Solar sail
A solar sail is a vast, thin sheet that lets sunlight push a spacecraft, with no propellant. In 2010 IKAROS proved it in flight, and its sail generated electricity too.
A conventional rocket cannot fly without propellant. A solar sail needs none: sunlight presses on a thin sheet and pushes it, with nothing burned at all. IKAROS, launched in 2010, unfolded a polyimide square 14 metres on a side and 0.0075 millimetres thick, about 1.9 million times wider than it was thick. And the same film carried thin solar cells, so the light pushing the craft also powered it.
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