This event marks the launch of the Sputnik 3 satellite, a Soviet satellite launched on May 15, 1958. The satellite was part of the International Geophysical Year and was the first to measure the Earth's magnetic field. It was also the first satellite to measure the Earth's radiation belt
From Wikipedia
Sputnik 1, often referred to as simply Sputnik, was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to Earth for three weeks before its three silver-zinc batteries became depleted. Aerodynamic drag caused it to fall back into the atmosphere on 4 January 1958.