In a story written by the science fiction author Arthur C Clarke in 1964, the hero asks a group of students to hold their hands out to the sun. “What do you feel?” he asks. “Heat, of course. But there’s pressure as well — though you’ve never noticed it, because it’s so tiny. Over the area of your hands, it only comes to about a millionth of an ounce.”
The scientist is right: sunlight has no mass, but it has momentum and it exerts pressure — a property that will soon be harnessed when Nasa unfurls the largest solar sail it has ever launched.
Last month a rocket blasted off from New Zealand carrying an unmanned spacecraft called an Advanced Composite Solar Sail System (ACS3)
