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New Horizons: Countdown to Pluto

We're about to get our first ever close-up look at Pluto as NASA's New Horizons spacecraft closes in on the distant frozen world.

Pluto was still a planet when New Horizons was launched on its historic mission in 2006.

The mission will help fill in all the blanks about this odd-ball world with its weird orbit and swarm of moons.

"This is raw exploration, and I'm looking forward to being surprised," says the mission's lead scientist, Dr Alan Stern of the Southwest Research Institute in Boulder Colorado.

"We've not only never been to the Pluto system, we've never been to this new third class of planets out in the Kuiper Belt [a ring of frozen debris and comets orbiting the Sun beyond Neptune].

On July 14 the small washing machine-sized craft will fly 12,500 kilometres above Pluto's unexplored surface giving us the best view yet of the dwarf planet.

Until now, all we've known about Pluto has come from fuzzy images from the Hubble Space Telescope and ground-based observatories.

"The very best images ever made by any telescope on Earth or in Earth orbit were made by the Hubble, but they're only five pixels across.

"If you pixelate the Earth to that resolution you can't even find the continents, you can't tell that there are continents and oceans in a picture like that.

"We're going to bring the imagery to a point that we can bring out football fields on the surface of Pluto with New Horizons."

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What will it study?

The mission will help us understand how Pluto and the Kuiper Belt formed, and how that fits in with the early evolution of the solar system.

"New Horizons is carrying the most powerful set of cameras and spectrometers and other instruments ever brought to bear on the first reconnaissance of a new system like this," says Stern.

He says the spacecraft will study the atmospheres, surface features, geology and environments of Pluto, its many moons, and other Kuiper Belt objects.

It will also study Pluto's atmosphere and search for an atmosphere around its moon Charon.

New Horizons will also search for rings and additional moons around Pluto.

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The mission so far

Zipping through the solar system at 53,000 kilometres per hour, New Horizons was the fastest spacecraft ever launched.

When it blasted off on January 19, 2006, it took just 19 hours to reach the orbit of the Moon - it took the Apollo astronauts three days to fly to the Moon.

From there it flew past Jupiter to take advantage of the giant planet's immense gravity, ramping up its velocity to 83,000 kilometres per hour and flinging it out beyond Neptune.

The Jupiter encounter was a dress rehearsal for the main event, says Stern.

"It really was a shakedown cruise because Jupiter really was the only thing we passed along the way," he says.

In April this year, New Horizons sent back its first colour images.

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Where are we now?

After a journey of over nine years and four months New Horizons is finally approaching Pluto.

'We are 99 per cent of the way there," says Stern.

And already the science is well underway.

"Our telescopic imager ... has detected what appears to be a polar cap at the north pole of Pluto.

"We are now looking for new moons, we can see additional surface details, and we are beginning to train our spectrometers to study Pluto's atmosphere and its surface composition.

"Of course it reaches a crescendo around the 14th of July when we're in the heart of the system and flying through it."

After visiting Pluto and its moons, New Horizons will continue exploring the Kuiper Belt.

At this stage, the team are interested in two 50-kilometre-wide objects, known as PT1 and PT3, which were found by Hubble.

"We have to choose between them because they're in different directions," says Stern.

Unfortunately PT2 doesn't get a look-in.

"PT2 was an early good candidate but as we studied its orbit more we found out that it was just a bit beyond our fuel supply," says Stern.

Hubble is still searching for more potential targets for New Horizons to explore.

"We're just fastening our seat belt, we're along for the ride, and we're going to see what we discover," says Stern.

"All the early space missions 50 years ago, 40 years ago, the first to Mars, the first to Venus, the first to Mercury and Jupiter and so forth made enormous unanticipated discoveries.

"The big lesson of these first-time missions is to expect the unexpected."

Pluto vs the planets+ Pluto was discovered in 1930. It was classified as a planet until August 2006 when it was demoted to a dwarf planet by the International Astronomical Union.+ It is tiny! With a diameter of 2322 kilometres, it is about a sixth of the size of Earth.+ It is tilted at 120 degrees and rotates on its side like Uranus. At the moment its north pole is facing the Sun.+ It takes 248 Earth years to orbit the Sun. Its orbit is highly elongated and tilted compared to the traditional planets -- so much so that it was closer to the Sun than Neptune between 1979 and 1999.+ It has a surface temperature of minus 230 degrees Celsius, and a thin atmosphere that freezes onto the ground in winter and turns back into a gas in summer.Both the atmosphere and surface of Pluto is made up of mostly nitrogen, with trace amounts of methane and carbon dioxide.+ It has five known moons. The largest moon Charon doesn't orbit around Pluto like the Moon orbits Earth, instead they orbit each other around a common centre of gravity.

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