Spaceflight is a high-risk business. Spacecraft break down all the time and when that happens funding and careers evaporate. Back in the late 1960s, NASA decided to double the odds of success and send two spacecraft on one mission. Voyagers 1 and 2, for example, were the spacecraft that returned the first detailed pictures of the outer planets of our solar system and introduced us to the neighbourhood. Launched in 1977, both are still flying.

Any spacecraft must have three components: a payload, an engine and a fuel supply – by far the heaviest component. But what if we could do away with the onboard fuel supply and replace it with an external fuel supply? Say light itself?

Can you push a spacecraft with light? Video: Physics Girl

The idea of solar sail technology has been floating around for decades. Indeed, the notion of a solar pressure can be traced back to 1610 in a letter that Johannes Kepler wrote to Galileo. 

But it was only in the 20th century that solar sails began to be considered as an achievable engineering reality. Broadly, solar sails fall into two categories: those using light from natural sources – the sun and ambient starlight in space; and those using coherent light from lasers.