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Your large spaceship is damaged beyond repair. You manage to get it close enough to a planet so that you can release emergency shuttle craft. You have very little control over the ship and won't be able control it much longer.

The story demands that the ruined ship still be in orbit or close by (but not on planet) for millenia. Where do you leave the ship so that it doesn't:

  • rain down on you while re-entering the atmosphere in the shuttle craft
  • crash land onto the planet over the following years (I don't mind a few pieces making arkfa...landfall but the ship should preferably stay in space)
  • slingshot itself into outer space
  • coast on, into the sun ala BSG style.

The planet is:

  • slightly smaller than Earth
  • has a planetary rings
  • two small shephard moons chasing each other through the rings
  • a small moon within its Roche limit
  • either one or two larger moons

Where is the best place to park it for the foreseeable longterm before a towtruck can be arranged and how long could it realistically stay in this sort of position without constant maneuvers before orbital mechanics pick it up and take it to the impound yard? Note: I don't actually intend to tow it anywhere at anytime.

Is crashing it into one of the larger moons the only option or is there a way to possibly keep this ruined hulking artifact hanging forlornly in space.

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    $\begingroup$ A moon within the roche limit won't stay a moon for long. That's kind of the definition of the roche limit. $\endgroup$ – ths Mar 13 '17 at 23:31
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    $\begingroup$ @ths, yip. exactly! that's why I specified it was within the Roche limit. so that no one suggested we crash land the ship on IT. $\endgroup$ – EveryBitHelps Mar 13 '17 at 23:35
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    $\begingroup$ Just a note, but sundiving requires absolutely absurd amounts of delta-V. In a lot of cases, it's easier to go outward in a solar system than inward. From my brief looks into the subject, it's about 30 km/s Delta-V to get to the Sun from Low Earth Orbit. Getting to Pluto is a little under 12 km/s, and reaching Solar escape velocity is a little over 12 km/s $\endgroup$ – Andon Mar 14 '17 at 0:52
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    $\begingroup$ A moon can last indefinitely inside the roche limit provided its tensile strength exceeds the tidal forces it experiences at that altitude. What it can't do is form there through accretion. $\endgroup$ – papidave Mar 14 '17 at 19:18
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    $\begingroup$ Leave it in a hanger in Zarniwoop's electronically synthesized universe. Just don't deliver any lemon soaked napkins there and it will be quite content to sit and wait for you. $\endgroup$ – Toby Mar 15 '17 at 12:12

11 Answers 11

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The orbits of larger moons should be stable and mostly clear of hazards. I'd recommend placing your ship at one of their Trojan points. Just pick the one most convenient. Closer to planet, not already occupied by something, already occupied something that is useful...

The actual mechanics can get more complex, but I see nothing in your question suggesting you need to bother with more complex solutions.

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    $\begingroup$ The points are properly called Lagrangian points. Trojans are objects that are chilling out at a Lagrangian point. It should be noted that these orbits are not stable. Those Trojans will eventually be ejected from the lagrange point, but other asteroids will come to replace them. $\endgroup$ – BobTheAverage Mar 14 '17 at 0:06
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    $\begingroup$ @MarcusYoder Yes, I know, that is what the last paragraph refers to. Points where the Trojans are can be called Trojan points, IMHO. And are. It is actually more accurate than lagragian points as not all lagrangian points are trojan points. $\endgroup$ – Ville Niemi Mar 14 '17 at 0:27
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    $\begingroup$ Trojan points tend to collect pieces of rock, so they might not be the best choice, if you'd like to retrieve your vessel in one piece. $\endgroup$ – berendi - protesting Mar 14 '17 at 10:42
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    $\begingroup$ @MarcusYoder Actually... apparently Trojans and Greeks are stable. It's other 3 Lagrange points that are unstable, not Trojans and Greeks. en.wikipedia.org/wiki/Lissajous_orbit - L1, L2 and L3 are unstable, L4 and L5 are stable. $\endgroup$ – M i ech Mar 14 '17 at 17:54
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    $\begingroup$ +1 for L4 and L5 stability. If you put something there, it won't necessarily stay at that point, but will effectively "orbit" the Lagrange point itself. Jupiter is big, so it has a bunch of Trojans and Greeks at its L4 and L5 points. The earth-moon system could have something smaller, though, as long as its mass << the lunar mass. $\endgroup$ – papidave Mar 14 '17 at 19:21
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A simple extremely high orbit, such as the aptly named Graveyard Orbit - These orbits are really high and designed so that the satellites will be there for a very, very long time. An orbit should be able to be accomplished outside the ring system (And thus, outside major influence from the moons).

For a more permanent solution, depending on how small the moons are, you could land on one of them. HOWEVER, a spacecraft not designed to land will almost certainly be destroyed in the process.

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    $\begingroup$ not necessary destroyed - there is a body with almost 0 effective force on the surface. $\endgroup$ – MolbOrg Mar 14 '17 at 1:19
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The answer depends on what close means.

If your spacecraft can make a "modest effort at station keeping" park it in a halo orbit near the first or second Lagrange point between the planet and the sun. The Earth-Sun-L1 point is 0.01 AU from Earth. If your spacecraft cannot make any effort at station keeping, those orbits will eventually decay.

If one of the moons is far more massive than the rest, you could orbit that moon, or station-keep in the planet-moon-lagrange points.

You could simply orbit the planet very closely or farther away than its other bodies. If you are too far out, you would be orbiting the sun instead of the planet.

You could orbit the sun at a 90 degree angle to the elliptic and slightly closer or farther away than the planet. The pilot would come close to the planet, set the orbit, and then eject. The ship would have an orbital period slightly longer than the orbit of the planet. It would cross the elliptic twice a year. Each of these crosses would have a small chance of changing its orbit due to interaction with the planet.

You could orbit the sun at a 90 degree angle at the same distance as the planet. This would give you the same orbital period as the planet. You would have to make sure that your orbit crosses the elliptic when the planet is not around. If the pilot ejects at the right time, sufficiently far from the planet, the life boat can use the ship's momentum and make a smallish course correction to get to the planet. This solution involves chilling out in a life boat for a month or two.

You could get lucky. Space is big. The odds of actually crashing into anything in a given year are astronomically small. Eventually it will happen, but eventually might be a million years from now. If everything that could crash into something did, we wouldn't have any near Earth asteroids.

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    $\begingroup$ And, we might not have an earth. $\endgroup$ – Billy S Mar 14 '17 at 0:49
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If this is a story, putting the ship in orbit around the sun offers storytelling options. It would not be readily available anytime but you state you do not need it for millennia. Do you want to just look at it and think about it?

If not, a cool thing would be to put the ship in a huge comet-like orbit to keep it out of trouble with moons, rings etc. If you want to reclaim it, you would need to be ready for when it comes back. In this scenario you would have to time your intervention to retrieve it for when the ship's orbit brings it close to the planet.

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  • $\begingroup$ An interesting alternative scenario. +1 $\endgroup$ – EveryBitHelps Mar 14 '17 at 18:06
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    $\begingroup$ This is actually really good. A comet-like orbit around a planet would yield a period of a couple of months, and high inclination will keep it away from disturbing moons. $\endgroup$ – Joshua Mar 16 '17 at 2:24
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I'll assume that at least one of the moons has no residual atmosphere or orbital debris. Park your spaceship in orbit around that moon, say halfway down into that moon's gravity well, for stability.

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    $\begingroup$ Safe and correct assumption $\endgroup$ – EveryBitHelps Mar 14 '17 at 13:30
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    $\begingroup$ But not necessarily a safe place. Our Moon for instance is gravitationally extremely "lumpy", the low orbits are extremely unstable (on the order of weeks or months) and the high orbits are perturbed by the Earth and the Sun. It is uncertain whether you could put an inert object into any orbit around our Moon for a millennia. $\endgroup$ – tylisirn Mar 15 '17 at 15:02
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The best possible way is to put the spaceship on stable orbit around the planet, an orbit that would not get it close to any of the moons. Crash-landing on any of the moons or the planet itself is not a good option, because such a crash would be so violent it's very unlikely that salvaging the ship afterwards would be possible.

However, if that is only your space engines that are malfunctioning, but landing systems are somehow in good order, you can try to land it safely on the planet.

P.S. I would advise against "parking" spaceship in Lagrangian points. Trojans are generally less stable than normal planets and moons.

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Sure, park it in an orbit around the planet.

The planet is good enough to keep all that stuff flying around for probably millions of years, it will be good enough to home your millennium falcon.

5000 km above upper layers of atmosphere and it will stay there for a long long time.

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The Lagrange Points have been mentioned. L4 and L5 (leading and trailing the large moon) are in stable equilibrium. If something big enough comes along, the ship would be pulled from its position but there is a "pull" toward the center of that point that would correct any small tugs against the spacecraft.

Another method is to land/crash it on the moon. It won't be going anywhere then.

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    $\begingroup$ Landing it on a moon without an atmosphere unfortunately exposes it to even more micrometeroids than when you put it in an orbit. When you salvage the craft in a few years, you might have a lot of additional hull breaches to fix. $\endgroup$ – Philipp Mar 14 '17 at 12:25
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    $\begingroup$ @Philipp When you salvage the craft in a few years... well, the question says it will take more than a few years: The story demands that the ruined ship still be in orbit or close by (but not on planet) for millenia. Would it make much difference where you land it after milennia? (Honest question) $\endgroup$ – xDaizu Mar 14 '17 at 13:24
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Since they have the ability to select where to put the ship it clearly has some propulsion. A landing on one of the shepherd moons strikes me as possibility. They are small, it won't take much thrust to actually land there and even if it's a crash it won't do much damage.

A quick analysis will show they've been around for quite some time shaping the rings, thus landing on them is unlikely to result in unexpected forces tossing the ship away.

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The real difficulty in this type of situation is determining orbital stability quickly.

On Earth we have made observations for a long time and found most of the relevant orbiting bodies and measured their orbital parameters very precisely, and given some supercomputer time could simulate an orbit for thousands of years to determine likely stability.

If it is a star system that is not well charted or observed for any length of time you are going to have difficulty finding smaller orbiting bodies or any items on comet like long elliptical orbits. At best I would expect you could find major planets and moons maybe smaller items around the planet being orbited, but all of the orbital parameters will be very rough numbers extrapolated from the short time the ship has been in the system making observations. This is going to be especially problematic if your ship has suffered damage to sensors or computer systems or is moving through the system very quickly.

You could find a generally clear zone and set up what you thought was a long term stable orbit, but with the likely large amount of error in the measurements of the system, the probability of some long orbit comet, or asteroid/meteor shower, or simply the errors in orbital parameters adding up to major problems, goes up the longer you leave your ship in place unassisted.

In this situation, I would say landing on a sizable planet or moon would be your best bet for long term stability, so land on the large moon outside the ring system. Based on size of the moon you could bet that it has been there for a while and the larger the body the less likely it is to be perturbed by the error in your measurements of the systems orbits.

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Unfortunately, when the ship was parked, a splash down for a large craft with little or low gravity capability was the only answer. Time is now the enemy. Biometric memory ingrams though cloning only provides the key to open the lock. Not to find what you lost. By now there are lots of keys with different agendas for the outcome of this technology, but the race is on and yet no-one even knows it's there??

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