Pluto is bigger than scientists thought — bigger across.
At a briefing Monday at the Johns Hopkins Applied Physics Laboratory — mission control for the New Horizons spacecraft — NASA announced that it's now more certain of Pluto's diameter than ever.
According to the latest photos of the hitherto unvisited dwarf planet, Pluto measures 2370 km (about 1472.6 miles) across, give or take 20 km (12.4 miles). It was previously measured to be 2368 km (1471.4 miles) across, with the same margin of error. So we're talking about roughly a mile. That might not sound like a big deal, but it could mean a lot. Scientists already know what Pluto's mass is, so changing the diameter — even by just a mile — changes the calculated density of the planet.
According to principal investigator Alan Stern, that small diameter change means that Pluto is less dense than thought. It could be that there's more ice in the ice-to-rock ratio of Pluto's composition. The confirmation of Pluto's size also reaffirms that it's the largest known object in the Kuiper belt (though not the most massive — that would be Eris, the dwarf planet that helped dethrone Pluto from planetary status).
The 3 billion-mile, 9½-year journey from Cape Canaveral, Fla., culminates Tuesday when the spacecraft zooms within 7,767 miles of Pluto at 31,000 mph.