The first artificial satellite, Sputnik I, was launched by the USSR in 1957 and spurred the dormant US space program into action, leading to an international competition popularly known as the “space race.” Explorer I, the first American satellite, was launched just months later, in January 1958. In the decade that followed, the US and the USSR launched approximately 50 space probes between them to explore the Moon. What project is said to have marked the end of the space race? Discuss