit is about the only way to accomplish that - maybe some kind of high altitude ballon/plane could (loon?) but they’re so comparatively easy to shoot down that it makes it a very different kind of situation.
You might be surprised to learn that Enhanced LORAN recently became operational around the UK's coast, specifically because satellite-based PNT is so susceptible to interference and jamming.
Not at all. It's only being installed in specific, high value areas within a specific jurisdiction. And mainly as a backup. Notably by a party which doesn't control GPS (albeit a close ally).
LORAN has also been used near airports in developed areas for a long time.
That isn't the 'base case' though.
The US military initially developed and launched GPS because of the reasons I stated, and it is still widely used as a base case for exactly those reasons.
It seems like you are looking at it only from one side.
Undersea cables are probably more expensive than satellites today, but we'll still continue to put them in. And nope, someone can't just walk in and cut a cable sitting at 5000m under the surface.
Detecting a StarLink terminal is relatively easy from the ground, and someone can just walk in and demolish it once they locate it.
Basically, all tech has pros and cons.
If you could get to space with a fusion engine, then why would taking satellites into space on said rocket be any different then it is for us (to build GPS)?
Wouldn't LTA blimps work BETTER in higher gravity for flying?
A limit often ignored for high-gravity balloons is, the pressure gradient inside the balloon reaches a point where it tears it apart. When the atmospheric gradient is compressed to some point, the distance between the bottom of the balloon and the top can create enormous forces on the fabric.
So depending on the gravity we're talking about, blimps are out!
Which actually leads me to thinking that a space-adapted race really doesn't want to bother with planets and their big ass gravity well.
Resource extraction from asteroids or moons is a lot easier than carting it out of a big gravity well. Building stations in zero G rather than having to worry about orbital degradation and the like. Atmospheres get in the way of solar energy collection.
Earth is probably only useful as a vacation destination. Unless of course all those UFO reports are actual physics-defying antigrav drives with little green men.
The nice thing about gravity wells is they naturally concentrate things along density gradients.
The bad thing about gravity wells is they naturally concentrate things along density gradients.
That civilization could have invested in rail transit and tunneling instead. Positioning isn’t so hard on fixed roads, although fixing the, in the first place under oceans could be a problem. They might figure out triangulation using their planet’s magnetic field or something. It’s also completely possible that life isn’t viable at all on non-Earth like planets.
Interesting thought. I think ground-based GPS wouldn't be too difficult though - we already have most of earth covered by GSM/3G/LTE, and with updated towers you could have something as precise, if not more, as GPS. Of course the coverage wouldn't be 100%, and navigating in ocean would be more difficult.
Planes would be replaced by trains and aquaplanes for sure. Our modern fastest trains (TGV, Maglev) are only half as slow as the fastest commercial planes. Also, you might have rocketry on such a planet, just not for orbit, and for things that right now we use jets for.
The biggest issue with be probably no detailed aerial maps, and in later stages - no space mining, so such civilisation would be limited to resources on their own planet.
Also, I'm imagining that such a civilisation would send out more signals into space to encourage someone to come and visit them, and hopefully dropship resources from orbit :D
Imagine two civilisations living like that in symbiosis - one on the orbit, able to drop things to the one that is lower, but being able to extract only information / art / mental labour / energy from below.
All projectiles become much shorter range weapons. Maybe once they've got gunpowder they can finally fight at range, though each shot would require a lot more gunpowder relative to the same shot on earth. Maybe it sort of washes out if you figure the inhabitants are all stronger and more sturdy as a result of the gravity.
Oddly enough nuclear rockets aren't particularly powerful and tend to be extremely heavy. Their strength is that they're highly efficient, so they can just keep going with relatively little fuel. Chemical rockets, by contrast, tend to be extremely high power but also extremely inefficient. Here's a few comparisons:
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NERVA [1] / Nuclear / 1969 / 246kN thrust / 18,000 kg mass, 841s ISP (seconds of specific impulse - higher is better/more efficient, a little is a lot) / The only completed possibly launch viable nuclear rocket engine, as far as I know.
F-1 [2] / Chemical / 1959 / 7,770kN thrust, 8,400 kg mass, 263s ISP / Powered the Apollo rockets
Merlin [3] / Chemical / 2007 / 981kN thrust, 470 kg mass, 282s ISP / Powers the SpaceX Falcon 9 in a group of 9
Raptor [4] / Chemical / ?? / 2,640kN thrust, 1,600 kg mass / 327s ISP / Powers the SpaceX Starship in a group of 33
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So what really matters in a rocket, for getting off Earth, is its thrust to weight ratio. NERVA isn't inefficient because it's dated (which was part of the reason I included the F-1), but simply because nuclear itself has an inherently poor thrust to weight ratio. However it just keeps going and going and going, which makes it absolutely awesome for travel once you're already in space.
It's even "fast" in space, because of how travel in space works. You don't just keep thrusting in space; instead you make a limited burn and then coast to where you're going, making a final reversal burn towards the end. So even if it takes hundreds of times as as long to reach a higher cruising velocity, it'll end up getting to the destination long before a chemical rocket, for any sufficiently distant destination.
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[1] - https://en.wikipedia.org/wiki/NERVA
[2] - https://en.wikipedia.org/wiki/Rocketdyne_F-1
once they left the gravity well of their planet getting to other planets with nuclear rockets was comparatively trivial?
We are actually on that planet. Spacecraft have what is called delta-v, which is basically a measure of what orbit changes they can perform given the amount of fuel they have onboard. For example getting from the ground to LEO has one measure, and getting from LEO to moon orbit has another.
It varies somewhat by the specific rocket to get into space (due to drag and effects of higher gravity), but once you are there it's basically the same for all spaceships.
It takes around 9.6km/s (no relation to gravity, just a coincidence) of delta-v to get into LEO, however once you are there it's fairly cheap to get around the solar system. To get from Earth LEO to a captured orbit around Mars needs a delta-v of around 5km/s - yes, less than to get into Earth orbit. To get out further to Neptune would need around 12km/s of delta-v.
For anyone interested in the history of rocket propellants, I highly recommend "Ignition!" by John D. Clark[0]. It has plenty of chemistry if you're into that, but even if you're not (like me) it's an enjoyable read.
[0] https://library.sciencemadness.org/library/books/ignition.pd...
As a former chemist, I thought this book was a great example of "applied chemistry".
The theoretical aspects are challenging enough. But then you realize just how difficult the practical application of the theory can be. Sure, a mixture of fuming nitric acid and hydrazine will produce enough propulsion, but how do you dump tons of it into an engine without it just exploding?
There are few things that get certain types of chemists and engineers excited like being able to find out - and not being in trouble with ‘the bosses’ if it explodes a few times along the way.
The section on building high-precision detonation speed timing apparatuses (and occasional explosive deconstruction of same) made me realize how uncomfortably close "information we require" and "catastrophic consequences of collection that information" are in the field.
Just one of dozens of amazing passages in this book (page 48):
"... its density was a little better than that of the other acid, and it was magnificently hypergolic with many fuels. (I used to take advantage of this property when somebody came into my lab looking for a job. At an inconspicuous signal, one of my henchmen would drop the finger of an old rubber glove into a flask containing about 100 cc of mixed acid -and then stand back. The rubber would swell and squirm for a moment, and then a magnificent rocket-like jet of flame would rise from the flask, with appropriate hissing noises. I could usually tell from the candidate's demeanor whether he had the sort of nervous system desirable in a propellant chemist.)"
I think it's a bimodal distribution. On the one hand you have the unflappable who just calmly watch what happens. On the other you have the far too flappable who is already out of the lab and making good time out of the building and the state.
Exactly this. Either they have the right stuff, or they don't.
Although for some reactions, the latter is the appropriate reaction.
There is an excellent YouTube channel that explains the V2 rocket in detail. Almost down to the last screw. Highly recommend. https://youtube.com/@RocketPlanet?si=DdgyQ8HFnswrZgZr
His videos about the Steam Pot [0] and the mechanical system that scheduled each stage of the rocket [1] are extremely good:
Seconded! That is such a wonderful book.
we lived on a planet where gravity and chemistry would make orbital rockets possible
It's kind of insane luck. Bit heavier planet and we wouldn't be able to have a single satellite before building nuclear engines.
I'm not sure how nuclear engines would help?
A nuclear reactor is a bit like an ion drive: great for long distance space travel, but not great for getting off a planet.
Unless you mean the kind of nuclear engine that consists of detonating atomic bombs behind you? See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
For non-manned launches and those that can be hardened to withstand extraordinary g-force, something akin to setup that resulted in the missing (900 kg) borehole cap of Operation Plumbbob may do the trick.
Acceleration to 66 km/s is probably a little bit overkill, even.
Orion drive should work fine.
A nuclear reactor is a bit like an ion drive: great for long distance space travel, but not great for getting off a planet.
What are you basing this on? NERVA was for getting off the planet. It had a thrust of ~250 kN. In comparison, a SpaceX Merlin engine has a thrust of ~900 kN, while ion drives have <1 N of thrust.
High fixed (non propellant) drive weight compared to chemical rockets makes it pretty inefficient due to the gravity well - thrust/weight ratio vs time matters a lot when you’re quickly climbing out of the well. And it is very difficult to do that quickly with nuclear without exceeding our materials science abilities and causing a nuclear accident.
Additionally, atmospheric density and friction matter a lot in these situations, and getting out of high density atmosphere and ‘up’ as quickly as possible pays large dividends.
Thrust to weight of a nuclear engine is fairly poor, so they are best suited for upper stage or in-space work. A heavy-planet rocket might use chemical propulsion in a lower stage just like we do and a high-energy nuclear upper stage (or two) where the really high Isp would be quite useful.
Maybe something like NERVA
Is there an equivalent to the Drake equation that includes a factor that describes planets small enough to escape?
Very depressing to me to think about how vanishingly rare smart, spacefaring life might be. But on the flipside of that, there may be a little corner of the universe where multiple spacefarers contemporaneously live within a few light years of each other. That might be cool from a space opera point of view but it'd probably end up being dominated by a space fascist enslaving everyone.
Few lightyears is something like a hundred stars. Probably zero chance to nurture more than one unprobable life.
The upper limit on the size of the universe is infinite. So we can’t exactly rules stuff out for simply being improbable.
You can make a fairly strong statistical argument there are no spacefaring species in our galaxy. Even at 1% the speed of light a species could fully colonize the galaxy in 10 million years, 0.1% the age of the universe.
That we see nothing implies intellgent life is rare, short lived, or we're early in the age of the universe. For example, red dwarfs will last trillions of years compared to the sun's 5B lifespan.
1% the speed of light is stupidly fast to colonize a galaxy. It only sounds reasonable in the same way startup claim if we can just capture 1% of the X market, while forgetting numbers much smaller exist.
Let’s assume super tech they can build that somehow allows vastly faster speeds than we can today ~120 km/s worth of DeltaV. Half that is spent slowing down so we’re talking 0.02% c.
Now let’s assume half the time is spent in flight and half the time is spent colonizing stars before launching ships. So now we’re down to 0.01% C. Suddenly 1 Billion years is a more reasonable estimate and even that takes super tech we don’t have any idea how to build and assumes nothing fails.
Several more advanced civilizations could be colonizing the galaxy today that are still 10 billion years from finishing.
Fusion rockets would be able to get over 1% c. I think the theoretical maximum for fusion is around 10%.
Fusion rockets might be able to get a sufficiently large spacecraft over 1% c. If you’re instead sending multiple generations of craft and waiting until each succeeds then it’s 1/n %c etc.
Colonization using a massive habitat capable of extreme redundancy and asteroid mining could be a completely viable solution to colonization. But such a structure wouldn’t be light.
Hitting 1% c could very well take megastructures that a civilization would rather spend on redundant craft etc. We can dream, but we’re nowhere close to being able to say what’s actually viable.
That we see nothing implies intellgent life is rare, short lived, or we're early in the age of the universe.
Or it implies that not everybody's first instinct on seeing a vast galaxy is to try to take it over ASAP.
If you want resources for quality of life— Gas giants are a thing. If you're an explorer driven by curiosity, then take only samples, leave only memories, right.
If you want money— Century-long shipping times with civilization-scaled fuel costs tend to eat into profit margins.
If you're worried about survival— genuinely worried about survival, on a level personal enough to motivate action, not just academically or for fun— then the focus is on people you know and care about; all of those are here.
In fact, I suspect the ones that see other stars and immediately think "Mine mine all mine!" probably have a higher chance of nuking themselves before they even get out of their star system.
That we see nothing implies intellgent life is rare, short lived, or we're early in the age of the universe.
> Or it implies that not everybody's first instinct on seeing a vast galaxy is to try to take it over ASAP.
Or it may simply imply that intelligent life is good at hiding and does not want to be seen e.g. the dark forest hypothesis.
That's why we typically only talk about the observable universe, which is very much finite.
fascist
This word is wild. Very interesting.
It basically means "evil bad guy".
If I had to describe fascism in one sentence it would rather be "take social darwinism 'the strong rule the weak' and add a pragmatic leader who helps your group crush those who hold you back and rule everyone else"
You know, exactly the kind of ideology you don't want a neighboring nation to have, no matter how you judge their actions morally
That's definitely not even close to reality.
National Socialism and Italian Fascism were all about the strong taking responsibility for the nation, in a philosophical sense.
These were social welfare states that provided for the "people" far more than modern American liberalism (the standard operating produre for free economies of scale in the modern world), for example.
Now, obviously I'm not saying this was a good system, but you're so far off base that it's ridiculous.
The discourse around this complex historical movement is profoundly anti-intellectual. I know this because I grew up in the same society you did, the one where I also learned and used fascist as a pejorative without any further information.
Now, as someone interested in having sophisticated ideas about systems, I'm not really in a mental state where I'm going to take appeals or emotion or shortcuts that shut down thought seriously. Regardless of the context, I still want to try and reflect reality as closely as possible in my minds eye.
How many liberal, or even communist thinkers have you read? Personally, countless. As for fascists, almost none. It's a taboo, the works aren't translated, etc -- but it's there, and has intellectual underpinning that is more complex than mindlessly calling people you don't like "fascists".
This can't be conscionable to any earnest intellectual. Imagine sitting here and tolerating people pejoratively calling people "communists". It's so stupid.
Edit: I get in trouble here because there's something really interesting going on with controversial topics: all you need to do is make a choice and your model of reality is much more accurate than the presented model of reality. It's the easiest way to take Ws out of discourse, nobody has good ideas when the id has the reins.
Imagine sitting here and tolerating people pejoratively calling people "communists".
Also tolerated - by calling them "Russians".
The logic goes, if there's nothing innately wrong with Communism then Russians (and, to extent, Chinese) must be up to blame.
That's pretty much it - if you check your history it's clear that neither Russia nor China adopted Communism in practice - they both went for authoritarian committe rule with power struggles as some kind of "neccessary" middle state while they work their way towards Twue Communism.
It was still way more socialist than what California or South European left wants.
Soviet Union had socialized assigned housing, affirmative action policy and equalized wages right from the start. Still, it is largely ignored by socialist LARPers of today.
Australia was and likely still is more socialist than any US centralists wants.
The 1900's Harvester agreement indexed the minimum wage to am eight hour work day with a week sufficient to feed, house, and clothe a worker and their family.
The Whitlam years saw free university education for anyone that merited by high school (and equivilency) exams, health care has been universal - now with a split of both public and private, pharmacy companies are capped on their generics so that costs are reasonable, differences are picked up for those that can't afford medication (for almost all prescriptions), etc.
For the record (and the GP is using the term somewhat out of bounds), there are definitions of fascism that are rather tighter than 'evil bad guy': https://en.wikipedia.org/wiki/Ur-Fascism
That's a stronger definition, but it's still weak because it isn't primary source material. This guy isn't a disinterested political historian. Imagine taking as definition the views of Giovanni Gentile on Marxism.
I wouldn't call any of these groups "fascist" except in the loose pejorative sense
The closest governments to actual Fascism that I can think of are governments like modern China or even Singapore, and I don't mean that in a pejorative sense. They're just very fascist in character, i.e. national interest, social welfare, strong-arming of capital, etc.
Rockets are not the only way off a planet. If humans had spent space program amounts of resources on railguns or another method of locomotion there's real possibility it would have been successful too.
Rockets are most convenient for Earth's variables so engineers optimized for them.
Railguns also become much harder in a larger gravity well. Bigger planets generally have thicker atmospheres as well. Your payload will end up disintegrating at the velocities required even on Earth.
Vacuum inside the tube?
Balloon stage followed by a rail gun might work.
The Earth is larger than Venus, but its atmosphere is 90 times denser than ours.
There's a lot more variables that just gravity.
I think mostly the Drake equation shows a lack of imagination about the forms life might take. Every time you add a term to it, you're baking in additional assumptions.
Both lack of imagination of what life can be, and supporting factors we take for granted. I dismiss drake equation as fully useless as it doesn't provide useful upper nor lower bound.
Maybe. But you still have to explain the observation that the night sky is empty of signs of life.
Keep in mind that human technology is pretty close to being good enough to detect not just foreign civilisations (via eg radio waves), but signs of life itself: studying the spectra of light reflected by exoplanets can tell you what chemical elements are in their atmosphere, so you can detect atmospheres that are far from chemical equilibrium, like earth's oxygen rich one.
We emitted radio waves for only a few decades. But earth had oxygen for billions of years. So that widens the window of time of development that we could detect.
That might be cool from a space opera point of view but it'd probably end up being dominated by a space fascist enslaving everyone.
Fascism already barely works on earth, and gets out-competed. See https://tvtropes.org/pmwiki/pmwiki.php/Main/FascistButIneffi... Similarly with slavery. (See https://www.econlib.org/library/Columns/LevyPeartdismal.html to go off an slight tangent.)
In space, slavery is even less useful. That's mostly because humans are even less useful: we are already doing pretty much all of our useful space exploration with robots, and sending humans is just for bragging rights. Keeping space slaves alive costs you more than they ever could conceivably do for you.
Of course, aliens might have biologies that are much better adapted to surviving in space, maybe?
The Galactic Emperor is a monarchist, thankyouverymuch! And It treats all of Its loyal subjects quite well, with no discrimination against the water-based ones. Vs. if you have the misfortune to visit the Andromeda Galaxy...
If you have a sufficiently tall* first stage, and use hot staging, then you can make it work on even on an extremely large Earth.**
*First stage may need to extend well above the atmosphere.
**No, that's for-sure not a Randall Munroe book in my hand.
This is the same way you make a train capable of traveling from Los Angeles to New York in 1 second. A sufficiently long train.
I think the speed of sound in steel (or whatever the train is made out of) is slower than that.
But I guess you could cheat by having multiple engines along the way that all accelerate in lockstep.
Related theoretical question for those who are of the physics mindset - if I had a long (very long like 1 light minute long) bar of metal and I pushed on one end, I'm assuming the other end would not move instantaneously because that would imply some part somewhere inside the bar was moving faster than the speed of light. So I'm assuming that the bar would just compress slightly and for a period of time in between when I pushed on one end and when the other end moved the bar would be slightly shorter. That's fine if that's the case.
But what if the thing I push on is a quantum particle? Does this same thing happen at the smallest scales? If one end of a quark is pushed on does the other end move instantaneously or is there a small(!) delay?
Probably the answer is just "that's not how quarks work" but I've always been curious.
If one end of a quark is pushed on does the other end move instantaneously
Quarks don't have an "other end." To the best of our knowledge, particles are points.
Well, at that level, you are talking about clouds of probability or structures in the wave function or something like that.
So an object of zero volume will arrive in New York.
Interactions between particles such as quarks are mediated by fields filling the space between them (such as electromagnetic field and gluon field). Ripples in these fields propagate at speed less than or equal to c.
This is a classical picture, but the quantum picture is similar: evolution is generated by a local Hamiltonian constructed out of field operators attached to every point of space.
So, both classically and quantumly, relativity demands the existence of fields filling space to propagate causal influences at finite speed.
There's no such thing as instantenously "pushing" on a particle. E.g. electrons can be accelerated by electromagnetic fields. If the field changes, the electron feels a force and is accelerated according to a = F/m (handwaving away relativity). When you macroscopically push against a rigid body, what happens at the particle level is your constituent atoms' electrons (and protons) interact with each other through the electromagnetic field.
When you tap on the bar it creates an acoustic wave that will propagate at the speed of sound in the material.
For subatomic particles, the most intuitive way to think about things is to adopt the "fields are real" mindset. Here fields are the underlying reality, and particles are just a pattern of waves excited in the fields. Disturbances in all fundamental fields we've discovered propagate at the speed of light, and we have pretty solid reasons for believing no future discovery will contradict that, as it would break causality in a fundamental way.
On your long bar, the push propagates at the speed of sound in the material. Look up Slinky drops on YouTube.
The other answers are correct that it's an acoustic wave, but sometimes it helps to see a demo "proving" it:
Or you could just put the engine in the front. There's no rule a train can't arrive before leaving after all.
Surely for a train to "travel" from LA to NY, it "starts" when the front of the vehicle passes a line in LA, and "finishes" when the front of the vehicle crosses a line in NY.
Just build two fronts. The Germans did it with their ICE trains.
You’re going to have a hell of a difficult time trying to construct anything that tall on a high-g planet. The taper ratio between the base and the top would have to be enormous – likely a sizeable fraction of the radius of the planet! Though I guess it would have to be anyway so you have somewhere to attach all those first-stage engines…
If you're allowed to build it tall enough, just don't even light the first stages. Launch the final stage directly from a high enough altitude that it can escape on its own.
Well, yeah, but building something tall enough to reach the synchronous orbit is impossible even on Earth, there’s no material with even a thousandth of the compressive strength required. Space elevators are only possible because they’re tensile structures and the “bottom” that supports the weight of the entire structure is up there in a low gee environment.
Remember that just getting outside the atmosphere is the almost trivial part of rocketry compared to the problem of having to then accelerate to >= orbital speed fast enough to not fall down!
And anyway you’d have to dismantle the planet to build your launch tower, which I guess would solve your problem, in a fashion. Though – whatever you turned your planet into would just have an annoying tendency to rapidly collapse back into a ball.
If you really have a lot of time for the project (starting early in the star's burn?), you might try using photovoltaics to move a lot of mass across the surface to ahead of where tides would accumulate, slowly speeding up the day/night cycle. The faster you spin, the flatter your geoid and you should probably stop accelerating before your entire equator region goes interplanetary.
On earth, you could use active support to build your tall structures, then you don't need exotic super-materials. See eg https://en.wikipedia.org/wiki/Space_fountain
You could just perch the first stage at the top of a sufficiently tall mountain.
Could, but at least on Earth the difficulties outweigh the gains enough to make it too expensive. https://youtu.be/4m75t4x1V2o?si=6FzbQYrLtl7Zfe0J&t=157
You just need to launch off of the highest point on the planet! (Kerbal Space Program approved).
We don't talk about ground logistics though.
** And assuming sufficient propellant exists on your planet :)
I’m trying to decide whether you’re describing a rocket or a space elevator. If you build a tower that extends to somewhere near geostationary orbit, you can pretend it’s a rocket stage delivering a delta V of zero, and you can “hot stage” a tiny little stage off the top, and voila, you’re in orbit.
Of course, once you’ve managed to build this, the rockets are basically optional. :)
On a something like a gas giant with a hydrogen atmosphere surrounding a rocky core, would it be possible for the vessel to be hydrogen breathing until it reaches the edge of space and then ignite a stage to carry it out of the gravity well? Or if a nitrogen or CO2 atmosphere is thick enough, to fly aerodynamically or even float until it reaches a point where the gravity is appreciably lower than at surface level?
No. To float you would need a gas lighter than hydrogen, which isnt a thing. And powered flight (wings) without oxygen would be trickey, requiring more of a rocket motor than an aeroplane engine.
Yes, you could use a balloon filled with vacuum, but lifting something the size of an orbital rocket in a hydrogren atmosphere would require a vacuum chamber at least the size of a city, possibly the size of a small state. It would probably be easier to build a tower.
Sorry if I wasn't clear, but I brought up the question of floating with respect to nitrogen/CO2 atmospheres (thinking Titanlike or Venuslike) not hydrogen.
It's a moot point, but I still want to point out that "a gas lighter than hydrogen" is a thing: it is simply hydrogen at a higher temperature, i.e. a hot-hydrogen balloon, analogous to a hot-air balloon.
Well, maybe if there's no oxygen nearby.
If you have a planet with a hydrogen atmosphere, it's a pretty good bet there is no (free) oxygen nearby.
Fair point
I do not want to ride in a hot hydrogen balloon :)
Why not? Hydrogen inside a hydrogen atmosphere is perfectly safe.
It's hydrogen inside an oxygen atmosphere that's the problem.
(So on Jupiter, you wouldn't want to ride in an oxygen balloon, ie you wouldn't want to ride in a hot air balloon there.)
How do you "fill something with vacuum"?
With a vacuum pump ;)
No. To float you would need a gas lighter than hydrogen, which isnt a thing.
The atmosphere gets denser further down. You just need a negative pressure vessel, or to heat the hydrogen, like a hot air balloon. At 1 (Earth) atmospheric pressure the gravity of most Gas giants is quite low.
On rocky planets gravity doesn't get much lower in the orbits we're concerned about. For example the ISS still experiences 90% of the gravity we experience at the surface. Reaching orbit is mostly about reaching a speed where the arc in which you are falling never intersects the surface.
But you can absolutely use an aircraft to gain height and speed, and then launch a much smaller rocket from that aircraft (where the speed is the primary advantage, and is what rockets use most of their fuel for). This setup is used by Virgin Galactic's SpaceShipTwo. There is also Virgin Orbit's LauncherOne, which is a small rocket that launches from a modified Boeing 747. On Earth it's just about not worth the additional complexity, but on planets with stronger gravity but comparable access to powered flight this might be the preferred method of reaching space.
One important factor might be the speed of sound. Subsonic flight is much easier for aircraft than supersonic flight. In an atmosphere with a much higher speed of sound, like say hydrogen, aircraft could reach much higher speeds and thus would be a much more advantageous launch platform for rockets. Assuming you already solved the issue of powering those planes of course.
I've never really understood this, why is it easier for a plane to reach that speed than a rocket? Is it sort of just another rocket stage?
An air-breathing jet engine doesn't need to carry oxidizer, which in a rocket is most of the propellant weight. It also has access to unlimited reaction mass, so it can be much more energy-efficient in producing thrust (it is more efficient to produce thrust by accelerating a lot of mass by a little, than by accelerating a little mass by a lot, but a rocket can't take advantage of this because it would need to carry all that extra mass. A plane can use ambient air for this purpose)
This all adds up to a plane needing to carry many times less mass to gain the same altitude and speed as a rocket, at least within relatively dense atmosphere.
The basic physics of https://en.wikipedia.org/wiki/SABRE_(rocket_engine) have been vetted, which is an air breathing rocket engine. I don't know how much difference trying to liquify H2 vs O2 is though.
The oxygen is the majority of the mass (but not volume!) in a stoichiometric hydrogen engine, so the mass savings would be less I think. The RS-25 (space shuttle main engine) runs at a higher fuel ratio. Should work very similar to SABRE in general - the concept of a high speed atmosphere collector and precooler is pretty universal to any gas, and hydrogen has an extremely high heat transmission rate.
On a smaller gas giant you could build a floating platform (like a giant zeppelin) and launch from there. Because gas giants are so large the “surface” gravity at the altitude such a platform would be floating at is not as high as you might expect. On Uranus and Neptune it’s actually lower than 1G.
However, past Jupiter size the mass keeps increasing while the radius doesn’t, so even from a floating platform you’re contending with multiple G’s.
Since it's barely mentioned in the answers, and was my first thought -- nuclear thermal rockets are something to think about too, at least in theory:
Not so much for takeoff! Most rocket designs better than chemical rockets trade off thrust for specific impulse. That's an improvement in orbit, since delta-v is delta-v. But imagine a 10kg rocket- it's receiving ~100N of gravity. If your engine doesn't put out 100N of thrust you'll just sit there on the launch pad. As you pick up speed you no longer have to deal with that (after all, LEO has basically the same gravity and doesn't have to burn against gravity at all) but when you're launching off something other than a point mass, some of your thrust has to go towards ensuring you don't hit the planet, or you will not into space today.
The practical designs we have for NTRs are solid core, which after long effort got up to a thrust to weight ratio of 7:1, meaning they could in principle carry up to 6 times their weight and accelerate up in Earth's gravity rather than down. Chemical rockets can get 70:1. No one ever had plans to use NTRs in lift platforms- instead they could serve as more efficient upper stage engines, for orbit-orbit transfer burns and the like. In principle there are engines which are technically NTR and offer much better performance, but no one's ever gotten a working prototype. Also you probably wouldn't want to launch with an open cycle rocket, since the open part describes how the radioactive fuel is ejected out the rear. Unfortunately, with the technology we have, we have to make tradeoffs between efficiency and thrust. For the lift stages chemical rockets are, for now, unrivaled.
(Unless of course your nuclear propulsion is of the more, shall we say, entertaining variety. Project Orion has its proponents...)
When discussing potential alien civilizations, one can’t discount the existence of civilizations which exist on substantially more radioactive planets.
If the background radiation of earth was 100x higher, would we care about an Orion launch? Or a small nuclear exchange…
I can't help but think that any species insane enough to use Orion drives in the first stage probably already found a way to blow itself up before it gets to that point.
And maybe I'm taking Terra Invicta too seriously but maybe they would wait until they figure out nuclear fusion and have more options.
I once got to briefly discuss project Orion with Freeman Dyson at a book signing. IIRC (it was a long time ago) he said that :
- he thought it could be made to work
- all big engineering projects (dams, skyscrapers etc) kill people
- putting all that radiation into the earth's atmosphere couldn't be justified
The more fuel you have to pile onto the rocket, the less the weight of the engine matters.
Using the chart in the accepted answer, launching with chemical engines takes 50 thousand tons at 3x gravity and 3 million tons at 4x gravity.
Now consider a theoretical engine that has a 7:1 thrust to weight ratio at 1G but sips fuel. Take a 25 ton engine, strap 10 tons of fuel to it and 1 ton of payload. Watch it go to orbit on a single stage.
A real NTR doesn't save nearly as much fuel, but it can still be useful in certain ranges.
NTR is a very inefficient use of nuclear fuel. What you want is a NSWR: https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket
A true nuclear rocket. Just like a chemical rocket is a controlled explosion, NSWR is a controlled (cough) nuclear explosion.
NTR have high specific impulse but relatively low power to weight, this makes them good in space and poor for getting out of the gravity well as discussed here. They are efficient at using reaction mass but not for power to weight.
From the article:
Early publications were doubtful of space applications for nuclear engines. In 1947, a complete nuclear reactor was so heavy that solid core nuclear thermal engines would be entirely unable[23] to achieve a thrust-to-weight ratio of 1:1, which is needed to overcome the gravity of the Earth at launch. Over the next twenty-five years, U.S. nuclear thermal rocket designs eventually reached thrust-to-weight ratios of approximately 7:1. This is still a much lower thrust-to-weight ratio than what is achievable with chemical rockets, which have thrust-to-weight ratios on the order of 70:1.
What technological advancements would be impossible for a civilization that can't go to space?
Those aliens have no gps and worse internet. Flight travel and shipping is more expensive too. Weather forecasting is more difficult. They never create Starlink.
Internet is almost entirely terrestrial. GPS (for civilian use) could be replaced with a land-based network of transmitters (like GSM network).
and shipping
I want to mention that this would only be for heavier than air based airborne shipping. Liquid based shipping is unaffected by gravity. Archimedes' principle has the buoyancy force as the weight of the displaced liquid. The gravitational effects cancel out. Also, dirigibles would be possibly more useful here as, again, gravity cancels out.
Something neat I remembered, great comment all the same, thank you.
Satellites mostly.
In Project Hail Mary one of the exoplanets is 8.45 Earth masses and the residents are able to attain space flight.
https://www.reddit.com/r/ProjectHailMary/comments/s5n7j4/eri...
Wonderful book! One of my all time favorites. Though just cause they did in a book doesn't validate the science of doing it.
You're suggesting that Andy Weir did not science the shit out of that?
Well, he gave the aliens some hand-wave-y super-material called Xenonite.
Jazz hands!
Can anyone ELI5 what the issue is; I understand/assume larger Earth increases gravity so more for rocket to overcome, but why doesn't that also affect jet aeroplanes?
Or does it, it's just that this is space.SE so naturally they're asking about rockets specifically?
but why doesn't that also affect jet aeroplanes?
Jet engines pull in air and expel it out the back, creating thrust. The energy to do so comes from fuel, but almost all of the reaction mass is air.
Rockets don't have this luxury; they must bring all the reaction mass with them. This causes a big problem of diminishing returns. Adding more fuel means you can burn longer, but also makes the rocket heavier so it doesn't accelerate as much with the same thrust.
The result is that the fuel required goes up exponentially with the desired delta-v, as expressed by the rocket equation .
if reaching escape speed is impossible, building a work rocket is impossible.
Planes get lift from the pressure difference between top and bottom of the plane.
A higher gravity planet pulls harder on air, increasing the pressure from any given mass over any given area, which IIRC doesn't affect this difference directly.
Indirectly, a higher density atmosphere (which is technically a different question to pressure; look at Venus for example), will lead to higher drag, needing more engine thrust to maintain any given speed. Lift depends on speed, but is easier to design around.
By the same token, a space faring civilization based from the Moon or Mars is much more feasible, and a large argument for colonizing either imo, also rarely discussed nowadays.
Spacefaring would be much easier if we were Martians but going up, then down to colonize Mars just to start launching rockets back up from there seems mostly pointless? Isn't it much easier to colonize some asteroids, or Mars' moons?
The idea is that you can pick up cargo, fuel, and rocket-building minerals directly from Mars.
more depressing is that a space elevator might never see the day, since the material required for it is difficult to make
and even if it did exist, I have no idea how that thing would be put in place
if I remember, in the mars trilogy, it's assembled in high altitude, low gravity, and then put in place?
but gravity is lower on mars so rockets work better?
anyway, for earth, assembling a space elevator in space, meaning putting tough cable in orbit, would require so many launches and would emit a lot of CO2 in the process.
also the cable might be progressively thicker starting maybe at 1/3 of the distance, to bear the entire weight of the lower cable that is the most affected by gravity, while the rest of the cable would have a progressively centrifugal force away from earth to compensate, so maybe the cable would not need to be thick everywhere.
maybe that question was already asked
You can use active support to make a space elevator without super-materials. See https://en.wikipedia.org/wiki/Space_fountain
I have been pointing out for years that space elevators are feasible from a class of asteroid called a "fast rotator". They do not need to be very big either.
This thread made me curious, what's the most amount of stages a rocket has been launched with?
There are a couple of four-stage rockets. For example the Proton has a couple of four-stage variants, and India's primary workhorse, the PSLV, has four stages in all configurations.
Five stage rockets are a lot more exotic. There is the Minotaur V, which was launched exactly once, and India's ASLV, which they abandoned after a couple launches due to budget issues.
I'm guessing you mean stages to LEO, but technically Apollo was a six-stage rocket.
1. Saturn V first stage
2. Saturn V second stage
3. Saturn V third stage
4. Lunar module descent stage
5. Lunar module ascent stage
6. Service module for Earth return.
Um doesn’t balloon assistance become increasingly effective in that case? Use your plentiful surface energy blow up a balloon and float it up past the upper atmosphere.
But they explicitly exclude that from this question:
For our purposes, let's not explore alternative or hybrid launch systems or boost systems (such as balloons, planes, laser beams, space elevators etc.). Just stick to chemical propellant rockets.
Floating a balloon to the upper atmosphere doesn't make a meaningful difference in escaping the planet, it saves you a few percent of the energy but you still have to do most of the work to bring it up to escape velocity. Going to space isn't about getting high, it's about getting fast.
Though you might be able to get past a substantial portion of the atmosphere, and that would help you get past a lot of sources of friction.
Getting off a planet, even a heavy one, that doesn't have an atmosphere would be relatively easier, because you could 'just' build very long, flat rails to accelerate along.
Interest thoughts, but forgot one very practical calculation, unfortunately not easy to calculate. I say about shock-wave, which is known from practice on Earth, and for Earth limit rocket starting mass about 10k metric tonnes at sea level.
What it mean, shockwave from supersonic engine exhaust creates literally powerful pressure on construction, so on mentioned scale, nothing will withstand it long enough.
If it is possible to create much stronger materials, as I know at the moment, is unknown and we cannot forecast.
Sea level is important, because, at the moment I only remember TWO space rockets, which started from much different position, and high altitude (air) launch have very different atmosphere properties, which could be solution to shockwave problem (but have other limitations).https://en.wikipedia.org/wiki/Northrop_Grumman_Pegasus https://en.wikipedia.org/wiki/LauncherOne
Vacuum-dwelling spherical cows are immune to shockwaves.
Ooh I absolutely did not click on the link yet, but I love this question!
(and the answers were just as amazing as I expected!)
And now let’s break all the numbers by mentioning it’s likely the aliens are not dumb enough to make rockets so inefficient for their task. Nuclear at minimum would be used.
Do you have an explanation for how nuclear would actually be better? As far as I understand it’s terrible for orbital insertion levels of thrust.
The post about the 1.55R⊕ planet made me curious and I thought this was an interesting discussion
I wonder if air breathing rockets would change this much.
You can escape any gravity with teleportation, but it's easier said than done.
Or maybe we're just a dumb civilization/species? Maybe it's also dumb to assume our intelligence is "normal".
https://en.wikipedia.org/wiki/Missile_Gap by Charlie Stross features this kind of concept.
I wonder if this was part of the inspiration for Outer Wilds, where the system’s planets are so small that they could be explored with wooden spaceships.
Unless you are in a black hole you can get off any planet theoraatally
This was a delightfully weird question! I'm sure it makes sense to calculate this before landing on another planet, though.
It’s easier to build a space elevator for a low gravity planet as well. So if some day we find a species living on a heavy earth, even throwing them a rope may be difficult.
Though I don’t suppose we’ll be visiting any aliens with chemical rockets regardless. We don’t have that kind of patience.
If Randall Munroe's name is not on the answer, it's not the answer.
Now try being from a water planet, and getting to escape velocity!
One of the biggest hypothetical great filters is massive war. Higher engineering requirements for rockets (or even simple projectiles such as cannons or arrows) would set limits on the rate of increase of warfare technology. It's possible other means of diplomacy would advance at sufficient speed to preempt population annihilation from global war.
I'm curious what effect an increase of gravity may have on heavier-than-water displacement craft (canoes and other modern boats). I think probably none, since you're dealing with density, not weight. Except for any increase in density of early building materials and cargo/supercargo. But it's been long enough from physics I'm unsure.
I think atmospheric density is more dependent on magnetic field than gravity.
War may be the exception, not the rule.
Just because we're built for it doesn't mean other species will be.
If evolving in a different environment, they might be built for cooperation. That is, in a certain environment the only species that can evolve enough to go interplanetary might be a species that learned to co-exist internally and externally, otherwise the environment would have kept them down.
You heard of what chimps get up to? Ants? Microbes? They don't just have wars; They have raiding parties, take slaves, serve as battlefield medics, compete in intrafactional and interfactional rivalries that slowly boil over… Hell, even trees actively release toxins to try to kill other nearby plants.
On a long enough timescale, war is almost certainly highly (and lethally) maladaptive.
But in a non-post-scarcity environment with social contact, creatures whose bodies disagree with entropy tend to learn that violence is an effective tactic for taking others' calories/oil and nutrients/minerals.
Maybe there's exceptions. I hope so, anyway.
All those species evolved on the same planet, with the same constraints.
War works well for them, so they evolved to get better and better at it.
But war is resource-intensive and costs lives. Lives are easily replaced on Earth.
It’s not hard to imagine a planet where going to war would be mutually assured destruction on a species level, even for ants and microbes.
Interesting thought, but how would that work?
If the environment is so hostile that life keeps appearing, failing to find a foothold, then getting crushed by statistics, then if some resilient life does eventually develop, it might be able to survive in such a hostile environment only through internal/external cooperation or symbiosis.
For instance, if two or more extremophiles evolved together but remained separate species. They might even require one another’s contribution to successfully procreate. And successful procreation might be rare.
That sort of life, if it evolved to consciousness, would be averse to any form of damaging competition.
One poorly timed selfish move and the hostile environment wins: everybody dies.
This cooperation imperative would be built into their biochemistry, same as war is built into ours.
You’d probably still find insane or outlier members of their society, who are radically uncooperative or individualistic. But they would be rare and containable, otherwise their species couldn’t exist.
....Could be something like extremophilic archaea here on Earth? Not sure how they treat each other, but they're usually quite friendly (beneficial, or harmless— never pathogenic or parasitic) to us mammals– Something about branched separation in biochemistry and highly diverse ecological niches making resource competition less of a thing, I'd guess.
But that's not really "mutually assured destruction on a species level", so much as more to gain by working together– Which honestly is better.
That is very hard to imagine, how do you reckon that would be possible? Does the planet only support a couple of anthills and then all resources are consumed? How would ants even appear on such a planet?
Atmospheric density is very much affected by gravity. I'm not sure magnetic field has any appreciable effect at all on the density of the earth's atmosphere. Why would it? The vast majority of the atmosphere isn't charged, so doesn't interact directly with magnetism.
Magnetic field protects from solar wind that strips atmosphere.
It's often said but with heavy evidence to yhr contrary e.g. Venus. Venue's negligible magnetic field is almost non-existent yet its atmosphere is many many times thicker than ours.
In fact, earth actively loses material to space because is has a magnetosphere, polar outflow of oxygen for example.
Venus loses a lot of material, too.
One of the reasons Venus still has a dense atmosphere is because its atmosphere is mostly composed of a relatively heavy compound, CO2, which is harder to lose than lighter gases like H2, N2, and O2.
If you turned off the earth's magnetic field today, then presumably the atmosphere would be gradually stripped, away over millions of years. Similar to what happened to Mars. But it would not make any immediate difference to the density.
Not immediate, no.
But magnetic fields don't usually just switch off. If the planet didn't have one to begin with, then it probably doesn't have much of an atmosphere for long enough for advanced life.
The thing I often think about is while the demands for an orbital class vehicle quickly become untenable, ICBM's stay viable for a lot longer. I don't know if MAD is more or less stable without the prospect of space exploration.
I would argue that situation is usually more stable with less secrecy, so a lack of spy satellites would not be beneficial.
Absence of comm satellites would also help fragment the world and make the idea of a surprise attacks more enticing.
Not beneficial, but I imagine you’d just see a lot more spyplanes.
Spy planes can be shot from the ground, or with another plane. This is closer to an act of hot war :(
Orbital space (around modern Earth) is ex-territorial, so killing a spy satellite would be seen as an act of aggression, not legitimate defense. This holds back "kinetic action" in near-Earth space.
That's a historical accident of arrangements on earth (so less useful in the Fermi-paradox / filter debate), and could easily have gone differently. Ie air space could also have been seen as ex-territorial.
Assuming something like air space territories in these other cultures, extending them out to infinity seems... tricky. Wouldn't it imply that the ownership of various celestial bodies changes with time?
My recollection of how it evolved on Earth was the soviet's more asked forgiveness than permission and eisenhower basically shrugged and said "whatever, at least our spy satellites can go over you too"
Buoyancy is indeed not affected by gravity, you're correct.
Depends how compressible the fluid you're floating in is right? Note that's 'fluid' rather than just liquid.
As you increase gravity, with fully compressible fluids the buoyancy scales the same as weight, you wouldn't sink lower. But with any incompressibilty you'd need to displace proportionally more (ie sink down more) to counter the increasing weight.
(I think)
The water would also weigh more. Buoyancy is the force of the water around the volume you displaced being pulled down into that space, exerting pressure that pushes you up. So you'd float just as well.
Actually, the compressible fluids would become denser, and make it easier for you to float (assuming you're relatively incompressible). At the extreme end, you could swim in pressure-liquified air (assuming you survive being crushed, of course).
Replies are accurate (more precise?), it is more true to say that buoyancy in a virtually incompressible liquid is not affected by gravity because even if gravity is increased, the water experiences it the same as the buoyant mass.
This is technically wrong. Increase in gravity does affect buoyancy because air density changes with gravity. The reason is that the column of air above you is compressed by gravity. With very large gravity, all the atmosphere could be compressed down to possibly a few km.
If you can't shoot them, surely you can keep stabbing them until you develop bio-warfare? Then you can both go extinct and the filter keeps working.
You may find the concept of Superhabitabilty interesting.
The hypothesis suggests that larger planets with more mass and gravity than Earth would be more favorable to life. It’s certainly possible that there is a lot more life out there on planets where getting into space is nearly impossible with conventional chemical rockets.
We may be living on a comparatively barren rock, but the tradeoff of that we are actually able to get into orbit.
https://en.wikipedia.org/wiki/Superhabitable_planet
Imagine visiting such planet. They'd think you are gods because, how did you get up there? But you can't really land because it would be one way trip for your tech.
So you just hang around and talk with radiowaves, sending them pictures of their world from above they could never see otherwise.
If you have the power to cross the stars, surely taking off from a steep gravity well wouldn't be a problem.
But I do like the idea that you wouldn't be able to.
So instead you slingshot your orbital craft past the planet, using its gravity well itself to build up speed— And you release a cable ahead of you, that swings down through the atmosphere to zero surface velocity at the point of your perigee, so your away team and their new friends can attach it to a glass elevator and be smoothly hoisted into space.
Different problems entirely. You don't need a lot of thrust to get to high velocities. But you need a lot of thrust to leave a planets gravity well. On a planet, your thrust needs to win not only gravity, but also any atmospheric losses. E.g. on a 3g planet, you'd need thurst in excess of 3g's to leave.
But to reach say 0.25c, a tiny ion engine over a long enough time would suffice. an engine that wouldn't even get you off of earth.
Tsiolkovsky says otherwise, by a factor of over 10^663 (not even counting relativity):
https://www.wolframalpha.com/input?i=1%2Fe%5E%2874900km%2Fs%...
…Seriously. I tried to figure out just how much xenon you'd need to make that work. But you'd need to be able to store it in something like 15-dimensional space to even fit it within the diameter of the observable universe. And even if your ion engine and Hubble-scale fuel tank weighed less than the mass of the lightest quarks, the amount of propellant you'd need for it to reach 0.25c is still well over 10^600 times the combined mass of the entire observable universe, and would also collapse somewhere around 10^600 times the diameter of the observable universe into a single black hole:
https://www.wolframalpha.com/input?i=2G%282.2MeV%2Fc%C2%B2%2...
Of course, this also shows the intent of my original comment: Energy density matters, and somebody packing enough to casually cross interstellar densities isn't going to struggle with a planetary gravity well unless Idk they're doing like a low-tech off-grid trend or something.
Also, skyhooks!
Maybe light sail and beaming energy from the home planet?
Rockets are out, but you could drop a space-elevator line down
Higher gravity means space elevator might be infeasible as well. It's barely feasible on Earth.
No advanced civilization would conclude they are dealing with "gods" just because they see someone with presumably better technology than themselves.
In fact, if they are anything like humans, they have probably already realized that species that live on a lower-gravity planet could escape that planet using the same chemical reactions that are available to them also.
Well, you could send remotely controlled probes. And there might be some volunteers for a one-way trip, too. There are certainly volunteers for one-way trips to Mars right now, and by the time humanity would be an interstellar species, our population size would have gone up by several orders of magnitude; so even if volunteers are rarer as a proportion, they would be much more numerous in absolute terms.
Is this really a big factor?
Not being multiplanetary seems like the least of our existential problems here on Earth, and will continue to be that way for awhile.
At the same time, chemical rocket efficiency becomes totally irrelevant for a slightly more advanced civilization than us.
You can't build a space elevator before getting to the orbit first.
A jet engine capable of leaving a deep gravitational well must have a big ratio of thrust to weight. If a chemical rocket is too weak, a nuclear jet engine is the only remaining option. Would you be comfortable running it in the thick atmosphere of a densely inhabited planet?
A civilization just a few decades ahead of us is (theoretically) almost unimaginable. Bio augmentation, true AIs, who knows what advances in fundamental physics knowledge... Just to start.
What I'm saying is that whatever engineering and environmental limitations we currently perceive are probably irrelevant.
That might be true, especially if your 'for awhile' talks about millennia at most.
But it's an extremely relevant concern in the context of the Fermi paradox.
Yeah, but those evolved on high-gravity planets are smarter (and maybe stronger) since they too must calculate thrown-object trajectories but have to do so faster. Our brains use just enough energy, but no more, to keep us alive. Being smarter than we are would be a waste .... but if we HAD to think faster, we would evolve to match.
So maybe they'd figure it out.
Humans are (nearly?) the only animals that got really into throwing stuff, especially throwing stuff with heft and precision. (As a corollary: you can train seals to balance balls, and apes might through excrement; but only (some) humans can juggle.)
If throwing things well had been much harder, perhaps no animal would have ever bothered?
So, the known quantities that term refers to tend to be steps more like planetary habitability and abiogenesis, which might prevent complex life from getting established in the first place. But it sounds like you mean some kind of cataclysmic event which wipes out an already existing industrialized civilization.
What, specifically, are the "Great Filter" scenarios which being multiplanetary is actually supposed to help with?
Supernovas? GRBs? Simple asteroid impacts? You can usually see those coming from millions of years in advance. And surely building a couple layers of solar sail material to shield the planet, stockpiling ozone generators to repair the damage quickly, gently nudging the asteroid, or simply digging some holes/eating a gas giant and weathering the storm, would be easier and save vastly more people than establishing a sizable population in another star system.
The other "Great Filter" idea which seems to be memetically adapted for proliferating in modern discourse is the idea of a locust-like swarm of technologically advanced aliens that kill any industrial civilizations which do emerge. But in that case, presumably settling multiple star systems is the opposite of what you'd want to do; You'd be better off quieting your emissions to shrink your footprint than spreading even more biomarkers around at high blueshift.
Frankly, I think this entire idea of needing to "become multiplanetary and survive great filters" is more mainstreamed now largely due to one specific individual fancying himself a savior of humanity. SpaceX builds interesting machines, but I liked it better when it was people like Sagan, Aldrin, and Zubrin getting excited about Mars.
But even then, I'm not sure if the idea of colonizing more planets in order to survive planet-scale catastrophes really jives with how people think— Plenty of us already live within splash radius of the Pacific Ring of Fire, Yellowstone Caldera, tornadoes, tropical cyclones, land below sea level… and yet there's no billion-dollar emergency backup cities in Antarctica to "make San Francisco into a multicontinental city and survive great quakings".
The assumption is that we have a problem getting anything off the planet. All these would require some good rocket engineering.
I agree with everything else here a lot.
The article limits itself to chemical rockets. They work well enough on Earth so that's what we are using, but we can do better. Replace chemistry with nuclear, and use the air in the atmosphere as a reaction mass. On Earth, that would cause more problems than it would solve, that's why we don't do that despite having the tech to do it. But on a higher gravity planet it may be what we would do. Harder, but not impossible.
It is interesting how we got nuclear technology that would allow for way more capable rockets at the same time we perfected chemical rockets enough to get to orbit. So much that we could have been able to escape a 10g planet almost as soon as we have escaped Earth.
If you want to use nuclear technology to get to orbit on a planet with an atmosphere, you pretty much have to use bombs. See https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
More conventional nuclear propulsion has similar trade-offs to an ion drive: great for long distance travel when you are already in space, but useless to get off a planet.