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Members of the New Horizons science team react to seeing the spacecraft's last and sharpest image of Pluto before closest approach later in the day, Tuesday, July 14, 2015, at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. NASA's New Horizons spacecraft was on track to zoom within 7,800 miles (12,500 kilometers) of Pluto on Tuesday. (Bill Ingalls/NASA via AP)[/caption]
We've made it to Pluto by NASA's calculations, the last stop on a planetary tour of the solar system a half-century in the making. The moment of closest approach for the New Horizons spacecraft came at 7:49 a.m. EDT Tuesday. It culminated an unprecedented journey spanning 9½ years and 3 billion miles.
Based on everything NASA knows, New Horizons was straight on course for the historic encounter, sweeping within 7,800 miles of Pluto at 31,000 mph. But official confirmation won't come until Tuesday night, 13 nerve-racking hours later. That's because NASA wants New Horizons taking pictures of Pluto, its jumbo moon Charon and its four little moons during this critical time, not gabbing to Earth.
NASA marked the moment live on TV, broadcasting from flight operations in Maryland.
The United States is now the only nation to visit every single planet in the solar system. Pluto was No. 9 in the lineup when New Horizons departed Cape Canaveral, Florida, in 2006 to shed light on the mysterious icy world, but was demoted seven months later to dwarf status
The New Horizons probe of NASA also settled one of the most basic questions about Pluto - its size.
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This July 11, 2015, image provided by NASA shows Pluto from the New Horizons spacecraft. On Tuesday, July 14, NASA's New Horizons spacecraft will come closest to Pluto. New Horizons has traveled 3 billion miles over 9½ years to get to the historic point. (NASA/JHUAPL/SWRI via AP)[/caption]
Mission scientists found Pluto to be 2,370 kms in diameter, somewhat larger than many prior estimates.
Images acquired with the Long Range Reconnaissance Imager (LORRI) aborad New Horizon probe were used to make this determination.
The result confirms that Pluto is larger than all other known solar system objects beyond the orbit of Neptune, the US space agency said in a statement.
"The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest," said mission scientist Bill McKinnon from Washington University in St Louis.