The Pluto reconnaissance flyby mission



Although Pluto was discovered in 1930, our modern view of this distant world only began taking shape in 1976. Prior to that time, the technology for studying this faint, 14th magnitude object was simply too immature. Virtually all that was known before 1976 was that Pluto circles the Sun in an unusually eccentric 248-year orbit (ranging from 29.5 to 49.4 AU) that is tilted far (17°) from the ecliptic—the plane in which the other planets orbit, that Pluto's rotation period is 6.39 days, that its intrinsic surface color is reddish, and that its rotational lightcurve is the strangest and most variegated of the planets.

Since 1976, however, the pace and diversity of discoveries have increased dramatically. First, methane ice was discovered on Pluto's surface, which gave a clear indication that Pluto was formed in the outer solar system rather than simply ejected to it from another region. Next came the discovery of Pluto's large satellite Charon (usually pronounced “Sharon”) in 1978. Lying just 19,400 km from Pluto (<1 arcsecond as seen from Earth), Charon orbits Pluto every 6.387 days—Pluto's rotation period. Thus, unlike any other satellite in the solar system, Charon orbits Pluto at its synchronous orbit.