The Vela defense satellites were launched in the 1960s to monitor a treaty that forbade nuclear explosions in space. The Vela satellites did not detect any nuclear explosions. However, a serendipitous by-product of this mission was the discovery of celestial gamma-ray bursts (GRBs): short and intense pulses of gamma rays arriving from random directions in the sky. Several times a day a cosmic explosion takes place somewhere in the universe, and its echoes reach us as a burst of gamma rays. The burst appears suddenly, lasts for a few seconds, and then fades and disappears forever. The lack, until recently, of any long-lasting counterparts to these short bursts hindered their study, making them one of the most puzzling phenomena in modern astronomy.