Ok, so let me see if I have this right. If I have 1 gallon of gas in my flying saucer and I want to go one light year, I know I can burn a 1/2 gallon accelerating because I need 1/2 gallon to decelerate. So the acceleration I get from half a gallon of gas allows me to coast from completing my initial burn at speed X.

So, let's say my destination 1 light year distant.

If I burn the whole gallon of gas I would accelerate long... hmm... ok... I get it now.

My flaw was in thinking I would be able to burn twice as much fuel accelerating because I was going half the distance. But I can't do that because I'd use it all and wouldn't be able to decelerate and I'd have to settle in for a nice long journey. So, no matter how far I'm going, it'll take half my fuel to accelerate and half my fuel to decelerate, so I can only travel speed X because my gas tank only holds a gallon of unleaded.

However! If I put a refueling ship in orbit around the rogue planet that will come out, match speeds with me, refuel as I blow past, I can burn all my fuel on my acceleration and refuel for my deceleration along the way. I can't think of why that might be more practical than just having a big extra tank I'd use to accelerate and lose it once I hit cruising speed.

Ok, so using the planet as a half-way point to refuel is probably dumb for a one shot trip out and back. But if I want to explore, I'd be better off using the rogue planet as a base for refueling so I wouldn't have to come all the way back.

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The Brain Science Podcast

Unless you have immortality, near-C drive, or both, almost any trip to another star will be one-way. Neither are likely near-term developments. (unless you believe this guy.)

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
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Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
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Frank Drake says yes.

He was a guest lecturer last year in an Astrobiology class I took. He said "habital zone" needs to be completely redefined as even interstellar planets might support life. He pointed out that the majority of Earth's biomass exists below the surface (I think by orders of magnitude greater than us surface dwellers). I asked the dumb question "do you means worms and gophers?" He meant microbial life. And he pointed out that interstellar planets can remain warm below the surface for billions of years.

Thinking about it, you don't need to go very deep on Earth before the crust is hotter that you would expect from the Sun alone. I think even deep mine shafts can be uncomfortably warm, and I doubt the Sun has anything to do with it.

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[quote=FriedPhoton;1170694]I find the concept of interstellar planets interesting. They'd make convenient way-stations if we ever were able to travel beyond the solar system, assuming any just happened to be along the way.QUOTE]

I think visiting a rouge interstellar planet could be an interesting precursor to visiting another star, instead of a way station. If there was one at say, twice the distance of the heliopause then an unmanned flyby could be launched with existing technology, although it would be fearsomely long-term (40 to 50 years at a guess). If we're lucky enough to find one so close by then we could launch a mission to a truly extra solar planet within my life time. A giant planet with its own system of moons could be even more exciting, as the moons might have some liquid water and chance for life. Anyone know how hard it would be to spot something jupiter-saturn sized at 100 to 200 AU distance?

I would guess the odds of one being that close are very slim to none.

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There should be an upper limit to the density of these free-floater planets from the MACHO studies?

The odds are unknown.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Yes, if it were something like Europa. A world with a geologic active core that could keep liquid water protected against a insulating layer of ice.

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The art of thinking and reasoning in strict accordance with the limitations and incapacities of the human misunderstanding.

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If an astronaut doesn't need good grammar, niether does you.

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It could be a moon like Europa, getting energy from tidal stress from its primary, or perhaps a large planet with an active core generating its own heat.

A thick atmosphere makes better insulation than ice, BTW.

__________________
"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Interesting article discussing modelling the ejection from proto stellar systems of earth-sized planets with moons. Estimated to be 70 billion in the galaxy, density 9E-04 per cubic parsec. However this is just a minority of free floating planets, as it is specifically those planets that retain their moon during ejection:

http://arxiv.org/PS_cache/arxiv/pdf/...709.0945v1.pdf

Correction, sorry that would be 70 MILLION....

Sorry about the slow reply; I seem to have lost contact with this thread.
I think you're right for any sort of "realistic" mission involving a prolonged period of low-velocity coasting.
But a refuelling stop is energy-efficient for a relativistic constant-boost journey, though it inevitably prolongs the journey time. You save on the big mass ratios required for constant boost over long distances.

As an example, a ten-light-year journey using a photon rocket to boost at 1g throughout, with a midcourse turnover, takes a mass ratio of 150 and an onboard time of 4.85 years.
Making the same journey with a refueling stop midway gives you a mass ratio of 49 for each of the two halves of the journey, so you use about 2/3 of the fuel; total on-board travel time is 7.55 years, but we need to add refuelling time.

Grant Hutchison