Pluto and Charon: Double Planet			PIA00827
			This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope (HST).
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Image Credit: Dr. R. Albrecht, ESA/ESO Space Telescope European Coordinating Facility; NASA
This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope (HST). The image was taken by the European Space Agency's Faint Object Camera on February 21, 1994 when the planet was 4.4 billion km from Earth; or nearly 30 times the separation between Earth and the sun. Hubble's corrected optics show the two objects as clearly separate and sharp disks. This now allows astronomers to measure directly (to within about 1 percent) Pluto's diameter of 2320 km and Charon's diameter of 1270 km. The Hubble observations show that Charon is bluer than Pluto. This means that both worlds have different surface composition and structure. A bright highlight on Pluto suggests it has a smoothly reflecting surface layer.		A detailed analysis of the Hubble image also suggests there is a bright area parallel to the equator on Pluto. This result is consistent with surface brightness models based on previous ground-based photometric observations. However, subsequent HST observations will be required to confirm whether the feature is real. Though Pluto was discovered in 1930, Charon wasn't detected until 1978. That is because the moon is so close to Pluto that the two worlds are typically blurred together when viewed through ground-based telescopes. (If our moon were as close to Earth, it would be as big in the night sky as an apple held at arm's length). The new HST image was taken when Charon was near its maximum elongation from Pluto of .9 arc seconds. The two worlds are 19,640 km. Pluto typically is called the double planet because Charon is half the diameter of Pluto (our Moon is one-quarter the diameter of Earth).
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