Explorer 1 discovers the Van Allen radiation belt

What happened

Explorer 1, the first American satellite, launched from Cape Canaveral at 10:48 p.m. EST on 31 January 1958, atop a Jupiter-C rocket built by Wernher von Braun's Army Ballistic Missile Agency team, with the Jet Propulsion Laboratory managing the spacecraft and mission. Weighing just 30.66 pounds, 18.35 pounds of which was instrumentation, Explorer 1 carried a cosmic-ray detector designed by University of Iowa physicist James Van Allen. Unlike Sputnik, the American satellite returned real scientific data. Van Allen's instrument registered a far lower cosmic-ray count than expected in some parts of the orbit; he theorized the detector had been saturated by intense radiation from a belt of charged particles trapped by Earth's magnetic field. A second satellite two months later confirmed the belt existed, and it was named after him.

How we know

NASA's own mission history and its Explorer 1 fast-facts page document the launch details, weight, and instrumentation; the Van Allen belt discovery is described in the University of Iowa physics department's own account of the mission Van Allen led.

Sources

• NASA Science. Explorer 1 · Primary source (author-declared)science.nasa.gov · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)
• NASA. Explorer 1 Fast Facts · Primary source (author-declared)nasa.gov · Cited as a "primary" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)