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While interstellar space travel is still in the realm of science fiction, scientists say that if an alien ship came within range of Earth at near light speed, we would be able to detect it.

This is according to a pair of defence engineers who say that any spacecraft travelling that fast would leave distinct light signatures in its wake.

But their study also reveals the tricky physics of travelling at near light speeds, calculating that a collision with single dust grain would release energy equal to 5291lbs (2,400kg) of exploding TNT.

A pair of defence engineers say that any spacecraft travelling close to the speed of light would leave light signatures in its wake because it would interact with the cosmic microwave background (pictured). The panels show how scientists predict the CMB signature would change as the spacecraft increases speed

California-based Raytheon engineers Ulvi Yurtsever and Steven Wilkinson claim that spaceships travelling at near light speeds would interact with the cosmic microwave background (CMB).

CMB is the light left over from the earliest moments of creation that has been stretched across the entire cosmos as the universe expanded.

A fast-moving spacecraft would collide with cosmic microwave photons, say the engineers, creating highly gamma rays, according to a report inMIT Technology Review.

Raytheon engineers suggest that spaceships travelling at near light speeds would interact with the cosmic microwave background (CMB). Pictured is a Narn spacecraft from Babylon 5

The researchers have estimated that each cubic centimetre of space contains over 400 microwave photons.

This means a spaceship would collide with thousands of billions of photons every second creating something known as electron-positron pairs.

Each of these pairs would cause drag on a spaceship.

'Assuming an effective cross-sectional area of say 100 square meters, the dissipative effect is about two million Joules per second,' said Yurtsever and Wilkinson in their study.

The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second). 

Seconds last longer when travelling at high speed so the energy dissipation is higher too at around 10^14 Joules per second.

'In general one can imagine the same interactions that occur in a particle accelerator to occur between relativistic spacecraft and interstellar matter,' write the authors in their study.

As well as drag, a single cosmic dust grain with a mass of 10^-(14) grams would release about 10,000 megajoules worth of energy if it the spacecraft.

The engineers explain that is the same amount of energy released by 2,400 kilograms of exploding TNT.

'Our assumption that matter-matter interactions can be dealt with when civilisation can build relativistic spacecraft may prove false and may be a barrier that will prevent space travel [at relativistic speeds],' write the engineers.