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Looking into fleshing out my world story. One of the things I had in mind was an infrequent but consistent shipment of supplies to a planet from Earth. For simplicities sake, we'll use 10 as the number of years between shipments, but this number could vary. The key is that a lot of time passes between shipments, time enough that a missed shipment could be catastrophic or at least mysterious. My world is sort of western, slightly mystic, but with hopefully realistic explanations. I think of Firefly a little bit, how they land on a planet that looks like it came from the old west, yet they also fly to massive space stations. The technology level would be similar.

So my main question is what would/could cause so few visits from Earth (or any other large Entity/Government), given that Earth was not in ruins?

Please let me know how I can make this question more answerable.

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    $\begingroup$ Maybe they planned it like that. Earth isn't just going to send all it's resource to another planet just because it wanted it. The thing you'd have to cover would be why private companies won't be doing shipping in the mean time. That can be covered by the cost of FTL putting the cost too high for non government entities. Which could also partially explain why so few shipments are sent. $\endgroup$ – A. C. A. C. Sep 21 '17 at 17:38
  • $\begingroup$ You effectively want to limit the number of shipments. Easiest way is to limit the number of available ships as in this answer. $\endgroup$ – Trilarion Sep 22 '17 at 8:43
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    $\begingroup$ International travel is expensive so you ship everything in bulk - it's simply cheaper to send everything at once instead of sending half-empty ship. Making shipment regular makes the scheduling easier as people may book space in advance. Yeah, sure if someone really want to send a shipment pronto they could spend a few billion dollars to send one but everyone else is using slow Interplanetary Post Office. Think about how post office operates - it has regular schedule (although based on days rather than decades). Just scale it. $\endgroup$ – Maciej Piechotka Sep 22 '17 at 8:49
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    $\begingroup$ Two separate rocket launches are more expensive than one. Heavy rockets are more expensive than light ones. Somewhere those two graphs cross at the most economic size/frequency combination. Maybe that's every ten years. $\endgroup$ – Grimm The Opiner Sep 22 '17 at 9:24

18 Answers 18

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It could most simply be a matter of launch windows.

Things in space tend to travel in orbits, these orbits follow the different planets/moons and periodically lineup to allow low energy transfer orbits between the different planets. Sometimes these orbits involve interactions between multiple different bodies to line up.

For a simple example both Earth and Mars are moving around the Sun, every 780 days they both line up such that you can move from Earth to Mars (or vise versa) at the lowest required energy orbit. The further the planets move from this ideal alignment the more energy/fuel it takes to move between them, at some point making it effectively impossible.

Other more complicated planetary alignments occur much less often, the Voyager 2 probe took advantage of a 175 year planetary alignment to travel past Jupiter, Saturn, Uranus, and Neptune.

Your 10 year timing could be because that is the only time that the planets line up allowing a ship to easily travel to your outpost from Earth.

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    $\begingroup$ I like it, could be an easy explanation without politics or economics $\endgroup$ – Premier Bromanov Sep 21 '17 at 21:17
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    $\begingroup$ For a story using conventional propulsion and limited to a single solar system, this seems like the single most obvious reason - to the extent where a story of that sort which used a different explanation should maybe almost explain why this explanation was used. +1 from me $\endgroup$ – Ethan Kaminski Sep 22 '17 at 3:44
  • $\begingroup$ But I don’t think there’s any launch window from the Earth that opens up every ten years. $\endgroup$ – Mike Scott Sep 22 '17 at 9:41
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    $\begingroup$ The most common interplanetary transfer is the Hohmann transfer. Launch windows come up once in each synodic period relative to the departure planet. For synodic periods relative to earth, check this table. The longest for a planet is 2.1yrs to Mars. But near earth asteroids can have much longer periods between windows - so maybe that would be a good place for an outpost. $\endgroup$ – Jack B Sep 22 '17 at 11:44
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    $\begingroup$ The OP should be mindful that this is predicated on fuel limits, so if the ships do not have fuel limitations or specific impulse limits (i.e. they are torchships), then the explanation is weakened. If they had a torchship that could just burn for 1/2 the trip and then reverse burn the other half, then why do Hohmann transfers or gravity slingshots? $\endgroup$ – Yorik Sep 22 '17 at 19:15
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There aren't many ships and it is a long way.

If you only have one ship on a route and it takes 10/2 years to travel there it will only make a shipment every 10 years. Multiply the distance by the number of ships on the route if you don't want to be familiar with a single ship. Space is big, and going fast is expensive.

There isn't anything worth shipping back.

Earth doesn't need anything specific from your cowboy world, so they rarely check in. They don't really want everyone to die, so they still send some support, but it's less hassle to make one big shipment than a bunch of little ones. In the unlikely event the cowboys turn up something worth sending back a few years lag isn't considered a problem.

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    $\begingroup$ Hmm, i wonder if 1600s/1700s america could give some examples of this type of thing, like how expensive and how frequent shipments to any given place were $\endgroup$ – Premier Bromanov Sep 21 '17 at 20:38
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    $\begingroup$ @PremierBromanov I understand most colonies saw ships every year to collect things the empires wanted, and the travel times were only months. Maybe spice from India before that or caravans. But traders really hate shipping empty loads, and most things work on a single year cycle. $\endgroup$ – user25818 Sep 21 '17 at 20:52
  • $\begingroup$ So maybe hitting more of a trader mindset where there's profit to be made, rather than investment. The size of the settlement and lack of resources could lead to that $\endgroup$ – Premier Bromanov Sep 21 '17 at 21:16
  • $\begingroup$ Most straightforward answer. $\endgroup$ – Trilarion Sep 22 '17 at 8:42
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Assume that spaceflight will be either slow or very expensive. Perhaps there is a "jump point" in the outer system, and the most economic flight from the colony world to the jump point is a slow Hohmann transfer orbit.

  • A fast courier or a warship takes a year from Earth to the colony.
  • A passenger liner takes two years, but it can carry little cargo.
  • A freighter or a combined freighter/colonist transport takes five years.

So for economically marginal colony worlds, there is one tramp freighter going back and forth. It starts from Earth with a load of colonists and cargo, travels five years to the colony, unloads, and returns five years later with exports from the colony and a shopping list. Then this freighter, or another one from the same company, repeats the schedule.

If a competitor wants to get into the business, the first flight of that ship would have to load speculative cargo without the benefit of the shopping list with import orders. And the colonists might not have anything to pay that new ship, unless they want to break their existing contract or they have enough surplus. At some time in the future the economy of the colony world might get that far. It isn't there yet.

So every ten years the scheduled ship from ColonyCorp arrives, with the stuff that was ordered ten years ago. Finally, the new turret lathe. And the new DNA sequencer for the colony hospital. And there must be twenty crates of refined molybdenum to pay for it.


A similar scenario would be possible with a shorter flight time, assuming that the colony takes ten years to produce "one shipload" of export goods. Refined metals? Pelts? They are in a contract with one shipping company to come every ten years, take the promised exports and deliver the ordered imports (as above).

The colonists could say at any time "the next load will be ready in sooner, come back in four/six/whatever years." But if they don't hold their promise, they will have to pay a penalty.

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  • $\begingroup$ I like the idea of building up an export shipment. Almost an offering to Earth. Question, why would larger ships travel slower through space? $\endgroup$ – Premier Bromanov Sep 21 '17 at 20:40
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    $\begingroup$ @PremierBromanov It isn't size it is efficiency. Every gram of engine is at least one less gram of cargo. $\endgroup$ – user25818 Sep 21 '17 at 21:07
  • $\begingroup$ @PremierBromanov, a warship might be both much larger and much faster than a colony transport. But as the other comment pointed out, it will cost quite a lot. $\endgroup$ – o.m. Sep 22 '17 at 4:57
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    $\begingroup$ @PremierBromanov In space, using fuel speeds you up or slows you down, and then you coast. A slow freighter could save a lot of money by just speeding up to the minimum required and then drifting for a long time. The military is much less interested in saving fuel. $\endgroup$ – chrylis Sep 22 '17 at 17:12
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    $\begingroup$ @PremierBromanov Say we have an inexpensive method of travel that works in spacetime as we know it. We need to take the time of actually traveling the full distance, but it's cheap af. Meanwhile Fancy McMilitary Cruiser can take advantage of a wormhole network, but it's extremely expensive to outfit the ship and power the drive to tolerate the wormhole. Alternatively, it uses some technology to fold space, decreasing actual distance traveled, but drastically increasing the amount of fuel one needs to bring, taking up what would otherwise be significant cargo space and reducing profits. $\endgroup$ – Thebluefish Sep 22 '17 at 17:18
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Harvest season for the planet's only export. Put the target planet in an elliptical 10-year orbit. Harvest occurs just after the planet passes its closest point to the sun. It won't grow anywhere else because it requires such a wide temperature variation. If the planet's orbit is on a different plane than the others, it would require a massive amount of fuel just to get there and back. The planet's just not worth visiting any other time.

It could also be tourist season for a similar planet. Paradise one year in 10, and for the rest of the time it gradually cools down until nitrogen starts condensing out of the air. Maybe the townies spend winter underground, or maybe they abandon it each fall and come back the following spring.

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The https://en.wikipedia.org/wiki/Mars_cycler, a short-period version of which is championed by astronaut Buzz Aldrin is a proposal that accomplishes exactly this for you.

enter image description here

It's essentially an orbit which crosses both Earth and Mars' orbits, at the times that those planets happen to be nearby. This means all the heavy shielding and life support and structure only needs to be put into that orbit one time. You only need local shuttlecraft to rendezvous with them as they approach Earth, to retrieve and deliver the (probably much lighter) payload, which will then reach Mars with minimal fuel.

The payload can even be launched to Mars from closest approach without any rendezvousing. Or Mars can rendezvous with its own shuttlecraft, and exchange that payload for its own payload.

The possible cycling orbits between Earth and Mars listed on the table at that page give you cycle periods of about 2.1, 4.3, 6.4, 8.5, 10.7, or 12.8 years.

So a cycler in the 10.7 year orbit answers the question perfectly. One such orbit, with Aphelion radius of 2.49, has an advantage over all other such orbits in that it has the lowest Earth-Mars transfer time of only 75 days... though the Mars-Earth transfer time is over ten years!

A single craft in such an orbit would mean you'd get stuff from earth only every ten years.

From a storytelling point of view, this has many advantages.

We've experienced with NASA that it's possible for a great goal to be achieved (interplanetary travel) and then disinterest to pull us back from that for decades. You could get to a point where they had established the Mars Cycler, put a colony on Mars, started off the terraforming systems, but then, after a number of high profile disasters and such, had pulled back.

Maybe some people went to Mars on the Cycler, but Mars failed to rendezvous... after their 75 days of life support ran out, they died, and their bodies were collected from the Cycler by the Earth team. That'd sour interplanetary relations, and also discourage all but the most desperate future people from wanting to go to Mars.

Asteroid mining had failed because more disasters, Mars isn't going to be habitable even with the Terraformer running for a few centuries, and for Earth, really, only near-space travel is profitable.

The Mars colonists didn't want to come home, probably partly because the cycler takes a decade to come back! Plus, it probably doesn't even have life support in for longer than 75 days: it's just a big box in a specific orbit.

Faster cyclers were planned, with life support, but never got the funding to put up, and Mars only has shuttles, not interplanetary craft. So they're still there, manning the Terraformer for a few hundred years, until Mars becomes fully livable and profitable again.

In the meantime, neither planet feels it worth their expense to invest in interplanetary manned craft, nor to put up another Cycler for the Mars colonists; it gets proposed, but never funded because "we have cheaper bridges to build, that would benefit more people and save more lives, right here on Earth!"

Now, what I don't know are things like what the velocity to match is, at either end (that'll be a limiting factor). I also don't know if, at rendezvous, whether the sun is between Earth and Mars. Picking an orbit where that's true would be ideal, because (unless there are radio relay satellites orbiting the Sun) that would mean that when it doesn't turn up, they can't ask Earth why it didn't show up.

Instead, it just wouldn't be there. And then everyone would start to wonder if Earth had abandoned them. People would believe that, and would get angry. Some would argue for destroying the Terraformer, to spite Earth; others would oppose that, since the Terraformer would be for the benefit of their own future generations. And so on. Drama! :D

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    $\begingroup$ Ironically, the energy cost for a Mars-Luna mission is less than the cost for an Earth-Luna mission. You could do Mars to Luna with a shuttle. +1 for the project got started and people list interest. $\endgroup$ – pojo-guy Sep 23 '17 at 3:38
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Time travel, but the boring kind

So this answer will make a couple of assumptions but bear with me. The setting I imagine is that your colony is somewhere far away from Earth, but within reach of rockets. Eventually.

So Earth ships off a large colony ship, maybe even a generation ship, and then proceeds to also send several back up resource ships behind it while it is still in transit. The colony ship lands and starts building a base, and gets extra supplies or personnel arriving on each of the resource ships.

Because the transit time is so long you could have several rockets in flight before the next rocket from Earth is set to launch. This kind of delay would mean that if Earth missed a launch window it could actually be several years before that delay actually propagated and was noticed at the colony.

Practical Example

Assume that it takes 30 years for a rocket to reach the colony world, and the launch window opens up every ten years. So Earth sends out a colony ship, then ten years later a supply ship, and then ten years later another supply ship. By the time they send the second ship the colony ship would be just landing.

Lets also assume that in with the supplies on the two in-route supply ships is a fair bit of news so that the colony can be kept abreast of the happenings back on Earth. Because there is such a wide gap between when a rocket is launched with supplies and when it arrives, all of the news that the colony gets is vastly outdated. If something were to happen that would make it impossible for another rocket to be sent that way they would not find out about it for several decades. And that is if the problem is even recorded and sent to them in the last supply rocket.

Imagine getting your usual shipment, and being told that things back on Earth are going great and to expect more of the same in the next supply drop. Then that supply drop just never arrives. Did something happen to the rocket in transit? Or did something happen to the people sending it? Ten years is both a very short amount of time for a global tragedy to hit, and also a more than enough, depending on the tragedy.

Until the next Landing Day comes along and they can see whether another rocket was sent out your colonists would have no way of knowing whether they were on their own or not. Maybe something happened and Earth couldn't send any more supplies. Maybe the colony has been written off, or completely forgotten about. How does an entire planet react in the face of that kind of uncertainty?

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    $\begingroup$ This fits in pretty nicely with some ideas i had regarding the absence of shipments $\endgroup$ – Premier Bromanov Sep 23 '17 at 18:03
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    $\begingroup$ If it is not FTL, you would usually get radio signals earlier than that. (Assuming you don't have something which blocks radio.) $\endgroup$ – Paŭlo Ebermann Sep 23 '17 at 23:16
  • $\begingroup$ Agree with @PaŭloEbermann, transmission of information should be much easier/cheaper/faster than cargo. $\endgroup$ – shiri Sep 24 '17 at 8:30
  • $\begingroup$ Communications are the big weakness to this scenario, admittedly. Still, there are bound to be reasons why simple radio waves aren't being used. Solar storms causing interference, broken down com relays, too high energy requirements given the distance. $\endgroup$ – D.Spetz Sep 25 '17 at 17:21
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The time lapse is due to the nature of the supplies

Your supplies might only be sufficient for shipping once every 10 years.

Perhaps there is an exclusive supplier or group of suppliers on Earth that provides supply xyz, and they are only able to design, grow, build, mine, assemble, culture or otherwise process that supply in the needed quantities in that amount of time.

Having the supply be only available from Earth and not other planets can add to the critical effect from a missed shipment.

Possibilities:

A certain type of art for which only one artist is famous.

Rare and difficult to modify and breed animals that take 10 years to mature on earth that can also survive as adults on the receiving planet but for some reason the juveniles cannot.

Experts graduating from a highly specialized training/doctorate program that only accepts new attendees once a decade in order to keep one entire class learning together to create the most coherent team possible in newology, which is in very high demand on the new planet. And if that team goes missing...oh boy!

Endless other possibilities....

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    $\begingroup$ The consumer could be partial to periodical cicadas which have cycles of 13 or 17 years. $\endgroup$ – Andrew Morton Sep 24 '17 at 11:16
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    $\begingroup$ This could also apply to the nature of the transport vessel, which could be a massive ship in orbit, and it takes a very long time to get material from the planet into orbit over multiple launch cycles $\endgroup$ – Richie Frame Sep 24 '17 at 22:31
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    $\begingroup$ e.g. McDonald's McRib sandwich $\endgroup$ – barbecue Sep 25 '17 at 0:22
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Nuclear fuel for the reactor.

Reactor is needed to keep life support systems running, say, producing oxygen. It takes a lot of energy.

Even if there were local deposits of Uranium, the enrichment facility might be too big an endeavor to land and deploy.

It's heavy, dangerous to transport, and expensive. The launches are rare, and one supply run can last the colony 10 years easily.

There's no point stockpiling on it: natural half-life of the fuel means it decays over time and becomes worthless eventually. You need a fresh supply, or at worst one that's been waiting past 10 years - anything older will be way sub-par, so the colony may stagger the use, maintaining a stockpile for one shipment's worth, using up older fuel and renewing the stockpile with newest delivery. Or not - in that case, the fuel at the end of 10 years will not just "run out", but it will lose enough oomph to cause brown-outs, need to conserve power, and a general crisis.

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    $\begingroup$ I don't have a problem with this answer in general, but given that the half-life of Uranium is 4.5 billion years, it's probably a bad choice for an example. ;-) $\endgroup$ – Doug R. Sep 22 '17 at 13:29
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    $\begingroup$ He3 would work as well. Not that heavy to ship, but that is exactly why they don't bother with local production. $\endgroup$ – MSalters Sep 23 '17 at 14:55
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There are multiple possible reasons, you just need to choose the one that fits for you:

In-System Travel

  • Low-Thrust: You may have a low-thrust drive that boosts continuously, but takes a long time to get you to where you need to go.

  • Use of Efficient Transfer Orbits: Others have mentioned that, due to bulk, cargo vessels may need to use efficient orbits to be cost-effective, while other types of vessels, which can boost continuously, may be able to make the trip in shorter time-periods.

Insterstellar Travel

  • Slow FTL: Depending on the method of FTL or the period in history when the story is set (i.e., early colonial when FTL technology hasn't had time to mature), you may have a "low" maximum velocity (i.e., max v=10 c). The colony in question may be (10 * v)/2 light years from Earth.

  • Drive Efficiency: As with sea-going vessels, larger vessels can't travel efficiently at as high a velocity. I.e., a quick Google search shows that an ocean-going supertanker has a max speed of around 15 knots, while some naval destroyers can sustain speeds approaching 60 knots for several hours (although they have lower cruising speeds). A space-going cargo ship may take 5 years to make a trip that a courier can do in 6 months.

  • Trade Routes: No matter how you look at it, trade in space (especially interstellar) isn't going to be cheap, especially for newer or smaller colonies (or at least, early in the colonial period it won't be); there may not be an economic benefit to having a freighter going directly from Earth to the colony and back. Freighters may do routes that are a 10-year round trip. For the world at the far end of the route, it's 5 years each way to Earth. Or the critical cargo from Earth may need to be transshipped from world to world, from ship to ship, until it gets to the colony.

Combination Reasons

This is one that I happen to have a fondness for. FTL travel by itself is cheap, easy, and fast. It's the STL part of the trip that's slow and relatively expensive. Whether it's jump points, wormholes, or just that you need to be in a relatively flat gravitational metric prior to engaging your drive, a ship has to be quite a way from a star before it can engage it's stardrive...which is virtually instantaneous, but leaves the ship a similar distance from the destination. I.e.:

  • Solar System: "No-fly" zone for the stardrive varies, but for a ship the size of a cargo ship, it's 0.4 light years.

  • FTL Drive: Cheap, virtually instantaneous, but in order to make the necessary calculations for a cargo ship, it has to be "at rest" relative to the nearest large object (i.e., the nearest star). In other words, a cargo ship must decelerate in order to jump, but a courier might be able to do jump at speed, or "on the fly" (if the benefit outweighs the risk).

  • STL Drive: Low-thrust drive may be most economical for cargo; 0.1g acceleration for 1 year brings the ship to a velocity of 0.1c. 3 years' cruising time, 1 year to decelerate, is an average cruising velocity of 0.08c for a total cruise time of 5 years (if my calculations are correct). Instantaneous jump, then another 5 years to the destination.

In this type of scenario, travel time would be roughly equivalent whether destinations are 1 light year apart (entire trip is done STL) or 1,000 light years apart. Distance doesn't matter, just gravitational gradient at origin and destination.

Update:

I just thought of this...in this type of situation, cargos may be sent out to the edge of the "no-fly" zone on uncrewed solar-sail vessels (or flung out there by some sort of mass driver). Cargos would be collected out there and shuttled through periodically by personnel stationed at the collection point, then on the far side, they're sent back down the gravity well. What you end up with is a fairly continuous pipeline of cargo and permanent station personnel out at the collection point. Could make for some interesting stories about them...

People who are traveling between established destinations (i.e., Earth and an established colony) would travel on a rapid-transit in-system ship on this side, transfer over on a dedicated vessel, then travel down the well on the other side on a dedicated in-system ship.

Personnel and cargo would switch from a system ship to an FTL ship that is essentially a tin can with limited maneuvering ability for the FTL jump, similar to a Guild Heighliner in Dune.

The Handwaving Effect

Is the reason crucial to the story (or will it be crucial to future stories)? If not (i.e., if the story is strictly about what happens when a ship is overdue...and then becomes overdue enough that people start to worry), then the fact that there won't be another ship for 10 years is all that matters. You can simply explain it away with "a change in stellar geometry," a rogue star entering the region, or whatever. You can even just leave it as a fact and not bother to explain it.

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The planet has a small research station or colony on it and its remoteness makes it very difficult and costly to reach. Consequently those there must live off of the land and will have to have recycle and repair facilities that are very effective. Nevertheless after a period of time it will be necessary to bring supplies in for materials that can’t be replaced or repaired locally such as advanced technology items that can’t be made locally (perhaps they have no silicon chip manufacture capability or similar). There might also be a need to replace or rotate crew / inhabitants from the planet and provide raw materials for local use that were not readily available on the planet or at that location such as Helium3 for their fusion reactor? For all these reasons a supply ship is sent on an infrequent basis as you describe.

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10 years according to whom?

Time is relative, and it does funny things as you approach the speed of light. Particularly, it tends to dilate.

A ship might travel near instantly from its own perspective, but for an outside observer that trip looks like 10 year, all of that thanks to time dilation.

Either that or it actually takes 10 years to travel, but that brings the problem of your cargo and crew being able to survive that long.

If you have just the one ship for a particular route (because e.g. not profitable enough; space ships are just rare; route is too dangerous; your own reason here) that would certainly be a perfectly reasonable explanation.

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  • $\begingroup$ I think that the assumption that it's a 10-year round trip based on the colony's frame of reference is implicit in the question. Whether that's a local year or an Earth year is not, although it might be relevant. $\endgroup$ – Doug R. Sep 25 '17 at 19:16
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The Sun has captured a passing Star (or vice-versa), and now they are a Binary Pair orbiting around each other. Each one kept its own Solar System.

The stars orbit each other in a highly eccentric elliptic pattern, with a 20-year period. So, they're closest every ten years. This closest is still far enough to leave the planets' orbits undisturbed, but close enough to send a ship that takes one or two years to get there (aim for where it will be, obviously).

Any other part of the orbit is just crazy far.

Now, this all falls apart if FTL travel is available. Maybe it's a secret colony, and FTL travel is easily detectable, so they have to fly the old way.

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    $\begingroup$ I think a binary pair like that creates a whole lot of new problems to solve. I'm not even sure if we could "capture" a star without our planetary orbits intersecting $\endgroup$ – Premier Bromanov Sep 23 '17 at 18:02
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    $\begingroup$ A 20 year period would mean this second sun is nearer than Saturn (on the major axis of the eclipse). $\endgroup$ – Paŭlo Ebermann Sep 23 '17 at 23:14
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There is a launch window from Earth to Mars every 800 days or so if you want to do conventional ballistic tajectories. If the orbits of the two planets were even closer, that would make the two planets more similar in habitability, and it could easily take five years or longer for the inner planet to make one orbit more than the outer. Of course the orbits can't be too close or they become instable.

I haven't seen the calculations how long the average orbital period in the habitable zone of lighter or heavier stars would be, but probably different from our 365 days.

Other possibility: A multi star system, where the differences are simply huge and you need swingbys with larger planets in both subsystems to make the tranfer oeconomically feasible.

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Great question, some thoughts:

Space is full of radiation, even if it took 5 years there and back you'd need 2 years for a retro fit after every round trip. Also you need to get the supplies up from earth into orbit, there are only so many launch windows and standard rockets only take a couple of tonnes each. It would take 2 years just to get the next load into orbit.

What does earth needs, we are going to run out of rare earth metals, in advance of that the price will rise, so earth will want to import lots of rare earth metals for electronics etc. These are inert in the sense that they can happily sit in a container for years. So massive single shipments make more economic sense than speed.

What does earth ship back? People - the population of earth is expanding and we need space for them, another planet needs workers. The cost of shipping people would be huge so it only makes sense to send them back on the cargo crusisers once every 10 years. A poor african worker isn't going to have 1,000,000 dollars laying around for a space fare on a dedicated transport, they would though slum it in cargo for a few thousand.

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The planet's star throws around just too much radiation. Like our Sun when it's in a bad mood, but all the time. Or at least too frequent and unpredictable to be worth the risk.

The planet itself is safe, it's got enough of an atmosphere and magnetic field to protect everyone inside it.

But it's not large enough to provide an adequate shadow to approaching and departing ships. The star just shines around it. So usually it's just unsafe to travel to or from the planet.

Once every ten years, the orbit aligns with a gas giant in the same star system, and the star is eclipsed. The gas giant provides a large shadow, allowing ships to approach and depart the shadowed planed.

You can pick the time length that better fits your story. The eclipse could last weeks, for a very comfortable visit. Or less than an hour, with incoming ships rushing in to be stranded for ten years, while the departing ships (that came ten years ago) rush out.

Somehow the planet needs to be worth the hassle. The shorter the eclipse, the bigger the hassle, the bigger the needed reward. But even if the eclipse lasts a year, it's still a large hassle to be stranded the other nine. (Or maybe it's a feature? Colonized by jaded misfits who loved the idea of escaping the greater community?)

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    $\begingroup$ Hmm, I can't imagine a spaceship traveling always in the shadow of the gas giant ... that looks like a really non-stable trajectory. $\endgroup$ – Paŭlo Ebermann Sep 23 '17 at 23:25
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It would almost need to be a problem of launch windows, as Josh King stated; nothing else has the required regularity.

But launch windows work only for intra-system travel, when you need to rely on grav assists from planets, and therefore go slow. For interstellar travels, you need a lot of delta-vee-equivalent. It makes little sense to have interstellar propulsion and still need slingshotting.

So this way you're forced to suppose a pipeline of shipments, each of them requiring twenty or thirty years to reach the outer Solar System with conventional means before they can start the long voyage between stars; or an even longer pipeline of sublight shipments, but in that case the travel time will be several thousands of years.

As a compromise, we could add a limitation on the interstellar travel technology: say that it is based on a sort of gravity lensing effect, so that only a few places are actually reachable, and the arrival position is a function of the distance between the "portal generator" and some large mass. By focusing on the Moon from Earth, for example, you can reach a point on the same line, at a distance that is a function of the Moon's mass and distance; the Moon acts as a "focusing lens". There is some wriggle space, at a cost, but not so much as the energy expenditure increases exponentially.

And to reach this far away star, the only mass that can be used is Jupiter, which is in a suitable position for a few days only, once every 11.8 years (or rather, the reverse: of all the places that can be reached by focusing on the Sun, Mars, Jupiter, Saturn etc., and have been probed, the only one near enough to a star is the one in the Earth-Jupiter focus).

If the generators could be placed on a space station capable of independent motion, then by placing the station in the correct position with respect to Jupiter, Saturn, or the Sun, any point within several light-years could be within easy reach - but the mass and energy requirements of the generators prevent this from being possible.

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How about shear mind boggling distance, you have FTL, it's nearly instantaneous, but that "nearly" is the killer. If you use jump points then it's a matter of transfer time between jumps, i.e. if you have to make four jumps to get from Earth to your colony world then it's two years and change between jump points and a bit more for in-system maneuvering at pick up and drop off. If on the other hand you use real space FTL doing ten times the speed of light that's great for going to say Barnard's Star you'll be there in about 6 months but Canopous is still 30 odd years away so a 10 year delivery schedule is not unreasonable for a distant colony.

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The EM drive works. (Never mind how badly that upsets physics, the fact that it appears to work seems like enough for basing a story on.) The cargo ship is unmanned, speed is not a priority but the EM drive means it gets there for free. However, the EM drive takes a lot of power, the max attainable thrust (instead of the tyranny of the rocket equation you have the diminishing returns of adding power making it heavier) is tiny.

Thus your ship crawls between the worlds. There's one ship, it takes 10 years to make the round trip. (Or there's two and it takes 20 years.)

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