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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Are you implying the replies above yours were not serious?
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.
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.
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.
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.
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?
Are you implying the replies above yours were not serious?