NASA is testing a propulsion method that shouldn't be possible, according to tried-and-tested physics principles. But now at least one source of error has been ruled out.
This is about the EM drive, a strange machine that supposedly generates thrust even though it shouldn't. If it works, it could enable spaceships to control attitude and acquire speed using only energy and no propellant. That would be even better than an ion engine because spacecraft, once in space, wouldn't need to carry any propellant. Like an ion engine, the EM drive would not be suitable for launching spacecraft, because it can't generate lots of thrust very quickly.
The EM drive is basically a conical cavity in which microwaves are made to bounce around. A force toward the narrow end has been predicted and measured. If you don't understand, that's OK: this violates Newton's Third Law, one of the most important principles of physics. This law states that if an object is pushed, it pushes back. In layman's terms: if you're sitting in the driver's seat of a car, you can't set the car in motion by pushing the steering wheel. You're pushed back in your chair just as much as you push the wheel forward. Yet this is how the EM drive is supposed to work.
The EM drive also violates conservation of momentum, another big one in physics. If a rocket goes one way, some propellant usually goes the other. If you jump up, the Earth moves down a tiny amount. In the EM drive, nothing goes the other way and that stinks.
Supporters of the EM drive concept say general relativity makes up for this, but this is outside yours truly's jurisdiction so try and make up your own mind.
Meanwhile, Chinese scientists have measured a non-zero force in 2012. So have NASA scientists in 2014. But the NASA results were a thousand times smaller than the ones from China. Moreover, a control experiment in the NASA research that wasn't expected to yield any thrust, did so anyway. All of this screams 'Cold fusion!' but the NASA guys have been undeterred.
The other day they came up with new experiments, done in a vacuum. They did this because one possible source of error in the previous experiment could have been thermal convection, in other words heat causing air to move around. Once more they measured a force and this time no convection could be to blame.
Still the EM drive remains problematic and there is a lot of heated discussion going on at Nasaspaceflight.com. The NASA scientists also take part there and have been known to use insights from the forum in their work.
In any case, even if the EM drive would work, using one would require having a bulky nuclear reactor in your spaceship. In a space environment you could care less about the radiation. But for the EM drive to be useful the reactor should weigh no more than the propellant it replaces. So there's another problem to be solved. :-)
Herbert Blankesteijn is a technology journalist from the Netherlands who has written for many prominent Dutch newspapers. He presented and directed television and radio programmes and has 10 books to his name. Herbert is interested in nascent fields such as 3D printing, drones, robotics and the private space business.