Little Pluto is a little bigger than anyone imagined.
At the approach of NASA's historic flyby of Pluto, scientists announced on Monday that the New Horizons spacecraft has nailed the size of the faraway icy world.
Measurements by the spacecraft set to sweep past Pluto on Tuesday indicate the diameter of the dwarf planet is 2370 kilometres, plus or minus 19km. That's about 80km bigger than previous estimates in the low range.
Principal scientist Alan Stern said this means Pluto has a lower density than thought, which could mean an icier and less rocky interior.
New Horizons' 4.8 billion-kilometre, nine-year journey from Cape Canaveral, Florida, culminates Tuesday morning when the spacecraft zooms within 12,500km of Pluto at 49,900 km/h.
Mission managers said there's only one chance in 10,000 that something could go wrong, like a debilitating debris strike, this late in the game. But Stern cautioned: "We're flying into the unknown. This is the risk we take with all kinds of exploration."
"It sounds like science fiction, but it's not," Stern said as he opened a news conference at mission headquarters in Maryland.
Discovered in 1930, Pluto is the last planet in our solar system to be explored. It was a full-fledged planet when New Horizons rocketed away in 2006, only to become demoted to dwarf-planet status later that year.
New Horizons has already beamed back the best-ever images of Pluto and its big moon Charon on the far fringes of the solar system.
"The Pluto system is enchanting in its strangeness, its alien beauty," said Stern, a planetary scientist at Southwest Research Institute in Boulder, Colorado.
With the encounter finally at hand, it all seems surreal for the New Horizons team gathered at Johns Hopkins University's Applied Physics Laboratory. The energy there was described as electric.
Project manager Glen Fountain said New Horizons, at long last, is like a goods train barrelling down the track, "and you're seeing this light coming at you and you know it's not going to stop, you can't slow it down."
"Of course, the light is Pluto, and we're all excited," Fountain said.
Besides the revised size of Pluto - still a solar system runt, not even one-fifth the size of earth - 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. New Horizons detected lost nitrogen nearly a week ago.
As for pictures, the resolution is going to increase dramatically. Until New Horizons, the best pictures of Pluto had come from the Hubble Space Telescope.
Hubble did its best from earth orbit, but managed to produce only crude pixelated blobs of the minuscule world.
The New Horizons spacecraft is the size of a baby grand piano with a salad bowl - the dish antenna - on top. It will come closest to Pluto at 1149 GMT Tuesday (9.49am AEST Wednesday). Thirteen hours later flight controllers will learn if everything went well.
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.
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