Although Vanguard's solar-powered "voice" became silent in 1964, it continues to serve the scientific community. Ground-based optical tracking of the now-inert Vanguards continues to provide information about the effects of the Sun, Moon, and atmosphere on satellite orbits. Vanguard I marked its 50th year in space on March 17, 2008. In the years following its launch, the small satellite has made more than 196,990 revolutions of the Earth and traveled 5.7 billion nautical miles, the distance from Earth to beyond the dwarf planet Pluto and halfway back. Original estimates had the orbit lasting for 2000 years, but it was discovered that solar radiation pressure and atmospheric drag during high levels of solar activity produced significant perturbations in the perigee height of the satellite, which caused a significant decrease in its expected lifetime to about 240 years.