This is a recently-declassified US government photo of a hydrogen bomb detonating in space. The test, called Starfish Prime, set off the nuke 250 miles above the Earth's surface. NPR explains that the US did so see if the Van Allen radiation belts around the Earth had military uses:
The plan was to send rockets hundreds of miles up, higher than the Earth's atmosphere, and then detonate nuclear weapons to see: a) If a bomb's radiation would make it harder to see what was up there (like incoming Russian missiles!); b) If an explosion would do any damage to objects nearby; c) If the Van Allen belts would move a blast down the bands to an earthly target (Moscow! for example); and — most peculiar — d) if a man-made explosion might "alter" the natural shape of the belts.
Video at the link.
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