24 June 2015

Charon emerged from Pluto's shadow when U.S. Naval Observatory astronomer James W. Christy, working with colleague Robert Harrington, discovered it on June 22, 1978.

Pluto was discovered in 1930, but the icy world's large companion remained hidden for 48 more years due to the close proximity of the pair, which made them appear to blur together in observations. Charon and Pluto are separated by about 12,000 miles, with Charon measuring about 790 miles in diameter– slightly more than half Pluto's size. Scientists sometimes refer to the objects as a "double planet" due to their sizes and close proximity. Christy noticed that Pluto looked elongated in images, as if a blur moved around the planet at a rate of about 6.4 days, the time it takes Charon and Pluto to complete their "pas de deux" rotation about their common center of gravity. Christy studied archives of images of Pluto, which confirmed he had found Pluto's first moon.
Charon's anniversary comes as NASA's New Horizons spacecraft makes its approach to the Pluto system. The spacecraft's closest approach to within 7,750 miles of Pluto and 17,900 miles of Charon will occur on July 14, 2015, when scientists hope it will snap photos clear enough to depict surface objects on Pluto as small as 200 feet across. That is smaller than the length of a 747 jet.
New Horizons took the first color image of Pluto and Charon on April 9, 2015. The Ralph color imager made the image from a distance of about 71 million miles. NASA's Goddard Space Flight Center in Greenbelt, Maryland, contributed Ralph's spectral imager, the Linear Etalon Imaging Spectral Array, known as LEISA.

Image: The Ralph imager on New Horizons took the first color image, seen here, of Pluto and Charon on April 9, 2015. Clearly visible are Pluto and Texas-sized Charon, the smaller dot.