Pluto and Charon, the odd couple

New images reveal the tiny planet and its moon in detail.

Eighteen years ago this month, Jim Christy - an astronomer at the US Naval Observatory in Washington - found that what looked looked like a pear-shaped Pluto was in fact the combination of two bodies - Pluto, and its hitherto undiscovered moon. He named his discovery Charon - not after the ferryman who transported souls across the River Styx to Pluto's Underworld, but after his wife, Charlene. Those in the know pronounce it "Sharon", instead of the classical "Kar-on".

The discovery of Charon cleared up one mystery about Pluto - its mass. Astronomers believed they had originally tracked the planet down as a result of its gravitational pull on Uranus and Neptune. But observations of the orbit of Charon revealed that Pluto is only one-500th the mass of the Earth - you'd even need 30 Plutos to make up the next smallest planet, Mercury. Now astronomers believe that the apparent discrepancies in the paths of Uranus and Neptune were simply errors of measurement.

Pluto and Charon are a strange couple. Each is tiny: Pluto measures 2,284km across, while Charon's diameter is a mere 1,192km. The pair look like a double planet, especially as Charon is 20 times closer to Pluto than the Moon is to the Earth. This proximity has raised huge tidal bulges on both bodies, and the pair are now locked facing one another.

From the "far" side of Pluto, you would never see its moon. Christy's discovery of Charon came at a fortuitous time. For just a few years in Pluto's 248-year orbit about the Sun, the two bodies are angled to us in a way that they periodically eclipse each other. The eclipses started in the mid-1980s, and astronomers were able to monitor the changing brightness of the system as the bodies covered and uncovered one another. This allowed them to make the first crude maps of Pluto and its moon, revealing that Pluto has bright polar caps - probably made of methane ice.

Now the Hubble Space Telescope has scanned Pluto, revealing details as small as 160km across. The images look remarkably like Mars viewed through a small telescope, showing broad dark and light patches. These may be regions of frost which move across the planet according to its seasons.

We may be in for an even closer view. In 2001, America's space agency NASA hopes to launch the first space mission to Pluto - the Pluto Express. It would consist of two small spacecraft targeted to arrive at Pluto three to six months apart in 2013. But time is of the essence. Pluto, which has a very oval, tilted orbit, is currently at its closest to the Sun - inside the orbit of Neptune. The (relative) warmth has spurred the tiny planet into activity, producing a thin temporary atmosphere. If we are to see Pluto at its most exciting - and perhaps even explore its atmosphere with the Russian Drop Zond probe that the Pluto Express may carry - then the mission must get off the ground without delay.

The sky in June

Venus, so long a feature of our evening skies, plunges down into the twilight this month. On June 10 it passes in front of the Sun ("inferior conjunction"), after which it will re-emerge as a morning star.

Mercury and Mars are both morning objects, rising one and two hours before sunrise, respectively, by the third week of the month. But they will be hard to spot in the dawn twilight.

Jupiter is now up before midnight, in the star-packed (but low) constellation of Sagittarius. And its fellow gas-giant, Saturn, starts to put in an appearance in the south-east from about 2am.

Star-wise, we are in a transition period betweeen the rather lacklustre constellations of spring and the brighter constellations of summer. Arcturus, the brightest star in Bootes (the Herdsman), is at its highest this month - find it by extending the handle of the Plough downwards. This distinctly orange star, fourth-brightest in the sky, is close to the end of its life. Having used up almost all of its nuclear fuel, it has now become a bloated red giant star almost thirty times bigger than the Sun.

Diary (all times BST)

June 1 9.47pm Full Moon

8 12.06pm Moon at last

quarter

10 Venus at inferior

conjunction

16 2.36am New Moon

21 3.24am Summer

solstice

24 6.23am Moon at first

quarter

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