NASA’s New Horizons mission has answered one of the most basic questions about Pluto—its size. Mission scientists have found Pluto to be 1,473 miles (2,370 kilometers) in diameter, somewhat larger than many prior estimates. ... This result confirms what was already suspected: Pluto is larger than all other known solar system objects beyond the orbit of Neptune.

Pluto’s newly estimated size means that its density is slightly lower than previously thought, and the fraction of ice in its interior is slightly higher. Also, the lowest layer of Pluto’s atmosphere, called the troposphere, is shallower than previously believed.

New Horizons observations of Charon confirm previous estimates of 751 miles (1208 km) kilometers) across.

Nix and Hydra were discovered using the Hubble Space Telescope in 2005. ... Nix is estimated to be about 20 miles (about 35 kilometers) across, while Hydra is roughly 30 miles (roughly 45 kilometers) across. These sizes lead mission scientists to conclude that their surfaces are quite bright, possibly due to the presence of ice.

What about Pluto’s two smallest moons, Kerberos and Styx? Smaller and fainter than Nix and Hydra, they are harder to measure. Mission scientists should be able to determine their sizes with observations New Horizons will make during the flyby and will transmit to Earth at a later date.

Pluto’s diameter also affects the size of its atmosphere, which New Horizons has learned is bleeding off into space at a faster rate than expected.

The nuclear-powered probe was in position to pass dead center of a 60-by-90-mile (97-by-145 km) target zone between the orbits of Pluto and its primary moon, Charon, at 7:49 a.m. EDT on Tuesday

Most of New Horizons’ data will be stored on the spacecraft and transmitted back to Earth after the probe passes beyond the Pluto system. Flight controllers expect to receive just a short message from New Horizons around 9 p.m. on Tuesday that it survived the Pluto encounter.

Lead scientist Alan Stern said there was a one-in-10,000 chance that a debris strike could destroy New Horizons as it nears Pluto.

Measurements by the spacecraft set to sweep past Pluto on Tuesday indicate the diameter of the dwarf planet is 1,473 miles, plus or minus 12 miles. That's about 50 miles bigger than previous estimates in the low range.

Three new discoveries were revealed Monday ... Besides the revised size of Pluto ... scientists have confirmed that Pluto's north pole is indeed icy as had been suspected. It's packed with methane and nitrogen ice. And traces of Pluto's nitrogen-rich atmosphere have been found farther from the dwarf planet than anticipated. ...

Pluto is the largest object in the so-called Kuiper Belt, considered the third zone of the solar system after the inner rocky planets and outer gaseous ones. This unknown territory is a shooting gallery of comets and other small bodies; every time one of these wayward objects smack one of Pluto's five known moons, the ejected material ends up in orbit around Pluto, thus the debris concern. An extension of the $720 million mission, not yet approved, could have New Horizons flying past another much smaller Kuiper Belt object, before departing the solar system.

When Pluto was discovered in 1930, a competition was held to find a name for the new planet. Eleven-year-old Venetia Burney chose Pluto because it was dark and far away, like the god of the underworld. She received a £5 note as a reward.

• Day length: Pluto takes much longer to complete a full rotation than Earth does, making a day there equal to nearly six and half Earth days. A Pluto year is about 248 Earth years.
• Composition: Basically, Pluto is a huge chunk of rock covered in a relatively thin layer of ice, with nitrogen ice on the surface - and very possibly, water ice between that and the rock.
• Atmosphere: Pluto doesn't have an atmosphere like ours. It's about 100,000 times less dense than ours, because the planet is "sweating". It's mostly nitrogen, with traces of methane and carbon dioxide, that sublimate – or move from solids directly to gases – off the surface to produce this thin "atmosphere".
• Distance: On average, Pluto is about 40 times further from the Sun than the Earth (it varies between 39 times and 50 times because its orbit is elliptical). That's between 4.4 and 7.4B km from the Sun.
• Seasons: Pluto's seasons are extreme and long. Earth's seasons are caused by its tilt (23.5 degrees). To give you some perspective, Pluto's tilt is 120 degrees!
• Orbit: Pluto and its largest moon, Charon, orbit in such a way that the same moon face is always seen from Pluto's surface – just like Earth and its Moon. But Charon is also always in the same spot in Pluto's sky, meaning that if you're on the wrong side of Pluto, you'd never see the moon.

Six months after New Horizons launched and with more than 40 Kuiper Belt objects on the books, the International Astronomical Union made a controversial call to reclassify Pluto as a “dwarf planet.” Astronomers have since discovered about 2,000 more Kuiper Belt residents out of a population estimated at hundreds of thousands. Ironically, it was the discovery of the Kuiper Belt that provided the scientific motivation and money for a mission to Pluto. Scientists believe the Kuiper Belt holds fossils from the formation of the solar system 4.6 billion years ago. New Horizons will conduct its science on the fly, much like NASA’s Pioneer, Mariner and Voyager missions of the 1960s to the 1980s, when exploration of the solar system began.

Timing is crucial. New Horizons will have just 30 minutes to conduct the most important part of the mission, including photographing Pluto and Charon, determining what the icy worlds are made of and scanning Pluto’s atmosphere - all done while the probe and Pluto finally cross paths.

The probe will arrive at the dwarf-planet carrying the ashes of discoverer Clyde Tombaugh. Tombaugh died on January 17 1997, nine years and two days before New Horizon’s launch, but one of his final requests was for his ashes to be sent into space. A small container carrying his remains is affixed to the insider of the upper deck of the probe bearing the inscription: “Interned herein are remains of American Clyde W. Tombaugh, discoverer of Pluto and the solar system's 'third zone'” It is fitting that when New Horizon first set off in January 2006, carrying Tombaugh’s ashes, Pluto was still considered a planet. But just a few months later it was downgraded to a dwarf-planet or ‘plutoid’ and is now known unceremoniously as ‘asteroid number 134340’