Pluto’s only moon, Charon, is over half the diameter of Pluto, making it the biggest moon in relation to its parent planet in the Solar System. They are only 20,000km (12,430 miles) apart, and are caught in a gravitational headlock that scientists call a dual-planet system. They are so similar in size that they can be thought of as a double planet, as shown below.
NASA/ESA Hubble Space Telescope has obtained the clearest pictures ever of our solar system’s most distant and enigmatic object: the planet Pluto. The observations were made with the European Space Agency’s Faint Object Camera. The ninth and last real planet known, and the only planet that has not been visited by a fly-by spacecraft, Pluto was discovered just 60 years ago by the American astronomer Clyde Tombaugh, who was searching for the source of irregularities seen in the orbits of Uranus and Neptune. It has become apparent since then that Pluto is a very peculiar object. Its orbit is tilted and is more elliptical than the orbits of any of the other planets in the solar system. Pluto also rotates upside down with its North Pole below the plane of the solar system in the opposite sense of the Earth and most of the other planets. Pluto is smaller than our own Moon and also denser than any of its neighbors in the outer solar system. But, perhaps, its most fascinating property was uncovered only 12 years ago when a huge companion “moon” called Charon was detected from ground based photographs. Subsequent investigations have shown that Charon is about half the size of Pluto making it the largest known satellite relative to its planet in the solar system. In fact, because of this, Pluto is often referred to as a double planet. The rotation period of the Pluto-Charon system is a mere 6 days. A recent Faint Object Camera image of Pluto and Charon is shown in the upper right hand frame of the accompanying photograph. This image is the first long duration HST exposure ever taken of a moving target. In order to avoid smearing of the images, ground controllers had to pre-program the HST spacecraft to track Pluto extremely accurately and compensate exactly for the “parallax” introduced by the combined motions of Pluto, the Earth and HST in their respective orbits. Pluto is currently near its closest approach to the Earth in its 249 year journey around the Sun, and is approximately four and a half billion kilometers away.