On October 4th 1957, the Soviet Union launched Sputnik 1 into low Earth orbit.
Sputnik 1 was a comparatively tiny satellite, only 58 cm in diameter, and it only reached a height of, at most, a few hundred kilometres above the Earth, but it was the first artificial satellite to reach orbit.
The successful launch of the satellite triggered the Space Race, as both the Soviet Union and the United States attempted to increase their space presence. The US, in particular, was taken by surprise by the Soviet success, and this at least in part resulted in America's attempts to put a man on the Moon.
Image: By U.S. Air Force photo (www.nationalmuseum.af.mil; exact source) [Public domain], via Wikimedia Commons