Another amazing property of the material graphene has been discovered. It seems these sheets of carbon one atom thick can turn light into action, maybe forming the basis of a fuel-free spacecraft.
Graphene was discovered accidentally by researchers playing with pencils and sticky tape. Its flat structure is very strong and conducts electricity and heat extremely well. Yongsheng Chen of Nankai University in Tianjin, China, and his colleagues have been investigating whether larger arrangements of carbon can retain some of these properties. Earlier this year they published details of a “graphene sponge”, a squidgy material made by fusing crumpled sheets of graphene oxide.
While cutting graphene sponge with a laser, they noticed the light propelled the material forwards. That was odd, because while lasers have been used to shove single molecules around, the sponge was a few centimeters across so should be too large to move.
The team placed pieces of graphene sponge in a vacuum and shot them with lasers of different wavelength and intensity. They were able to push sponge pieces upwards by as much as 40 centimeters. They even got the graphene to move by focusing ordinary sunlight on it with a lens.
But how was this movement happening? One explanation is that the material acts like a solar sail. Photons can transfer momentum to an object and propel it forwards, and in the vacuum of space this tiny effect can build up enough thrust to move a spacecraft. In May, the Planetary Society in Pasadena, California, launched a small solar sail to test the technology. But the forces the team saw were too large to come from photons alone.
The team also ruled out the idea that the laser vaporizes some of the graphene and makes it spit out carbon atoms.
Instead, they think the graphene absorbs laser energy and builds up a charge of electrons. Eventually it can’t hold any more, and extra electrons are released, pushing the sponge in the opposite direction. Although it’s not clear why the electrons don’t fly off randomly, the team was able to confirm a current flowing away from the graphene as it was exposed to a laser, suggesting this hypothesis is correct.
Graphene sponge could be used to make a light-powered propulsion system for spacecraft that would beat solar sails. “While the propulsion force is still smaller than conventional chemical rockets, it is already several orders larger than that from light pressure,” they write.
“The best possible rocket is one that doesn’t need any fuel,” says Paulo Lozano of the Massachusetts Institute of Technology. He thinks a graphene-powered spacecraft is an interesting idea, but losing electrons would mean the craft builds up a positive charge that would need to be neutralised, or it could cause damage.