Zontar said:
John Markley said:
Depends on the amount of mass you're accelerating and how efficiently your propulsion uses energy, but the short answer is: a lot. There was a study in the 1970s by scientists at the British Interplanetary Society into the idea of unmanned ships that could make one-way trips to nearby stars, propelled by fusion engines running on deuterium/helium-3 reactions. The design they came up with could reach 12% of lightspeed after about four years of acceleration. At launch, it would mass 54,000 metric tons- of which 50,000 would be fuel! That would be enough to get the other 4,000 tons to Alpha Centauri or Barnard's Star in only a few decades. (Though not enough to STOP once they got there...)
JESUS, a 25:1 fuel to mass ratio for a one way trip with only enough gas to get moving and come to a stop? Please for the love of god tell me someone found an alternative, more efficient means of moving stuff in space then fusion. I'm not letting the dream of humanity escaping this system die because of pesky physics.
Nope, no coming to a stop. That four 4,000 tons will keep careening along 12% of light speed forever. Luckily, there are other possibilities, yes.
One is simply to just go slower- on the time spans planets and stars live, it hardly matters if your journey takes decades or millennia. (Though that's problematic if you're carrying live humans, obviously.) You might use a fusion reactor to power some other form of propulsion like an ion engine, which accelerates at a much more leisurely pace but makes more efficient use of propellant mass.
Possibility two: Even better propulsion, either by producing a fusion drive better than the one imagined in the BPS study or by using some other, even more powerful method. You could, conceivably, use matter-antimatter annihilation to produce even more energy per gram of fuel than fusion, and either blow the product of that reaction out directly as exhaust or use it to propel some other reaction mass. Alternately, you could have a fusion drive that uses small quantities of antimatter to ignite a fusion reaction in pellets of fusion fuel.
(This may or may not be practically feasible, but then again it might be; it does not run into any hard-and-fast limits like the speed of light, at any rate.)
Possibility three: Don't take your means of propulsion with you, or at least not all of it. Equip the ship with a light sail, and propel it with a giant laser that stays in our own solar system. Would allow for a much smaller, easier-to-accelerate ship. You might even be able to use the laser back home to slow down at the destination, using multiple sails.
Downside: Depends on the folks back home being able and willing to keep the giant laser running for decades, centuries, or longer. So don't use if you're fleeing from some sort of impending apocalypse.
Also, as albino boo says, the sail would be vulnerable to impacts. I'm not sure about how large the holes would be, though; depending on the speed, what the sail is made of and how dense it, etc. matter impacting the sail might punch through it pretty cleanly and keep going, taking most of its kinetic energy with it, which would limit how much kinetic energy would be available to damage the sail. (Think of a gunshot wound- a bullet that simple passes through your body and keeps going more or less intact is usually less damaging than hollow-point round that expands and slows down inside you.) So you might have a gradual degradation over time, rather than a sudden catastrophe
Possibility four: Some currently unforeseen breakthrough that significantly changes our understanding of physics. I wouldn't bet on this, but can't rule out the possibility.
