Atomic surfing: A pulse-propulsion craft would detonate small nuclear bombs behind it, and could surf the shockwaves all the way to Mars in as little as one month (artist's impression pictured).
Credit: NASA
SOLAR SAILS
Instead of catching a light breeze, a solar sail catches sunlight – and the minute force of photons bouncing off can move a craft through space without fuel. The sail itself is a reflective sheet 100 times thinner than paper. It might sound too delicate to move anything, but both the NASA Jet Propulsion Laboratory and the Planetary Society, based in Pasadena, California, plan to test real prototypes in space soon. Like solar thermal propulsion, this method leaves the spacecraft helpless in the dark.
DARK ENERGY WARP DRIVE
As any sci-fi fan could tell you, a warp drive creates a bubble of normal space-time and expands or contracts the space-time around it to move a spacecraft faster than the speed of light. According to a 2008 study from string theorists at Baylor University in Texas, the power required might come from manipulating dark energy, the mysterious force accelerating the expansion of the universe. Exactly how to harness it, though, is anyone's guess. Even then, the researchers say the spacecraft would consume the entire mass of Jupiter just to move 10 m at light speed, so it wouldn't get any points for fuel efficiency.
ANTIMATTER ENGINES
How could antimatter engines whisk a spacecraft to Mars in six weeks? At first glance, it looks like a conventional rocket – the products of matter-antimatter collisions would heat a gas and propel it through a nozzle. But researchers at Pennsylvania State University say it's a case of less being more: just one gram of antimatter could provide as much energy as 1,000 external fuel tanks currently strapped to NASA's s pace shuttle prior to take-off.
Get it right
1 gram of antimatter annihilated with 1 gram of matter produces 2*c^2 of energy. C^2 is 9E+16, thus 0.002*C^2 = 180 TJ (teraJoules). A Space Shuttle main propellant tank contains ~750 tons of hydrogen/oxygen propellant. 1 kiloMole (18.015 kg) of H2/O2 when combusted produces 241.83 MJ (STP) thus 750,000 kg of the stuff is 100 TJ. Just 1.8 Shuttle ETs full of H2/O2 would be the equivalent of 2 grams of energy.
Perhaps you meant kilograms?
Ships that self-lift by their own bootstraps much to be desired
but there seems to be no clue how they'll do that. "Vacuum-powered spacecraft" and those powered by "Artificial Gravitational Fields" and "Dark Energy Warp Drive" seem to be in this category.
Antimatter, nuclear explosives, and solar thermal all use on-board heat and share a common limit, that of how much low-grade heat they can get rid of. Only some of the heat can be converted into propulsion power, the rest having to be dumped through a radiator. Non-pulsed nuclear fission powering a VASIMR engine, an alternative that was not mentioned, would also have that limitation. The VASIMR is soon to be tested on the space station, I seem to recall.
--- G.R.L. Cowan
external detonation has advantages
One plus of nuclear pulse drives is that very little of the heat is retained by the pusher plate. Most of the plasma flies off into space taking the heat with it. Nuclear Salt-Water Rockets also dump heat too rapidly for it to be a major issue because of the high venting rate of propellant. External fusion-pulse drives, like the system recently described by Winterberg, would also vent most of the heatload directly into the void.
Alternatively one can leave all the nasty mass/heat issues at home and ride a beam of light/ions/pellets to relativistic speeds. Slowing down becomes problematic without a "starbase" at the other end, but advances in superconductors might allow very high efficiency magnetic sails by the time we're pushing off to the stars. Or rather, being pushed off...