On the eve of NASA's historic flyby of Pluto, scientists announced Monday 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 radius of the dwarf planet spans 736 miles, plus or minus six miles. That's 20 miles to 30 miles bigger than previous estimates. The diameter would be twice that.
Principal scientist Alan Stern says this means Pluto has a lower density than thought, which could mean an icier interior.
New Horizons' 3 billion-mile, 9½-year journey culminates Tuesday morning when the spacecraft zooms within 7,767 miles of Pluto. Managers say there's only one chance in 10,000 something could go wrong, like a debris strike.