Vanguard I celebrated its 50th birthday this year as both the first solar-powered satellite and the oldest artificial satellite still orbiting Earth.

The satellite has traveled 196,990 revolutions of the Earth or 5.7 billion nautical miles in the past 50 years, equivalent to the distance from the Earth to beyond the planet Pluto and halfway back.

The United States launched Vanguard I on March 17, 1958, from Cape Canaveral, Florida, as part of the opening salvoes in the Space Race. The Soviets had earlier launched Sputniks I and II, while the U.S. had managed to get Explorer I into orbit.

The 3-pound, 6-inch-diameter Vanguard I is small compared with the 200-pound Sputnik I launched by the Soviets, and earned the nickname "the grapefruit satellite" from then-Soviet Premier Nikita Krushchev. However, the satellite has outlasted all its Soviet and U.S. predecessors.

The U.S. Army, U.S. Air Force and U.S. Navy collaborated to track, build and launch Vanguard. Official responsibility for the satellite fell to the Naval Research Laboratory (NRL), which based the design on German V-2 and Viking rockets used to probe Earth's upper atmosphere.

Vanguard's solar technology paved the way for other U.S. satellites that have launched since — the early satellite's solar cells operated for about seven years, while conventional batteries powering another onboard transmitter lasted just 20 days.

The satellite fell silent in 1964 after its last solar cells died, but continues to allow scientists to discover the effects of the sun, moon and atmosphere on satellite orbits. NRL researchers used many of the lessons learned from the original Vanguard tracking system to build a Space Surveillance System that can detect unannounced, radio-silent satellites passing over the U.S.

Scientists originally estimated that Vanguard would have a life expectancy of about 200 years. Now the estimate stands at 2,000 years, meaning that Vanguard should see many more anniversaries to come.

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