Don’t buy stock in impossible space drives just yet

Some microwave resonators left over from early accelerators at CERN.

John Timmer

Yesterday, Wired UK had geeks everywhere salivating at the prospect of a purely electrical space thruster system—one that should be impossible based on what we know about classical physics. The article notes that NASA engineers have now tested one of these devices and found that it appears to produce thrust without using any fuel. Although there are ways that non-classical physics can make things work, there are enough red flags raised by material in the initial report that the news should be greeted very skeptically.

The limit that space thrusters face is purely classical: to push something along at a higher velocity, you need to push against something else. So typical thrusters push against the mass of the burning fuel that they're explosively expelling behind them. Even the most sophisticated, efficient thrusters—ion drives—act as particle accelerators that shoot ions out in the opposite direction of the way they're accelerating. As a result, any form of thrust that we've used requires that the spacecraft carry some mass that then gets shot out from the spacecraft. This adds weight to the launch vehicle and sets a finite limit on how much propulsion can be generated during the spacecraft's lifetime. Which is a bit frustrating, given that the high-efficiency solar panels on many spacecraft can give them a surplus of energy. It's just not energy we can convert into thrust—or at least we think we can't convert it into thrust.

The Wired UK article details how a variety of groups have suggested that it might be possible to use electricity to generate some thrust via a decidedly non-classical mechanism. The device involves a radio frequency resonant cavity, which takes microwaves as an input and uses them to create an oscillating electrical field. These cavities are used in particle accelerators like the Large Hadron Collider, where the electric field helps boost the energy of the circulating ions. At the LHC, however, they actually provide some ions for the resonant cavity to push around, which would be equivalent to supplying this thruster with fuel. Yet the backers of this device suggest it's pushing against the swarm of virtual particles that quantum mechanics indicates are constantly popping in and out of existence in empty space.