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Pluto - A frozen rock
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Pluto is a tiny icy rock skirting the outer solar system at a 
distance as far as 7.5 thousand million kilometres from the 
Sun. At that distance the Sun appears as little more than a 
bright star, and the temperature falls to under -230°C, cold 
enough for a surface of frozen methane to have formed on Pluto. 
Observations of Pluto are still blurred because of its great 
distance but recent pictures indicate that the surface has more 
regions of contrast than any planet excluding the Earth.

Pluto is in many ways the oddball planet in the solar system. 
It is a tiny world, smaller even than our moon, and its 
location and composition do not fit the current models of how 
the solar system formed. These facts along with its highly 
elongated orbit have lead some scientists to believe that it 
may not be a planet, but rather an escaped moon of Neptune. 
This theory would help to explain some of the peculiar orbits 
of Neptune's other moons.

Pluto has a single moon, known as Charon, which orbits at a 
distance of 19 thousand kilometres every six days. The 
Pluto-Charon system is the most closely matched planet-moon 
pair in the solar system, with Charon measuring around half 
the size of Pluto. This similarity seriously effects Pluto's 
orbit, and both objects are locked around a common centre of 
mass like a dumbbell, with the same face fixed towards each 
other. Astronomers have used the gravitational properties of 
the system to calculate the masses, and the repeated eclipses 
that occur to try to map the surface.

Apart from the basic properties of the planet, very little is 
known about Pluto and its moon. This may change with the 
arrival of a probe known as Pluto Express. Scheduled to arrive 
at Pluto around 2007 two small probes will pass within 15 
thousand kilometres of Pluto, providing the first high resolution 
mapping of its surface.

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Images
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1.  The Pluto-Charon System
(http://www.solarsystem.f2s.com/pluto/img1a.jpg)

A rare image of tiny Pluto with its moon Charon, which is 
slightly smaller than the planet. Because Pluto has not yet 
been visited by any spacecraft, it remains a mysterious 
planet.

Public domain image courtesy of NASA.

2. Hubble Space Telescope image of Pluto and Charon
(http://www.solarsystem.f2s.com/pluto/img2a.jpg)

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 kilometres from Earth (nearly 30 times 
the separation between Earth and the Sun). 
 
© Calvin J. Hamilton