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WILL THE REAL PLUTO PLEASE STAND UP

No probe has ever visited it. Nor has it ever been photographed in any clear detail by a telescope: Pluto is one of the last great mysteries of the solar system. But now, at last, mankind is all set for a close encounter with the dwarf planet

Cosmic explorers like Alan Stern need one thing above all else: endless patience. For while ships here on Earth only need a couple of months to complete their journeys, it can take space-faring vessels decades to reach their objectives. A lot can happen on Earth during  that time – and that can include an astronomical revolution turning the whole mission on its head…



IS IT RIGHT THAT PLUTO WAS DOWNGRADED TO A DWARF PLANET?
When the New Horizons space probe took off in 2006, under the leadership of NASA engineers, its target, Pluto, was considered the ninth planet in the solar system. Shortly afterwards, the astronomical community downgraded its status and, since then, it has officially been known as a dwarf planet. But why? Pluto’s size and shape has remained the same, so the reason for the redefinition is down to its neighbourhood.

“A series of discoveries has thrown everything we know about the architecture of our solar system out of the window,” explains Stern. Scientists have slowly realised that Pluto is in fact the guardian of a gigantic and equally unexplored region on the very edge of our solar system. It’s an environment that no one knew anything about 20 years ago. An environment composed of a ring of ice and rock more than a billion kilometres wide, beginning just behind Pluto. This debris zone receives only one to two thousandths of the amount of light that reaches the Earth. The Kuiper belt – as its known – is therefore so dark that even a second Earth would be hidden from all but the best telescopes. Pluto is so far away it takes 250 years to orbit the sun at a speed of 17,000 kilometres per hour. But, on the 14th July 2015, the New Horizons probe was due to bring light to that distant darkness and reveal this world to humanity for the first time ever.

Five other ‘Plutos’ have already been discovered in the Kuiper belt, with one – called Eris – being even larger and heavier than the original. This was the death knell for Pluto’s status as a planet. And an end to the discoveries isn’t in sight: “I have 107 objects with the status ‘possible dwarf planet’ on the list. And another 428 are ‘maybes’,” says Mike Brown.

The planetary scientist from the California Institute of Technology is one of the astronomers who was instrumental in helping along the expulsion of Pluto from the family of ‘genuine’ planets. Since the middle of 2006, the official definition has been thus: only those bodies that have cleared their orbit of other objects can be called planets. After 76 years, the number of the sun’s companions has shrunk to eight.

“Laughable and unscientific,” is how Alan Stern describes the decision of the International Astronomical Union to relegate the destination of ‘his’ probe to the second tier. “The further a planet is from the sun, the larger it must be to clear its orbit,” he grumbles. “Even the Earth couldn’t fulfill the definition of a planet out there. It’s like calling a river a stream, just because it’s surrounded by other major rivers and mountains.”

HOW DO YOU ACCELERATE A CRAFT TO 60,000KM/H?
Pluto’s mysterious nature is one of the reasons for the continuing disputes between scientists. We know more about foreign stars and galaxies that are light years away than we do about the dark edge of our own cosmic home, where the sun is a mere point of light. To date, the best photos of Pluto (all others are graphics) are made from just a handful of coarse pixels, each of which represents an area of 500 by 500 kilometres. You can imagine how useful a similar map of the Earth would be, consisting of just 30 coloured squares…

Even the size of Pluto isn’t clear: it’s only during the past few years that astronomers have agreed on a diameter of 2,306 kilometres – give or take 20 kilometres or so. Pluto is, therefore, only about two-thirds the size of Earth’s moon and half the size of the smallest planet, Mercury. Its entire surface area is smaller than South America. But nobody knows what the New Horizons camera will see when it finally arrives in July – after a nine-year journey to get there.

The enormous amount of time is not down to laziness: when the probe was shot into space, using an Atlas V rocket on 19th January 2006, it achieved speeds of almost 60,000 kilometres per hour – the fastest spacecraft ever launched. It passed the moon after nine hours. It began to orbit Mars after 78 days and Jupiter after little more than a year. But, from then on, the trip was a game of patience: Saturn in 2008, Uranus in 2011 and finally Neptune in 2014 – all in an almost straight line.

Now the New Horizons is racing along nearly five billion kilometres from the Earth, through the minus 220°C cold of interplanetary space. The piano-sized probe has safely travelled 32 times the distance from the Earth to the sun. Contact with the probe is now severely delayed: it takes more than four hours to control it via radio.

Despite still being a few million kilometres away, the instruments have deployed and have sent the first ever ‘close-ups’ of Pluto back to Earth. They’re no more than a few white spots, but they prove that the New Horizons is functional and in position to hunt down its unimaginably distant goal. “The spaceship is in spectacularly good shape,” confirms Stern.

On 14th July 2015, the probe was due to pass within 10,000 kilometres of Pluto – half the distance at which navigation atellites orbit Earth. In contrast to them, New Horizons will only fly over like a spy plane once, accurately measuring and mapping the surface and atmosphere using  seven integrated instruments. The data collection will be so large that the transmission to Earth will take 16 months at 700 bytes per second. “We can’t brake hard or swivel in orbit because the gravitational pull of Pluto is so low,” says NASA technician Fran Bagenal.

But the $700 million mission won’t be over just yet: “We’ll fly directly into the Kuiper belt,” continues Bagenal. And then the engineers’ hard work begins in earnest. That’s because, rather than just letting the probe fly onwards, they need to use the remaining 50 or 60kg of fuel to steer towards one of the 1,000 known objects in the Kuiper belt.

A total of 16 thrusters make the spaceship accelerate, brake or flip over in a desired direction. The final mission is yet to be confirmed. “We’re going to wait and see,”  explains Bagenal. There’s enough time, as a decision doesn’t need to be made until 2016. After 2025, the probe will run out of juice and become an abandoned celestial body. With a clear route, it could still travel for millions of years through space.

IS PLUTO HIDING AN OCEAN BENEATH ITS ICE?
Regardless of what the probe does next, the most important thing remains the ‘King of the Kuiper’. Alan Stern is expecting big surprises, as Pluto is an often underestimated world. “Life may even be possible there,” the mission head explains. In 2011, researchers discovered that a fluid ocean could circulate under the dwarf planet’s covering of ice, heated by radioactive potassium. Organic molecules could explain the strange colourful spots on the grainy Hubble Telescope images. So every day the tension increases in the NASA control centre in Maryland, even for Alan Stern: “Contact is imminent. Nine years I’ve been waiting for this moment.” Perhaps the New Horizons will spark a new revolution: to resurrect and restore a fallen planet.

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