The Vela satellites were built to catch secret nuclear tests, but they accidentally recorded flashes from deep space that opened a new branch of astrophysics

The Vela satellites were not built to study stars. They were built to watch for nuclear weapons.

In the 1960s, after the United States, the Soviet Union and the United Kingdom signed the Partial Test Ban Treaty, American defense planners faced a blunt verification problem. A nuclear detonation in the atmosphere, underwater or in space was banned, but a test beyond easy reach of ground-based sensors might be hard to catch. The answer was a fleet of satellites carrying x-ray, gamma-ray and neutron detectors, designed to look for the telltale signatures of a nuclear blast.

Then the satellites saw something that was not a bomb.

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