Pluto - the smallest, coldest and most remote planet - made its closest
approach to the Sun in 1989, about a year before the launch of the HST.
With the help of HST, astronomers hope to monitor changes in its thin atmosphere
and icy surface as it moves outwards on its next 247-year orbit around
the Sun. In this image, taken in July 1994 when Pluto was about 4,400 million
kilometers (2,600 million miles) away, the large, northern polar ice cap
was clearly revealed for the first time. Ground-based observations of the
spectrum of Pluto indicate that the predominant icy material on the surface
is frozen molecular nitrogen, N2
To show up the surface features more clearly, the contrast in this
image has been exaggerated. The regions at mid and southern latitudes that
appear dark are actually quite bright, reflecting 30-50% of the light falling
upon them.
For 48 years after the discovery of Pluto in 1930, astronomers failed to see its moon, Charon, because the pair was very difficult to separate with the telescopes and techniques then available. It was not until 1978 that Charon was spotted by James Christy of the US Naval Observatory. Pluto is less than one fifth the size of Earth and Charon is just over one half the size of Pluto. The two are 19,640 kilometers (12,200 miles) apart. As shown here, the HST has no problem in revealing Pluto and Charon as distinct objects with a wide separation between them. However, no surface features can be made out on Charon.
Camera: Faint object Camera with COSTAR
Technical information: Taken through blue filter (410 nm). Resolution
is 12 to 15 resolution elements over the disk of the planet, almost 5 across
a diameter.
Credit: S. A. Stern (SWRI), M. W. Buie (Lowell Observatory),
L. M. Trafton (university of Texas at Austin), NASA, and ESA
