Sputnik was the first artificial Earth satellite. The Soviet Union launched it into space on 4 October 1957, orbiting for three weeks before its batteries died. It was a sphere with four external radio antennas to broadcast radio pulses. Its radio signal was easily detectable by radio antennas. It was also capable of measuring the thickness and temperature of the high upper atmosphere. The satellite transmission was monitored by radio operators all around the world. It burned up in the atmosphere.