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

Suggested Topics
Start your day with The Independent, sign up for daily news emails
ebooks
ebooksAn introduction to the ground rules of British democracy
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
SPONSORED FEATURES
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Games Developer - HTML5

£28000 - £45000 per annum: Recruitment Genius: With extensive experience and a...

Recruitment Genius: Personal Tax Senior

£26000 - £34000 per annum: Recruitment Genius: This is an opportunity to join ...

Recruitment Genius: Assistant Product Manager

Negotiable: Recruitment Genius: Due to on-going expansion, this leading provid...

Recruitment Genius: Shift Leaders - Front of House Staff - Full Time and Part Time

£6 - £8 per hour: Recruitment Genius: This is an opportunity to join a family ...

Day In a Page

A Very British Coup, part two: New novel in pipeline as Jeremy Corbyn's rise inspires sequel

A Very British Coup, part two

New novel in pipeline as Jeremy Corbyn's rise inspires sequel
6 best recipe files

6 best recipe files

Get organised like a Bake Off champion and put all your show-stopping recipes in one place
Mullah Omar, creator of the Taliban, is dead... for the fourth time

Mullah Omar, creator of the Taliban, is dead... again

I was once told that intelligence services declare their enemies dead to provoke them into popping up their heads and revealing their location, says Robert Fisk
Margaret Attwood on climate change: 'Time is running out for our fragile, Goldilocks planet'

Margaret Atwood on climate change

The author looks back on what she wrote about oil in 2009, and reflects on how the conversation has changed in a mere six years
New Dr Seuss manuscript discovered: What Pet Should I Get? goes on sale this week

New Dr Seuss manuscript discovered

What Pet Should I Get? goes on sale this week
Oculus Rift and the lonely cartoon hedgehog who could become the first ever virtual reality movie star

The cartoon hedgehog leading the way into a whole new reality

Virtual reality is the 'next chapter' of entertainment. Tim Walker gives it a try
Ants have unique ability to switch between individual and collective action, says study

Secrets of ants' teamwork revealed

The insects have an almost unique ability to switch between individual and collective action
Donovan interview: The singer is releasing a greatest hits album to mark his 50th year in folk

Donovan marks his 50th year in folk

The singer tells Nick Duerden about receiving death threats, why the world is 'mentally ill', and how he can write a song about anything, from ecology to crumpets
Let's Race simulator: Ultra-realistic technology recreates thrill of the Formula One circuit

Simulator recreates thrill of F1 circuit

Rory Buckeridge gets behind the wheel and explains how it works
Twitter accused of 'Facebookisation' over plans to overhaul reverse-chronological timeline

Twitter accused of 'Facebookisation'

Facebook exasperates its users by deciding which posts they can and can’t see. So why has Twitter announced plans to do the same?
Jane Birkin asks Hermès to rename bag - but what else could the fashion house call it?

Jane Birkin asks Hermès to rename bag

The star was shocked by a Peta investigation into the exotic skins trade
10 best waterproof mascaras

Whatever the weather: 10 best waterproof mascaras

We found lash-enhancing beauties that won’t budge no matter what you throw at them
Diego Costa biography: Chelsea striker's route to the top - from those who shared his journey

Diego Costa: I go to war. You come with me...

Chelsea's rampaging striker had to fight his way from a poor city in Brazil to life at the top of the Premier League. A new book speaks to those who shared his journey
Ashes 2015: England show the mettle to strike back hard in third Test

England show the mettle to strike back hard in third Test

The biggest problem facing them in Birmingham was the recovery of the zeitgeist that drained so quickly under the weight of Australian runs at Lord's, says Kevin Garside
Women's Open 2015: Charley Hull - 'I know I'm a good golfer but I'm also just a person'

Charley Hull: 'I know I'm a good golfer but I'm also just a person'

British teen keeps her feet on ground ahead of Women's Open