On October 4, 1957, the Soviet Union successfully launched the first artificial satellite into orbit. Sputnik I (the name means "Fellow Traveller") was about the size of a basketball and weighed about 183 pounds (about 85 kilograms). It carried two radio transmitters which broadcast beeping signals at 20.005 and 40.002 megacycles, and which could easily be detected with amateur radio equipment.
Overshadowing the technological achievement for people at the time was the geopolitical significance of the event. The Soviet Union's effort was three months ahead of the United States launch of Explorer I, and outweighed the intended payload of that satellite significantly. For many in the United States, the launch of Sputnik I was seen not as a scientific achievement, but a demonstration of military capability. If the Soviets could launch a significant payload into orbit, the reasoning went at the time, then they could easily reach the western hemisphere with a nuclear weapon. For its part, Tass, the Soviet news agency, proclaimed the achievement as evidence of the superiority of socialism.
The event triggered the Space Race, in which the United States and the Soviet Union attempted to best each other in the exploration of space. This eventually culminated in the Apollo 11 mission to the Moon.
http://www.nytimes.com/partners/aol/special/sputnik/; includes transcripts from the New York Times's original articles from the week following the launch.