## fictional planetary system

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plytho
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### fictional planetary system

I'm trying to imagine a fictional planetary system with multiple life bearing planets and moons. Due to some constraints travel and communication between these planets would only be possible when the planets are closest to each other.

I'd like to make a model of such a system, generating the interplanetary distances over time. If been trying to find a simulator online but without success. Does anything like this exist?

Another thing I'm trying to figure out is what the constraints could be. Why would travel and communication be limited unless the planets are close. Cosmic radiation seems like a possible explanation for the lack of communication. But how would it be different from our solar system and how could travel be difficult over large distances?
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tomandlu
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### Re: fictional planetary system

plytho wrote:I'm trying to imagine a fictional planetary system with multiple life bearing planets and moons. Due to some constraints travel and communication between these planets would only be possible when the planets are closest to each other.

I'd like to make a model of such a system, generating the interplanetary distances over time. If been trying to find a simulator online but without success. Does anything like this exist?

Another thing I'm trying to figure out is what the constraints could be. Why would travel and communication be limited unless the planets are close. Cosmic radiation seems like a possible explanation for the lack of communication. But how would it be different from our solar system and how could travel be difficult over large distances?

IM - very - HO...

Modelling the orbits should be fairly straightforward - I've got a spreadsheet for that if it would be helpful (it will basically say that for a sun with mass x, for a planet with a distance of y from the sun, the orbital period will be z). I'd ignore the planet's own masses, as you're heading into a three-body problem there... (the basic equation, with G as the gravitational constant and using x, y, z as specified above, is, I believe (in kg, m and s):

SQRT((4 * PI^2 * y^3) / (G * x)) = z

Could you have a very fierce sun (as you've hinted), and inhabitants can only travel between the planets when one is in the shadow of another? Also, this would act as a tunnel for communications, which would normally be impossible due to over-ionisation of the upper-atmosphere. If you gave your planets a thicker, denser atmosphere than earth, that might account for the aliens being able to survive on the surface, but not in space.
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Neil_Boekend
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### Re: fictional planetary system

How can they know of the other planets' life if communication is not possible? If they can see they can communicate with laser beams. Initially there will be much trouble due to language differences, but with enough time those can be ironed out.
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tomandlu
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### Re: fictional planetary system

BTW there's another thread somewhere, where the difficulties in developing navigation or an understanding of the solar system on a planet with permanent cloud-cover was discussed...
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Neil_Boekend
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### Re: fictional planetary system

Travel over large distances is difficult. We send interplanetary missions at the opportune moment because else the fuel cost multiply massively (to the extent that we don't have rockets that can carry the fuel). Carrying lifeforms would only exacerbate that problem.
For long distance missions we even use gravity assists in order to save fuel. That means not only the Earth and the target need to be in the correct places, but also all gravity assist planets.
See this image for the gravity assists for the Voyager missions:

(Click for the whole article. It is interesting, especially in case you want to write more a story based on it.).
Each bend in a trajectory (except the spiral path due to solar orbit) means that it had some kind of power propelling it. If it's near a planet that means that it's a gravity assist. Voyager 1 had 1 gravity assist and one "accidental" one and Voyager 2 had 3 of them.
This was not done because gravity assists are easy or fun. Yes they are fun but mostly they are done because it saves massive amounts of fuel, due to the rocket equation.
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Izawwlgood
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### Re: fictional planetary system

I love the idea of life arising on two moons, and there being a period of time where they are aware of one another (city lights, telescopes) but still incapable of reaching one another. Fairly improbable on the scale of evolution, but then, not wholly impossible.

Unfortunately, a planetary event that would have transported life between the two moons, helping along the pace of development, would probably destroy anything more complex than a spore. It's no banana raft to Madagascar.
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plytho
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### Re: fictional planetary system

What I'm thinking of is a system where one planet has intelligent life and other planets and moons have non-intelligent life. The people of the original planet would only be able to travel to/contact other planets when they are within some sort of 'jump distance'. This way the cultures on the newly colonized planets would develop independent from each other. The planets in smaller orbits would be able to interact more frequently while the outer planets would only be in contact once every decade or century.

To make such a system I need to figure out how those two constraints are possible. (Preferably without heavily influencing other aspects of the system.)

Neil_Boekend wrote:How can they know of the other planets' life if communication is not possible? If they can see they can communicate with laser beams. Initially there will be much trouble due to language differences, but with enough time those can be ironed out.

Perhaps they haven't figured lasers out yet and their radio is drowned out by cosmic radiation over larger distances?

Neil_Boekend wrote:Travel over large distances is difficult. We send interplanetary missions at the opportune moment because else the fuel cost multiply massively (to the extent that we don't have rockets that can carry the fuel). Carrying lifeforms would only exacerbate that problem.
For long distance missions we even use gravity assists in order to save fuel. That means not only the Earth and the target need to be in the correct places, but also all gravity assist planets.
See this image for the gravity assists for the Voyager missions:

(Click for the whole article. It is interesting, especially in case you want to write more a story based on it.).
Each bend in a trajectory (except the spiral path due to solar orbit) means that it had some kind of power propelling it. If it's near a planet that means that it's a gravity assist. Voyager 1 had 1 gravity assist and one "accidental" one and Voyager 2 had 3 of them.
This was not done because gravity assists are easy or fun. Yes they are fun but mostly they are done because it saves massive amounts of fuel, due to the rocket equation.

I'll definitely check out the full article to figure out if that might help with the travel constraint. Although at first sight it seems a though the outer planets would be 'too reachable' for the setup.
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PM 2Ring
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### Re: fictional planetary system

plytho wrote:I'm trying to imagine a fictional planetary system with multiple life bearing planets and moons. Due to some constraints travel and communication between these planets would only be possible when the planets are closest to each other.

[...]

What I'm thinking of is a system where one planet has intelligent life and other planets and moons have non-intelligent life. The people of the original planet would only be able to travel to/contact other planets when they are within some sort of 'jump distance'. This way the cultures on the newly colonized planets would develop independent from each other. The planets in smaller orbits would be able to interact more frequently while the outer planets would only be in contact once every decade or century.

Communication

One way to limit communication by radio would be to place your solar system near the path of an astrophysical jet emitted by a galactic black hole. The jet itself will create radio noise, and so will the interaction between the jet and the solar wind of your solar system. OTOH, I guess the gamma rays associated with such jets may be a bit too much to allow life to develop in your system.

Travel

Are these people traveling using rockets, or are they using some form of teleportation, as the phrase "jump distance" implies?

If they're using rockets, then the most natural restriction on interplanetary travel is the fuel required (as Neil mentioned). Rockets move by the reaction of throwing stuff (reaction mass) out the back of the rocket. The thrust developed is equal to the amount of mass ejected per unit time times the speed with which it's ejected. In theory, you could eject any kind of mass if you can eject it at an adequate speed. In practice, the easiest way to achieve this is to make your fuel highly explosive and to direct the explosion products in the right direction. But in any case, a rocket needs to carry a substantial amount of reaction mass to allow it to accelerate (and decelerate).

Lifting fuel out of a gravity well isn't easy. At our current level of space technology, most of the fuel used by an interplanetary rocket is the fuel used in just getting above (most of) the Earth's atmosphere. So these rockets need to use very low-energy trajectories to get to other planets. Fortunately, you don't need a lot of fuel to get from one planet's orbit to another, but the process is time consuming. The simplest way to do that is known as the Hohmann transfer orbit - you burn some fuel at your starting orbit, coast to your destination orbit on an elliptical trajectory without burning any fuel, and then when you arrive you burn more fuel to match your rocket's orbit with that of your target planet. You also need a little more fuel at each end to break out the orbit of your starting planet and to enter an orbit around your destination planet.

A Hohmann trajectory is (half of) an ellipse, with the starting planet at one end of the long axis of the ellipse and the destination planet at the other end of the long axis. Note that the sun is on that long axis. You can't simply take off from planet A & travel to planet B around the time of their closest approach - that would consume vastly more fuel.

This diagram from the University of Arizona’s Lunar and Planetary Laboratory may be helpful.
Spoiler:

Note that the alignment of the two planets is fairly critical: you have to set off when the angle between the planets is correct (the launch window) or you won't be near your destination planet when your trajectory touches its orbit. If the transfers are between perfectly circular orbits the launch window repeats with a simple period. Elliptical orbits make things a little more complicated: different launch windows have somewhat different transit times and fuel requirements, and the launch windows don't occur with a simple repeating period.

Below are some tables that show the time taken for various Hohmann transfers between planets in a simple solar system of 10 planets where each planet is in a perfectly circular orbit and each planet has double the orbital radius of the previous planet.The tables list the orbital radius of each planet, its orbital period (IOW, the length of its year), the transit time for a Hohmann transfer (IOW, half the period of the Hohmann ellipse) and how often the launch window occurs. The fourth planet of the system has been chosen as the reference planet - its orbital radius is 1 standard distance unit, and its orbital period is the standard year for the system; all time data in these tables is given in terms of that year.

The 1st table lists data for journeys between the innermost planet at radius 0.125 & the rest of the system. The 2nd table gives data for journeys to or from the reference planet at radius 1. Etc.

Hohmann Transfer Data

Code: Select all

Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt
0.1250   0.0442 -------- -------- -------- -------- --------
0.2500   0.1250   0.0406   0.0684   0.0547   0.0918   0.1464
0.5000   0.3536   0.0873   0.0505   0.0937   0.2599   0.3536
1.0000   1.0000   0.2109   0.0462   0.1179   0.5286   0.6464
2.0000   2.8284   0.5476   0.0449   0.1315   0.9291   1.0607
4.0000   8.0000   1.4810   0.0444   0.1388   1.5076   1.6464
8.0000  22.6274   4.0941   0.0443   0.1426   2.3323   2.4749
16.0000  64.0000  11.4465   0.0442   0.1445   3.5019   3.6464
32.0000 181.0193  32.1877   0.0442   0.1455   5.1578   5.3033
64.0000 512.0000  90.7750   0.0442   0.1460   7.5005   7.6464

Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt
0.1250   0.0442   0.2109   0.0462  -0.5286  -0.1179  -0.6464
0.2500   0.1250   0.2471   0.1429  -0.3675  -0.1325  -0.5000
0.5000   0.3536   0.3248   0.5469  -0.1835  -0.1094  -0.2929
1.0000   1.0000 -------- -------- -------- -------- --------
2.0000   2.8284   0.9186   1.5469   0.1547   0.2595   0.4142
4.0000   8.0000   1.9764   1.1429   0.2649   0.7351   1.0000
8.0000  22.6274   4.7730   1.0462   0.3333   1.4951   1.8284
16.0000  64.0000  12.3908   1.0159   0.3720   2.6280   3.0000
32.0000 181.0193  33.5117   1.0056   0.3926   4.2642   4.6569
64.0000 512.0000  92.6393   1.0020   0.4033   6.5967   7.0000

Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt
0.1250   0.0442   1.4810   0.0444  -1.5076  -0.1388  -1.6464
0.2500   0.1250   1.5488   0.1270  -1.3140  -0.1860  -1.5000
0.5000   0.3536   1.6875   0.3699  -1.0572  -0.2357  -1.2929
1.0000   1.0000   1.9764   1.1429  -0.7351  -0.2649  -1.0000
2.0000   2.8284   2.5981   4.3753  -0.3670  -0.2188  -0.5858
4.0000   8.0000 -------- -------- -------- -------- --------
8.0000  22.6274   7.3485  12.3753   0.3094   0.5190   0.8284
16.0000  64.0000  15.8114   9.1429   0.5298   1.4702   2.0000
32.0000 181.0193  38.1838   8.3699   0.6667   2.9902   3.6569
64.0000 512.0000  99.1262   8.1270   0.7440   5.2560   6.0000

Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt
0.1250   0.0442  11.4465   0.0442  -3.5019  -0.1445  -3.6464
0.2500   0.1250  11.5799   0.1252  -3.2984  -0.2016  -3.5000
0.5000   0.3536  11.8482   0.3555  -3.0153  -0.2776  -3.2929
1.0000   1.0000  12.3908   1.0159  -2.6280  -0.3720  -3.0000
2.0000   2.8284  13.5000   2.9592  -2.1144  -0.4714  -2.5858
4.0000   8.0000  15.8114   9.1429  -1.4702  -0.5298  -2.0000
8.0000  22.6274  20.7846  35.0028  -0.7340  -0.4376  -1.1716
16.0000  64.0000 -------- -------- -------- -------- --------
32.0000 181.0193  58.7878  99.0028   0.6188   1.0381   1.6569
64.0000 512.0000 126.4911  73.1429   1.0596   2.9404   4.0000

Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt
0.1250   0.0442  90.7750   0.0442  -7.5005  -0.1460  -7.6464
0.2500   0.1250  91.0405   0.1250  -7.2943  -0.2057  -7.5000
0.5000   0.3536  91.5724   0.3538  -7.0039  -0.2890  -7.2929
1.0000   1.0000  92.6393   1.0020  -6.5967  -0.4033  -7.0000
2.0000   2.8284  94.7853   2.8441  -6.0305  -0.5553  -6.5858
4.0000   8.0000  99.1262   8.1270  -5.2560  -0.7440  -6.0000
8.0000  22.6274 108.0000  23.6737  -4.2288  -0.9428  -5.1716
16.0000  64.0000 126.4911  73.1429  -2.9404  -1.0596  -4.0000
32.0000 181.0193 166.2769 280.0221  -1.4680  -0.8751  -2.3431
64.0000 512.0000 -------- -------- -------- -------- --------

As you can see, travel in the inner system is fairly straightforward: transit times aren't too bad and launch windows are reasonably frequent. However, journeys between the outer planets take ages and the launch windows are rare.

It is possible to achieve cheaper orbits via gravitational slingshots, etc, especially if your system contains one or more gas giants. OTOH, the launch windows are significantly rarer. See Interplanetary Transport Network for details.

FWIW, here's the Python 2 code I used to generate those tables.

Code: Select all

#!/usr/bin/env python

''' Hohmann orbit data for simple circular orbits

Keplers 3rd law, in natural units
R^3 = T^2

R_H = (R_0 + R_1) / 2

Synodic period (how often the launch window occurs)
1/P_S = 1/P_1 - 1/P_2

delta v on depature
delta_v_0 = sqrt(mu/R_0) * (sqrt(R_1 / R_H) - 1)

delta v on arrival
delta_v_1 = sqrt(mu/R_1) * (1 - sqrt(R_0 / R_H))

See http://forums.xkcd.com/viewtopic.php?f=59&t=111861

Written by PM 2Ring 2015.05.23
'''

return r ** 1.5

def delta_v
(r0, r1, rh):
return r0 ** 0.5 * ((r1 / rh) ** 0.5 - 1)

rads = [2.0**i for i in range(-3, 7)]

def hohmann_table(rp0):
r0, p0 = rp0
print
print
'  Radius   Period  Transit   Window  deltaV0  deltaV1  deltaVt'
fmt = '%8.4f '
fmt_short = 2 * fmt + 5 * (* '-' + ' ')
fmt_long = 7 * fmt
for r1
, p1 in rp:
if r0 == r1:
print fmt_short % (r1, p1)
continue

rh
= (r0 + r1) / 2
transit_time
synodic
= abs(1.0 / (1.0/p0 - 1.0/p1))

dv0 = delta_v(r0, r1, rh)
dv1 = -delta_v(r1, r0, rh)
dvt = dv0 + dv1

print fmt_long
% (r1, p1, transit_time, synodic, dv0, dv1, dvt)

for i in
(0, 3, 5, 7, 9):
hohmann_table(rp[i])

Update

I decided to modify those tables to include figures for delta-v, to give a rough idea of the fuel requirements for the various Hohmann transfer orbits. At the start of a Hohmann trajectory your rocket's orbit around the sun matches the orbit of the starting planet. You do a burn to change the rocket's solar orbit to the Hohmann ellipse, and when you reach the destination you do another burn so that the rocket's solar orbit matches that of the target planet. deltaV0 is the speed change required at the start of the Hohmann trajectory, deltaV1 is the speed change required at the end of the Hohmann trajectory, deltaVt is the total speed change. These speed changes are given in arbitrary units. They are not exactly proportional to the fuel required, since they don't take into account the changes in the rocket's mass - obviously it gets easier to accelerate a rocket as the mass of fuel it's carrying decreases.

CBusAlex
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### Re: fictional planetary system

If you have two gas giants, each with a tidally locked moon, and have only the planet facing hemisphere of the moon be habitable, you could probably set up the orbits such that there is never (or almost never) a direct line of sight between the habitable parts of each moon. This would explain the lack of radio/laser communication.

gmalivuk
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### Re: fictional planetary system

If the moon is close enough that the gas giant blocks a significant portion of the sky, it will orbit quickly enough that every point on the near side gets a pretty complete view of the sky every day or so. If it's far enough to orbit slowly, the gas giant only blocks a small portion of the sky.
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### Re: fictional planetary system

Izawwlgood wrote:I love the idea of life arising on two moons, and there being a period of time where they are aware of one another (city lights, telescopes) but still incapable of reaching one another. Fairly improbable on the scale of evolution, but then, not wholly impossible.

Unfortunately, a planetary event that would have transported life between the two moons, helping along the pace of development, would probably destroy anything more complex than a spore. It's no banana raft to Madagascar.

A mutual seeding event from a third locale should get you there. Assuming similar rates of development, such a period seems at least reasonably possible.

plytho
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### Re: fictional planetary system

Tyndmyr wrote:A mutual seeding event from a third locale should get you there. Assuming similar rates of development, such a period seems at least reasonably possible.

That's kind of what I had in mind. One ancient life bearing planet being smashed to smithereens, spreading the seeds of life to a bunch of planets in the habitable zone.

Thanks PM 2Ring for the Hohmann and delta-v calculations, that stuff is awesome.
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CBusAlex
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### Re: fictional planetary system

gmalivuk wrote:If the moon is close enough that the gas giant blocks a significant portion of the sky, it will orbit quickly enough that every point on the near side gets a pretty complete view of the sky every day or so.

Sure, but if Moon A gets its view of the sky containing Moon B at a time of day when Moon B is behind its planet (or really in any part of its orbit where the habitable zone isn't facing Moon A), and their orbits were of the right length that this happened every day*, then they would have no way to signal each other.

*I don't know if this is possible, and don't even know where to start doing the math for it, but conceptually it seems like if the moon around the outermost planet completes exactly one more orbit per year more than the moon around the innermost planet, then both moons should see each other in the same place relative to their planets each time they look.

Nicias
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### Re: fictional planetary system

What if the moons orbits were eccentric enough to take them in and out of their planet's various radiation belts? This might make enough radio noise that radio communication between them was only possible when they were both near apoapis.

Of course real large moons of large planets have very small eccentricty. The large moon with the largest eccentricty is ours (0.05). The Gallilean satelites are all less than 0.01 or so. Titan is 0.03.

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### Re: fictional planetary system

CBusAlex wrote:
gmalivuk wrote:If the moon is close enough that the gas giant blocks a significant portion of the sky, it will orbit quickly enough that every point on the near side gets a pretty complete view of the sky every day or so.

Sure, but if Moon A gets its view of the sky containing Moon B at a time of day when Moon B is behind its planet (or really in any part of its orbit where the habitable zone isn't facing Moon A), and their orbits were of the right length that this happened every day*, then they would have no way to signal each other.

My point is that Moon B wouldn't be behind its planet long enough for it to be there the entire time Moon A gets a view of Moon B's gas giant. Either it's close enough to spend a large fraction of its orbit behind the planet, in which case it doesn't stay there for very long, or it's far enough away that it orbits very slowly, in which case only a small fraction of its orbit is actually obscured by the gas giant.
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Tyndmyr
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### Re: fictional planetary system

plytho wrote:
Tyndmyr wrote:A mutual seeding event from a third locale should get you there. Assuming similar rates of development, such a period seems at least reasonably possible.

That's kind of what I had in mind. One ancient life bearing planet being smashed to smithereens, spreading the seeds of life to a bunch of planets in the habitable zone.

Thanks PM 2Ring for the Hohmann and delta-v calculations, that stuff is awesome.

Also a decent justification for rings, which, if the two planets are on opposite sides of them, could be fun as an added complication/looking cool.