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Quartz
10-24-2007, 05:11 PM
So you've got your colony ship headed to Alpha Centauri or whereever. You've managed to accelerate it to a decent speed of 0.1c. But how's it going to decelerate when it gets there? How do you bleed off all that energy safely? How far out do you have to start? I know you can slingshot past a planet and thereby accelerate; can you do something similar to decelerate?

Let's assume that the colony ship has a mass of a megatonne and cannot survive significant atmospheric exposure, so no aerobraking a la 2010.

Turek
10-24-2007, 05:31 PM

Assuming you fired them for 30 days to get you to that speed and you used no other method to accelerate (ie you didn't use a gravity well), you'll have to fire them 30 more days in the opposite direction to slow you down.

Exapno Mapcase
10-24-2007, 05:31 PM
You need as much energy to slow a ship down from its highest speed to zero as it took to get it there in the first place. Therefore, most fictional voyages require a turnover at the halfway point.

If you assume an acceleration of 1 g for the first half of the voyage, you need a deceleration of 1 g for the second half. It requires the same amount of fuel. This brings you to a relative speed of zero at your target. You then need additional fuel to maneuver around and to planets in that system.

This is fantastically costly of fuel and requires planning years ahead of time. That's why space battles aren't real and starships aren't being built. There are a few schemes using solar sails and the like that allow for a very slow buildup of acceleration using non-ship sources of fuel that may be physically feasible. Everything else depends on unobtanium.

Randy Seltzer
10-24-2007, 05:33 PM
Heinlen, in Have Spacesuit, Will Travel, had an alien ship accellerating for the first half of the trip and decelerating for the second. The characters felt powerful g-forces pinning them to their seats constantly for some period of time, and then weightlessness for a few minutes at exactly halfway through the journey. During this period, the ship presumably turned around: nose towards its origin, tail (and thrusters) towards its destination. Then the characters experienced exactly the same powerful g-forces for exactly the same amount of time, and in their frame of reference, the forces had the exact same vector.

This should theoretically work. If you flip your ship halfway, and apply the same force, you'll arrive at your destination with a velocity of 0.

Upon preview: What they said.

10-24-2007, 05:35 PM
Same way you accelerated, only you point the ship the other way.

The rest is details. Admittedly the details are why it won't work, but why bother with that?

Tris

Ximenean
10-24-2007, 05:48 PM
A slingshot can be used for deceleration, you just approach the planet in the same direction it is moving rather than the opposite. That would transfer some energy from the craft to the planet.

Exapno Mapcase
10-24-2007, 06:26 PM
Of course, you're not going to get much effect if you try to fly by a planet at 0.1C.

Bryan Ekers
10-24-2007, 06:28 PM
Parachutes!

The Great Sun Jester
10-24-2007, 06:34 PM
Or just toss out a really big anchor ...

Shagnasty
10-24-2007, 06:39 PM
You dedicated all resources you have to get to Alpha Centauri to save mankind from an impossibly damaged Earth. You figured out a way to get this ship stopped over the last 1000 years or so as you arrive at a favorable velocity. Now what do you do?

It is points like this that make me a huge science buff and not a big science fiction fan.

scr4
10-24-2007, 06:43 PM
You need as much energy to slow a ship down from its highest speed to zero as it took to get it there in the first place.
Unless the spacecraft is carrying all the fuel/propellant from earth. In which case it'll take less energy to slow down, because the spacecraft is lighter.

Of course if you think of it backwards, this really isn't a good thing. Let's say you have a 100-ton manned capsule, and you need 1000 tons of fuel to accelerate the capsule alone to 0.1c. But if you need to stop at the end of the journey, you need to accelerate this whole 1100-ton assembly (capsule+fuel) to 0.1c, which will probably take 10,000 tons of fuel. The capsule will then use the 1000 tons of fuel to slow down.

scr4
10-24-2007, 06:54 PM
You dedicated all resources you have to get to Alpha Centauri to save mankind from an impossibly damaged Earth. You figured out a way to get this ship stopped over the last 1000 years or so as you arrive at a favorable velocity. Now what do you do?

It is points like this that make me a huge science buff and not a big science fiction fan.
Some science fiction authors have tried to address this issue, or at least show just how hard it is to do this. Robert Forward, for example, envisioned a spacecraft with a 2-piece light sail, propelled by a laser beam from the solar system. At midpoint the outer sail detaches, so now the laser is reflected by it and hits the front of the spacecraft, slowing it down. Of course the power requirement and optical resolution of the laser is insane - I think he depicted a solar-powered laser located near Mercury, and an enormous Fresnel lens floating somewhere beyond the orbit of Jupiter.

And some other authors have taken the view that manned interstellar flight will not happen unless the laws of physics as we know it is violated. I think Arthur Clarke pretty much said manned interstellar travel won't happen until a reactionless drive (violating Newton's 3rd Law) or a vacuum-energy drive is invented.

Bryan Ekers
10-24-2007, 06:55 PM
In a more serious attempt at an answer, send out unmanned ships ahead of the colony ship, each carrying a full tank of fuel, and arrange precise docking points along the way where the colony ship can dump its empty tank and replace it for a new one. If I get around to it, I'll crunch some numbers for a plan involving ten refueling ships launched at one-year intervals, preferably after each is fully refueled in orbit so they don't waste half their payload just getting off Earth.

iamthewalrus(:3=
10-24-2007, 07:05 PM
Another possible answer is that you don't.

You would not be going at 0.1c. If you have a propulsion system that can get you to a meaningful fraction of the speed of light, you don't need a generation ship. An actual generation ship would be moving much slower. That doesn't really make it easier to stop; you still don't have the fuel. But it makes it easier to get off.

You don't need to slow down the whole colony ship, you only need to slow down a very small personnel/supplies transport, and it only has to go a short distance to the planet of destination. A colony ship has a whole lot of mass dedicated to an environment that you won't need once you hit dirt. Sure, it'd be nice to have the ship up there to escape to, but there's definitely no realistic way to refuel and go anywhere, nor is there a reasonable expectation of indefinite survival aboard ship or of any external rescue. I think one of the major assumptions made when planning such a venture would be that the colonists will sink or swim on the planet as it is, no turning back. Such a ship would have escape pods, not a plan to put the entire thing in orbit.

scr4
10-24-2007, 07:12 PM
In a more serious attempt at an answer
:confused: Are you implying the replies above yours were not serious?

... send out unmanned ships ahead of the colony ship, each carrying a full tank of fuel, and arrange precise docking points along the way where the colony ship can dump its empty tank and replace it for a new one.
That doesn't make sense. This would require your colony ship to accelerate, then decelerate to stop at the first docking point, accelerate again, etc. If you can do that, you might as well accelerate once, coast to the destination and decelerate once. It would be faster and require no refueling.

Keeve
10-24-2007, 07:14 PM
You don't need to slow down the whole colony ship, you only need to slow down a very small personnel/supplies transport, and it only has to go a short distance to the planet of destination. A colony ship has a whole lot of mass dedicated to an environment that you won't need once you hit dirt. .... Such a ship would have escape pods, not a plan to put the entire thing in orbit.Granted that you don't need the gigantic support system of the mother ship, but escape pods won't be enough either. There will certainly be a certain amount of prefab buildings and other supplies to get the new society started.

Omphaloskeptic
10-24-2007, 07:22 PM
... send out unmanned ships ahead of the colony ship, each carrying a full tank of fuel, and arrange precise docking points along the way where the colony ship can dump its empty tank and replace it for a new one.That doesn't make sense. This would require your colony ship to accelerate, then decelerate to stop at the first docking point, accelerate again, etc. If you can do that, you might as well accelerate once, coast to the destination and decelerate once. It would be faster and require no refueling.You wouldn't want the fuel tanks to be stationary. You'd send one out to coast at, say, v/2 (half your manned ship's maximum speed v), timed so that your manned ship would meet it right when it had emptied its fuel tanks in decelerating to v/2. That way you don't waste fuel accelerating this tank from v/2 to v and then back to v/2.

scr4
10-24-2007, 07:23 PM
You don't need to slow down the whole colony ship, you only need to slow down a very small personnel/supplies transport, and it only has to go a short distance to the planet of destination. A colony ship has a whole lot of mass dedicated to an environment that you won't need once you hit dirt.
If you're going to a world that is already terraformed, that might be true. But otherwise, I think you'd design the colony ship so that most of it - life support, habitation modules, etc. - can be removed and taken down to the planet. That leaves the engine section, you can let that fly into deep space - except this engine is very useful for bringing the rest of the ship to a halt. So you might as well bring the entire ship to a stop, intact, then disassemble it and take what you need down to the planet.

10-24-2007, 07:31 PM
no aerobraking a la 2010.
:(

It's like you saw me coming or something.

SandyHook
10-24-2007, 07:31 PM
No idea in the world if this is feasible, but.....

Space isn't a total vacumn. Between here and anyplace out of the solar system we might make an attempt at there is, by my back of the enevlope calculations, somewhere between a butt-load and a butt-load and a half of hydgron atoms floating around.

Is it possible to harvest these and use them as fuel?

scr4
10-24-2007, 07:40 PM
You wouldn't want the fuel tanks to be stationary. You'd send one out to coast at, say, v/2 (half your manned ship's maximum speed v), timed so that your manned ship would meet it right when it had emptied its fuel tanks in decelerating to v/2.
OK, I see what you mean - that should work. It's not a huge saving in mass though, you still need to bring all the fuel for acceleration, plus a non-negligible fraction of what you need for deceleration. And the less fuel you carry for deceleration, the faster the resupply ships have to be.

The Hamster King
10-24-2007, 07:41 PM
No idea in the world if this is feasible, but.....

Space isn't a total vacumn. Between here and anyplace out of the solar system we might make an attempt at there is, by my back of the enevlope calculations, somewhere between a butt-load and a butt-load and a half of hydgron atoms floating around.

Is it possible to harvest these and use them as fuel?Behold, the Bussard Ramjet (http://en.wikipedia.org/wiki/Bussard_ramjet)!

It probably won't work, for reasons explained in the Wikipedia article. But it's a clever idea!

Bryan Ekers
10-24-2007, 07:45 PM
:confused: Are you implying the replies above yours were not serious?
Just my own.
That doesn't make sense. This would require your colony ship to accelerate, then decelerate to stop at the first docking point, accelerate again, etc. If you can do that, you might as well accelerate once, coast to the destination and decelerate once. It would be faster and require no refueling.
Why would it have to stop? The fuel ships are pointed at Alpha Centauri, all accelerating up to some set speed that lets them still keep some useful amount of fuel in reserve. Figure this:

Fuel Ship 1: Leaves Year 1. Very slowly accelerates up to .1c, then coasts
Fuel Ship 2: Leaves year 2. Very slowly accelerates up to .09c
Fuel Ship 3: Leaves year 3. Very slowly accelerates up to .08c
Fuel Ship 4: Leaves year 4. Very slowly accelerates up to .07c
Fuel Ship 5: Leaves year 5. Very slowly accelerates up to .06c
Fuel Ship 6: Leaves year 6. Very slowly accelerates up to .05c
Fuel Ship 7: Leaves year 7. Very slowly accelerates up to .04c
Fuel Ship 8: Leaves year 8. Very slowly accelerates up to .03c
Fuel Ship 9: Leaves year 9. Very slowly accelerates up to .02c
Fuel Ship 10: Leaves year 10. Very slowly accelerates up to .01c

Colony Ship: Leaves Year 11. As it accelerates, faster than any of the fuel ships, it catches up to each of them and relieves them of any unburnt fuel. Planned carefully enough, the colony ship won't have to "stop", though it might have to lose some velocity to match each fuel ship. Rather than burn braking fuel, maybe it just launches a unbreakable diamond tether and reels the fuel ship in. In any case, by the time it catches Fuel Ship 1, the plan is for them both to be cruising at or near 0.1c and be halfway to the destination. Colony Ship tops up its tanks from FS1's reserve, then begins the long breaking routine.

Of course, the only fuel likely to make any of this practical is anti-matter. Isaac Asimov wrote and essay called Sail On in which he calculated that a single ship could likely carry all it needed to make the trip (I don't recall the numbers, but it was a fairly piddling quantity of anti-matter, combined with a few tons of hydrogen that could supply an extra kick, being pushed into fusion by the anti-matter).

DrFidelius
10-24-2007, 07:50 PM
Stasis fields and lithobraking...

DSYoungEsq
10-24-2007, 08:05 PM
"I just flew in from Alpha Centauri, and boy are my arms tired!"

Shagnasty
10-24-2007, 08:10 PM
No idea in the world if this is feasible, but.....

Space isn't a total vacumn. Between here and anyplace out of the solar system we might make an attempt at there is, by my back of the enevlope calculations, somewhere between a butt-load and a butt-load and a half of hydgron atoms floating around.

Is it possible to harvest these and use them as fuel?

This hits on a different if not terminal problem that many experts have noted on this whole idea. At speeds any significant fraction of C, the tinniest little speck of anything will just destroy the entire ship. The space shuttles have already had problems with that and they the shuttles only fly at about 17,000 mph which is only the tiniest fraction of the speed any such ship would need to go to reach the nearest star within even a few thousand years.

Humans are smarter than most in the ape family but we still have a very limited lifespan and both bodies and brains with significant needs that not incoincidentally are designed for life hear on human earth. I have tons of optimism for all technology including inter-solar system travel but inter-stellar travel just seems laughable at best and probably impossible. That may explain the dearth of reputable UFO sightings recently.

scr4
10-24-2007, 08:49 PM
Planned carefully enough, the colony ship won't have to "stop", though it might have to lose some velocity to match each fuel ship. Rather than burn braking fuel, maybe it just launches a unbreakable diamond tether and reels the fuel ship in.
I don't think it would work for the acceleration phase. The resupply ships would be moving slower than you, so you lose speed when you capture it with your tether.

It should work for the deceleration phase though, like Omphaloskeptic said.

Ike Witt
10-24-2007, 09:27 PM
Rather than burn braking fuel, maybe it just launches a unbreakable diamond tether and reels the fuel ship in. In any case, by the time it catches Fuel Ship 1, the plan is for them both to be cruising at or near 0.1c and be halfway to the destination. Colony Ship tops up its tanks from FS1's reserve, then begins the long breaking routine.
Sorry, but anything in deep space flight that can be disrupted by Zoidberg is going to get a pass.

Der Trihs
10-24-2007, 10:18 PM
Behold, the Bussard Ramjet (http://en.wikipedia.org/wiki/Bussard_ramjet)!

It probably won't work, for reasons explained in the Wikipedia article. But it's a clever idea!But a variation on the idea might; you should have read to the bottom. Using a magnetic field to gather fuel might not work - but using a magnetic sail (http://en.wikipedia.org/wiki/Magnetic_sail#Interstellar_travel) to produce drag and slow the ship might.

OtakuLoki
10-25-2007, 01:48 AM
Stasis fields and lithobraking...

Okay, that's got me laughing out loud!

ETA: Of course, after you hit the planet at 0.1 C, it's going need a millenium or more to return to being desirable real estate....

Quartz
10-26-2007, 03:58 AM
Behold, the Bussard Ramjet (http://en.wikipedia.org/wiki/Bussard_ramjet)!

Not a lot of good when you're trying to slow down.

Malacandra
10-26-2007, 04:07 AM
Not a lot of good when you're trying to slow down.

Why not? Turbojets can use thrust deflectors to brake, why shouldn't the ramjet? Or why can't you tweak the field to pump the fuel into the other end of the ramjet so it's just firing forward in the first place. Alternatively, consider the possibility that you're using a fission pile as your prime mover, and all the ramscoop field is collecting is reaction mass, not fusion fuel. You can squirt that out in any direction you like. Indeed, just collecting the interstellar hydrogen would provide a little friction braking (just as it imposes an upper limit on the speed of the ramjet in the first place).

essell
10-26-2007, 04:40 AM
If you assume an acceleration of 1 g for the first half of the voyage, you need a deceleration of 1 g for the second half. It requires the same amount of fuel. This brings you to a relative speed of zero at your target.

Possibly wrong nitpick, wouldn't this bring you to zero speed relative to your starting point, not the target? Relative galactic motion and all that...

Mangetout
10-26-2007, 04:43 AM
Why not? Turbojets can use thrust deflectors to brake, why shouldn't the ramjet? Or why can't you tweak the field to pump the fuel into the other end of the ramjet so it's just firing forward in the first place. Alternatively, consider the possibility that you're using a fission pile as your prime mover, and all the ramscoop field is collecting is reaction mass, not fusion fuel. You can squirt that out in any direction you like. Indeed, just collecting the interstellar hydrogen would provide a little friction braking (just as it imposes an upper limit on the speed of the ramjet in the first place).
If you eject it forwards, some of it is might be slowed down by the interstellar medium in front of you enough for you to collect it and use it again, making braking all the more efficient.

Except for the tiny details of making a ramscoop field (don't think we have any idea how to do that at the moment, or if it's even possible).

But if this is a long term voyage, how is a fission reactor going to be maintained? Where are you going to get new fuel rods?

10-26-2007, 05:20 AM
I should point out that, assuming your fuel has non-zero mass and that your maximum rate of acceleration/deceleration isn't limited by your puny earth bodies, then you shouldn't stop and turn around halfway. Because you're carrying less fuel for the deceleration phase, you can decelerate more rapidly than you accelerated due to your lower mass. This allows you to keep accelerating past the halfway point, because you know you can slow down faster.

Exactly how far past halfway you go depends what proportion of your vessel's mass is the fuel expended on your complete journey.

Malacandra
10-26-2007, 05:26 AM
If you eject it forwards, some of it is might be slowed down by the interstellar medium in front of you enough for you to collect it and use it again, making braking all the more efficient.

Except for the tiny details of making a ramscoop field (don't think we have any idea how to do that at the moment, or if it's even possible).

But if this is a long term voyage, how is a fission reactor going to be maintained? Where are you going to get new fuel rods?

There I think we have to start doing the math, as to how much power we're getting out of the pile, how much fuel we can take along with us, how long the voyage is going to take... I don't know if there's even a theoretical chance of, say, a round trip to Alpha Centauri on one load of fuel; IANARS. If we can get there and find that the system has an asteroid belt then things start to look up, as we can go prospecting for radioactives (using our back-up light-sail to run a solar furnace) and incidentally shovel up a shitload of dust or ice to use as reaction mass to start the journey back. :)

Mangetout
10-26-2007, 06:41 AM
I don't know if there's even a theoretical chance of, say, a round trip to Alpha Centauri on one load of fuelThat's one potential problem with fission - even if you take spare fuel rods, they decay before you've had the chance to use them.

MrFloppy
10-26-2007, 06:56 AM
Well if it is taking 1000 years to get there, I would think that much faster ships invented 500, 600, 700 years after your departure would catch up with you and use ther tractor beams to slow you down :)

Malacandra
10-26-2007, 07:18 AM
That's one potential problem with fission - even if you take spare fuel rods, they decay before you've had the chance to use them.

Sounds like a case for using a breeder reactor then - assuming there is some kind of rod we can use that is initially low-activity, slow-decay, and which we can bombard with neutrons shortly before use to make it high-activity, quick-decay. How are we fixed for converting U238 to plutonium, and then burning the plutonium? (Or converting U238 to U235.)

OtakuLoki
10-26-2007, 08:20 AM
That's one potential problem with fission - even if you take spare fuel rods, they decay before you've had the chance to use them.

Wubba? Just what are you using in your piles?

235U has a half-life of 7.038·108 years - if your trip is going to last long enough that you're going to signifigantly reduce the amount of 235U in your fuel cells, it's time to re think things.

The more common 238U has a half-life of 4.46·109 - ten times longer.

Yes, Pu has shorter half-lives, but I think that U would be the better fissile material.

I don't think there's any need to consider a breeder reactor, so you can reprocess your fuel. That's just too much mass, IMNSHO, to carry along.

Malacandra
10-26-2007, 08:59 AM
Wubba? Just what are you using in your piles?

Preparation H. It's wonderful stuff.

Back on topic - so 235U is not all that radioactive to begin with, but is wicked crazy for fissioning once you mix in some neutrons? Sounds like our interstellar project is back on track, then. :cool:

Quartz
10-27-2007, 12:47 PM
Why not? Turbojets can use thrust deflectors to brake, why shouldn't the ramjet?

Because as you slow down the ramjet scoops less and less fuel.

Malacandra
10-29-2007, 09:40 AM
Because as you slow down the ramjet scoops less and less fuel.

Sort of like acceleration in reverse. What's the problem again?

Ludovic
10-29-2007, 10:02 AM
But with the hydrogen scoops, when you're trying to accelerate, you have to accelerate the hydrogen from zero momentum, whereas when you're trying to slow down, the hydrogen is already going in the direction you want to be going. I wonder if you couldn't just put out really wide sails that would run into the interstellar hydrogen and provide fuel-less deceleration.

The same thing would go for fuel ships you send out beforehand: if you make them of sufficient weight, and used a really long elastic band (to prevent shock to the inhabitants,) you would achieve additional braking by the mere fact of running into them to gather the extra fuel.

Malacandra
10-29-2007, 10:13 AM
But with the hydrogen scoops, when you're trying to accelerate, you have to accelerate the hydrogen from zero momentum, whereas when you're trying to slow down, the hydrogen is already going in the direction you want to be going. I wonder if you couldn't just put out really wide sails that would run into the interstellar hydrogen and provide fuel-less deceleration.

You could. You'd achieve a lot more deceleration by using it for braking thrust, though, rather than just deploying a "Bussard parachute". :)

The ramjet obviously needs some on-board fuel just to get going - like any other ramjet vehicle. And there's no reason why it can't use the fuel scoops to refill its tanks before actually using the engine to brake - then you have a fuel reserve for when you drop below ramscoop speed.

Stranger On A Train
10-30-2007, 09:40 PM
So you've got your colony ship headed to Alpha Centauri or whereever. You've managed to accelerate it to a decent speed of 0.1c. But how's it going to decelerate when it gets there? How do you bleed off all that energy safely? How far out do you have to start? I know you can slingshot past a planet and thereby accelerate; can you do something similar to decelerate?You can, but the change in velocity limited at best to something less than twice the speed of the planet. The actual analysis is somewhat more complicated than this but if you're approaching any planetary sized object at 0.1c then the effect of a gravity swing-by is obviously negligable. Something the size of a galactic mass singularity would be necessary to effect a significant reduction in speed without subjecting the ship and its delicate human cargo to stresses far in excess of material limits.

Unless the spacecraft is carrying all the fuel/propellant from earth. In which case it'll take less energy to slow down, because the spacecraft is lighter.

Of course if you think of it backwards, this really isn't a good thing. Let's say you have a 100-ton manned capsule, and you need 1000 tons of fuel to accelerate the capsule alone to 0.1c. But if you need to stop at the end of the journey, you need to accelerate this whole 1100-ton assembly (capsule+fuel) to 0.1c, which will probably take 10,000 tons of fuel. The capsule will then use the 1000 tons of fuel to slow down.Actually, even given a propulsion system with the very high specific impulse of Isp=10,000 with sufficient thrust to actually move a massive object (roughly that expected from D-D or D-T fusion) you're talking about a ratio of deceleration to acceleration propellant of somewhere on the order of 10000:1 or more (neglecting the mass of your actual cargo under the assumption that it is negligable compared to propellant mass) to attain that speed. Unless you can come up with some propulsion system that is massively more effective (i.e. a higher effective exhaust velocity for a given thrust level) than any known or speculated mass propellant (see the Tsiolkovsky rocket equation (http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation) which very succintly lays out the effectiveness of a rocket motor) you just can't carry enough fuel to reasonable decelerate. You can stage your system as iamthewalrus(:3= suggests (which is analogous in concept to multistage orbital boost systems used today) but the payload mass is such a small fraction of the overall starting mass--even a fraction of the mass required for deceleration--that it doesn't really help much. Prepositioning propellant to be captured en route doesn't really help, as you have to accelerate and/or decelerate them too, as well as the added complexity of rendezvousing with them. And external thrust source eliminates the problem of carrying propellant, but no feasible system for interplanetary, much less interstellar, distances has ever been credibly proposed. (Massive interstellar lasers are a patent absurdity given both the divergence of a laser beam and the inefficiency of laser, and as for Bussard ramjets, the less said the better; they belong in Larry Niven stories, not plausible speculation for interstellar transit.)

The only conceivable propellants to accelerate to relativistic speeds using any conceivable scheme are lightweight or massless fundamental particles. When you get that matter-antimatter photon drive all worked out let me know, and I'll give you a hand with the patent application. Aside from that, you're going to find yourself limited to a tiny fraction of a percent of c. (And before someone pulls out the Orion nuclear pulse propulsion, take a look at both the effective velocity and geometric efficiency of such systems; you'll find that while providing magnitudes greater thrust per propellant mass than any conventional chemical propellant, they're actually horribly inefficient and effectively limited to about a percent or two of c even given an unlimited supply of propellant bomblets.)

And as Shagnasty notes, there are many other problems with transiting at anything like relativistic speeds. Not only will the slightest speck of dust have the relative energy magnitudes greater than the largest thermonuclear weapon ever built, but you also have the problems of thermodynamic and resource sustainability; you have to provide enough energy for a recycling lifesystem to operate (and replace resources that are not perfectly recycled in usable form) and yet radiate away unrecoverable waste heat into space, which will require massive radiating surfaces for any complex system. You'll end up with a ship the size of a small moon, at least, and pushing before it a literally massive shield to deflect oncoming particles.

We may one day travel to other star systems, but baring the development of some kind of highly speculative "warp drive" or the discovery of usable trans-spacetime-plenum conduits (or whatever space opera term you care to use), "we" should be loosely defined as "our highly modified successors"; semi-sentient robots, or organic organisms capable of surviving the environment of interstellar space without complex plumbing, or whatnot. It's unlikely, to a point of certainty, that we'll be running around in colorful spandex uniforms shouting "Fire at will!" and banging the occasional Orion slave girl. Colony ships moving at sublight speeds are only marginally less likely than USS Enterprise.

Stranger