Sputnik 1 was the first artificial satellite successfully placed in orbit around the Earth. (The Russian word "Sputnik" means "companion," "satellite" in the astronomical sense.) In 1885, in his book "Dreams of Earth and Sky," Konstantin Tsiolkovsky had first described how such a satellite could be launched into a low altitude orbit. Coming at the height of the Cold War, the launch caught the West by surprise, and began the space race by galvanizing interest and action on the part of the American public to support an active role in space research, technology, and exploration.

Sputnik 1 was launched on an R-7 (ICBM) booster from Baikonur Cosmodrome at Tyuratam (370 km southwest of the small town of Baikonur) in Kazakhstan, then part of the former Soviet Union, on 4 October 1957 at 10:28:04 pm, Moscow time. It was the first in a series of four satellites in the Soviet Sputnik program, a contribution to the International Geophysical Year (1957-1958). Three of these satellites (Sputnik 1, 2, and 3) reached Earth orbit.

The Sputnik 1 satellite was a 58.0 cm (14.7 inches) diameter aluminum sphere that weighed 84 kg (184.3 lb) with four whip-like antennas that were 2.4-2.9 meters long. The antennas looked like long "whiskers" pointing to one side. The spacecraft obtained data pertaining to the density of the upper layers of the atmosphere and the propagation of radio signals in the ionosphere. The instruments and electric power sources were housed in a sealed capsule and included transmitters operated at 20.005 and 40.002 MHz (about 15 and 7.5 meters wavelength), the emissions taking place in alternating groups of 0.3 seconds duration. The downlink telemetry included data on temperatures inside and on the surface of the sphere.

Since the sphere was filled with nitrogen under pressure, Sputnik 1 provided the first opportunity for meteoroid detection (no such events were reported), since losses in internal pressure due to meteoroid penetration of the outer surface would have been evident in the temperature data. The satellite transmitters operated for three weeks, until the on-board chemical batteries failed on 26 October 1957, and were monitored with intense interest around the world.

The orbit of the then inactive satellite was later observed optically to decay 92 days after launch (4 January 1958), after having completed about 1400 orbits of the Earth over a cumulative distance traveled of 70 million kilometers. The orbital apogee declined from 947 km after launch to 600 km by 9 December.

The Sputnik 1 booster rocket also reached Earth orbit and was visible from the ground at night as a first magnitude object, while the small but highly polished sphere, barely visible at sixth magnitude, was more difficult to follow optically. Several replicas of the Sputnik 1 satellite can be seen at museums in Russia and another is on display in the Smithsonian National Air and Space Museum in Washington, DC.

Sputnik 1's launch is the basis of selecting 4 October - Sputnik Day - as one of the L5 National Holidays.

https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1957-001B