On March 17, 1958, Vanguard 1 became the second artificial satellite successfully placed in Earth orbit by the United States. It was the first solar-powered satellite. Just 152 mm (6.0 in) in diameter and weighing just 1.4 kg (3.1 lb), Vanguard 1 was described by then-Soviet PremierNikita Khrushchev as, "The grapefruit satellite."
Vanguard 1, and the upper stage of its launch rocket, are the oldest artificial satellites still in space, as Vanguard's predecessors, Sputnik 1, Sputnik 2, and Explorer 1, have decayed from orbit.
The Air Force proposal was not seriously considered, as Atlas development was years behind the other vehicles. Among other limitations, the Army submission focused on the vehicle, while a payload was assumed to become available from JPL, and the network of ground tracking stations was assumed to be a Navy project. Meanwhile, the NRL proposal detailed all three aspects of the mission.