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I have a world where humans have developed interplanetary space travel that allow them to travel to other planets. There is an issue though- the speed is slow, sublight; as an example it will take at least a week to travel to Pluto, at which point your ship’s fuel tanks have run dry.

My humans need a planet, comet or other body that is far enough outside the solar system that they can explore other planetary bodies. The planetary body needs to be within a weeks distance of Pluto (7.5 billion kilometres away) that can be accessed frequently from ships from the solar system that are exploring new systems.

Any help as to what planet, comet or other planetary body would be within 7.5 billion kilometres of Pluto (this could include artificial bodies)?

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    $\begingroup$ if you are suggesting the light from the Sun takes one week to reach this is slightly in error. That would put planet Pluto one light week distant from the Sun, when, in actual fact, it is approximately fuor light hours distant. In asking what planets, comets or other planetary bodies are within 7.5 billion kilometres of Pluto that includes the entire solar system itself. I presume you may be looking suitable locations for colonies beyond Pluto where lies the Kuiper Belt and the Oort clouds. Lots of ice dwarf planets and comets out there. $\endgroup$ – a4android Nov 3 '18 at 3:14
  • $\begingroup$ @a4android: Judging by the punctuation, I suspect that the OP is saying that his ships will take a week to reach Pluto and then be unable to recharge due to insufficient sunlight. Though I'm not sure how a body further away would be better in that respect $\endgroup$ – nzaman Nov 3 '18 at 10:09
  • $\begingroup$ @a4android I never specified that the ship or colony would run on solar power... $\endgroup$ – Boolean Nov 3 '18 at 11:54
  • $\begingroup$ @a4android I think 'sunlight' is a typo for 'sublight' $\endgroup$ – TimeTravellyParadoxySciFiSmeg Nov 3 '18 at 17:12
  • $\begingroup$ @TimeTravellyParadoxySciFiSmeg autocorrect “corrected” sublight to sublight $\endgroup$ – Boolean Nov 3 '18 at 20:35
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I'd say you're not really ready to go beyond Pluto. Remember that these things are orbiting the Sun. Distances are changing! So if it's in range of Pluto now, it may not be in a few decades (and you mention "colonies" so I assume you want this hanging around for a while).

It looks to me like your next step is to some of the Plutinos -- these are things in the same resonance with Neptune as Pluto is (2:3).

Maybe Orcus is your first target? It's been called the "anti-pluto", it has sort of the oppposite orbit.

Once you've got enough plutinos covered, things further out that go out of range of one plutino may come in range of another. So maybe you're ready to go further in the Kuiper Belt.

After that you're probably stuck until you improve your space drive.

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I am not convinced that humans will settle Pluto in any meaningful fashion. Pluto is too far from the sun, making it cold and hard to power. It would make more sense to dismantle Pluto and turn it into space stations in L4 and L5. But if you want to find a place outside Pluto's orbit, there is always Eris and some of the other dwarf planets. Eris is about the same size as Pluto (more mass; slightly smaller volume).

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    $\begingroup$ Which L4 and L5 are you referring to if you are dismantling Pluto? $\endgroup$ – kingledion Nov 3 '18 at 12:10
  • $\begingroup$ I thinknit's not a full dismantling. $\endgroup$ – Renan Nov 3 '18 at 18:25
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Distance is not a good measure of 'distance' in space travel

Pluto is 7.4 billion km from the Sun at aphelion, and 4.4 billion km at perihelion. Neptune, the next closest planet is between 4.4 and 4.5 billion km from the Sun at all times. What this means is that there are times when Pluto is about 7.5 billion km from Earth; and there are times when Pluto is over 11 billion km from Neptune. The planets are all moving in relation to each other. The distance between two objects varies over time; the straight line distance at any given time has little to do with how hard it is to get from one planet to another.

An object set into motion will stay in motion, says Newton's laws. In order to move from one place to another, you need to generate enough velocity to break out of one orbit and insert into another. These are usually called 'burns' from burning a rocket engine, and are best described in terms of delta-v, the total amount of velocity change needed to move from one orbit to another. The delta-v is in turn proportional to the amount of fuel (energy) and propellant (reaction mass) needed. Energy and reaction mass are the true quantities in which 'distance' in space should be measured.

To get from one orbit to another, you need a transfer orbit. A commonly used one is the Hohmann transfer orbit, which is the transfer orbit using the minimum amount of energy. On the other hand, a transfer orbit that yields a seven day travel time will take a lot more energy burning. But, the relative energy it takes to move from planet to planet will stay roughly the same.

As it turns out, due to the shape of the orbits, the Hohmann transfer delta-v to Pluto is less than that to Neptune. Further more, the delta-v to Alpha Centauri, or any other point outside of the gravitational pull of the Sun is about 12.3 km/s, which is marginally more than the 11.7 km/s delta-v to Neptune. So, if you have enough fuel and propellant to get to Neptune, you pretty much have enough fuel and propellant to get anywhere you want. Of course, at some point you have to worry about the Milky Way's delta-v, but the travel times to stars at sublight speeds mean you have bigger problems than just fuel and propellant.

Conclusion

You don't 'run out of fuel' in space travel. You have a set budget of energy and reaction mass, and you burn that to get into transfer orbits. Presumably, you should burn into your transfer orbit, then spend days to months (to years) coasting to wherever you wanted to go, depending on how much energy and reaction mass you burned in the first place.

The minimum energy and reaction mass needed for a given burn are well established, withing the solar system. Any craft headed to Saturn (10.3 km/s of delta-v) or beyond has close to enough energy and reaction mass to make it anywhere within the gravitational bounds of the sun. So the answer to your question is: If you can colonize Pluto, you can colonize anything in the Solar System

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    $\begingroup$ Except OP's drive is getting to Pluto in seven days, not seven years! Not sure this explanation works. $\endgroup$ – TimeTravellyParadoxySciFiSmeg Nov 3 '18 at 2:17
  • $\begingroup$ @TimeTravellyParadoxySciFiSmeg Noted! I adjusted my answer. The explanation is still valid, the delta-v's just have to be much higher. $\endgroup$ – kingledion Nov 3 '18 at 2:30
  • $\begingroup$ The delta-v could be your entire travel time. For example, accelerate at about 1G for a few days, decelrate at 1G for a few days. Travel time about a week, distance about Pluto (I think I've done the calculations right), amount of fuel used: insane. You can't spend much time coasting because ... reasons ... maybe you still need to burn fuel to power your deflector shields to protect you from bombardment while travelling at ludicrous speed. TBH I was just taking OP's drive for granted as is. $\endgroup$ – TimeTravellyParadoxySciFiSmeg Nov 3 '18 at 2:34
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    $\begingroup$ @Renan - I mean that you are spending half the trip accelerating, and the other half decelerating. You have to do this to get to Pluto in one week. Hohmann transfer is not going to work here. $\endgroup$ – TimeTravellyParadoxySciFiSmeg Nov 3 '18 at 17:18
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    $\begingroup$ @TimeTravellyParadoxySciFiSmeg There's a name for what you're describing. It's a Brachistochrone Trajectory. $\endgroup$ – Ryan_L Nov 3 '18 at 17:25

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