Comment Can't be fuel-free forever (Score 3, Interesting) 265
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 (arxiv.org/abs/1505.04254).
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.
So they'd need to carry hydrogen and split off its electrons or something to neutralize the charge.