Time-sequence of Pluto and moon Charon
Credit: Gemini Observatory
Pluto and its moon Charon are shown in this sequence of four infrared images obtained on different nights during June 1999 at Gemini, utilizing the University of Hawaii's infrared camera QUIRC and adaptive optics (AO) system, Hokupa'a. The AO system uses a flexible mirror to compensate for distortions caused by the earth's atmosphere and allows images which are as sharp as possible to be recordedat this wavelength.
Pluto, the most distant planet in our solar system, was discovered by American astronomer Clyde Tombaugh in 1930. Charon was discovered in 1978 when James Christy, an astronomer at the U.S. Naval Observatory, noticed that Pluto appeared slightly elongated compared to stars in the same image. The new Gemini infrared images, easily separate the two objects, illustrating the rapid advance in imaging sharpness at ground-based observatories during recent years.
Charon orbits Pluto every 6.4 days at a distance of 20,000 km. Although the orbit actually is nearly perfectly circular, as viewed from the earth it appears highly elongated with maximum northern and southern separations of 0.9 arc-seconds (1/4000 of a degree, equivalent to 1/2000 of our moon's diameter as viewed from Earth). Thus at different phases of its orbit Charon appears to be different distances from Pluto.
Pluto is twice the diameter of Charon (2,300 km vs 1200 km). Largely because of this it reflects more sunlight and therefore is brighter. Although Pluto and Charon are small and are 6 billion kilometers distant from earth, in these AO images they do not quite appear as "point sources" of light, as do the distant stars. By careful comparison of their images with the those of stars in the same field of view the diameters of Pluto and Charon can be estimated from these Gemini images.
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