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this post was submitted on 13 Apr 2026
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According to Wikipedia this planet has an estimated surface gravity of 12.43 m/s^2 with a margin of error of about 2 m/s^2. That's only up to 50% higher than Earth's 9.8 m/s^2 (on the high end of the error margin) so it probably would be possible to get into orbit.
That said we don't actually know much about it for sure. We don't know if it's a terrestrial planet for example. It could be composed mostly of gases and liquids like Neptune.
(Not a rocket scientist or mathematician, but I spent 100s of hours playing KSP RP-1)
Just doing some estimates using data from the wikipedia page:
The dV (delta-V) needed to get into low Earth orbit is around 9.4km/s.
The dV for K2-18b might be around 19km/s, more than double that of Earth's.
It's practically impossible I think, you would need such a massive launch vehicle. For double the dV, you would need exponentially more fuel assuming current rocketry tech (fuel+oxidizer tanks and engines). There wouldn't be any single-stage or two-stage rockets that could do this. With a 3 or 4 stage rocket maybe? But you would be sending nearly 100% fuel off the launchpad with virtually zero payload.
Check out the "tyranny of the rocket equation". The more propellant you need to lift heavier rockets, the more propellant you need to lift that extra propellant and so on and so on.
I tried to factor in:
spoiler
Since the atmosphere is so thick and takes up a lot of mass, I've picked 500km as the low orbit altitude (comparing to Earth's ~100km Karman line, it makes you appreciate how thin our atmosphere is ).
Rotational assist - I'm assuming it's tidally locked since it orbits so closely to its star (33 day years), and so you wouldn't get the assist from rotation like you do on Earth:
Kerbal Space Program is such an amazing game that secretly teaches you physics.
those are the best!
I'm stealing this
Nice. It's from XKCD if you want the source.
What about something like nuclear pulse propulsion, or some kind of massive spin launch?
Nuclear propulsion, like Project Orion, would probably make it more likely they'd manage to get out of orbit. No idea on the math here, tho
https://en.wikipedia.org/wiki/Project_Orion_%28nuclear_propulsion%29
yeah there's also antimatter drives which give an even greater effective exhaust velocity (which is the speed of light). the highest possible achievable.
none have been built, so far
If it's tidally locked, no spin assist.
Missed that part but that doesn't preclude what I was saying, just requires “more” of it
Likely tidally locked
With a denser atmosphere, wouldn't that mean that you could get more lift from a traditional aerofoil than on earth? And if so, wouldn't that technically make it easier to start from a high enough altitude that at least some of the gravity is mitigated?
Let's say you do the same on Earth. If you fly to the top of the atmosphere you are 100 km above the ground. That's a 1/60 of the distance to the center of the Earth. You don't have to fight air resistance but gravity is almost the same, if I'm not wrong, less than 1% of difference.
Yeah I realized that right after I made that comment. If the gravity is strong enough to hold a gas on the planet, it'll definitely have a prominent effect on something denser like a solid.
Or ask Randall Munroe How many model rocket engines would it take to launch a real rocket into space?
Wouldn't that be a non starter for life? One side would be perpetually baked and the other would be frozen.
I guess there could be a planetary Goldilocks Zone in the dusk area
I figured that area would be full of extremely violent megastorms due to the heat differential.
Oh interesting that is a good point
Pretty late response, but if the atmosphere is that dense and hydrogen rich, you could reduce the start amount of fuel by a lot and capture it directly during flight? Needs some elaborated technology and low angle flight vector but could work, does it?
It's probably still a lot harder though. You're not just heavier, but also slower which means you'll spend more time fighting gravity. And all the extra fuel you bring for that makes the rocket heavier which means you need even more fuel to launch the fuel. Higher surface gravity likely means a thicker atmosphere too which is a big issue and a more massive body also has a faster orbital velocity. Although in this case the larger diameter might counteract that a bit because higher orbits have slower velocities.
My point is that this would probably still be a lot harder than just building a 50% bigger rocket. If you've ever tried launching from Eve in Kerbal Space Program you know the pain. Although in that case you also have to fly the entire rocket there first which is its own challenge.
Aw man. This is already a significant portion of my day.
It would actually be impossible for them to get to orbit using chemical rocketry, like we use. They could theoretically do it with nukes.
Chemical rocketry limits
Nuking your way to orbit
I assume it's not just about the gravity, but also the much larger radius of the planet would mean much larger distance from the surface, and thus much more fuel needed.
That's not how.......what???
F = G * (m1 * m2) / r^2
Note that radius is both squared and the dividing term. More distance = less gravity
Escape velocity does scale with (square root of) radius so its not a dumb thought.
And I'm not a rocket surgeon but I could imagine earth rockets might be operating near some physical limits that make a 50% increase (or whatever) infeasible.
https://en.wikipedia.org/wiki/Escape_velocity
Wikipedia says
energy = GMm/r.if
g=GM/r²thenenergy = mgr, proportional to r given g is constant.apologies
My previous comment was wrong, I derivated while integrating.I stated an assumption and was contributing to the conversation. Even if that assumption is incorrect, there's no need to be a dick about it.
It seems like a larger atmosphere would result in a longer duration exposed to atmospheric drag, thus requiring more fuel to overcome it.
That's, uh, not really how that works. A taller atmosphere would mean you have to go through more of it, but unless it's not a terrestrial then the atmosphere won't be that much taller.
If it is a non-terrestrial planet, it's unlikely anyone would be building rockets on there to begin with.
If it has a higher gravity would the atmosphere technically be lower since it will squish up closer to the planet?
And your username would also be relevant.
You're sort of right. The change in distance from the surface is insignificant, but a spacecraft orbiting a bigger planet has to travel further with each orbit so its speed must be faster to avoid falling out of orbit, even if the gravitational acceleration at its orbital height is the same.
I've been wondering what a hypothetical perfect habitable planet for spacefaring would look like. Could you have one where a plane line the SR-71 Blackbird or an even less capable aircraft could simply "fly" into orbit? Or what about something Earth-like but with a flat plateau at 15,000 m where you could launch rockets from?
I think Mars, assuming you terraform it, would be pretty close to that on both counts. Space planes might still be difficult, but the delta V is much lower and Olympus Mons would pretty much sit above the atmosphere.
Holy shit, I hadn't considered that you could use Olympus Mons as a launch site cause it sticks so high up.
The best part about it is that it's an extremely gradual slope completely unlike the mountain ranges on Earth, so you could haul stuff up there on trucks or trains easily.
The problem is you can't have mountains like that on tectonically active planets (a mountain that big on earth would sink into the mantle), which is kind of a prequisite for a long-term magnetosphere so its unfortunately not something a species could likely ever have except as a result of terraforming a world like mars and setting up some kind of artificial magnetosphere.
Is there a lower density limit for having a magnetosphere though? A habitable planet with 1.5x earth radius and the same mass would be much easier to get off of.
I guess that could work? Earth is actually the densest planet in the solar system so our baseline mass > size ratio might actually be a bit abnormal.
I was sure you were bullshitting, but no. Its true.
Iron and nickel core + enough mass for gravity to start compress the planed and we are just little more dense than Mercury.
Our core is also just huge, the outer core is larger than the moon and mercury.
If that's true, how did Olympus mons get there in the first place? I thought it was a volcano.
Mars was geologically active but its core cooled.
Classic planes require an atmosphere to generate lift. There's an outer limit where that would be a viable mechanism, and on Earth it's still far below LEO. Still too deep in the gravity well for ion thrusters to be viable. It requires chemical rocket fuels to bridge that gap.
Maybe someday fusion propulsion will break that limitations, but for now the best you can do is reduce the amount of fuel needed by flying to the upper atmosphere and reaching hypersonic speeds before kicking into rocket fuel propulsion.
Then after orbital injection, switching to ion thrusters to move around, and solar sails for exiting orbit into interplanetary/lunar routes.
Mars is better for launching rockets into deep space than Earth because it has a lower gravity field and also thinner atmosphere.
Orbital speeds would be very hard to reach compared to low Earth orbits. Also a much deeper gravity well to escape for travel.
Or Uranus.
Or your mother’s.