NASA's "Impossible" Space Engine Recently, there was some news that said that the researchers at NASA have come across some impossible kind of space engine which does not require any fuel. I have read at a few places like here, here and here. They all say that this engine seems to violate the laws of physics including conservation of momentum.
The original paper is here.
Is the statement that it violates conservation of momentum true?  
 A: It is possible to generate thrust using EM radiation such as a laser or microwaves. Discussed in this XKCD blog.
However this relies on momentum being transferred from the photons of the EM radiation to the object being propelled. This is not possible if the microwaves are completely sealed within a container as indicated in the article.

This paper will not address the physics of the quantum vacuum plasma 
  thruster...

Until the physics used is addressed I would be very skeptical of these claims.
A: It seems like the previous answers were based on the abstract or third-party articles about the abstract, but I gather from this Wired article that the full paper provides more details (behind a paywall, so I can only speak for what the article says).
The described tests were on the Cannae drive, the inventor of which believed required slots in the drive to function. The "null test" previously described was a Cannae drive without slots, which actually did produce thrust, showing that the slots weren't necessary (much like the previously-tested EmDrive). The experiment's actual control, an inert load, didn't produce any thrust. Tellingly, when they reversed the drive, the thrust reversed as well.
They were testing in a vacuum, seemingly as insulated from the outside world as possible.
The previous Chinese tests used much more power - kilowatts to NASA's watts. If this is a true effect then it's expected NASA's experiment would produce less thrust.
I wouldn't bet money on this, but I also wouldn't discount it yet.
A: One of my mentors likes to say, "Nothing resembles a new effect quite so much as a mistake."
Conservation of momentum is a fundamental principle of mechanics supported by hundreds of years of experimental evidence since the language needed to discuss it was codified by Newton.
Certainly it's the case that electromagnetic radiation carries momentum and can be used for thrust, though the efficiency is terrible.
The news articles appear to be based on a conference presentation, whose abstract is available from NASA's technical reports server but whose content is not. But the little snippets of information that we can find are pretty damning. Consider:


*

*"Manual frequency control was required throughout the test." That means if the experimenters stopped fiddling with it, the effect went away. I remember nearly exactly the same comment from one of the folks involved in the bubble fusion fiasco a decade ago.

*"Thrust was observed on both test articles, even though one of the articles was designed with the expectation that it would not produce thrust." So there was no difference between the experiment and the control. The entire reason you do a control measurement is so that you know what the zero is. So either (a) the result from their thruster was zero, or (b) the group doesn't understand their measurement, or (c) both. If I were as smart as Kyle Kanos I'd have stopped reading here.

*The experiment is a reproduction of a similar test by a Chinese group, which saw more than 1000 times as much thrust. If you gave me a stove element and said "this device produces 1000 W of thermal power" and I put in my stove and measured half a watt, I'd tell you it was broken. But space.com's report has the audacity to say

The NASA scientists determined that the Cannae Drive produces 30 to 50 micronewtons of thrust — less than 0.1 percent of that measured by the Chinese team, Wired UK noted, but nevertheless suggesting that the technology works. [emphasis mine]  

Hanlon's Razor suggests we should not attribute to malice that which can be explained by incompetence.
In a comment alemi links to a fuller article. That article contains a fairly comprehensive description of the torsion pendulum thrust measurements, but essentially zero evidence for the remarkable claim in the abstract that the force "is not attributable to any classical electromagnetic phenomenon and therefore is potentially demonstrating an interaction with the quantum vacuum virtual plasma." Furthermore that article's only citation, the test by Yang Juan and collaborators claims in its abstract that the thrust is explained by classical electromagnetism. What a mess.
A: The first thing that should jump out to anyone is the following excerpt from the abstract of the recent NASA paper:

Thrust was observed on both test articles, even though one of the test articles was designed with the expectation that it would not produce thrust. Specifically, one test article contained internal physical modifications that were designed to produce thrust, while the other did not (with the latter being referred to as the "null" test article).

If this doesn't scream "There's something screwy here..." to you, then I have a bridge to sell you.
Also, Randall Munroe took a shot at this:

