They achieved the feat by observing Charon as it passed in front of a star last July -- an event that has been witnessed only once before in the past quarter-century.

Under this so-called occultation process, light from the star is dimmed and refracted as it passes through an object's atmosphere but is barely touched if the object has no atmosphere.

Using the high-powered European Southern Observatory (ESO) telescope located in the dry mountains of Chile's Atacama Desert, the astronomers determined that Charon has a density of about 1.71 of that of water.

That indicates the moon is an icy body, with rock comprising about half its volume.

The density is also very similar to that of Pluto's, and could back theories that the planet wacked into a large space object, causing a large chunk to break off and eventually be enslaved as a satellite.

The occultation also yielded a remarkably accurate measurement of Charon's size.

It has a diameter of between 1,206 and 1,212 kilometers (753.75 and 757.5 miles), give or take five kms (3.1 miles). It is almost half the size of Pluto's 2,300 kms (1,437 miles).

Pluto, discovered in 1930 by the American astronomer Clyde Tombaugh, is the outermost of the recognised planets, although a new contender for that title emerged last year in the form of an object called 2003 UB313.

Charon was discovered in 1978. It orbits so close to Pluto, at a distance of less than 20,000 kms (12,000 miles) that some astronomers have wondered whether they should be classified as a double planet system rather than mother and satellite.

The pair circle the Sun at a distance ranging from 4.5 to seven billion kms (2.81-4.37 billion miles), and take 248 Earth years to complete a single orbit.

A mission by the US space agency NASA to explore Pluto and Charon is scheduled for liftoff later this month, with a launch window opening on January 17. The 700-million-dollar unmanned probe, New Horizons, will take nine years to reach its goal.

The two studies, headed by Amanda Gulbis of the Massachusetts Institute of Technology (MIT) and Bruno Sicardy of the Paris Observatory, appear on Thursday in Nature, the British weekly science journal.

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