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In my world a moon is reached and colonised but only in space stations for a long time. They gain the ability to reach the surface and then story happens. It needs to be something that can be bypassed so the main characters can get down to it. Some important things to know are:

  • it is similar to Titan (in composition, with more earth like conditions, water, oxygen etc)
  • it is about half the size of Earth
  • it is on the cold edge of the habitable zone
  • the gas giant its orbiting is similar to Saturn
  • it must have conditions where (non extremophile) life can live
  • I do not want anything to be able to get down: no probes, robots, etc

What could cause a civilization to be unable to land (physically) on a moon's surface but build many space stations around it?

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    $\begingroup$ This is a pretty broad question, with probably dozens of possible answers. For example, the colonists might not have a descent vehicle with the delta-v required. Or, the moon could have recently been the subject of a massive collision which rendered the surface unstable for a long period of time. The colonists could have superstitious or religious reasons that have to be overcome. There could be political reasons as well - perhaps the government of the orbiting colonies has forbidden it because they like the status quo, and a revolution will be required. What kind of solution do you need? $\endgroup$
    – Dan Hanson
    Apr 24, 2020 at 20:13
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    $\begingroup$ @DWKraus edge of habitable zone of star. yes it has life but id prefer if that isn't related to why they don't colonize it, id prefer they cant even land anything. as for composition take titan (the moon) and make it more earth like, but still covered in clouds and haze $\endgroup$
    – Topcode
    Apr 24, 2020 at 20:15
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    $\begingroup$ @dan-hanson i need a solution where it is unable for anything to successfully land on the surface, they try but have to fail until eventually they figure it out. $\endgroup$
    – Topcode
    Apr 24, 2020 at 20:17
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    $\begingroup$ Another example: Perhaps they discovered that the moon has primitive life, and they won't send people down due to fear of contamination, or until scientists have determined its safe. Or, the moon has some kind of toxin in the atmosphere, and there's a long program of detoxification underway, which is not compatible with colonization.. Once the toxins are scrubbed, the people can live there. We really need some parameters for what you are looking for, or we could come up with ideas forever. $\endgroup$
    – Dan Hanson
    Apr 24, 2020 at 20:17
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    $\begingroup$ If you can live comfortably in space (in a habitat), they why bother with a planet with their messy weather and gravity? $\endgroup$
    – NomadMaker
    Apr 25, 2020 at 11:54

25 Answers 25

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They have no ships that can withstand reentry.

Your people arrive in a huge starfaring ship, built in orbit. Descending through an atmosphere would rip it to pieces. So they remain in orbit around this new planet.

To get down to the planet, they will have to build a vehicle capable of doing that. Your people might lack the raw materials to build such a vehicle, or skills to build it, or both. They might have tried a few times which ended in disaster and then decided it was not worth trying again. Until someone who is born who wants to try again, and succeeds.

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    $\begingroup$ +1 but... silly them. How could they spend heaps to get to a destination only to stumble at the last step? (not saying it's impossible, I'm saying the answer will benefit from addressing possible causes of their inability. Otherwise, the disbelief levels goes beyond what a suspension can withstand) $\endgroup$
    – Adrian Colomitchi
    Apr 25, 2020 at 0:13
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    $\begingroup$ Maybe they needed generations to get here, and, people being short-term focused, nonessential infrastructure got trashed / cannibalized for the important things. Use landing craft parts to fix your broken life support, and you'll arrive at the planet in okay condition but nowhere near ready to land. $\endgroup$
    – parasoup
    Apr 25, 2020 at 1:13
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    $\begingroup$ Maybe the project was designed from the beginning to be based in orbit, later bootstrapping itself into a land-based colony. The landers and other equipment would be a lot of extra weight you could delete from the cruise. $\endgroup$
    – Cadence
    Apr 25, 2020 at 3:15
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    $\begingroup$ This is the interplanetary equivalent of flying into JFK, then not being able to find transit for the last 50 miles of your journey... $\endgroup$
    – smci
    Apr 27, 2020 at 5:03
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    $\begingroup$ @AzorAhai: you can build a relatively functional space station by welding a few cargo containers together and adding sufficient life support systems. Maybe repurpose a few RCE thrusters for orbital corrections. Depending on how you do it re-entry could require any number of components that you don’t have available when building a space station. Heat shielding, parachutes, high power rockets, portable power sources etc etc... $\endgroup$
    – Joe Bloggs
    Apr 28, 2020 at 8:36
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The planet already hosts life. Nobody but extremely clean and well-trained exobiologists are allowed to land there to prevent cross-contamination. This is a political rather than physical constraint, so the primary thing that changes is the prevailing political parties and red-tape.

Of course, one of the reasons for avoiding cross-contamination is preventing unforeseen interactions between two different biospheres...

Can you say interstellar plague?

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  • $\begingroup$ Also perhaps continuous and intense jet stream activity and a very uneven surface $\endgroup$
    – Slarty
    Apr 25, 2020 at 22:27
  • $\begingroup$ So, in other words, they have the Prime Directive. $\endgroup$
    – In Hoc Signo
    Apr 27, 2020 at 19:24
  • $\begingroup$ @MarvintheParanoidAndroid: Not necessarily. Even if it’s only single celled life any potential mission would need to be incredibly careful. $\endgroup$
    – Joe Bloggs
    Apr 27, 2020 at 20:46
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The air will kill you.

There was some airborne hazard which rendered the surface poisonous to humans but not the native fauna. Take your pick of germs, atmosphere, radiation, anything that could be fixed by creating a sealed environment like a space station.

The options were (a) stay where we are in orbit or (b) land and live in bubble cities on the surface. Unfortunately we didn't have the materials for that.

Then recently we invented a vaccination / genetic modification that made us immune to the hazard. Then we landed and started building cities.

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    $\begingroup$ while this is a good answer, it doesn't fit my purpose as i am looking for a mainly physical or chemical reason that would stop everything, rather than only stopping manned missions. $\endgroup$
    – Topcode
    Apr 24, 2020 at 22:00
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    $\begingroup$ Cool. You should edit your question to make that more obvious. $\endgroup$
    – Daron
    Apr 24, 2020 at 22:04
  • $\begingroup$ Do you need to stop ALL missions or just most of them? I don't see why we'd bother sending a lot of robot missions if we knew the place couldn't be coloised yet. Sounds like there is something on the planet you want to keep secret from the characters until they colonise the surface. $\endgroup$
    – Daron
    Apr 24, 2020 at 22:06
  • $\begingroup$ well you will notice that a) i state physically implying that its not a toxin b) i state unable to land, not unable to survive and to answer your other comment yes all missions have to be stopped, but there has to be a solution that can be found. since you must know here is the basic plot: they keep sending robots to the ground but none of them make it, then they make a solution and one makes it through but gets odd results, turns out there was an alien, the protags go down and find it ect. if you cant be bothered to actually read the entire question and answer properly then ok. $\endgroup$
    – Topcode
    Apr 24, 2020 at 22:59
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    $\begingroup$ In order to get the answers you want it is important your question be understood. If someone misunderstands the question you should reconsider the wording rather than being sarcastic. Remember we have a lot of people on Worldbuilding who don't speak English as their first language. I do speak English as a first language and didn't gather from the question you want to prevent robots, probes et cetera. $\endgroup$
    – Daron
    Apr 25, 2020 at 11:37
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The moon's surface is full of pockets with enriched fissionable material (think https://en.wikipedia.org/wiki/Oklo).

The spacecraft (or robots) are made of many materials, some of them reflecting enough neutrons back into the pockets so they heat up considerably when spacecraft approach anywhere near them. This causes excessive radiation, if not nuclear power excursions (think Chernobyl).

The pockets are underground and randomly distributed. It's basically a mine-field labyrinth for explorers, whether humans or machines.

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  • $\begingroup$ +1 because pockets of enriched uranium require developed life (oxygen atmosphere). $\endgroup$
    – fraxinus
    Apr 26, 2020 at 15:22
  • $\begingroup$ Uh... that’s gonna make life on the planet pretty unlikely. $\endgroup$
    – Daniel B
    Apr 27, 2020 at 20:18
  • $\begingroup$ @DanielB "It's life, Daniel, but not as we know it." :-) We don't know what effect higher background radiation would have on natural selection. Maybe there are large patches of ground without natural reactor pockets. But if encountering just one of them is a loss of mission event, that would end all exploration quickly. $\endgroup$
    – Jens
    Apr 28, 2020 at 20:14
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There are regular, dangerous storms and winds in high atmosphere. These tend to knock down ships and disable them with a mixture of wind and lightning.

However, the MC have spotted a predictable pattern, and think they see a way past the storms.

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If the ship was a generation ship, the prospective colonists may have gotten used to being inside a habitat.

If there are plenty of resources available to them from outside the planet that are easily available, then why should they brave the hazards of a planet rather than stay in easy-to-build habitats?

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  • $\begingroup$ “Go somewhere the atmosphere isn’t kept in by walls?? Are you crazy??!” $\endgroup$
    – Joe Bloggs
    Apr 28, 2020 at 8:38
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The moon is habitable for a reason unknown to the colonists.

There is some type of artificial shield around that planet, here's why:

Life survives on Earth because it's magnetic field protects it from solar wind.

Titan doesn't have a magnetic field,(maybe not large enough to have a molten iron core) thus it is not protected. https://www.nasa.gov/jpl/cassini-catches-titan-naked-in-the-solar-wind

An artificial shield surrounds the moon, keeping the ozone layer in, bad radiation out, and maybe protects against asteroids too(Titan's atmosphere should burn up most though)

This intense charged field messes with the electronics of anything that passes through it.

The colonists can't detect this field until some accidental event causes it's reveal. (Solar flare causes it to sparkle?)

A electric field at a certain frequency applied to the skin of the craft allows it to pass through the shield.

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Catastrophic Surface Conditions

While the colonists were en-route to the moon, an asteroid beat them to the punch and landed first. The moon just had the local equivalent to the Chicxulub impact and is at the start of a full-on ecological and meteorological catastrophe. Volcanic activity is constant, acid rain is everywhere, massive storms are spinning through the atmosphere, ash is burying everything, etc. Most important to our colonists is the large quantities of ash in the upper atmosphere, which can severely damage craft passing through it.

So, after some time, everything will settle down and the surface will be safe to visit again, but until then, it's best to stay in orbit and wait it out there.

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The moon is extremely dangerous. It's a hell.

Edit: This contradicts some requirements of the OP. Sorry.

It's like a devilish combination of Titan and Io. Volcanoes, electric storms, magnetic storms, hail storms made out of hydrocarbons, glaciers or similar moving ground, earthquakes, strong winds, poisonous and corrosive fogs with fluoride components, and so on. Just invent a collection of really dangerous things.

There are some places where life is possible. But they are rare and well hidden. Even there steel, plastic and glass are corroded and break down. The sentient beings have a nomadic life because the places change slowly. They are connected by a network of narrow somewhat safe but always changing paths, so that a civilization can live on the moon and have trade, like an archipelago.

The sentient beings are small. Let's say about an inch. So for them it is a big world anyway even if they can live only on a tiny part of it. It is impossible to live there for humans, even protected by spacesuits because they break down by the magnetic storms and are corroded by the toxic cocktail the air is on the moon. Probes sent to the moon are destroyed, too, and are soon covered in the hydrocarbon hail and dissolved.

Now, to explore the moon the civilization needed to find out a solution to each of the things that make the moon dangerous. Protect against magnetic vortices, shield against lightnings, make resilient against strong winds, find a way to communicate through the fog, invent anticorrosive coatings, and so on.

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  • $\begingroup$ This seems to defy the OP's requirements of "earth-like conditions where non-extremophiles can live". $\endgroup$
    – Nuclear Hoagie
    Apr 27, 2020 at 13:59
  • $\begingroup$ @NuclearWang, yes, you are right. I read the other answers and then I got the idea of a hell moon and forgot the requirements. $\endgroup$
    – nalply
    Apr 27, 2020 at 14:24
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My suggestion is that you look at Asimov's Nemesis, where the colonists on the space station Rotor were in a somewhat similar predicament, having to decide whether they want to build a base in the nearby moon Erythro, or head to the local asteroid belt or whatnot.

If memory serves, some factors were:

  • They had lived in a space station for most of their lives. May be gotten complacent? Who would want to leave the comfort of a space station for the barren, alien moon? Ok, the young ones in particular would :-)
  • The leader felt that moving to the moon would compromise his power base.
  • The occasional visitors to the moon had developed "strange symptoms" (later explained, I am not gonna spoil the story) that scared off many.
  • Anyway, if there is an asteroid belt "near by", ready to be mined for supplies, the point of not needing that much delta-vee to exploit that resource instead is strong.
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  • $\begingroup$ I realize that this is more about unwilling as opposed to unable. $\endgroup$
    – Jyrki Lahtonen
    Apr 26, 2020 at 15:45
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    $\begingroup$ well, really what i need is for them to not send anything down, whether that be by choice or impossibility $\endgroup$
    – Topcode
    Apr 26, 2020 at 15:48
  • $\begingroup$ That @Topcode wants looks like more than Arthur Clarke's 2010 Space Odyssey. A powerful civilization (or inteligence, whatever) no allow to land. $\endgroup$
    – Rodolfo Penteado
    Apr 26, 2020 at 19:04
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This is a microgravity-adapted species (or offshoot) accustomed to zero and near-zero gravity thanks to genetic engineering centuries earlier. Even half a g would be too much for them to endure beyond a few hours.

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  • $\begingroup$ again like most answers this only solves part of the problem, probes would still be sent down in this situation $\endgroup$
    – Topcode
    Apr 26, 2020 at 16:58
  • $\begingroup$ Even your edit didn't make that clear; by 'physically' I understood the biological component, and 'anything' didn't change that. $\endgroup$
    – rek
    Apr 26, 2020 at 21:05
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Lightning. The world is subject to epic levels of cloud-to-cloud lightning. The problem is the plasma trail left by entering the atmosphere acts as conduit for this--any spacecraft attempting to aerobrake becomes the target of repeated very powerful lightning strikes--not only do you have to shield your craft from those energies (and the impact point of the lighting burns away with the hits) but heat shields aren't exactly known for appreciating lightning.

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The moon was once inhabited by another civilization and surrounded by a lot of artificial satellites and space stations, maybe a thousand times as many as surround the earth in the present. After the civilization fell, all the remaining debris turned into high-speed dust due to constant random collisions over the many millennia. By now the orbit is so cluttered with fine metal particles, that any vehicle that tries to go through takes significant damage before getting close to the atmosphere. The particles are no longer recognizable as artificially created due to their small size, but they are very slowly decaying towards the surface. Maybe some of the particles are even radioactive remnants of orbital nuclear fission plants, which irradiate the ship and disrupt any sensors.

In any case, all the attempted expeditions had to be aborted before getting too close to the planet, though some heavily shielded expeditions might have gotten through to the surface - and some of them might not have gotten back up through the space dust.

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  • $\begingroup$ but how can they establish the orbital colony if there is so much spacesdust? $\endgroup$
    – Topcode
    Apr 26, 2020 at 16:52
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    $\begingroup$ @Topcode They're in a higher orbit, landers descending through the lower orbits are at high risk of damage and destruction, but the colonies are safe. $\endgroup$
    – Ruadhan
    Apr 27, 2020 at 9:14
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    $\begingroup$ a.k.a. Kessler syndrome $\endgroup$
    – Chronocidal
    Apr 27, 2020 at 11:49
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The fictional moon doesn't have much mass and therefore its orbit is quite fragile. Therefore, the force of landing on it could disrupt the orbit of the moon and kill everyone. The space stations are built symmetrically around the moon as to preserve the exact center of gravity, the moon's orbit, and the balance of everything in the region.

In this fictional scenario, maybe extreme measures are taken to preserve the center of gravity as you approach the moon, such as a drone on the exact opposite side the moon that will mirror your presence.

Or maybe it's a network of drones working together on the opposite side of the moon, and their job is to calculate and compensate for the additional mass on the inhabited side of the moon, and then automatically shift around the uninhabited side to maintain a mathematically balanced center of gravity for the entire moon, to preserve an ideal orbit. Adding additional people, food, tools, and so on, would require extra calculations, more insurance, and the drones would need to carefully acquire more "weight" to balance out new inhabitants.

Back to reality, I wonder if NASA even considered this, because who knows how fragile our moon's orbit really is? Maybe we're already doomed, in barely-measureable slow motion, because someone landed on the moon carelessly (jumping on it) which disrupted the orbit enough to dismantle our entire solar system.

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    $\begingroup$ moon doesn't have much mass and therefore its orbit is quite fragile? Based on which physics law? $\endgroup$
    – L.Dutch ♦
    Apr 26, 2020 at 9:13
  • $\begingroup$ @L.Dutch-ReinstateMonica p=mv (momentum) just a new vector of force that's alien to the moon in question. Perhaps the fictional moon is mostly hollow or made of a material that doesn't have much mass. My understanding is all steady orbits are extremely precarious and it's a zillion-to-one miracle we have a predictable calendar with predictable lunar cycles, etc. It probably wouldn't take much of an explosion (or meteor impact) to disrupt our actual moon's center of gravity, which would probably wipe out life on Earth as we know it. But don't take my word for it, maybe ask an astronomer :-) $\endgroup$
    – PJ Brunet
    Apr 26, 2020 at 9:22
  • $\begingroup$ In real life, rotational inertia surely adds a lot of stability, but in a perfectly balanced system, even the slightest imbalance could eventually become a "wobble." Which is why little weights are added to your wheels, this gives you a smoother ride and protects your car from damage. $\endgroup$
    – PJ Brunet
    Apr 26, 2020 at 9:30
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    $\begingroup$ while i like the idea of this answer it seems a little too extreme $\endgroup$
    – Topcode
    Apr 26, 2020 at 16:56
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    $\begingroup$ For your last point, NASA has measured the moon's orbit quite carefully and noted that it's spiralling very slowly outwards about 22mm a year. Eventually the moon will leave earth's sphere of influence and become a dwarf-planet. but this will take about 50 billion years, so it's a non-factor, the sun will have become a Red Giant in 5.5 Billion years and swallowed the earth and moon before that can happen. $\endgroup$
    – Ruadhan
    Apr 27, 2020 at 9:26
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Unexpected Terraforming Requirements

When they arrived the atmosphere had an issue that requires Atmospheric processors to be built and deployed in order for safe landing and occupancy to be possible. Unfortunately this meant some basic industry was needed in orbit which took time to construct.

Afterall they weren't expecting a lengthy terraforming effort when they set off, how could they have known they'd need that kind of equipment?

Landing colony efforts would be destroyed in the processes.

A Similar Situation in Scifi

A similar example of the atmospheric processors is Acheron (LV-426) from the Aliens franchise:

enter image description here

Acheron,[3] formerly known as LV-426, is one of three known moons orbiting Calpamos in the Zeta2 Reticuli system, 39 light years away from Earth. The moon was given its common name by the early human colonists who settled there. The main colonist base, Hadley's Hope, housed 158 people.[4]

Acheron had a thick debris laiden atmosphere concealing most of the surface, and making it inhospitable for those visiting. When they did initially land they took damage and had to conduct repairs. Take this and dial it up to 11.

Of course, while most of LV-426 was unmapped and innaccessible, they still had that initial outpost, so..

Make The Processors Orbital

And who says the processors need to land themselves? They can sit in low orbit, perhaps processing the upper layers unable to venture lower, or perhaps they fire lasers at debris and particulates to fuse it? Wide angle UV to catalyze chemicals that prevent landing? Maybe they work by dispersing chemicals into the upper layers of the atmosphere over long periods? Magnetic fields?

You could even arrive around the moon with the processors and deploy them on arrival, just say they take 50-100 years to do their job.

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  • $\begingroup$ again, like most answers this really doesn't solve the problem, i have stated that i want nothing to be able to reach the surface, no probes, nothing. please read the entire question. $\endgroup$
    – Topcode
    Apr 26, 2020 at 16:54
  • $\begingroup$ @Topcode there is no mention of probes or anything in the question. The Q just says they can't land, you need to edit the question to be clearer. E.g. we can't land a settlement on Mars yet, but we can send probes or drop packages. Also, my answer does answer your Q, they arrive and realise nothing can reach the surface, so they have to build something that can, which takes a looooong time. Also with a little imagination why couldn't the processors be based in low orbital? Perhaps they work using lasers or something. $\endgroup$
    – Tom J Nowell
    Apr 26, 2020 at 17:03
  • $\begingroup$ well i say " want nothing to be able to reach the surface" meaning i want nothing to be able to reach the surface. i did not think "nothing" was that hard to understand $\endgroup$
    – Topcode
    Apr 26, 2020 at 17:15
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    $\begingroup$ @Topcode no need to be hostile, see my updated answer. Nobody need touch the surface for it to work, or even know what it looks like $\endgroup$
    – Tom J Nowell
    Apr 26, 2020 at 17:16
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The large moon could have a very high gravitational force

such that the members of civilisation could not survive it once on it's surface. Orbiting in space stations, however, this force of gravity can be cancelled out due to the rotational velocity of the stations travelling around the moon. Over an extended period of time, perhaps the individuals may evolve to be able to survive that gravity and therefore eventually descend!

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  • $\begingroup$ The periodic bolding of words or phrases seemingly at random is a bit weird, but good first post. $\endgroup$
    – Escaped dental patient.
    Apr 26, 2020 at 20:03
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  1. Moon's surface is extremely porous (e.g. Evaporated liquid left "shells" that look like a surface but are like bubbles ... house of cards), any craft landing will be buried at the centre of the planet and be unable to return to orbit.
  2. High concentration of a gas that corrodes the ship that attempts to land (e.g. Aluminum and Mercury are "not friends") but is not itself directly toxic
  3. High concentration of a toxic gas that isn't lethal to the life already present (e.g. Cyanide or arsenic)
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Kessler syndrome taken up a notch. There is so much orbital debris nothing can get past it.

Make the orbital debris in a high enough orbit that the atmosphere will never clear it, and then put the colony in an even higher orbit. I also suggest having the debris layer be very thick, so that even random collisions do not clear it, and there no chance for even a very agile probe to be able to avoid everything.

For extra credit have multiple layers orbiting in opposite directions, so that you couldn't even try to match speeds with the debris, because then you'd have to reverse direction to get past the next layer.

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There isn't fuel to get back into orbit

The people on the space station can create a vehicle that can survive landing. The main problem is that they couldn't shrink their fusion motors small enough to power the landing craft and have to rely on a chemical propellant to get it back to the space station. When they sent down the robotic scout missions, they found that there isn't anything that can be easily refined to fuel the rocket to get back into orbit. It is taking a long time to prepare the refining equipment to get around this problem.

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  • $\begingroup$ again, while there are many answers similar to this, there would be plenty of water, which can be refined into fuel, also i am mainly looking for nothing to get down, no probes, no ships, nothing. $\endgroup$
    – Topcode
    Apr 26, 2020 at 16:51
  • $\begingroup$ @topcode If you want nothing to get down, you are looking at a physical barrier of some sort. Either a planetary shield or ground based defenses that are good enough that everything gets shot down on the way in. Those are the choices. Everything else is stuff makes it down and the crew dies or stuff makes it down and can't get back. $\endgroup$
    – Futoque
    Apr 26, 2020 at 18:06
  • $\begingroup$ or weather, or terrain hazards, acids etc there are many different ways,, and there are already a few answers that i will proboally use that artent a physical barrier, in fact anly one answer is a physical barrier $\endgroup$
    – Topcode
    Apr 26, 2020 at 18:22
  • $\begingroup$ The weather, terrain, acids, poisons all allow people to go down, just not survive very long. After some thought, even shooting down the vehicles still allows people to go down if their ship is designed properly. $\endgroup$
    – Futoque
    Apr 26, 2020 at 18:28
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There's a theory that Titan's deep atmosphere can hold so much "moisture" (in this case, methane and ethane), so that when it rains the downpour often causes massive flooding. Amphibious and underwater life can adapt to this but building any human-habitable structure or placing long-lived probes would be practically impossible there. They would be torn off foundations and buried in the mud.

Reminds me to Ursula Le Guin's "The Word For World is Forest" where the soil was held together by dense vegetation and humans' colonization attempts failed at first when they stripped the forest away. Only after understanding (which was not so easy for technocratic minds) what happened could the colonization proceed.

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Extreme magnetic activity

The extreme magnetic pull of the moon is damaging electronics and metal mechanic components entering the atmosphere.
A space elevator needs to be built with no electronics in the cabin, the parts need to be mined on other objects and then delivered, the engineering also needs to be done and tested.

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Rocket equation and gravity

If our Moon were more similar to the Earth, it wouldn't be harder to get there, but to come back we would need to get there a Saturn V - and probably we would need to build in the Moon all the same facilities and crew from Cape Canaveral to be able to launch it. And sending a full fuelled Saturn V to the moon is tens of times harder than to send there a lunar module.

For the satellite in the OP it isn't so hard, but it is still way larger than the Moon, and sending there a launcher large enough to reach at least a low orbit to rendezvous with the returning vehicle might be out of the reach of the technology or the budget of the civilisation. At least, if such a satellite had been anywhere in the Solar System, sending there and back a crewed mission would have required several times the effort of the whole Apollo program.

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Energy barrier with no apparent source destroys anything that tries to reach the surface. Bypassed by discovery of a signal that allows for temporary disabling of the barrier. Maybe interstellar civilization previously set it up to protect the moon. Maybe ancient civilization on moon set it up to protect themselves.

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    $\begingroup$ Is an energy barrier science based? $\endgroup$
    – L.Dutch ♦
    Apr 27, 2020 at 19:38
  • $\begingroup$ Plasma shields are a thing, so, yes. This could also be a magnetic field that is just so strong it EMPs any ship that tries to land. $\endgroup$
    – Nosajimiki
    Apr 27, 2020 at 21:39
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Just spitballing an idea: they might be using the planet as a barrier against solar particles. Imagine if the sun had recently expanded into a red giant state and engulfed the inner terrestrial planets. Their inhabitants managed to escape into space and built space stations in the solar shadow of a nearby gas planet. This was preferable to building on the moon which would get uncomfortably close to the engorged sun.

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It really depends on how you define landing and how much do you accept that the science behind the problem is advanced.

If you define landing as "soft enough to survive the event" then as said by other, the Kessler syndrome is a good reason.
But it does not prevent that a probe is fired to the planet (we already did this so no problem here) with the plan to collect data until it crash on the surface: since you are near (relatively) to the surface and can time the launch on an opportunity window based on how many debris there are and gambling that the probe is small enough to pass the debris layer. Add that you can fire more than one probe and in fact something can "land" on the surface.

To solve this, you should have a Kessler syndrome so extreme that the debris layer is so thick and broad that is more similar to a solid structure that a cloud of debris. This way you can be unable even to fire a probe to crash on the surface because you cannot find a window for the launch: everything that will try to pass through the debris layer will be basically eroded way before they exit the debris layer.
A such dense debris layer, on the other hand, will probably render the planet uninhabitable since it will probably mask most of the sun light, so that life will be probably possible in some extreme conditions, a case you excluded. And it can be really problematic, but maybe possible, to overcome.

Another option is to have a situation inspired by the Space Odyssey cycle by Arthur C. Clarke: a powerful and advanced (beyond human comprehension) civilization that deployed some sort of device to destroy everything will attempt to land (probe, robot, everything) in order to protect something, be it life, resources or whatever. In this case you will have an option to bypass it and finally land. (Just do not use the solution used in the Space Odyssey final book...)

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