The Sputnik program was a series of unmanned space missions launched by the Soviet Union in the late 1950s to demonstrate the viability of artificial satellites. The name "Sputnik" ("Спутник") means "fellow traveller" in Russian.
Sputnik 2 was launched some months later, and carried the first living passenger, a dog named Laika. The mission planners did not provide for the safe return of the spacecraft or its passenger, making Laika the first space casualty.
The first attempt to launch Sputnik 3 on February 3, 1958 failed, but the second on May 15 succeeded, and it carried a large array of instruments for geophysical research. Its tape recorder failed, however, making it unable to measure the Van Allen radiation belts.
Sputnik 5 was launched into orbit on August 19, 1960 with the dogs Belka and Strelka (Russian for "Squirrel" and "Little Arrow"), 40 mice, 2 rats and a variety of plants on-board. The spacecraft returned to earth the next day and all animals were recovered safely.