in 1959, the us started to experiment with space-based nuclear sensors, beginning with the vela hotel satellites. these were originally intended to detect nuclear explosions in space, using x-ray, neutron and gamma-ray detectors. advanced vela satellites added devices called "bhangmeters", which could detect nuclear tests on earth by detecting a characteristic signature of nuclear bursts: a double light flash, with the flashes milliseconds apart. these satellites also could detect electromagnetic pulse (emp) signatures from events on earth.