Synthetic-aperture radar is making the Earth’s surface watchable 24/7

THE FIRST time that humans observed a battlefield from a celestial vantage point was in June 1794, scarcely a decade after the Montgolfier brothers had invented the hot-air balloon. The French Aerostatic Corps, a motley crew of chemists, carpenters and hangers on (sometimes literally), flew a tethered hydrogen balloon, l’Entrepenant, over the battlefield at Fleurus, in what is now Belgium. The spotters on board informed their comrades down below about the disposition and movements of their Austrian enemies by semaphore. France won the battle.