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I'm outlining some ideas for a game I'd like to write, that at its core is about interplanetary space-trading (amongst other things, but that's not relevant to the question).

For project-scope reasons I don't want to go down the route of implementing planetary landings, and want the entire game to occur in space. For this reason when the player will pick up or drop off goods, it's entirely done at an orbiting space station. Now, the commodities eventually have to end up on the planet surface anyway, so why would all trade occur in orbit rather than on the ground?

Similarly, goods that are exported would have to find their way up to the orbiting space station to be traded. This seems excessively expensive.

If the space-ship in question were massive, it might make more sense as the cost of getting that thing off the ground might be impossible. However, I envisage the player to be in a smaller space-ship (think: Firefly), and not some behemoth of a transport.

I am looking for "as real" answers as possible. My ships won't have magical shields, and I'm trying to keep it as close to reality as possible - only cutting corners where there is literally no other way of achieving something.

Thanks in advance!

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    $\begingroup$ Its a bit like designing a containership that can drive out of the water to deliver its goods directly in a center rather than the seaport. $\endgroup$
    – Demigan
    Jul 31 at 16:09
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    $\begingroup$ Alternatively, the cargo ship gets in orbit and the receivers come to it to retrieve their stuff. But if it's for a game and not for a book, you don't really have to explain or justify stuff that's not in the game. $\endgroup$
    – WGroleau
    Jul 31 at 19:20
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    $\begingroup$ The Last Mile Problem $\endgroup$ Aug 1 at 15:18
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    $\begingroup$ @Demigan Or like a semi (tractor-trailer) truck driving the goods directly to the shelves rather than unloading at the loading dock. $\endgroup$
    – Michael
    Aug 1 at 19:12
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    $\begingroup$ Incidentally, the game Empyrion actually models this reasonably well. You can make a cheap craft that's great for moving things in and out of orbit but is clumsy at anything else. And you can make a powerful warship that maneuvers well and has high speed but turns into a brick in atmosphere due to the lack of downward thrust. Making a ship that does both is possible but expensive and you'll end up making compromises elsewhere to fit all the engines and directional thrust you need. $\endgroup$
    – JamieB
    Aug 1 at 21:48

28 Answers 28

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Other answers have already covered the fact that surface-to-orbit travel is energetically unfavorable (and therefore expensive and awkward) and that making a ship that can operate in an atmosphere as well as in space is challenging (and therefore expensive and awkward), but I'd like to throw in some environmental and safety concerns.

Ships that can make relatively prompt interplanetary journeys are safety hazards, as they could easily hit the atmosphere at over 15km/s as part of an aerobraking manoever. At first glance, it is difficult to distinguish high velocity re-entry heading for a spaceport from a kinetic bombardment attack or nuclear missile delivery or other kinds of hypersonic weapon system. They're not going to let just anyone drop into an inhabited atmosphere at supra-orbital velocities. For worlds with no atmosphere the problem is even worse, because things can hit the ground going at that speed without having a substantial amount of energy bled off by the atmosphere first.

Anyone on a trajectory that threatens an inhabited world is going to be watched very, very carefully, and at the first sign of deviation from an agreed minimally dangerous trajectory they're going to be on the receiving end of loudness, lawyers and lasers. If they're lucky, they'll get it in that order, too.

Maybe anyone operating an interplanetary vehicle from a planetary surface needs to have a range safety system fitted for emergencies. For those of you who haven't heard of these and didn't follow the link, that's a remote destruct system that will blow you to pieces if it looks like you might pose a threat to the safety of people on the ground, for example.

Now, nobody wants to blow up an operating nuclear reactor in their own biosphere, and that means that you'll also be limited to the kinds of engine you can operate. Maybe the kinds that are allowed are more expensive or more difficult to run due to their lower performance and higher fuel or reaction mass demands.

When you land, you'll be landing at some kind of spaceport. This will have to be a long way from habitation, so that accidents during landing or takeoff don't threaten anyone, and the sheer noise of high power rocket engines doesn't damage anything or hurt anyone. A huge amount of potentially valuable land has to be set aside, and a lot of expensive infrastructure with refractory launch pads and nuclear fire suppression systems and nuclear fuel storage cells and traffic control systems and all the rest. Who's gonna pay for all that? No surprise: you're gonna have to, via extortionate fees.

So, to summarize:

  • You need a ship capable of atmospheric re-entry, which requires a lot of reinforcement and shielding
  • You need a ship capable of landing in a gravity well, which requires strong enough landing gear for the highest-G world you expect to land upon.
  • You need an engine capable of getting you back into orbit economically that is also not an environmental threat.
  • You need to persuade traffic control that you're a sensible and trustworthy pilot with a safe and reliable spacecraft that won't fall apart, crash, blow up or leak.
  • You might need to fit a remote destruct system that some planetbound bureaucrat can trigger at some perceived need.
  • You need to operate from a massive and expensive spaceport, probably at great expense.

Ooooor, you could just park at the high orbit or langrange point freight transfer station, and let dedicated ferry systems do the awkward bit of the journey, and just worry about flying in space, well away from all these excess-safety jurisdictions with big defense cannons. What's not to like?


If you were interested, there's a whole variety of non-rocket spacelaunch systems that include space elevators, skyhooks, orbital towers, orbital rings, launch loops and electromagnetic catapults, some of which also double up as way to get stuff back down to the surface as well as up to orbit.

Dumb reentry vehicles are a cheap way to being stuff down safely on worlds with atmospheres. Other specialized systems (maybe some kind of laser-ablative retro-rocket) would be needed on airless worlds, though.

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    $\begingroup$ /loudness, lawyers and lasers./ excellent! $\endgroup$
    – Willk
    Jul 31 at 16:47
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    $\begingroup$ There were so many amazing answers, it was really difficult to pick one - but I think this edged ahead with the loudness, lawyers, and lasers. I wish I could accept more than one answer! $\endgroup$
    – Moo-Juice
    Aug 1 at 6:57
  • $\begingroup$ Wish I'd seen this question before you posted my answer for me 😁 One addition: apart from the energy cost of landing/takeoff, there's also a bunch of time wasted when you can just dock in orbit. And hopefully less LLL 😋 $\endgroup$
    – Corey
    Aug 1 at 23:00
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    $\begingroup$ Why lasers? To blind the guidance systems? $\endgroup$
    – gerrit
    Aug 2 at 11:22
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    $\begingroup$ Correct. But as all the other answers overlooks a point. If the market is an interplanetary market some of the traded goods might be forwarded to another planet. There is not point to bring on the planet goods that might then need to be sent back in space. $\endgroup$
    – FluidCode
    Aug 2 at 15:26
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Building a "realistic" spaceship that can land on a planet and take off again, carrying significant cargo, and without dumping lots of radioactivity into the planet's atmosphere, is actually quite hard. The idea of long-range trading ships moving goods between space stations, and different means being used to take goods down to a planet, and lift goods from planets into space, is well-established in written SF, although it's a bit complex for TV or films.

The efficient way to move goods between the ground and orbit is a Space Elevator. This consists of a very long and strong cable, with one end attached to the ground, and the other attached to a large counter-weight. There is also a space station attached to the cable. When this is set up correctly, the space station is at stationary orbit altitude, and the counterweight at considerably higher altitude. The cable sweeps through space as the planet rotates, and the counterweight keeps the cable taut. Goods are moved up and down the cable via a vertical railway running along the cable. This is much cheaper than using rockets.

The SF novels that established the idea of a space elevator in the genre are The Fountains of Paradise by Arthur C Clarke, and The Web Between the Worlds by Charles Sheffield. Both are well worth reading to get a clear idea of this device, along with the Wikipedia page.

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    $\begingroup$ Not only is it cheaper but you can generate electricity with minimal pollution compared to most rocket fuels. $\endgroup$
    – John
    Jul 31 at 21:25
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    $\begingroup$ @John - The station itself (And even more the counterweight) are going to be in sunlight a significant amount of the planetary day, simply due to distance from the planet surface. This almost certainly means that you "export" electricity to the planet. Too, the climbers can use regenerative braking, which would also save additional energy on each end of the journey. $\endgroup$ Jul 31 at 22:50
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    $\begingroup$ @Clockwork-Muse only slightly more sun than the planets surface and space elevators may well need large amounts of electricity to reinforce and dampen the tether so the idea of exporting electricity is unlikely. $\endgroup$
    – John
    Aug 1 at 2:37
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    $\begingroup$ @John - Given this answer, an object in geostationary orbit will be in the umbra (the dark part of the shadow) an hour or less, and the penumbra (the rest of the shadow) less than twelve hours. Too, there's no pesky atmosphere in the way, which means you can use some of the spectrum+amplitude that's normally blocked. There's no weather system (clouds, rain, dust) either. $\endgroup$ Aug 1 at 19:05
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    $\begingroup$ Note that the upper part of the space elevator (above the geostationary orbit) can also be used to accelerate or decelerate space ships. So you might want to have the space station there instead on the geostationary point. (As an added bonus, there you can actually have some pseudo-gravity inside, which makes filming on earth easier.) $\endgroup$ Aug 1 at 23:54
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quarantine

The gravity issues are just engineering. But you could be carrying the plague. Invasive species. Running guns for cartels.

The only way to keep the planet safe is to hygienically deliver to a vacuum sealed location and inspect.

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    $\begingroup$ Oh, this is a great point as well! $\endgroup$
    – Moo-Juice
    Jul 31 at 17:38
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    $\begingroup$ I will add that the crew of the transport ship has just as much reason to avoid contamination by whatever awful bugs live on the planet. Some colonists might have been desperate enough to settle on Metaluna, but that doesn't mean that traders are willing to get infested with Metaluna Lung Termites while offloading a year's worth of frozen pizza (and lung-termite pesticide). $\endgroup$
    – Tom
    Aug 1 at 2:24
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    $\begingroup$ @Moo-Juice And import taxes. And all kinds of regulations that don't extend into outer space. Hence there are companies that are specializing in receiving interplanetary deliveries in space and then getting them through all the bureaucratic hoops and down to the ground. $\endgroup$
    – Vilx-
    Aug 1 at 17:44
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    $\begingroup$ @Vilx - since the station is almost certainly under the control of the local planet, import taxes and most other regulations would likely still apply. $\endgroup$ Aug 2 at 14:45
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    $\begingroup$ @Clockwork-Muse - We already have similar things down here on Earth - it's called "customs". And, for biological organisms - especially exotic and non-native ones - quarantine is also quite real. This is the same thing, just in space. $\endgroup$
    – Vilx-
    Aug 2 at 15:38
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Going up and down a gravity well costs, energy, resources and time:

  • energy because you need to shed orbital velocity to land and gain it back to get up in space again
  • resources because making something sturdy enough to not buckle under its own weight requires additional mass and that requires also additional fuel to be moved around. And each planet has its own gravity, meaning you would need a different design every time
  • time because doing all the maneuvering takes more additional time

Think of it to something similar to courier delivery: it's way more convenient for the courier to deliver on sidewalk instead of going up to the floor bringing the parcel and then getting back.

The transport ship can be optimized for space travel, while the movement in the gravity well can be optimized locally for the planet where it operates.

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  • $\begingroup$ Thanks for this response. So I am guessing they'd have alternate, more efficient ways of transporting commodities from the orbiting space station back to the surface (and back up again) - I wonder what that looks like... $\endgroup$
    – Moo-Juice
    Jul 31 at 10:13
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    $\begingroup$ @Moo-Juice a fork lift is optimized for loading/unloading a truck way more than a van can be. Something similar will happen in your case. $\endgroup$
    – L.Dutch
    Jul 31 at 10:14
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    $\begingroup$ @Moo-Juice: Don't think of it as "a more efficient way" of getting from the station to the surface. Think of it as "this is the only way that works and it's WILDLY inefficient, so nobody wants to put one on their trading ship." $\endgroup$ Aug 1 at 15:15
  • $\begingroup$ @Moo-Juice Down is easy with a little thrust. If the planet has a high enough tech level a space elevator to get up and down would be even better. $\endgroup$
    – Corey
    Aug 1 at 23:16
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In the shipping business, shipping things earns money. Storing things costs money, and waiting around costs money.

Ideally, if your business is moving things from place to place, your most desirable situation is:

  • You are on a predictable, profitable schedule that gets the items you have already been paid to ship from their source to their destination as economically as possible.
  • You already have your next set of items lined up at your destination so you can get them loaded as soon as you have hold space, and get back to your business of moving them to your next destination.
  • Either the person who paid you to ship these items, or the person who is receiving these items is contracted to take care of import duties, taxes, customs clearance, warehousing, and movement of the cargo to its final planeside destination, so you don't have to hang around to do any of that.

Delivering these items to a particular planetside destination costs you time you could be spending loading or shipping your next shipment. Waiting around in orbit to get something arranged also costs time. Landing puts wear and tear on your hardware that requires maintenance. Berthing fees at any spaceport, skyside or dirtside, costs money.

And while you're waiting around not moving things, you'll still have to pay your crew, because they usually can't be switched off and stored on the hull, so if they're not getting paid, they'll leave for someone who is paying and you'll lose time acquiring new crew.

So take advantage of the infrastructure that's already there. Get your cargo offloaded at the highport if your contract and local infrastructure allows it, and into the hands of the locals who have optimized their business into getting things from the highport to their ultimate destinations, so you can get back to doing what you do best; moving things from place to place.

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    $\begingroup$ Indeed, it's like real world container ships, which move huge quantities of cargo from one seaport to another, and don't care about how the product reaches the end customer from the port. Even in case of smaller ships, they don't deliver the cargo to the front porch of the end customer. $\endgroup$
    – vsz
    Aug 2 at 7:31
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Planes, trains and automobiles

enter image description here

Steve Coogan (left) and Jackie Chan (right) from "Around the World in 80 Days" 2004.

There are different types of spaceships, the same way there are different types of transport in the real world. You can walk, cycle, drive, get a train or bus, or go to the airport.

There is no vehicle that picks you up from your front door, then flies through the air to another country, and drops you off outside your holiday villa. Such a thing is possible in principle. If you were super wealthy you could build a landing strip at both of your properties. But even this is a hassle since you have to listen to planes landing when you are trying to entertain guests.

For the rest of us, it is rare to get an airplane to the shops or to work. The few people who do commute by plane still use other transportation to get from the airport to their workplace. The plane won't drop them off at the door.

Likewise, there are different types of spaceships for different jobs. The spaceships that move from one orbital habitat to another cannot take off or land from the surface. The kind that go from the surface to orbit cannot move long distances between planets.

We could in principle build a ship that does both. But it would be a waste to lug all that surface-to-orbit gear around between planets. Plus that gear was expensive, and it being idle when the ship is in space is lost opportunity cost. That's like having an airplane full of bicycles. They are not doing anything when the plane is in the air.

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    $\begingroup$ I am not sure what value the image adds to the answer. $\endgroup$
    – Philipp
    Aug 1 at 11:39
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    $\begingroup$ @Philipp It's Steve Coogan and Jackie Chan. $\endgroup$
    – Daron
    Aug 1 at 13:15
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    $\begingroup$ Which adds what value to the answer? $\endgroup$
    – T.J.L.
    Aug 1 at 17:08
  • $\begingroup$ @T.J.L. Steve on the left in the black hat. Jackie on the right in the white hat. $\endgroup$
    – Daron
    Aug 1 at 20:54
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    $\begingroup$ I think they were in "Around the world in 80 days" which had many sorts of transportation devices? Maybe? $\endgroup$ Aug 2 at 14:21
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Gravity - yes, yes. How droll. Expense; economics. Frugality. Long manifests. The gripping stuff games are made of?

I propose PIRATES!

pirates

source

Planets have inhabitants. Those inhabitants are hungry. They can get up into the atmosphere with their little craft but not into space. They know that ships coming and going from space to the surface and back have valuables on board. And the ship itself and its crew could be held for random or sold.

If your players had some hulking brute of a ship it could shoot its way through the pirates, chuckling derisively. That has its place but that is not what you want (at least not all the time). The little ships your players use could get down in the atmosphere but once there they are at risk of being outnumbered and outmaneuvered by desperate locals who see them coming and want what they have.

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    $\begingroup$ OK, have my +1 you entertaining bast... uh, person. $\endgroup$
    – Corey
    Aug 1 at 23:17
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There are two sets of compelling reasons - one based on physics, one based on societies and commerce.

On the physics side, entering an atmosphere is dangerous and hard. It needs to happen on a precise schedule and the object doing it needs to have all kinds of technological protections, such as heat shields, parachutes, and so on. It also would, we assume, have to take off again eventually, which would require a lot of rocket power and a lot of fuel. Obviously, this will still need to happen, but having specialized drop schedules designed and planned on orbital satellites, along with planned rocket schedules to bring tradable goods to the satellites, could offset many of these dangers and costs. Also, other hypothetical technologies like a space elevator could achieve the same goal. Meanwhile, the interplanetary trading ships themselves could be designed purely for convenience and efficiency in space, with no concern given to how they would fare in an atmosphere. You say you imagine the player's ship would be small, and I think that fits well - a small, spare ship designed for speed wouldn't want to be bogged down with heat shields, nor any of the infrastructure needed to function in gravity or connect with a planetary rocket to leave gravity.

A lot of this has been covered already in other responses, and it remains true. But I think there is a whole second set of reasons to do with how trade and commerce tend to operate in society.

Planets are huge. A lot of science fiction that imagines interplanetary relations tends to take the framework of nation-states and scale them up to be about whole planets instead. Personally, I find this to be an oversight. Even if we imagine a utopian future where an entire planet of people live in general peace with one another, only ever having large conflicts with other planets rather than internal ones, it is still the case that this is an entire planet. People living nearer to the poles will inevitably have different needs, culture, and luxuries than those who live near the equator; Different plants and animals will thrive on different continents, meaning that the laws of supply and demand apply not to the planet as a whole, but to each of the planet's regions. Even if these regions are not independent nations, they would function as independent marketplaces.

This means it is extremely profitable for satellite managers to act as middlemen between interplanetary traders. By adding an extra step between the supply of a given thing getting to the region that has a demand for it, extra costs can be tacked on, as well as controls placed upon how it is sent down. The aforementioned scheduled drops can be used as an excuse to hoard supplies and release them in trickles, keeping the price high.

This would also incentivize certain regions to send up their own satellites - if a satellite around one orbital plane can't easily send supplies down to your area, maybe you send up a satellite to a new orbit for faster deliveries. Also, trade always invites taxes and tariffs. Whoever sends up the satellites is most likely to see those revenues, whether its a nation state placing a tax or a corporation adding 'processing fees'. So, even if the regions of the planet aren't in conflict, they still would want to have their own platforms so that they best profit off the trade.

The other way of doing things, letting interplanetary ships land and then take off, would necessarily be less profitable. The hosting region would be shouldering the costs of sending the ships back into space, as well as the liability - rockets are dangerous, and if you lose someone else's trading ship through a rocket mishap, that's a big payout if you're a corporation and possible war if you're a nation. Letting the ships land in the first place would also be a liability, since if things go wrong a trade ship could turn into a very dense, fiery meteorite.

Furthermore, without the middleman of the satellite, traders would be going directly to a region, knowing much more about its local economy and thus striking much harder bargains. Also, whatever trade does occur would be subject to local laws, meaning more people can and will try to get a slice of the pie. The satellite gives one group direct control over the trade in a very advantageous way, while also reducing risks and additional costs. Basically, if any one company or nation did hold power on a planet, they would want to hold on to that power - and if two or more such entities coexisted, they would both need every advantage they could get, even if they weren't in open competition or conflict. This, along with all of the physics of orbits, means it is just so much more likely that planets would depend on satellites for interplanetary trade and travel.

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Put simply - that's the delivery/collection address.

Why? Because the planet's governors don't want unknown spacecraft from all over the star system constantly in-and-out of their atmosphere, for various reasons, including pollution, aesthetics, security, ecological and efficiency.

Any self-respecting planetary authority has an orbital relay station for such things.

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The interplanetary trade ships can't land on the planet

The trade ships are huge constructs built in space, never intended to land on a planet. When the goods arrive in orbit, smaller shuttles (from other companies) will arrive at the space station, to load and deliver things down on the planet.

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    $\begingroup$ I should point out that the OP says it's a relatively small ship "I envisage the player to be in a smaller space-ship (think: Firefly), and not some behemoth of a transport" $\endgroup$
    – n00dles
    Jul 31 at 15:09
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    $\begingroup$ Fine: the trade ships are tiny constructs build in space, never intended to land on a planet. When the goods arrive in orbit, then bigger shuttles (from other companies) will arrive at the space station, to load and deliver things down on the planet. $\endgroup$ Aug 1 at 15:17
  • $\begingroup$ Trade ships must be big. Like trucks. This is long distance transport, scale is everything. $\endgroup$
    – Goodies
    Aug 4 at 5:57
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They use an orbital space tether

The goal of cargo delivery is to get stuff there quick, but above all, cheap. You could land a starship and take off again, but that would be expensive. You could use the space elevator, but capacity is limited and you should see the cargo rates! Some does go down that way with subsidized rates to help even out momentum and center of gravity issues, but odds are, you're using the old fashioned standby: one of the network of orbital space tethers.

The tethers were what we built before the space elevator. They don't touch the ground, so they don't worry about weather or weathering. They're shorter, older, and less mechanically robust. (This doesn't mean they're too rickety to use for people any more; that's just a vicious lie. It's just the insurance rates make them more economical to use for cargo)

With an orbital space tether, in the old days you would pilot your toy spaceship up to the verge of space, hanging there in midair for a few seconds. Meanwhile, the long end of the tether has reached its lowest point, momentarily orbiting almost stationary against the ground, with its descent momentarily pausing in mid-air. (Like a thrown ball at the top of its arc, before it falls again) Thanks to Advanced Weather Systems Engineering (tm), these two events happen within meters of each other. Your toy spaceship grabs onto the tether, and comes along for the ride. When the tip of the tether swings around to the top of its arc, the ship lets go, and glides along in a stable low Earth orbit which it can then modify cheaply.

But the same process works in reverse. Your cargo comes in from the interplanetary and gets broken up into packets for each tether (since the boxes self-assemble into sturdy frameworks this doesn't take a lot of longshoremen to do). The packets wangle a course with the lovely people at Earth Traffic Control. (That's why the CEO gets paid the big bucks, because not all those payments want to be on the books) Eventually they make the perilous passage around the planet on a few newton-milliseconds per kilogram of delta vee (don't you love metric?), and now they grab on to the orbital tether that serves the locations you want, at the time you need, in order that they are more or less dropped from the Karman line.

They call it "parachuting", but the term is archaic. The probes use the same prayer flags you see in your building's ventilation system, which use a small amount of electricity to control their curvature precisely over the full extent of their surface in response to computer models of the air (note to self: remember to buy AWSE stock it comes out). The prayer flags interact with the vortices encountered or produced during falling to replicate (and surpass) the crude maneuvering allowed by control surfaces on an old-fashioned glider aircraft. This - plus a tiny charge of azide to help ensure they stop on a dime when they reach the destination - helps ensure that the package is delivered to the right house every time, even if the space tether's orbit misses it by hundreds of kilometers.

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  • $\begingroup$ This doesn't really explain why your space ship doesn't go down or up – it could just connect to the tether to be ferried down (and later up again). I'd recommend expanding this a bit. $\endgroup$ Aug 2 at 0:01
  • $\begingroup$ @PaŭloEbermann That's an awfully heavy shipping container. You would have to get the interplanetary drive back off the planet, even though it literally had no Earthly use being down there to begin with! $\endgroup$ Aug 4 at 0:11
  • $\begingroup$ Sure – please add this to the answer. $\endgroup$ Aug 4 at 19:46
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As prior answers detail, the energy cost is too much.

I will note one thing I did not see covered: parachute re-entry.

If I park a huge container in near-Earth orbit (like the current international space station), then we can have lightweight reusable delivery to the surface of payloads of many tons using re-usable heat shields (like those on our erstwhile shuttle craft) and large electronically guided parachutes to landing sights. Both of those can be re-packed and re-used, sent up as cargo like everything else.

But it would also be likely that most manufacturing on the planet would be conducted in space, on space stations, using asteroids as the raw materials. so about 75% of the space/earth traffic is going to be in the space->earth direction, not earth->space, and much of that earth->space traffic will be cargo using automated unmanned reusable engines that just drop a container in space that will be picked up later, and head back to Earth. Of course humans and animals could travel the same way; dropped at a pick up site. (Unless the space elevator works, but it may not).

My point is we need no engines to get down to the planet, just like the Apollo missions. parachutes and splash down in a floating vehicle worked just fine, and is cheap as dirt. Other countries have been using parachute landings on dry land for decades. With modern electronics and controls, I'd be surprised if we cannot effect this landing with pinpoint accuracy.

Instead of fighting the gravity well both down and up, for incoming traffic (cargo or living) you only need to fight it going up for 0.001% of the weight: The weight of the reusable parachutes and heat shield.

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    $\begingroup$ If you're in orbit, you need some propulsion to lower your orbit enough to intersect the atmosphere. The Apollo missions returning from the Moon came back on a trajectory that intersected the atmosphere, but needed propulsion to set that up accurately. It's also hard to make a precise landing at a predictable place, because the density of the atmosphere varies. $\endgroup$ Jul 31 at 15:10
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    $\begingroup$ @JohnDallman by the time you've got a mature space-based trading and manufacturing network, making hypersonic boost-glide vehicles should be pretty straightforward. Even quite dramatic changes in heading in the atmosphere would be possible then, so picking your landing spot shouldn't be a problem so long as the locals don't mind hypersonic re-entry vehicles bearing down on them. $\endgroup$ Jul 31 at 15:36
  • $\begingroup$ @JohnDallman Some guidance propulsion is easy, my point is you don't need a fully powered flight. And you can know the weather pretty precisely. The space shuttles landed on runways without much trouble. Guided falls of heavy loads on parachutes in low wind areas should be relatively easy; human skydivers manage to land on quite small target areas. A circle with a radius of one mile is about two thousand acres, put that in a desert and we can always wait a day if the weather report means the winds are unpredictable. In a story, just presume the technology has been developed. $\endgroup$
    – Amadeus
    Jul 31 at 18:18
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Most answers have focused on the technology limitations, I'd like to focus on something that's frequently ignored in Worldbuilding: economic practicality

Other answers have more than adequately pointed out that there are some great technological reasons why trade ships should stay in space. Frankly, the simple Earth-bound example of our container ships and oil freighters clearly demonstrates that limited harbor access due to the size of the ship is a reality.

But let's look at this from another perspective to give your story/game a richer base. Why don't any but the largest retail stores buy directly from manufactures? Answer: economic practicality.

Ignoring super-chains, which frequently have contracts direct with manufacturers,1 most retailers are forced to use distributors. And this is where your question gets answered.

Why wouldn't the ships land? Because the cost of aggregating and redistributing the incoming products of hundreds if not thousands of trade ships is much, much cheaper than doing it on the ground.

Distribution—the process of bringing in a wide variety of products and then redistributing them in a more useful manner that takes maximum advantage of shipping efficiency2—wants to take place where the lowest cost point for shipping exists. In the case of space-based shipping, that's always going to be in orbit where the cost to dock ever larger freighters is lowest and the cost of sending transshipped cargo can be maximized for the size and efficiency of surface-to-space transport.

In other words, the most efficient and cost-effective place to dock your version of Earth's container freighters is in space, where getting that ultra-expensive ship back into service with its next load of cargo can be done as quickly, as efficiently, and for as low a cost as possible.


1In fact, large chains like Home Depot and Walmart usually have contractual obligations with manufacturers guaranteeing that the manufacturer doesn't have an overwhelming dependency on the chain/store. In my own case, I had to prove as a small manufacturer that my revenues through Walmart didn't and wouldn't exceed 20% of my total business revenue. This was less altruistic than it might sound. Yes, it was thee to help protect me as a small business from the damage that can be caused by a whale rolling over, but it was also to ensure that if the Walmart whale did roll over and stopped buying my product, they wouldn't get sued for my bankruptcy. Frankly, the economics of a situation are often just as complex as the technological aspects.

1What this means is that shipping wants full trucks "coming in" to a destination and full trucks "going out" of a destination. Shipping directly almost always entails partial truck loads and having to send a truck to multiple locations (sources or destinations) reduces the profitability of shipping. The manufacturers want to ship a full truck to save all they can on shipping. The retailers want to receive as large a load as possible (or, in the case of less-than-a-truckload/LTL shipping, as much of a full pallet as possible) for the same reason. The best location for a distributor is where the number of full trucks is maximized. In your case, that's going to be orbit.

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    $\begingroup$ See also, "hub-and-spoke model" in public transport, for much the same reasons. $\endgroup$
    – Nij
    Aug 1 at 5:12
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    $\begingroup$ To add to this answer, if it's just as cheap to land goods in China from an orbital distribution center as it is Canada, then not only do you want a global distribution center, but it pays to put that distribution center in orbit because even though it's cheaper to build a global distribution center on the ground, once you land there, you then have to pay again for shipping to move stuff to other more regional distribution centers adding cost to shipping $\endgroup$
    – Nosajimiki
    Aug 1 at 16:12
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In your case, there are three main categories of reason as to why interplanetary shipping would use space docks in orbit to drop off goods as opposed to consistently dropping onto the planet with loads of goods

#1: Transit Logistics

As somebody that works for a courier, the courier does not have a truck for every delivery location going to every other delivery location in the area. That would be way too expensive, incredibly inefficient, and be a terrible use of resources. Instead, volume is sent to a regional hub where volume from the region is aggregated and processed in bulk far more efficiently before it is sent to where it needs to go.

Should it need to move outside of the country of origin, it goes to a designated export location and leaves the country from there, also consolidating points of exit (and entry) from the country. The only exception to this is if the origin and destination locations are the same -- they will leave it in the building and deliver it the next day most likely.

Couriers and post offices alike will do that. Those hub points are likely to be in the larger centres where there is more population (and volume to move) and good transit options to move that volume. Major regional cities are the order of the day, likely the largest city in the regional area.

This relates to your scenario in that your spaceport drop-offs are the regional hubs in this scenario. Your shipments will go into these space hubs and be processed by workers in the station. From there the volume will be distributed planetside in whatever way that particular planet handles incoming shipments.

Yes, this is trading goods and not mailing packages, but a similar premise of hub and spoke still applies -- a large enough company will have a main distribution outlet, and smaller ones in strategic locations to service customers readily.

#2: Economics

Space has a whole lot of nothing. It is much easier to accelerate a mass to speed, cruise for a bit, then decelerate back to near zero to dock with the spaceport.

In contrast, planets have gravity, and atmosphere to get in the way of everything. It takes fuel to push through all of that. Not only that, but every planet will be slightly different.

Some planets will have a higher (or lower) gravity. Different planets will have different atmospheric conditions -- some conducive for blasting out to space, and others less so. Some planets might even be actively dangerous to the pilot or the vessel. It's all so complicated overall having to keep track of what planets are safe to enter, what ones will kill you on entry.

Each world may even have their own planet-based spaceship standards, meaning that some places might not be possible to land because your spaceship is incompatible with their landing and/or launching infrastructure.

That does not even take into account the bio-hazards that might be brought into or out of the planet by accident. The liabilities around invasive species has got to be immense once you deal with an entire planet that may not have any form of defence against one.

No, better and cheaper to dock at a spaceport and let the planet's administration handle all of that mess and cost. Less expenses for the shippers/merchants in fuel to launch from the planet and lawsuits in case something went wrong with a planetside delivery. Plus, you can save some money on spaceship designs with some parts being universally compatible for docking with space stations.

This holds true with larger suppliers as well as the little independent agents.

#3: Game Logistics

AKA: Conservation of Details

If you are making a game about space deliveries, then do you need to know how the planets operate between the space station and the ground (or water) itself?

I would argue that if your game is about interplanetary mercantile logistics, there is no need to worry about how they either land imported volume or lift up exporting volume. The only other thing you really need to care about is what they would import and/or export. Of course, the other details of the game may change that.

The player isn't landing on the planet so they will not have to know the amount of fuel needed to land, or the shielding needed to survive entry onto the planet. All players will need to know is that E Resources are available per T time units and that they can process I units in delivered goods per T time units. Possibly with needing to spend Q time units in quarantine due to laws.

What you need to know is reasonable numbers to fill in the variables dependent on each planet. This might be the topic of other questions -- I would recommend a search to see if you aren't repeating a question. The Tyranny of the Rocket Equation is almost as well known as the Tyranny of the Square-Cube law.

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    $\begingroup$ Thank you for this detailed answer, this is definitely the definition of world-building. This answer, amongst others, have created interesting scenarios, just on the premise of my original question! $\endgroup$
    – Moo-Juice
    Aug 1 at 10:48
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For the same reason logistics centers are located outside of large cities.

After your traders deliver their cargo, some of these crates will be shipped somewhere else, perhaps on the planet below, or perhaps to another planet, or to another space station. In all these scenarios, it makes no sense to expend energy to land the entire ship and cargo on the planet, if most of it will be exported back into space.

If your game is about interstellar trading, then the space station would most likely be at the edge of the solar system, for the same reason. It makes more sense economically to have the local delivery done with ships designed for this purpose.

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Your drive isn't compatible with planets.

I'm thinking of the Honor Harrington novels by David Weber. Even freighters can pull a few hundred gravities, but the drive used in deep space is incredibly destructive to anything in the wrong place. You put one down at the spaceport outside town and the city is gone. If it doesn't burn out first it's going to carve a miles-deep crater.

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Distrust

You never know what fresh hell has developed in the opaque domes and tunnels that nestle among the craters or encrustments of a far-flung planetoid.

If you land, you might not be able to leave.

Maybe it's a retro sci-fi paradise designed to maximize human thriving. Or, just as likely, it's the Peacock Family In Space, a dim and dysfunctional hive of horror and decay.

If the place is locked into some kind of sustainability death spiral, there's a decent chance that somebody down there knows it and is on the lookout for naive passers-by who can be lured down to the planet, where their cargo, vessel, and bodies can be cannibalized to keep this hermetically-sealed atrocity going.

Even if it's not John Carpenter's Own Dome down there, the locals might still prevent you from leaving. Their plan might be to charge you with a crime and then impound your vessel for the purpose of extracting "fines" (aka ransom) from your trading company (via insurance) and/or your loved ones. In the real world, there are legal jurisdictions that depend for their survival on imposing specious fines on residents and travelers, and it's my understanding they use elaborate sets of rules to to heap on the penalties so they can extract as much money as possible from each victim.

So:

  • if you value your life--
  • if you value your freedom--
  • if your crew prefer to not become the sex slaves of a local tyrant--
  • if you don't want your eye sockets to be used as wombs by an alien organism that Paul Reiser intends to bring back to his heartless corporate overlords--

-- then never land. Don't accept any invitations to come down for a meal with the local administrator. Don't run down to the planet to see if you can score with an attractive local. Don't let the guy in the space suit squeegee your windows. Don't even listen to the local radio stations, and I am 100% serious. It's all hands to battle stations and every eye glued to a radar or porthole, from the moment you arrive until you're well and safely away, and if anyone or anything approaches the ship: vaporize it twice -- three times if it claims to need help.

Stay in orbit long enough to create proof of delivery, dump your cargo, and then get the hell out of there while you still can.

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All orbital platforms are essentially the same, planets vary widely.

Landing on different planets forces you to deal with many physical constraints that can vary widely between planets - how strong gravity is, how thick or dense the atmosphere is, what corrosive or breathable gases are present in the atmosphere, atmospheric weather events, biological organisms on the surface, the list goes on.

None of that matters when docking with an orbital port - the station is always in zero-G with no atmosphere and in a controlled environment. A ship that can dock at one orbital station can likely dock at almost any orbital station, since the physical parameters do not vary much at all. This isn't true when landing on a planet - being able to land on (and take off from) one planet doesn't imply that you can do the same elsewhere.

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  • $\begingroup$ "All orbital platforms are essentially the same" -- this finally gives me a good concept of the Star Trek term "standard orbit". For example: "Standard orbit is defined as a delta-v from planetary capture of 3200 m/s" means that this much delta-v gets you to the orbital platform every time, not counting course corrections. $\endgroup$ Aug 1 at 18:44
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Acceleration.

A ship that can't produce 1g of acceleration can never touch Earth, but could still be quite useful going between worlds. The more acceleration your ship can produce the bigger the percentage of the ship that must be devoted to the drive and it's power source. Thus a bulk freighter is better served by never landing.

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Technial difficulties

Long-range ships are built differently than "local" (inter-planetary, can't leave solar system) ships. They need specialised heavy-machinery, use different propulsion, can't enter atmosphere, etc.

(others have already proposed numerous explanations of this kind)

Biological problems

Others have mentioned quarantines, but also, if your world has different lifeforms, they might need different habitats, so this is simpler.

Politcs

An orbiting space platform is somewhat a neutral zone - it belongs to the planet, yes, but it's also far away from it that it acts like an independent place with it's own rules. It might even allow trading items between travelers that are outlawed on the planet.

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Teleportation

Add teleportation technology to your world. Every space station is equipped with a teleporter which beams the freight from the cargo ships to the station, and then from the station down to the surface. That means you don't need to implement docking either. The ship just needs to be close enough to the space station, and then you can visualize the goods exchange with a visual effect. Which is inexpensive for you to make and can be easily done in a way which is very readable for the player.

Why not teleport directly to the ground? Because the player's ship is too small to have an own teleporter. Or at least to have one which is powerful enough to beam whole freight containers from orbital height and through atmospheres. Only stations or very large ships have the energy and space for teleporters of that scale.

It worked for Star Trek. And having teleportation technology in your world could also help to explain a lot of other gamified quality-of-life features you will probably want in your game.

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For legal/compliance reasons.

That orbital station is more than a drop-off point. It's the planet's customs checkpoint. There's no practical way to enforce customs duties if ships can just land anywhere on the planet they want any time they want. Smuggling and contraband would grow out of control. Instead, your planets have designated customs stations that all offworld cargo goes through (similar to how real-world shipping ports work).

Also, it's immensely easier on your pilots if they can avoid entering the planet's atmosphere and thus their legal jurisdiction. Every planet has their own government and their own rules. It would be a massive headache to try and comply with all of them. Is this the planet that banned Taryak 4 coolant, or the planet that requires it? Do they pass on the right or the left? Did I install the infrared turn signal indicators? The native species can't see the yellow ones. Are their landing pads designed for standard gauge landing gear, or do they use that funky narrow gauge and my ship will fall off the pad? It's all a giant hassle, don't mess with it. Just drop off the cargo at the orbital station and let the local boys handle all the up/down travel and local rules.

Speaking of those local cargo haulers, don't forget that most of those are union jobs. You can't land at the local port unless you're a member of the local cargo hauler's union, but doing so would exclude you from being able to join the similar union on other planets. Scabs get shot down faster than enemy attack craft. You need to stay on good terms with the union workers. Let them keep their short-range transport monopoly, and they'll be much happier about hiring you to handle the long-range stuff for them.

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Time

You'll get paid the same amount whether you leave it in orbit or on the ground. You save time if you don't land. So...

I heard Uber Eats drivers prefer 'leave at door' deliveries over 'meet at door' ones for much the same reason. Time is money and delivery people usually get paid not by their time, but by delivery done.

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Many have already pointed it out, but I also believe that the best option is a space elevator with a conevyor belt line that moves commodities up and down pound-by-pound, or as the capacity of the belt allows it. The belt would be helped by a rotor which would rotate in either direction to compensate for the weight coming up or down. Being that weight is zero sum and such an important aspect of this machine, it will be difficult to smuggle things into the planet, as every pound is carefully assessed and charged for. This way, you also prevent alien vehicles from landing at your planet and polluting. No clearance needs to be provided other than just delivering and picking up payload at the orbital station.

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  • $\begingroup$ The rotor would be like a flywheel then, to provide more energy when you're lifting more than you're sending back down (which would be most of the time) ? That's a good idea! You can charge it up slowly from solar panels, then apply the power as needed. Although if you build an expensive space elevator you're probably gonna run it 24/7 to make your money back. Stuff going back down to the planet could be manufactured goods I suppose, or passengers, but probably a lot less than gets lifted up. Especially while we're building space infrastructure. Unless we mine asteroids. $\endgroup$
    – Greenaum
    Aug 26 at 2:09
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Loads of answers. I feel like this one should have been a very short comment, but SE is broken, so I'm going in more detail for my 2¢ on this:

Why interplanetary commerce should deliver at orbit:

It's the same thing with international commerce. Big ships dock at an international port, unload stuff, stuff gets more or less inspected, taxes are paid, load new stuff, go to the next destination

About spaceships that bring stuff from planet to planet

These big spaceships should be assembled in space (since we can't slide them into space in the same way we do with "seaships") and never touch surface. They should be huge.

They could be sort of like a mothership and the "containers" be sort of smaller standardized spaceships that dock to the mothership and to the orbiting docking station. Standardization is key as these smaller ships could be of many varying sizes, most I would say the same volumetric capacity as the ones we see today. These ones could carry the same kind of containers we use today.

About the transport between surface and orbit

  • Going up:
    • Attach a few Starship class rockets to it and you have liftoff
    • "Starship" boosters go back and are reused
  • Going down:
    • Easy peasy. You can just simply deorbit these smaller ships and land them SpaceX style or splashdown on the sea and docking at a traditional port.
  • Once on the surface:
    • Stuff gets transported around pretty much the same way they are today for delivery. Trains, trucks, ships, etc.

About the orbiting ports

There would be several stations like that orbiting specific orbits for making the delivery of goods spend the less fuel possible

Reusability and standardization are key for that to work properly

And about what kind of fuel? Well I guess you will have to get pretty creative on that one.

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Simply, gravity. Look at the Apollo moon missions. The ship that made it to the Moon had one small-ish engine, and a few quad-vernier engines around it, to adjust attitude. These were each about the size of a can of beans, or rather 4 cans of beans around a box.

Then look at Saturn V, then the biggest rocket ever. It's gigantic. Each stage is gigantic (all 3 of them) with multiple huge engines you could stand underneath. When they launched, observers were heavily shaken, and the noise was heard miles away.

I looked it up, the Apollo CSM's (the bit that went to the Moon) engine had 91,000N of thrust. That engine powered the ship from Earth orbit to Lunar orbit, and back.

Saturn V's first stage engines produced 35,100,000N of thrust. Just to get the whole thing up in the air and on the way to space, with two more stages to go yet.

So, 350x engine power (plus the next 2 stages!) to get up to space, compared to flying around once you're up there. Regardless of what new technologies you invent (unless you're gonna go with "magic", some previously unknown force you make up), it takes a shitload more energy to get up into orbit, than it does to fly around in space. So you'd need a very different kind of ship that could get back up there again.

Besides that, what's wrong with trading in space? People in one station might require things that another station has. Perhaps there are factories on the stations, which require components delivering, and finished product taking away to sell. Lots of commerce without bothering the planet. Let the station itself arrange a way up and down. Perhaps they've got a space elevator, or a railgun on the planet launches bulk material up where little ships catch it and relay it to the station. Any way you do it, a ship to make short relay trips under heavy gravity, to orbit and back, is differently optimised than a ship to travel great distances under no gravity.

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actually all the goods could be ore or refined metal purchased at asteroid mines, and trasported by some ships to space factories and space habitats to be used to manufature goods, and the manufactured goods transported to other space habitats.

Thus you could write a game where all the space travel and space trading is between asteroids, space factories, and space habitats, and nobody every lands on or takes off from a planet because nobody lives onlanets anymore.

Or possibly a lot of people do live in gravity wells (or planets), and the space traders pick up goods from space sations orbiting planets and taken to them other space stations orbiting other planets.

Possibly the space traders have interplanetary or interstellar cargo ships using Maxicorp engines, the most powerful space engines available. A cargo ship can transport 100,000 tons of cargo from planetary orbit to planetary orbit using Maxicop engines. But a cargo shuttle between a planet and its orbiting space stations can only carry a mere 10,000 tons of cargo either way, even using a Maxicorp engine. And some shuttles use less powerful engines.

So the interplanetary haulers transport 100,000 tons of cargo containers from orbital space station to orbital space station, unloading their containers at the destination space station. And the orbital shuttle cargo haulers take only 10,000 tons worth of cargo containers up or down between the planets and the space stations.

And middlemen in the space stations buy and sell cargo containers full of goods to and from the interplanetary haulers and the shuttle operators.

So the interplanetary cargo haulers never go down to the planets, but instead haul cargo between space stations orbiring planets.

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  • $\begingroup$ I feel like this answers the "how" (which isn't part of the question), but not the "why" (which is). $\endgroup$
    – Egor Hans
    yesterday
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There are plenty of real-world examples in supply chains, of transporting something across an ocean to a central warehouse, instead of directly to the end consumer. The reason, which is even more significant for space travel, is fuel costs and logistics.

If your space-warehouse is high enough, and has a firm network of connections to substations on its way to the end consumer, then you would massively save on fuel costs, which would unilaterally apply to both major star ships and smaller vessels—if they own a smaller transportation vessel (Serenity, Planet Express, etc.), then they likely are in a spot where fuel cost concerns are proportionately greater.

Once you're outside of a gravity well, you effectively just have to push and wait, and you'll get to your destination eventually; once you're in the gravity well, you need to reach the escape velocity of the well before you even begin to push.

If regular orders are retrieved from the Amazon Interstellar Ware-Station at L2, then a limited and calculable amount of fuel needs to be used to reach it, handling bulk quantities of orders at the same time and cutting resource costs down to a tiny sliver of what they would be for direct delivery.

Basically the same reason we don't order a pair of made-in-China Nike shoes and pick them up from a loading dock. It's good business!

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