Closest ever picture of Pluto as Nasa's New Horizons probe prepares for historic flyby

Nine years after it first left earth, Nasa's New Horizons spacecraft will this week become the first probe to reach Pluto, the mysterious icy world at the edge of the solar system, snapping photographs as it hurtles by at 30,000 miles an hour.

The mission marks a historic moment in space exploration, as New Horizons becomes the first space probe to reach Pluto in a journey that has taken sufficiently long that the status of the celestial body has been downgraded by astronomers from planet to dwarf planet.

"This is the frontier of space exploration," said Hal Weaver, project scientist on New Horizons at Johns Hopkins University in Maryland. "It's the first time we've explored an entirely new zone of the solar system."

Having journeyed through space for more than three billion miles, New Horizons will come within 7,800 miles of Pluto at 7.49pm Tuesday Hong Kong time. But because Pluto is so distant, radio signals from mission controllers take more than four hours to reach New Horizons.

The spacecraft will perform the flyby on autopilot, gathering detail about Pluto and its moons as it streaks past.

"It's like flying over a football field in a jet. To take pictures as we pass by, we have to swing round to keep Pluto in our field of view, and we have all that programmed in," said Weaver.

The scientists face an agonising wait for news that the spacecraft has survived the flyby. During the close encounter with Pluto, New Horizons will focus on taking photographs and making measurements that should shed light on Pluto's composition and its tenuous atmosphere. Only later will the spacecraft call home, with a signal that is expected to arrive at 9am HK time on Wednesday.