This event marks the launch of the Sputnik 4 satellite, which was launched on May 15th, 1960. The satellite was the fourth in a series of Soviet satellites, and was the first to be launched into a near-circular orbit. The satellite was used to study the Earth
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.