# A planet you can take off from, but never land back

Can there be a planet that space missions can take off from, but never land back?

It is easy to imagine that thicker atmosphere, large amounts of space debris or higher gravity could entirely stop space missions. But I'm thinking of a case where space missions could be developed, but by necessity they would always have to be one way trips.

Constraints:

• Planet should be able to support life that is similar to Earth.
• Locally evolved civilization should be able to construct rockets that can take off from the planet with enough payload to carry living beings and life support.
• It should be significantly more difficult to land back - hundreds of years difference in technological level needed.
• Some form of communication (such as radio) should be possible between surface and space.
• Technically, the Moon might qualify. The technology required to get humans from the Moon to Earth we could feasibly have done many decades before Apollo. Obviously, since no humans started on the Moon, there'd be no incentive to develop that technology until after the much, much harder task of getting humans to the Moon in the first place was being developed... Nov 7 at 16:42
• They have not developed powerful rockets, they launch like a bullet. The 'rocket' power is only strong enough for orbital changes - compressed gas and such, like a balloon. For some reason, they never developed parachutes - never thought about the idea. I mean, SOMEONE had to be the first person to think of the parachute. Just like it took us until now to think about induction stoves. Nov 7 at 16:58
• @AlexP I want in on that conversation! 😜 When we combine Edward Barcalo's decision to take someone else's invention and call it the Barcalounger with Philo Farnsworth's invention of TV (ignoring the many patent lawsuits that had merit) - the entire world obviously became indebted to exalted American scientific contributions! (And if you're not chuckling by now, I failed to present that correctly.)
– JBH
Nov 8 at 0:46
• Welcome to the planet California. You can take off any time you want, but you can never land. Nov 8 at 2:09
• @DarrelHoffman Not only is that an easier trip, but we can not too this day nor in the foreseeable future land a rocket on an Earth like world with enough fuel to take off again. Earth Rockets need to be massive and structurally minimal... not the properties you want when slamming into the Earth's atmosphere. This could also justify WHY you'd send 1-way missions anywhere though. It could take decades if not centuries to build up the infrastructure needed to make a space agency on the colony world able to send return missions. Nov 9 at 19:02

## It's suffering from massive Kessler Syndrome

The problem with this planet is, that there are only two very small zones along the poles where you don't get hit by anything when you launch. You need to launch pretty straight up above the debris cloud, and only then burn sideways. The cost of launching on this absolutely worst of all paths is super high, but the only way to even get into space without a collision.

However, trying to return means you have a much flatter trajectory and need to pass through the zone of debris - and that means you are guaranteed to get hit due to the density of debris.

• Steep descent trajectories are associated with higher shock loading and rapid temperature increases, too. That said, I'm not sure this meets the "hundreds of years of technological advancement to overcome" requirement. Nov 7 at 12:35
• @ Starfish Prime The tech development is the method to clear the Kessler Syndrome debris. Nov 7 at 16:51
• @JustinThymetheSecond WE are in space since more than 60 years. We haven't come close to develop a method to clear Kessler Syndrome safe of trying to prevent it in the first place. Nov 7 at 17:25
• Couldn't the debris have a natural source (e.g. a collision of two smaller bodies), rather than "space junk"? Even if it has a short lifespan in astronomical terms, it could still be extant at the right time to have the described effect. Nov 8 at 19:35
• A slight modification to this that would make it more realistic - the civilization have invented a Kessler syndrome clearer - for a small patch of sky. Big ground based lasers push the junk off into harmless trajectories for enough of a window for the rocket to get through. The reentry involves, as you say, traversing a much flatter path, which would be much harder to clear
– lupe
Nov 10 at 17:09

## Ancient planetary defense system.

The planet is in a star system with lots of asteroids. Those usually would make the planet uninhabitable. But millions of years ago the planet was colonized by a species of precursor aliens. The aliens installed a fully automatic planetary defense system which destroys anything on a trajectory heading for the surface. Mostly to protect it from asteroid impacts, but probably also to defend the planet against other spacefaring civilizations. This made the planet habitable and allowed the aliens to terraform the planet and create an ecosystem.

However, the alien civilization isn't there anymore. Circumstances that are not relevant here either killed them or caused them to abandon the planet. But although the precursor aliens are gone, they left the stable ecosystem and the automated defense system behind.

Over the past millions of years, intelligent life evolved, created a civilization and discovered spaceflight. After all that time, the defense system is still working and protects them from asteroid impacts (and occasional visitors from other spacefaring alien species). Unfortunately the system doesn't recognize the primitive vessels launched from the planet as friendly. It lets them launch, but it doesn't let them get back down. The ancient aliens might certainly have known a way to tell the system to not shoot down their own crafts, but that knowledge was lost to time. So anything that tries to return from orbit gets shot down by the defense system.

Why don't the current inhabitants try to get the defense system under their own control? The answer is that they can't. Their technological level is still far too low to understand how the system actually works. Perhaps they could destroy the system or parts of it permanently. But then they would also compromise their defense against asteroids and risk getting wiped out by the next bigger rock that comes around. So until they have the technology to provide asteroid defense themselves, that's not an option. The precursor artifacts could also be of cultural or religious significance for their society, making it politically impossible to get rid of them. They are, after all, literally a gift from their creators that protects them from harm. That's far more substantial than what other religions have to work with.

• I think this is the only real option that hits the spot. Other merely rocket-scientific issues seem likely to be overcome with less than hundreds of years of technological advancement, whereas alien automata can be arbitrarily complicated. Nov 7 at 12:36
• Oh hey, I’ve read that book! Nov 8 at 18:40
• I've played that water-themed game... Nov 9 at 4:07
• This reminds me a little of the background to Arthur C. Clarke's "The City and the Stars". But is it an actual plot point of an actual novel? If not, kudos for thinking it up. And upvoted, naturally. Nov 9 at 12:44
• "The ancient aliens might certainly..." Should that not be "would certainly"? "might" followed by "certainly" seems like an odd juxtaposition. Nov 9 at 12:48

## The planet has no atmosphere.

In a pure oxygen environment, humans can survive pressures as low as ~2PSI. The people on your planet can survive in underground pockets of gas, like we have here on earth. Unfortunately this has big ramifications for your spaceships. We use the atmosphere to our advantage during re-entry. Spaceships have massive horizontal velocities during orbit, something on the order of 8 km/s, and all of that velocity has to be removed before landing. Current spacecraft always use drag in the upper atmosphere to slow them down; and I'm not talking about parachutes. Simply ramming into the atmosphere generates tons of heat, enough to burn up most meteors. That's all the kinetic energy being converted into heat!

Without the atmosphere, though, the only way to lose horizontal velocities is to turn on those rocket engines again and propel yourself in the opposite direction. This takes nearly as much rocket fuel as it did to get into orbit! Our best existing spaceships had a payload capacity that's about 6% of the total weight of the spacecraft. The fuel can easily be over 80% of the mass.

So while obtaining orbit on your planet will be comparable to what we have today (easier in fact, because you don't have pesky air resistance or anything to deal with), landing would be pretty much impossible with anything conceivable made from today's technology. Due to the tyranny of the rocket equation, you need more rocket fuel to launch your fuel for landing (and you need to bring more fuel for that fuel, and so on, and so on), you would need incredibly efficient rocket engines that can take far more than twice as much fuel to orbit as needed while maintaining a reasonable payload fraction. Maybe some futuristic high-power ion-engines.

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Nov 8 at 17:37
• You don't need twice as much fuel, just a smaller mission payload. A 500 ton rocket can put a 100 ton payload into space. If 80 of those tons are more fuel and 20 are your mission capsule, you can get back down. Nov 8 at 22:30
• @Nosajimiki, at an ideal 6% payload, a 500 ton rocket can launch 30 tons of payload, not 100. If we assume the smallest reasonable rocket for people to safely de-orbit with is a Falcon 9 (not a very big rocket) at 500 tons, you need to launch a ~8,300 ton rocket to land! I can't say that's impossible but it's not reasonable with today's technology. Nov 8 at 23:56
• This isn't a convincing argument. Atmospheric losses due to drag are rather high during takeoff, and while you would need more fuel to reduce velocity and land again, this is not an issue that would bar landing from a technological perspective for "hundreds of years". This is Apollo era tech--we landed and took off propulsively from an airless moon and took off from Earth. Also, a pure vacuum environment outside enables high-ISP engines that you;d normally not want to operate in an atmosphere, like nuclear powered ones at minimal risk. Nov 10 at 9:56
• @Rafael Just no. Simply take a look at Δv budgets. The Apollo mission had around 16905 m/s of Δv and spent approximately 9300 of those getting to Earth orbit. On an airless Earth, it would only require around 7500 m/s of Δv, and since 15000 < 16905, a fully fueled Saturn 5 tipped with Apollo would be able to get to LEO (even a bit further) and back with little difficulty. This isn't even considering mass savings due to not requiring aerodynamics, parachutes, reentry ablators, etc. that such a mission would include, resulting in less payload mass required. Nov 11 at 18:56

The atmosphere contains a chemical that eats away the outer layers of the rockets upon reentry, when atmospheric compression turns the gas into a plasma.

The problem doesn't happen on lift off because the velocity are lower, and shielding the rocket to withstand the reentry can only be done with materials which make the rocket to heavy to reach space, until a few hundred years of material science development find the compound which is both light and sturdy for that application.

• Theoretically one could land slowly with rocket motors, but at the very least it would take many missions to transfer enough fuel to the orbit to supply a single landing mission.
– jpa
Nov 7 at 7:48
• @jpa I have one word, and a lot of explosions for you. Orion. Nov 7 at 12:35
• @StarfishPrime That is perhaps one of the best-written sentences I have ever read. Thank you for making my day! Nov 7 at 14:25

Other than an alien ex machina type of answer, like Philipp answered, I can't think of any pure astrophysical reasons. However, there there could be other reasons preventing them from returning.

Contagion Reason

For the first 3 successful missions to the moon, astronauts had to quarantine for 21 days. After Apollo 14, NASA realised that there was no reason to quarantine. But what if there was a reason, for example, near your planet there is space dust that is so harmful, that the worlds governments forbid any return missions? Then astronauts who leave, can never return. There is no reason to worry about the poisonous space dust naturally entering Earth, it gets neutralised when it burn and breaks up during entry. But that process would kill all of the astronauts on board, if a spaceship tried that.

Nowadays, with all of our modern medicine, there are (typically island) countries that have no rabies. In order to keep it that way, if you want to bring your dog with you, some places require up to 6 months of quarantine. If whatever space contagion could only be discovered by dissecting the dead body, then why would anyone want to return? They would have to be killed and examined right away.

Preparing the body for space is a one way process

Liquid breathing is a process that involves filling the lung with a liquid that has a lot of oxygen inside. Surprisingly, mammals can actually survive using this. There are many scientifically, militarily, and medically ways that liquid breathing would be beneficial. But it doesn't exists outside of laboratory experiments. That is because after the lungs are filled with the liquid, it can't be reliably dried to be able to breath air afterwards. You can bet there is lots of money up for the grabs, for whoever can develop a way for liquid breathing to be viable. Yet with today's technology, that doesn't exist.

What if going to space involved a one way body transformation? Then the moment you return, you would die. Maybe there is a lot of radiation, and you can do some skin transformation to survive that radiation. But then when you return, and there is not so much surrounding radiation, you would die of reverse- radiation poisoning.

Or maybe when entering space and you experience 0 gravity, your bones get extremely fragile. In 0G space that is not a problem, but the moment you return you would be crushed under your own weight and die.

• Is reverse radiation poisoning a thing? :P Nov 7 at 19:01
• @Seggan, It is not. But after a handwavium skin transformation, you can come up with a handwavium poisoning; maybe the skin requires heat from gamma rays to stay alive. Nov 7 at 22:59
• Space adaptation could definitely be a factor, as it is already a concern on Earth. If the beings in question do not have human physiology, it would be quite easy to claim that a microgravity environment causes a bunch of changes that make returning not just hard to adjust, but plain deadly. Could even be a cruel drawback of a rapid adaptation rate; they adapt to suit zero-G fast, but can't survive the crushing of fragile organs long enough to change back. Nov 7 at 23:32
• It's possible to invert this: some pathogen or pollution on the planet that the body tolerates when exposed to it from birth, but the adaptation quickly stops as soon as you are not exposed for a while. Kind of like common cold and COVID isolation :)
– jpa
Nov 8 at 9:56
• @Seggan Yes, its called Vitamin D deficiency. Humans require a certain amount of solar radiation to produce healthy amounts of Vitamin D. If you were modded to be say 100 times as resistant to radiation in space, you may on Earth only get 1/100th the Vitamin D production you need which will eventually result in heart failure. Nov 8 at 22:36

This is a LOT harder than you might think

I was having fun with the idea of a naturally combustible gas, maybe methane, occurring in low enough quantities in the atmosphere to allow the evolution of life but high enough quantities that, during re-entry, it would ignite. Clever technology (like really long bell housings) could conceivable allow for thrust while allowing the exhaust to cool so the atmosphere doesn't ignite on launch. But the lengthy re-entry process just dooms the planet.

Then I read @Rafael's answer (which I upvoted) and realized that the problem is easily, if not cheaply, circumvented. Just slow down, ease through the atmospheric boundary, and pop chutes.

The only idea I can think of comes from an episode of Star Trek Voyager

The episode in question is "Blink of an Eye." The gist of the episode is this: something is causing a time dilation field between the planet and the ship in orbit. Hours on Voyager are centuries on the planet. The planet evolves civilization from primitive humanoids to an advanced species that develops a way to visit Voyager.

The episode uses a technobabble cause for the time dilation: a tachyon field.

But a better solution comes from the movie "Interstellar."

The Endurance passes through the wormhole into another galaxy with a planetary system orbiting a supermassive black hole called Gargantua. The crew intends to investigate three planets, each previously explored by NASA volunteers, who shared positive reports for habitability. The first planet is an aqua planet with massive tidal waves and no dry land. Doyle drowns after failing to get into the probe and getting knocked off by one of the waves, and Amelia and Cooper fly back to the Endurance, only to find that decades have passed due to the time slippage caused by the planet's proximity to Gargantua. Romilly, having remained onboard, has aged 23 years. Cooper replays messages from Earth, learning that Murph is now his age and has become a scientist working with Brand.

Time Dilation Prohibits a Practical Return

In other words, there's a scientific basis for the kind of problem that really would make it impossible to return. The planet orbits close enough to a black hole that the simple procedure of entering orbit experiences massive time dilation. One thing the movie doesn't go into is that while the ship is on the far side of the planet compared to Gargantua (e.g., the planet is between the ship and the black hole), time on the planet moves much more slowly than on the ship. But when the ship orbits around to be between the planet and the black hole), time on the planet would move faster than on the ship.

One might conclude that orbiting the planet once would average out the time dilation and, therefore, allow a re-entry in sync with the time experienced on the planet. I don't have the equations to prove that, and I suspect it's incorrect. If I recall my astronomy correctly (I might not, it's been a honking long time), gravity is not linear with distance. It's logrithmic. That means the time dilation when the ship is between the planet and the black hole is worse than the dilation experienced when the planet is between the black hole and the ship.

While this solution may not guarantee the inability to return (or, more precisely, the meaninglessness of returning), it goes a long way toward meeting you needs in a suspension-of-disbelief manner.

• Indeed, there were a lot of problems with the handwavium of that Voyager episode; as is often the case where time travel implications start cropping up. I think time dilation making return possible but impractical is nevertheless one of the best answers here. Nov 7 at 23:43
• However, I believe the actual time dilations for survivable gravitational fields for the average humanoid would be far too small. Better would be if the planet was actually traveling at a sizeable fraction of the speed of light. In which case, its whole solar system would have to be also. Which, while requiring most likely some sort of Ancient Aliens setup, would AFAIK be possible... Nov 7 at 23:44
• ...aaand I've just gone and made it so return is only bad if you leave your entire solar system. Perhaps we need the handwavium after all. Nov 7 at 23:47
• @Alexander You've got some good points and I can only agree that SciFi's perspective on survivable time dilation tends to strongly favor telling a good story over "reality." Unfortunately, what many of us have realized is that there's nothing about physics (worldbuilding) that can prevent the return of a launched ship. That leaves answers like Trish's and HanMah's where it's a story problem (storybuilding) that prevents return. Might as well suggest that a military coup took place and the ship will be shot down if it tries to return ("your sacrifice is for the greater good!").
– JBH
Nov 8 at 0:21
• @JBH fwiw iirc gravity is proportional to $1/r^2$, so it'd be inverse quadratic, not logarithmic Nov 8 at 2:16

The planet has a very treacherous surface (steep cliffs, shallow waters dotted with jagged rocks, marshes, etc) which have only sparsely been made survivable by humans at a great cost and are merely connected to each other via a network of narrow roads and tunnels.

This setting would enable civilization to arise, the roads could be used by travelers and merchants on foot or with carts - later by vehicles with internal combustion engines and locomotives. Space-capable rockets could be built from the resources available, but unless ~10 meter CEP precise landing capabilities are developed, no landing is safe enough to attempt. Crashing down on boulders, sinking in swamps, breaking up on rocks or drowning in perpertually stormy seas is all but guaranteed when landing outside man-made territories - and any "fortunate" landing on the tiny, painfully expensively prepared safe areas would result in huge damages to infrastructure and human life already present there. And thus no landing capabilities are developed at all.

• This is a good idea with many possibilities: subsurface civilizations, hostile lifeforms, extreme surface weather. The accuracy difference between launch and landing is quite fundamental for many rocket types.
– jpa
Nov 8 at 9:50
• Spacex propulsively lands rockets on pads only 86 meters in diameter, which is not all that large. Is it really conceivable that humanity will be able to build functional spaceships but not a flat circle 86m wide? Nov 8 at 19:15
• @Rafael SpaceX can only do this in fairly calm weather. If you add constant storm conditions (with frequent unpredictable gusts) across the entire surface, it makes precision landing far more difficult. (It also increases the difficulty of launching, but not to the same extent.) Nov 8 at 20:00
• @Rafael SpaceX has only managed to do this by relying on decades of experience with spaceflight collected by various countries. Without incentive to even begin with the associated research the necessary knowledge might conceivably never get amassed. Nov 9 at 9:36
• @zovits I don't think so - assuming you want your space stuff to get people out of the planet and then back, that is incentive enough. Also, 100 years from first rocket launch I believe we will manage precision landing on the same takeoff path even in bad weather - so even if terrain is horrible, few centuries from first launch rockets will be able to get back to the same spot. Nov 9 at 14:53

## They figured out rockets before we did

The idea behind a rocket is not difficult. You need fuel, an oxidizer, and a way of controlling the burn. Perhaps some iron aged alchemist figured out kerosine rocket fuel and maybe they have a slightly lower gravity than we do making getting to space with an imperfect fuel much more doable. So, your Leonardo de Vince, Archimedes, or Pakal like guy figures out rocket fuel, and starts launching stuff into space, but his understanding of aerodynamics and material science is still way too primitive to figure out how to make a re-entry vehicle because he cant figure out why things burn up on re-entry or how to make them heat resistant enough to survive. Maybe they blame the burn up on the wrath of God, so they assume there is no scientific problem to be solved.

So instead of making the problem of re-entry harder than it is for us, you make the problem of getting to space easy enough that your civilization needs to wait hundreds of years to catch up with the 20th century science that made 2 way travel possible.

Philosophically, that's easy - like Heraclitus' river, the planet continuously changes, so you can never go back to the same planet you lifted off from. Unless time on it stops as soon as you leave.

But that's not what you meant, I suppose.

So. For a single flight (like the Apollo missions), it is doable - have a deep enough gravity well that no significant payload can escape the planet's gravity unless they exhaust the fuel needed for the reentry brake. This way, also, atmosphere density increases fast enough that aerobraking from the Karman height to the ground would result in a meteoric burn.

But if the above is not doable, or potentially infinite fuel resources are somehow available in orbit, so that you can always support a Space Shuttle-style reentry, then you need a planet you can't land on, ever. Hiding the surface wouldn't work - radar, parachutes and helicopter-like propellers would allow landing anyway.

So, we need to disrupt a very large, even armored landing capsule that could split open and release a helicopter at a suitable altitude, capable of hovering and choosing the landing zone with ease; or prevent that helicopter from hovering, or at least from landing without crashing.

The only way I can think of is violent, continuous cyclonic storms over the whole planet. Like Jupiter, but worse.

You can stay safe on the ground (maybe in a deep depression) as long as you like, and wait for some brief respite, and launch just then, in the ten or fifteen minutes' of relative calm in the eye of a storm. Let's imagine there is on average at most only one such period, in any one given area, every two or three days. When leaving, that's easy: you prep for launch, and wait. And wait. And wait. When you're sure you can, you launch - and you need about three minutes to rise to safety.

But when coming back, once you've committed to reentry, you do not know whether the safe area will be or even whether there will be one.

Chances of hitting the right five-minute window in three days, if the weather is truly unpredictable, are less than 0.2%. Even aborting the landing and retrying if things look ugly won't increase that very much.

And landing in the middle of a storm means crashing almost surely. You cannot hover at high altitude (also, it would be useless), you cannot use parachutes - they might even be worse - and the landing vessel cannot fly in the weather. Given those conditions, you simply cannot land.

Of course, the plausibility of such a hellhole of a planet is debatable, to say the least.

Planet inside a (Kerr-)Nordströn Black Hole

Charged black holes have 'two' event horizons, both of them being before the sigularity. Thus, when tou cross the second you are inside the black hole but with the singularity spatially away from you (spacelike distance) not in your future. This means that you could theoretically enter the black hole and go about your life inside it (i.e. you '''could''' have a functional solar system inside).

Now, the important part. When you are inside you can cross again the second event horizon that you crossed, which would actually lead outside, making you go outside the black hole but in a different universe (don't quote me on this). The reason being, from the inside they are white holes, effectively.

Charged solutions are not actually of physical interest, though, because the charge would in reality equalize quickly with all the matter orbiting the black hole, closing the Cauchy horizon. The stability of the system is concerning. Inside the black hole you could also find the naked singularity, which I don't know how anyone would regard in a novel, but, hey. It's not extremely believable, but not wholly unphysical either...

i.e. white holes might be your best bet.

• Wow. Welcome to the site. Please take the tour and refer to the help center for guidance as and when (it's rather long). If only supersymmetry hadn't been popularised, more of us might have been more familiar with this postulate. Excellent first post. Nov 8 at 14:44
• You can't exit a charged black hole, because the outside always lies in your past. You can pass the second event horizon and fall into the singularity though, but you'd just be squished. A white hole is not the same as a charged and possibly spinning black hole. Nov 8 at 15:15
• jila.colorado.edu/~ajsh/insidebh/penrose_rn.gif While you cant get out to the same universe, you can go out. Also, after crossing the second horizon you needn't fall into the singularity, there is a second 'reversion' o sign in the metric, leaving it with the same signs as in the outide, so you are spacelike away from it. There is a Penrose diagram of the process. Nov 8 at 17:39
• @EloyJesús much like the white-hole exit point of the maximally-extended schwarzschild metric, your traversable black hole is too delicate to exist in the real universe... they're slightly less likely than wormholes, which is quite an achievement! (also it should be kerr-newman or reisnerr-nordstrom metric for rotating and non-rotating versions respectively) Nov 8 at 19:53
• @StarfishPrime Yes, you are right, it's not something stable in reality. Most texts in GR* tend to skim through charged solution for this very reason. Where you to ignore the whole trouble of their coming into being and to put them in a region of space without any charge close/big enough to neutralize it, well, it would be stable enough for a novel, haha. But you are right, it's not something you expect to stumble upon. *Introctury ones, at least (SC, Thorne,...). Nov 9 at 12:21

Freakishly tall mountain above the winds.

This is where they launch from. It is up above most of the atmosphere. Wind speeds are high but the air is so thin that the wind is less able to tear things to shreds. Shredwinds is how it is lower down until you get below the surface, which is where your people live.

If you could land right on top of this mountain you would be ok. You could go home thru the tunnels, the same way you got up there. If you miss and run into the storms below, you and your ship will be torn to shreds.

• Is this a multiple-century technical challenge, though? Our world has seen less than a century of spaceflight and is already landing rockets with meter-scale accuracy. Nov 9 at 14:07

Many little planets

A three body system is chaotic, a 20 body system is pure chaos. Once you take off the only thing to do is thrust and get out of there ASAP.

Returning home is a suicide mission, you don't know what is going to hit you from where and setting a course to your home planet becomes an impossible task as you don't have a clue where it'll be in the next day or so

• Takes a lot of luck for a 20 body system to even survive in planetary timescales, but I guess there could be solutions to that (such as a lot of large asteroids that get captured by the planet every now and then and eventually end up burning in the star).
– jpa
Nov 8 at 9:53
• Technically our solar system is waaay more than 20-body, and we don't experience any problems with take-off and re-entry. And in your supposition, the majority of those bodies would collide very early in terms of solar system's age, reducing the amount to bearable. If you're speaking about asteroid rings, it's a partial Kessler Syndrome effect of natural occurrence, yet whatever of these hit the atmosphere (any kind of atmosphere, as it literally expands infinitely away from a gravitating body) it slows down, thus there still would be no obstacles below a certain point. Nov 10 at 10:34

An underwater civilization that's evolved on an airless moon orbiting a gas giant, with ocean beneath the frozen surface, like Ganymede or Europa, but bigger, with an earth equivalent surface gravity, but a tenuous atmosphere due to the temperature.

Once they breach the ice and get the idea to start launching satellites, they will have great difficulty getting them to return with negligible atmosphere to use for breaking plus most of the reasons for returning things are returning living people, who on earth can land in the ocean/taiga and wait to be picked up, even if off course, while for them the surface would be a deadlier environment than space.

Rapidly evolving pathogens

If the planet has really rapidly evolving viruses and bacteria, then population would evolve equally rapidly evolving immune system. But if you were to leave the planet for a months long trip, your immune system wouldn't get regularly updated, the pathogens upon your return would be too different to recognise and thus lethal.

This solution would need a planet to be on a smaller size, as otherwise even traveling on it would be dangerous. Or it should be really windy. It has to have a mechanism for whole population to be exposed to similar pathogens everywhere in small time windows.

The planet has catastrophic weather almost perpetually, except for a day or two every ~100 years

Even at our own level of technology, we have pretty stringent requirements for good weather in order to conduct a launch.

Suppose your hypothetical planet has horrendous weather, at least in the upper atmosphere if not on the ground, that makes launches doomed to failure a majority of the time. Due to weather patterns and seasonal changes and planetary influence and other hand waving, the weather on your planet is only safe for launches for a short window every ~100 years. Maybe it is predictable and scientists can plan for the big day, but maybe it's not and they have to keep everything ready to go at a moment's notice. After the window closes, the weather prevents any hope of landing, at least until some future advanced tech can protect their ships.

Assuming the people of your planet have a lifespan less than the frequency of these rare good launch/landing days, that's effectively a one-way trip.

The planet spins rapidly combined with very low gravity.

On lift off, it slings you off of the surface, facilitating easy access to space.

On re-entry there are two options, either hit it face on or try to catch it on the side.

In case 1 it would be like trying to jump from a moving truck, no way you could land safely.

In case 2 youd skip off like a rock skipping over water as gravity would be too weak to catch you.

I'm not doing the math, but I suspect a large asteroid might match the criteria. The required spin could be the result of a collision with another asteroid.

• Interesting idea, but I think it would simplify re-entry as long as you are on a prograde orbit. The atmosphere would have to be spinning rapidly also, so any air resistance would be reduced by the lesser difference in speed. You would probably need some rocket fuel to slow down at the moment of touchdown.
– jpa
Nov 7 at 17:48
• Landing at the poles is still feasible, you just need to match the rotation period and let gravity pull you down. This would indeed have to be a single solid rock like an asteroid and not a planet, since a planet that spins fast enough to have negative surface gravity would tear itself apart. Nov 7 at 18:39
• Would this pass the constraint "Planet should be able to support life that is similar to Earth"? Rapid spin and low gravity sounds to me as if there wasn't much usable atmosphere available. Nov 8 at 12:23

# Extremely Dangerous Megafauna

Perhaps your planet has skies littered with Dragons Sky Lizards whose domain covers the entire planet, that are extremely territorial. They are vicious and hardy creatures, that can and will easily decimate anything encroaching into its territory.

By necessity outgoing rockets are moving far too fast for them to be intercepted. Rockets coming in for landing though? Either slow enough they're lizard food, or fast enough they're pancake.

High gravity planet.

It supports life and all that, just not necessarily human-friendly. The natives are short and squat but they have figured out how to get into space through fairly conventional means.

The ships that get launched from here have to be very strong. Lots of structural reinforcement. All this is unnecessary in space, though, and just adds to the cost of any maneuvers or any other travel they want to do. So one of the launch stages actually jettisons the main hull, revealing a far lighter, thinner internal structure that is fine for space but absolutely cannot land. (There could be emergency landings via parachute but the vehicle destroys itself on touchdown because it can no longer support its own weight.)

So you can, maybe, if they included the emergency parachutes, "get back down" but "landing" is not an option, in the sense of ending up with an intact ship. And if there's no parachutes, you're not getting back down at all. (Perhaps the lighter module actually does not include the heat pads necessary for re-entry at all. They can get you up, but ditching all the weight means you can't come back down.)

• the issue is parachutes actually work better in high gravity. planes and winged landing craft also work a lot better.
– John
Nov 7 at 21:32
• @John Yeah that's why I'm thinking ultimately part of their weight savings plan is just not including heat plates. I'm kind of betting that the robustness of heat plates you need for re-entry is also a factor of gravity (and air density?), and with all the structural integrity issues from high-G, plus all the extra fuel it takes to lift off, "we couldn't include any gear to land with" may have been reasonable [within some cultural framework]. Really I don't think there is a "real" solution to OP's problem. We need something outlandish or kinda-close-enough. Nov 7 at 21:42
• I like this. The necessities of launching from a high-grav planet make your spacecraft incapable of withstanding the gravity of the planet if you return. I've not had any luck checking whether the apollo lander could withstand sitting on earth, (probably yes) but it was never intended to do so, and the moon's gravity is 1/6th of earth. It'd probably sit heavily on its suspension compared to how it would be on the moon. Nov 8 at 9:24
• another issue the space craft has to withstand a lot more than gravity to get to space, this only works if the ship is made in space, otherwise you don't actually save any launch weight, if anything it adds weight since it must have extra joints.
– John
Nov 8 at 19:02
• @John Yeah I don't see it as saving launch weight but rather, saving fuel for anything you want to do after launch, e.g., go to another planet, land on the moon, shift orbits, etc. Not much point if they're just going to a fixed orbit and sitting there, for sure. Build your launch vehicle to withstand 6 G on the planet (+ like an additional 2G for launch or whatever) but then the "travel section" only needs to withstand whatever acceleration you want for maneuvers. Nov 8 at 19:51

# Edge of a black hole

Your planet is very close to the photonsphere of an ultramassive black hole.

Were it a stellar black hole you and the planet would be spaghettified, but tidal forces are smaller for the bigger holes.

Due to conservation of momentum, if you take off either you or the planet will cross the event horizon (unless you do it from the poles, which might be prohibitively expensive).

In either case that will be your last contact with the planet.

# Travelers from the planet have access to 'resources' that inbound travelers do not

Perhaps the planet is very unpredictable due to its gravitational pull, or perhaps the inhabitants have shot so much junk into space that it is very difficult to find a way through. Either way, plotting a course is very difficult and perhaps the trash is even organized in a pattern that going out is generally easier than going back in.

Fortunately you are able to overcome this with heavy calculations just before and during the launch, but this is only possible with vast computational resources which you have on the planet but not on your spaceship.

Secondly you may need to observe the situation with very accurate sensors from the perspective of the launch. Something that obviously would only exist planet side.

Lastly you may need a stable&adjustable base to even adjust the approach angle in the final seconds of the launch countdown.

In short, there may be all kind of resources needed that are available planetside, but would be very hard to make available in the middle of space.

Option 1

#### Physiology

The planet has a strong gravity, more than double than Earth gravity. Not only reaching space from there is extremely expensive, but the aliens adapted to such gravity had to develop some clever tricks to adapt themselves to the loss of gravity. Trouble is that those tricks are not reversible. Those who reach the outer space are not able to go back without suffering some mortal physical damage.

Since this is the heaviest of the planets that developed some life forms, it is too dangerous even for the inhabitants of all the other known planets.

Option 2

#### The magnetic cloud

A cloud of ionised gas surrounds the planed. It scatters the light and creates constants magnetic storms that affect all the electronic equipment on board, any spaceship or missile passing through is blinded for a while. It may be safe to leave because going towards outer space the likelihood of bumping into something is small. But it makes it impossible to calculate a safe return route.

Astronauts lose muscle-mass every day while they are in a no/low-gravity environment. Only with a strict training regime during their stay they are able keep this process in check a bit. However even with such a training regime they are not able to walk or even stand after returning to earth. Full adaption to earth-level gravity can take up to a few months.

Perhaps creatures living on a planet with a higher gravity than earth will never be able to adapt again after a few months in a no/low-gravity environment.

# Severe Weather

Like, hurricane-severe. A dramatic axial tilt and short year gives your planet harsh, extreme, quickly changing seasons. That paired with the continental geography and atmospheric conditions cause near-constant gale force winds, monsoon rains, or hail the size of minivans. Don't forget lightning.

There are very few times a year when the weather is tame enough to launch, and the most advanced weather tracking technology can only predict those days a few days out. There is no way of knowing in advance when or where it would be safe to land.

More importantly, the lack of clear days to launch severely limits the frequency of shuttle tests and the knowledge they bring. Recovery of wrecked test craft for postmortem analysis is nigh impossible. Thus, what might take Earth a few years of progress would cost your civilization several decades.

You can't have such a planet naturally. If the planetary physics allows take off, it allows return. If you make it like in your question, a planet you can take off from and you can never return to -- face it, you're going to look a manic divorcee. ; )

I propose a very simple solution: just have the planet be orbiting its sun very quickly.

It's hard to land there because it's hard to hit a fast-moving target, especially when you have to arrive at the same velocity as the planet in order to not crash into it. But leaving the planet isn't any more difficult than usual, because "at rest" the rocket already has the same velocity as the planet does.

• Might work in a story, but from orbital physics standpoint there are limitations. Planet's orbital velocity depends on its distance from the star, so very fast orbit would require dim star and planet very close to it. And for simple missions where the spacecraft stays in orbit around the planet, the orbital speed of the planet around the star does not matter.
– jpa
Nov 8 at 17:34

Deliberately imposed restricions

The planet is under the yoke of the Galactic Empire.

The Empire doesn't allow its subjects to get too uppity with their own spaceflight. Return trips are forbidden.