There was quite a stir a week ago as NASA confirmed the success of an engine that runs without fuel. Your first reaction will be, “Where can I get one of those for my car?!” So perhaps I should call it a propulsion method with no apparent propellant. And it might be usable for spacecraft, but your Chevy is going to have to keep on sucking gas.

The first version of this system was something called the EmDrive by its British inventor, Roger Shawyer. The EmDrive produces thrust from electrical energy by bouncing microwaves inside a sealed container. Physicists said such a thing was impossible because it violated the law of conservation of momentum: to get something moving, you have to exert a force, whether it’s feet on pavement or hot rocket exhaust sprayed in the opposite direction—you gain momentum by taking it from something else. But Shawyer wasn’t deterred. He even got support from a Chinese team that built an EmDrive in 2013 and found that it produced enough thrust to potentially move a satellite around in space.

Non-Chinese physicists still weren’t buying it until an American named Guido Fetta built a microwave thruster of his own, persuaded NASA to test it out, and on July 30th, 2014 the NASA team unveiled its results: impossible or not, the microwave thruster did produce thrust using electrical energy alone—no propellant. The amount of thrust was much less than the Chinese results (Shawyer blames this on Fetta’s design) but still undeniable. The NASA scientists only reported their methods and results—they did not choose to speculate about how the thing works. But as Wired magazine points out, they implied that the microwave thruster may be pushing against the “quantum vacuum plasma”: a froth of the universe’s tiniest particles that, according to quantum mechanics, pop into and out of existence constantly in empty space. In that case, it’s not violating any laws. It’s also not impossible.

This is big news. One of the greatest challenges involved in space travel is the mass of propellant needed for any type of rocket engine. To be able to do without propellant is huge. An EmDrive thruster could be powered by solar energy or presumably, for interstellar travel, a nuclear reactor. The thrust produced is small, but steady, and over the vast distances of space it’s steady that wins the race.

To me, the aspect of the news that’s even more delightful is that it’s yet another instance of someone proving that the “impossible” is no such thing. I realize that the discovery of the laws under which the universe operates is at the core of advancing human knowledge. But when will people stop using the word impossible? I couldn’t begin to list all of the “impossible” things that have proven to be not only possible, but sometimes the next law against which other impossible things are measured.

For now, our best scientists still believe that such things as faster-than-light travel and time travel are impossible. As a science fiction writer, I don’t dare accept that because it would spoil too many great stories! But more than that, I’ve come to see that “impossible” just means “not yet” or perhaps “not within our current understanding”. I’d strongly urge scientists to remove the word from their vocabulary—there’s just too good a chance that they’ll eventually have to eat it.