January 31, 1958Reputable sourceWell documented
Explorer 1 — America reaches orbit
On the timeline · around January 31, 1958 ·
What happened
The first US satellite carried James Van Allen's cosmic-ray experiment and discovered the radiation belts that now bear his name.
Why it matters
The first satellite to return a major scientific discovery: space itself had structure.
Sources
- NASA. Explorer 1 (mission page) · Reputable source