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Vanguard I was the fourth satellite launched into orbit, and the second launched by the United States. (Others launched before were the Soviet Sputniks I and II and the American Explorer I.) Vanguard I was launched from Cape Canaveral, Florida, on March 17, 1958, just over 5 months after the launch of the first satellite, Sputnik. Vanguard I was also the first satellite to be solar-powered.
Vanguard I's greatest claim to fame is its status as the longest artificial satellite to be in orbit, having circled the planet more than 196,990 times since its launch. This adds up to a distance of 5.7 billion nautical miles, the distance from Earth to the dwarf planet Pluto and then halfway back. This is just over half the distance traveled by the space probe Voyager 1, now far outside the boundaries of the solar system, making it one of the farthest-traveled artificial objects in history.
On March 17, 2008, American scientists celebrated Vanguard I's 50th year in orbit. Vanguard I is the oldest satellite still in orbit. Its orbital path is an ellipse, 654×3969 km (406×2466 mi) in extent, inclined 34 degrees with respect to the plane of the elliptic. This puts it in Mid-Earth orbit (MEO). The satellite was initially launched on a trajectory that was thought to have it in orbit for 2000 years, but subsequent analyses, taking into account solar radiation pressure and atmospheric drag, have found it will remain in orbit for a total of "only" 240 years. This mistaken estimate is a result of the limited knowledge of space at the time of the satellite's launch.
Vanguard I was quite small for a satellite — a 1.47 kg (3.2 lb) aluminum sphere 152 mm (6 in) in diameter. For its diminutive size compared to a satellite launched earlier by the Soviet Union, Sputnik (weighing 83.6 kg (about 183 lb) and 2 ft across), then Soviet Premier Nikita Khrushchev called it "the grapefruit satellite". However, Vanguard I returned important information on geodesics and the dynamics of orbits, opening the window into space and paving the way for thousands of future satellites.