NASA's New Horizons makes its closest flyby of Pluto on Tuesday morning, but that doesn't mean we aren't already seeing exciting data from the spacecraft.
At a briefing held Monday morning at the Johns Hopkins Applied Physics Laboratory -- mission control for New Horizons -- the team announced that it's now more certain of Pluto's diameter than ever before.
And it's bigger than we'd thought! Well, bigger across. But more on that in a minute.
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 around a mile. That might not sound like a big deal, but it could mean a lot. See, 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 we'd thought. We'll have to change assumptions we've made about Pluto in order to allow for that loss of density. It could be that there's more ice in the ice-to-rock ratio of Pluto's composition than we'd thought.
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). But New Horizons is really our first foray into that strange, newly discovered region of the solar system. There's no guarantee that Pluto will keep its size title forever.