To fully appreciate how spacecraft can deploy a solar sail to harness the power of sunlight, let's travel back in time for a brief history of solar sailing. Almost 400 years ago, German astronomer Johannes Kepler observed comet tails being blown by what he thought to be a solar "breeze." This observation inspired him to suggest that "ships and sails proper for heavenly air should be fashioned" to glide through space.
Little did Kepler know, the best way to propel a solar sail is not by means of solar wind, but rather by the force of sunlight itself. In 1873, James Clerk Maxwell first demonstrated that sunlight exerts a small amount of pressure as photons bounce off a reflective surface. This kind of pressure is the basis of all modern solar sail designs.
In 1960, the large balloon-like satellite Echo-1 (pictured) felt these solar pressure effects loudly and clearly. "Photon pressure played orbital soccer with the Echo-1 thin-film balloon in orbit.... The shards were flung far and wide by sunlight."
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