Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chiralSPO on 11/08/2013 03:20:13
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Imagine we could cover the surface of a comet with photovoltaics, use sunlight to electrolyze the water in the comet, and store the resulting hydrogen and oxygen. I know insolation at perihelion would have pretty high flux, but I don't know the right constants and equations to use to calculate how much energy it would actually be (even assuming only 10% efficiency in energy storage). I don't know how many orbits it would take to store enough energy.
If we had an unlimited amount of time for electrolysis, and could split all the water in the comet, we could store enormous amounts of energy (up to 15 megajoules per kilogram of fuel).
Combining the kinetic energy of the comet, which is already enough to get it pretty far out from the sun (if the orbit has high eccentricity), and the energy released from the chemical fuel, could the entire comet escape from the solar system? Could it escape with a lot of kinetic energy left over?
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If we had an unlimited amount of time for electrolysis, and could split all the water in the comet, we could store enormous amounts of energy (up to 15 megajoules per kilogram of fuel).
Combining the kinetic energy of the comet, which is already enough to get it pretty far out from the sun (if the orbit has high eccentricity), and the energy released from the chemical fuel, could the entire comet escape from the solar system? Could it escape with a lot of kinetic energy left over?
I don’t quite understand the utility of doing something like this. It’s so much simpler and cost effective to do this on earth with water in the ocean or lakes and use photocell farms. What you haven’t taken into account is the means of transporting the equipment to the orbit and retrieving the energy in a stored and retrievable form, e.g. in batteries. Launching enough batteries into such an orbit and then retrieving them would cost millions and millions and millions of dollars and it wouldn’t be cost effective.
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If one was actually trying to make an interstellar spacecraft, then there would certainly be advantages of grabbing a comet as it would undoubtedly have many resources including water that would help support the crew for a very long journey. It would be impractical to try to build a 10 mile in diameter solid rock and ice spacecraft on Earth, then accelerate it into space.
It would be difficult to take a short-period comet like Haley's Comet (75 year orbit, Aphelion, 35 AU, 0.00055 LY), and accelerate it to escape velocity. Likely it would take several orbits, and thus several centuries of gradual increasing acceleration.
A better approach would be to use a long-period comet, still it might take more than one pass around the sun.
Some comets may, in fact, be hyperbolic comets or interstellar comets, that have a single pass through our solar system, before passing on elsewhere. It would still be a long trip, but if one could detect a hyperbolic comet early enough, it would be the easiest comet to hitchhike on to get out of the solar system. Directing the comet to the desired destination might be difficult, but it is easier to make a few degree alteration in course than to change the speed of an object, especially if one can utilize gravitational interactions.
There is discussion about comets being ejected from the solar system with certain planetary interactions. If that is possible, then perhaps the goal should be pushing the comet into a close interaction with one of the large outer planets.
As far as collecting energy from the sun, say one might be lucky to get 1 year worth of decent power generation out of say a thousand year orbit. The comet is least stable when close to the sun, but presumably one could be equally efficacious of using one's ion/plasma engine near the sun, rather than storing the energy for a long time.
Nonetheless, a secondary power source would be very useful during the period away from our sun. Most fusion energy experiments are being done with rare elements such as 3He, lithium, tritium, and etc. But, perhaps in the future we will have the technology to fuse ordinary single proton hydrogen (1H), thus gaining a significant amount of energy from the water on the comet, independent of being near the sun.
How fast would it be going? Assuming a hyperbolic comet, it would all depend on how fast it was traveling around the sun, but otherwise, the comet would likely drop down to very close to zero velocity when it reaches about 2 lightyears from our sun, and may need a bit of a nudge in the right direction.
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There aren't any coments with that large of an orbit that I'm aware of. I'm not even sure that it's possible. But taking any bound orbit and making it unbound to the point where a trip is timely would require far too much energy to be practical.
Otherwise it'd take many thousands of years to travel to another star system.
And there's no need of it taking a comet with you for water. A closed system is infinitely more practicle since water is conserved in such a system. After all, that's what Earth is. Yup. In every ounce of H2O you take in some of it was once pee!
Water can be recycled. Not very pleasant to think about but water is water no matter where it came from.
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Voyager 1 is supposed to pass by Gliese 445 (within about 1.6 light years) in about 40,000 years.
Capturing a hyperbolic comet would no doubt be a long way to travel. However, even with the fastest solar sail, or plasma/ion engine, it would still be a long trip to reach another star.
Imagine a hundred generations of astronauts living on the International Space Station for a period longer than the period from the the copper/bronze age to today. Everything would be recycled.
Yes, water and other resources would be recycled, unless it is needed for fusion or as a propellant. However, on a "generation ship", there would still be advantages of bringing raw materials with oneself, along with having some moderate manufacturing facilities. Plus, assuming the comet could be stabilized when away from the sun, having a large rock in front of one's space ship may offer some impact protection. Over thousands of years, one might be able to mine the comet/asteroid and build a larger space ship.
Plus, after a hundred generations on a small cramped ship, would one expect the astronauts to terraform a planet without any real life experience building anything?
There is some debate on the difference between long period comets and hyperbolic comets, but there are apparently about 67 proposed hyperbolic comets that are destined for a single pass through our solar system. Gravity assist (http://en.wikipedia.org/wiki/Gravity_assist) from planetary flybys might throw a long period comet into having escape velocity. See Voyager 2's velocity chart. It would not have made it out of the solar system without gravity assist. And, it may be easier to alter the direction of the comet to pass near Jupiter and other planets than it would be to double the velocity using rockets.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2Fthumb%2F2%2F2c%2FVoyager_2_velocity_vs_distance_from_sun.svg%2F500px-Voyager_2_velocity_vs_distance_from_sun.svg.png&hash=fd14724f066e0c487a380498d8270b56)
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Thank you Clifford. Yours comments are very insightful and informative!
You are certainly right that everything would have to be recycled, and I'll take this opportunity to say that the same is true of our own "spaceship earth." We MUST recycle everything material. In a more-or-less closed system (except for energy coming from the sun), eventually 100% of products will need to be 100% post-consumer. I know this is not an original philosophical point, but I wish it were more actively discussed these days...
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Quite true, we are parasites those days, not contributors.
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Hi there, how fantastic!
I just searched specifically for "Hijacking a Comet" and found this thread.
I have a question because i am the hijacking of a comet for interstellar travel is the centerpiece for a steampunk story i am writing at this moment (i love synchronicity)
Is it viable to use the ice within the comet to use with a nuclear reactor and create jets of steam to guide the comet, assuming that the crew has achieved a state of practical immortality.
Or is there a far more efficient and obvious way of achieving this?
Thanks, if anyone wants to help me fill out the ideas for this story, pm me and we can do it by email or whatever, i'll put your name in lights and send you a copy for your troubles O8)
Cheers, Jason