For the past year, it's been hard not to get caught up in the hype surrounding the 'impossible' EM Drive - the fuel-less propulsion system that defies Newton's third law, and yet continues to produce a small but unexplained amount of thrust in test after test.

Things got particularly interesting in December, when the Chinese government claimed they were already testing an EM Drive in space, and an anonymous source told the IB Times that the US was doing the same thing on board the military's mysterious space plane, X-37B. Five months later and those rumours are still circulating on blogs and news sites. So what's going on?

In case you've missed it, X-37B, or Boeing X-37, is a solar-powered, unmanned space plane that's been orbiting Earth for more than a record-breaking 678 days straight… but we still don't really know what it's doing up there.

Launched by the US Air Force in May 2015, the government has remained relatively tight-lipped about how long the plane will stay in space, and exactly what its purpose is.

Naturally, there's no shortage of conspiracy theories out there, covering everything from spying on the Middle East, space bombing (which isn't actually possible), and keeping tabs on China's ill-fated space station.

But the US military has always maintained that the mission is less about spying, and instead will "explore reusable space vehicle technologies in support of long-term objectives". 

Which is where the EM Drive rumours come in - the basic idea is that there's an EM Drive on board X-37B, allowing scientists back on Earth to remotely test the controversial propulsion system in space without gravity and friction interfering.

The EM Drive is a little like the Kardashian of the science world - it hasn't been proven to actually do anything as yet, but, still, we can't stop reading about it.

The appeal lies in the fact that the propulsion system theoretically works not by burning up heavy rocket fuel, but by bouncing microwaves back and forth to produce thrust.

On paper, that suggests a similar propulsion system capable of generating enough 'push' could get humans to Mars in a ridiculous 70 days, and could revolutionise space travel as we know it.

But that's if it actually works at all. 

Early tests, including a recent NASA study published in the Journal of Propulsion and Power, have shown that the EM Drive does produce thrust, but no one has been able to rule out that it isn't caused by an artefact or interference.

"Thrust data from forward, reverse, and null suggested that the system was consistently performing at 1.2 ± 0.1 mN/kW, which was very close to the average impulsive performance measured in air," the NASA team wrote in their paper.

"A number of error sources were considered and discussed."

But even researchers experimenting on the drive struggle to explain how it might work, seeing as it appears to defy Newton's third law of motion: that every action must have an equal and opposite reaction.

That means something has to be pushed out from one end in order to produce thrust the other direction. The EM Drive has no propellants to blast out, and so technically shouldn't be able to produce thrust.

One of the best ways to get to the bottom of all this would be to reduce the effect of variables such as friction and air pressure and test the drive in space - which is why the Chinese government and a US inventor, have all expressed interest in doing just that (and in China's case, may already be doing it).

But what about the US military and X-37B rumours? To be clear, the US government hasn't confirmed it's even interested in testing out the EM Drive in space at this early stage, and it's not clear whether or not they had access to a completed design back in 2015 when X-37B launched.

But there is some scant evidence behind the rumours. Other than the anonymous tip off reported by IB Times last year, a lot of speculation is based on a technology transfer contract approved by the UK Ministry of Defence and signed by Boeing back in 2007.

The contract itself doesn't give much away, but it does specify that it allows Boeing - the creator of X-37B - to research the EM Drive for 10 years.

There have also been rumours in the past that Boeing was working with the EM Drive's inventor, British engineer Roger Shawyer - although Boeing has later said they're no longer interested in the device.

What the US government has said is that X-37B is officially testing a Hall-effect thruster built by Aerojet Rocketdyne.

A Hall-effect thrust is another promising new propulsion system being investigated by NASA, which is capable of generating force of 60 millinewtons per kilowatt, an order of magnitude more than the EM Drive. But it still relies on propellants, so doesn't overcome the problem of future missions being weighed down by rocket fuel.

So, where does this leave us? The reality is there's not a whole lot of evidence to go on when it comes to an EM Drive being tested on X-37B, and so everyone needs to chill out until we hear something official, or see the results of some of these rumoured space tests make it to a peer-reviewed journal.

But the fact that the space plane is testing out at least one innovative propulsion system is a good sign for the future of space travel in general. And if any of these rumours do turn out to be true, it would be pretty huge. Let's face it, the quicker we can figure out if the EM Drive really works or not, the quicker we can move on with our lives.