Paul B. MacCready remembered well his Gossamer Albatross, the first human-powered aircraft to cross the English Channel, almost did not. Its pilot, amateur cyclist Bryan Allen, ran out of water in the middle of the trip and developed leg cramps due to dehydration. Human power was possible, but not yet practical, and Macready thought he could find a better way: solar power.

On this day, July 7, in 1981 MacCready’s Solar Challenger crossed over the English channel from France to Britain; the first time ever an aircraft powered entirely by the sun had done so.

MacCready learned made a number of important adjustments on the Challenger from his experiences with an earlier failed effort, the Gossamer Penguin. The Challenger materials — kevlar, teflon, mylar — all had high strength-to-weight ratios, and although it weighed considerably more than the Penguin its solar power array also gave it more power.