America's first satellite was built to test a rocket — it found a radiation belt instead

When Explorer 1 launched in 1958, physicist James Van Allen's Geiger counter instrument sent back readings that occasionally dropped to zero, puzzling scientists until they realized the detector was being saturated by radiation rather than detecting an absence of it. The satellite had found a doughnut-shaped band of charged particles trapped by Earth's magnetic field.

— James Van Allen, Explorer 1 and the discovery of the Van Allen radiation belt — 1958