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The most distant space encounter in history is happening now - give and take six hours

A spacecraft called New Horizons is flying past Ultima Thule, a rocky relic of the solar system's origins, four billion miles (and with a six-hour light time delay) from Earth

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The New Horizons spacecraft, which launched in 2006, is the first Nasa mission designed specifically to explore the outer solar system - a region that project scientist Alan Stern (above) calls "a scientific wonderland".

Washington

THE nerdiest New Year's party in the solar system is happening four billion miles from Earth, where a lone, intrepid spacecraft is en route to the furthest object that humans have ever explored.

There is no champagne in this dim and distant region, where a halo of icy worlds called the Kuiper Belt circles the outermost edge of the solar system. There will be no renditions of Auld Lang Syne (in space, no one can hear you sing).

But there is a mini van-sized spacecraft called New Horizons. And there is a puny, primitive rock called Ultima Thule, a rocky relic of the solar system's origins, whose name means "beyond the borders of the known world". At New Horizons' birthplace, the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, scores of space scientists gathered to study - and celebrate - the occasion. While their fellow Earthlings counted down to midnight, they were ticking off time until 12.33am, when the spacecraft would make its closest approach to its target.

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In images taken on Sunday night, Ultima Thule appeared as little more than an elongated white smear against the darkness of space.

But an amazed "oooh" rippled through the APL auditorium when deputy project scientist John Spencer projected the blurred image on a screen. Until then, no one had ever seen a Kuiper Belt object as anything more than a single pixel of light.

"This is just a first glimpse of what is rapidly going to get better from here on out," Mr Spencer said. "We're going to learn more very soon, and we just can't wait."

This is the biggest and busiest moment for the New Horizons team since its spacecraft soared past Pluto three years ago, capturing detailed close-up photos of the distant dwarf planet.

Helene Winters, the missions project manager, said that spacecraft operators have been subsisting on chocolate and sleeping on air mattresses at APL so that they can make the most of every minute until New Horizons reaches its target. Navigators have kept a watchful eye out for potential hazards, which can be hard to spot in this dim and distant corner of the solar system.

But because it takes more than six hours for light signals to traverse the vast distance between the Kuiper Belt and Earth, it is already too late to change the spacecraft's course. That same light time delay means that scientists will not know until Tuesday morning whether the flyby worked.

Ms Winters laughed when asked whether she thinks she will be able to sleep Monday night. "Ask me again tomorrow."

As midnight approached, the gathering at APL took on a festival atmosphere. Scientists and their guests munched on crudites in a room lit with sparkling blue lights. Small children up long past their bedtimes scurried between chairs and sneaked cookies from the buffet.

"This is like a dream come true," said Chuck Fields, a podcast producer from Indianapolis who drove nine hours to attend Monday's event. He was dressed in a blindingly bright blazer and tie bearing images of planets, galaxies and the sun. His wife, Dawn, wore matching pants.

Five-and-half-year-old Benjamin Holder pored over an image of New Horizons and the distant rock that it was due to encounter. "I used to have a cat named Toolie," he said. "But the Thule rock that you're looking for is not named after my cat."

His uncle Erik Lessac-Chenen stifled a laugh. As a member of the spacecraft's optical navigation team, he had devoted the better part of the past year-and-a-half to tracking down that "Thulie rock". Two nights ago, he and his colleagues were awake until 4 am, developing the final set of commands to send New Horizons before its encounter. Those commands - he hopes - will ensure that the spacecraft does not miss its shot to take an image of its target.

"Basically, it was like, 'This is where Ultima is. This is where you point. Good luck'," he said. "Now it's up to New Horizons."

The spacecraft, which launched in 2006, is the first National Aeronautics and Space Administration (Nasa) mission designed specifically to explore the outer solar system - a region that project scientist Alan Stern calls "a scientific wonderland".

Out in the Kuiper Belt, where sunlight is 0.05 per cent as strong as it is on Earth, and temperatures hover close to absolute zero, the primitive building blocks of planets have persisted unchanged for 4.6 billion years.

"This is history making what we're doing, in more ways than one," Mr Stern said. Not only is Ultima Thule the most distant planetary object ever explored, it is also likely the most primitive. Unlike planets, which are transformed by geologic forces in their interiors, and asteroids, which are heated by the sun, Ultima Thule is thought to have existed in a "deep freeze" since it formed.

"It is probably the best time capsule we've ever had for understanding the birth of our solar system and the planets in it," Mr Stern said.

The encounter with Ultima is among the more difficult feats that Nasa has attempted. The great distances from Earth and the sun mean that scientists must put up with a long communications lag and instruments must operate with very little light. Ultima Thule was discovered only four years ago, and its orbit and surroundings are still not well known. And New Horizons is a 13-year-old spacecraft; its power generator produces just a quarter as much wattage as a light bulb, which means that operators must carefully prioritise their use of remaining fuel.

The sheer speeds and distances involved boggle the mind. Ultima Thule is one per cent the size of Pluto, and New Horizons must get four times closer to image it. At the moment of closest approach, the spacecraft will be moving at a breathtaking 51,499 kmh. Its cameras must swivel to track Ultima Thule as it passes by; otherwise, all it will see is a blur amid the black.

"It's risky," Nasa's associate administrator for science, Thomas Zurbuchen, said on Monday evening. "There's a non-zero chance it's not going to work." But if it does, he added, scientists will have seen something entirely new. "And every time we do that . . . the solar system turns out to be more wonderful in some ways than we expected."

The New Horizons team has several experiments planned for the brief encounter. Particle and dust detectors will probe the environment around Ultima. The spacecraft's three cameras will take images in colour and black and white in an effort to map the tiny world and determine its composition. WP