We're one step closer to Star Trek, with NASA successfully testing an experimental Ion Drive in Earth orbit. In fact, since the Enterprise only had thrusters for low-speed maneuvers, this means we've got something even the guys with Warp Drive didn't think of.
It turns out that carrying tanks of volatile chemicals on your space ship and setting fire to them every time you want to move has a few problems. Just ask Apollo 13. And once you run out, that's it. The Ion Drive instead operates electrically, carrying tanks of utterly inert Xenon gas - a "noble gas", so called because you could poke its mother in the face with matches and it won't catch fire.
You'll still run out awful fast if you just squirt gas to move like some kind of interstellar balloon, which is why the Ion Drive uses electrical acceleration. An electric field is used to strip the Xenon of electrons, rendering it positively charged, then accelerate them out of the engine. This gives a tiny amount of gas a much larger momentum, and by Newton's laws, an equal but opposite change in momentum is imparted to the spacecraft.
The resulting acceleration is tiny, but constant, and can be maintained entirely under solar power - in other words, running the engines is now free. The first probe to use this system is the GOCE satellite, the Gravity field and steady-state Ocean Circulation Explorer. GOCE must fly in a dangerously low orbit to gather data with its fantastically accurate gravity sensors. So low that friction with the outer atmosphere will drag it down into an early, and remarkably fiery, grave - unless it's equipped with a revolutionary new engine.
The Ion Drive will operate to cancel out the Earth's effects on the satellite, keeping it in a constant balance between electrically powering out and plunging in. The Ion Drive isn't limited to station-keeping, however - small but solar-powered accelerations are perfect for interplanetary, or even interstellar missions. The GOCE engines can provide 20 milliNewtons of thrust - for a one-ton satellite, that's an acceleration of less than the width of a human hair per second squared, which is less than impressive. Unless you keep it on for a month, say, and end up moving at four kilometers a second - and with a little work, you can refuel anywhere there's an atmosphere.
Ion Drive http://www.sciencedaily.com/releases/2009/04/090406132821.htm