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I read a blog post explaining why interstellar travel is terribly impractical. The short answer is that stars are far away and travelling that far takes a very long time and ridiculous amounts of energy.

Anyway as a part of explaining the distances the blog gave examples of distances and one of them was the Oort Cloud where certain comets come from. Which extended to nearly halfway there. This reminded me that not only do these objects range far in-system on their weird orbits, they range far enough out that they could feasibly swap stars, and they are full of volatiles that can be used for life support or propellant.

So maybe you could rendezvous with a comet in the Inner System, use it as a place to live and propellant, rendezvous with another comet with a different trajectory, and so on, all the way to a neighbouring star system. Being able to resupply both propellant and volatiles such as water and oxygen en route should make interstellar travel much cheaper. And for most of the transit you would be passively travelling on a comet using it as protection from radiation, which should improve safety and comfort.

This would still require practical ship scale fusion power and probably "Cure for Cancer" level of medical technology to deal with long term issues of zero gravity and cosmic radiation. But both of these are AFAIK considered possible at the moment.

But given these assumptions is it plausible to use comets and the objects in the Oort Cloud as aids to travel between systems? And how much would it help? Would it make interstellar travel practical? I am not going to ask for strict science since much of what is "known" about the Oort Cloud is uncertain. And the third question is probably impossible to answer beyond opinion, but if you have a rationale why or why not, please share.

There seems to be some confusion in what I am asking and what I was already assuming in the question. Yes, I know that the cloud objects are sparse and unlikely to have perfectly convenient trajectories, so both time and delta-V needed would go up by a large factor. The question is; would the ability to supply propellants for propulsion and volatiles for life support outweigh that for a net improvement. Is there even a way to know or give a good guess?

As per Wil Selwood's answer: You can presume that they have done the groundwork of mapping the objects in the Oort Cloud and planning the hops and schedule in advance. It helps a lot with the cloud being relatively sparse and considering the investment of what would have to be a generation ship and thus very large and expensive, they'd do all the advance work they can. You can also presume that after reaching a comet they will dig in and resupply, wait in cover until scheduled burn time, then detach from the body with full supplies, and only accelerate the ship, not ship and comet.

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  • $\begingroup$ Using comets would be a handy source of material, but using a normal cometary orbit would still take tens of thousands of years to fall into the inner system. $\endgroup$ – JDługosz Jun 23 '15 at 14:18
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    $\begingroup$ @JDługosz Yes, the need to match speeds with the Oort cloud objects while in transit might put too much of a limit on the velocity. So this might be more of a way for a civilization with very good space habitat technology to incidentally spread from system to system than a practical mode of planned travel. $\endgroup$ – Ville Niemi Jun 23 '15 at 14:32
  • $\begingroup$ Yes, as discussed by Asimov etc. See this Q of mine. $\endgroup$ – JDługosz Jun 23 '15 at 14:39
  • $\begingroup$ @JDługosz That is a story I have not read. Technology seems different, but it would lead to a similar civilization and spread pattern, I think. $\endgroup$ – Ville Niemi Jun 23 '15 at 14:41
  • $\begingroup$ The idea is that the generation ship is not a goal in itself. The mobile worldlet is home, and it's easy to cast off the parent star once living that way. $\endgroup$ – JDługosz Jun 23 '15 at 14:54
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This would actually be a variation on the "generation ship" meme. You could "hop aboard" a comet and drift into the Oort cloud, but the speed of the comet will be so slow and the distances so vast that it might be generations before the crew approaches a "fresh" comet moving roughly the direction they want to go. Rinse and repeat for hundreds of generations, and in the end, you will be on a comet that is very tenuously attached to the solar gravity well, and with a small push, the comet can start drifting towards the next Oort cloud around Alpha Centauri. Generations later, you reach that gravity well and start drifting inwards, using their comets as stepping stones to the new sun.

Living aboard a comet would not be too bad. The comet can be hollowed out and the mass of water and volatiles that make up the bulk of the comet would be good shielding against radiation and even the occasional impact. There is probably enough D2 in the average comet to supply a small space city for a thousand years of fusion energy, so there is plenty of time to look for a new home. And the comet probably has sufficient stocks of the other elements to live a reasonably comfortable lifestyle, so long as you are not addicted to large amounts of metals. It seems reasonable to expect to find carbon compounds, nitrogen and even silicon in some quantities in a comet nucleus.

If you are impatient, it might be possible to bury a space colony in a comet and use the materials not only to run the ecosystem but also power a fusion engine to get enough deltaV to exit the Solar System and fly to Alpha Centauri directly. Once there, rendezvous with a new comet to replenish your stocks and start exploring. This cuts the time down to a more reasonable few centuries, rather than thousands of years.

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    $\begingroup$ Yes, probably should have mentioned outright that it would need to be a generation ship. And yes, propellant from the comets would help. But this is more an elaboration of the assumptions already in my question than an answer. +1 anyway since you explained it better than I did... $\endgroup$ – Ville Niemi Jun 23 '15 at 3:45
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    $\begingroup$ D2? If you mean deuterium, that's 2H (or possibly D). $\endgroup$ – a CVn Jun 23 '15 at 12:40
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I think this scheme mixes distance and delta-V. No matter how close two comets pass to each other, you don't just hop from one to the other. Instead your ferry has to match velocites. Also, space is empty. You can't expect to find a convenient comet going in the right direction.

The sum of velocity changes as you hop from comet to comet will be much more than the velocity change for an interstellar voyage. Is the convenience of living in a comet worth it?

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  • $\begingroup$ Yes, delta-V will go up enormously, the question is will it go up more than your propellant stores will. (So no I did not mix up distance and delta-V. I just was more concerned with the "tyranny of the rocket equation".) And yes, the "convenience" might be worth it. Or not. Having ready supply of volatiles and protection from radiation would make space more liveable. The question again is would the improvement exceed the increase in the time needed. $\endgroup$ – Ville Niemi Jun 23 '15 at 10:33
  • $\begingroup$ I guess when everybody on earth would stop going around by car senselessly the whole energy could get pumped into your comet engine and Scotty would let 'em speed out o' the system in no time! $\endgroup$ – kraftb Jun 23 '15 at 19:23
  • $\begingroup$ This should be marked as the correct answer. $\endgroup$ – Renan Oct 29 '18 at 12:58
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    $\begingroup$ Well, the apoapsis/aphelion will also be the point with the lowest velocity, so the minimum delta-V to "hop off" the comet at the far end. But, the velocity needed to "hop on" in the first place will be enough to you the same distance - the real question becomes "can you mine enough fuel/reaction mass from the comet in the meantime for it to be worthwhile?" $\endgroup$ – Chronocidal Nov 1 '18 at 11:32
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Hitching a lift on a comet is an interesting idea. It gives you a lot of fuel. However there are some pretty big downsides.

First Delta-V: Getting to a comet is hard, but doable, Rosetta proves we can do it. Getting out of the solar system is harder but also do-able see the Voyager probes, the trick here will be finding a comet with an orbit that's easy to modify to take you where you want to go. You need to find a reasonably eccentric comet with the argument of periapsis pointing at the star you want to head towards. You can then use the comet as fuel to make a big burn when near periapsis to push the apoapsis out until you break orbit around the sun and "whizz" off to your destination.

Second fuel: While you will have a reasonably large amount of it the rocket equation is a problem. Because you have so much fuel you need a lot of it to move it. The other problem is the fuel you have is going to be very low quality. It will be all mixed up, hydrocarbons, water, carbon dioxide, hydrogen, and dust. It will need processing very carefully before feeding into your engines. You will probably have more than one engine, each designed to burn/expel a different type of fuel. Making the big burn at periapsis is useful here because you can make use of solar energy as you make this burn.

A better option may be to link up with a comet in your generation ship process the comet into fuel. Then when you reach the right point of the orbit detach and make the burn. This way you are only trying to accelerate the mass of the ship not the ship and a dirty great comet.

Linking up to another comet when you arrive would done as a "if there happens to be a comet heading in the right direction" but should not be required as it would be impossible to know if such a comet existed in the target system with out already being there, getting the two orbits to sync up correctly would be extremely unlikely.

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  • $\begingroup$ Yeah, I guess it would be necessary to map the objects in the Oort cloud first. Good point with being better to accelerate just the ship not the ship and the comet, I kind of understood it, but never really thought it thru. I guess that is the point of asking questions? Thanks. $\endgroup$ – Ville Niemi Jun 23 '15 at 11:28
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Comet hopping really isn't as practical as it may sound. Comets are just ice balls, and they're not that big, so making a spaceship around one wouldn't work. The only thing they'd be useful for is hydrogen (For a fusion reactor) or water, te former of which is easier found on most gas giants, the latter recycled pretty much indefinitely. Even the 'rocky' comets have ice as a foundation that heats and escapes as jet streams when nearer to the sun.

Another factor of comets is grappling to them; they move at considerable velocity (When compared to a craft) so docking with them requires utmost time (A good launch schedule) and luck, enough to mean that getting to the exact place you need is more or less throwing a dart at a fly on a wall the size of central park. It is incredibly difficult.

So, honestly, the amount of effort it would take to go to a comet isn't worth the risk and cost when a spaceship could allow its inhabitants to go cryo and simply drift to the nearest system.

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Option 1: Comet Hopping

This option assumes that you build a base on a comet that is already heading the right way, and so on, until you arrive in the destination system. Earth to comet, comet to Kuiper Belt Object, KBO to somewhere in the Oort cloud, where the Oort clouds mingle switch to another Oort object, then work your way inwards.

Advantages:

  • Relatively modest delta-V for each comet-to-comet leg.
  • If you time it right, the ferry spacecraft could make multiple round trips between the old and new base. That means you need "less ship per person."
  • Fresh raw materials several times during the voyage.

Disadvantages:

  • The base must be disassembled and reassembled many times.
  • How can you assure that there is a suitable object whenever you need it?
  • You are moving at the speed of an outer system object. The middle legs will take millenia or tens of millenia.

Option 2: Comet Starship

Go to a comet or KBO as it passes through the inner system, install an engine, use the volatiles from the object for propulsion.

Advantages:

  • If the base is all ice/slush, you can turn nearly 100% into fuel. No need to waste mass on fuel tanks.
  • No need to find extra stepping stones.

Disadvantages:

  • Is it really better to take a random comet and not "custom-freeze" your ice ship with just the right mix of raw materials?
  • Is ice really a good building material?

Option 3: Mix and Match

Give the comet some engines/delta V to match orbits with the next comet.

  • Less dependence on "finding" a comet at the right time and place.
  • Combines the probems of both options.
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I'll try for a simplified form of o.m.'s answer:

To fetch a ride on a comet you have to match its velocity, so you gain no momentum from it. Now if you could convert a comet like the 67P/Churyumov–Gerasimenko into fuel, you would be able to obtain just as much fuel from Earth, by doing a series of orbital fueling maneuvers. This would greatly simplify and cheapen the process.

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  • $\begingroup$ I think you simplified too much. I can't understand what you mean. $\endgroup$ – Ville Niemi Nov 1 '18 at 7:52
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Summarizing the question: no one is doing any "hopping"

This is a cool idea, and a workable one, but I think the word "hop" in some answers is giving the wrong impression. When a spacecraft rendezvouses (<- weird derived form but apparently correct) with a celestial object it doesn't "hop" or "grab" on. It is more accurate to say it matches the orbit of the target object. Thus, the gain in this scenario is not some boost of speed, but simply the perk of not having to accelerate your fuel and consumables. The tradeoff is that the comet is almost certainly not on the most efficient trajectory for your needs, so you take a less efficient path in exchange for lessening the tyranny of the rocket equation.

So in a real-world scenario one would optimize this by gathering information such as:

  1. Target star
  2. Maximum transfer time (limited by consumables, or maybe just impatience)
  3. Maximum fuel (delta-V) carried by spacecraft alone
  4. Appropriate target comets (and when they will visit the inner solar system)

However number 4 is irrelevant in a narrative where the appropriate comet can happen to be visiting the inner solar system at the right time. If the trajectory is relatively close to how one might exit the solar system anyway, then it's pretty reasonable to assume time or resources are saved by not having to accelerate all of the consumables.

You have to do some work yourself

I get what OP is saying about leaving your chosen comet at aphelion and finding another comet at aphelion from a different star, but even in a narrative this strains believability (even if the oort cloud is near the outer reaches of some other solar system, space is incredibly sparse). The most likely scenario for interstellar travel using comets is simply accompanying a convenient one as far as you can while refueling/resupplying and then escaping the solar system by yourself using all of that fusion fuel.

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Lasers

Instead of using conventional fuel and stopping at the various comets, you can use laser sails.

https://en.wikipedia.org/wiki/Laser_propulsion

Your ships would be pushed by solar powered lasers near the sun. When further away where the sun is weaker, fusion powered lasers on the icy waystations would be used to push the ship even faster. A similar mechanism would slow the ship down at the other end.

https://youtu.be/oDR4AHYRmlk

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