CAPE CANAVERAL, Florida - Little Pluto is a little bigger than anyone imagined.

Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons spacecraft, taken on July 13, 2015, when the spacecraft was 476,000 miles (768,000 kilometers) from the surface and released on July 14, 2015. [Photo/Agencies]

On the eve of NASA's historic flyby of Pluto, scientists announced 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 1,473 miles (2,370 kilometers), plus or minus 12 miles (19 kilometers). That's about 50 miles (80 kilometers) 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'3 billion-mile (4.8 billion-kilometer), 9-year journey from Cape Canaveral, Florida, culminates Tuesday morning when the spacecraft zooms within 7,767 miles (12,500 kilometers) of Pluto at 31,000 mph (49,900 kph).

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. "Tomorrow morning a United States spacecraft will fly by the Pluto system and make history."

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.