Pluto is larger than scientists thought, New Horizons finds

Pluto still isn't a planet, but according to the New Horizons mission, it's slightly bigger than we thought.

As NASA's New Horizons spacecraft readies for its close encounter with Pluto on Tuesday, scientists said observations show the dwarf plant is slightly larger — and possibly icier — than they thought.

Pluto's radius is roughly 1,185 kilometers, give or take 10 kilometers, according to measurements from the mission, Alan Stern, the principal investigator, said at a news conference Monday. That's 10 to 25 kilometers larger than previously estimated.

Because scientists already had a reliable measurement of Pluto's mass, that also means the dwarf planet is slightly less dense than previously thought, suggesting its ratio of ice to rock is higher than expected.

New Horizons also detected nitrogen escaping from Pluto's atmosphere millions of miles away from the dwarf planet, far earlier than mission scientists at the Johns Hopkins Applied Physics Laboratory in Laurel expected to encounter it, but they still don't know why.

The findings were some of the first of what could be many surprises over the coming days as New Horizons nears the climax of a 3 billion-mile, 91/2-year journey to the edge of the solar system. New Horizons will pass within 7,800 miles of Pluto's surface just before 8 a.m. Tuesday, though scientists at mission control in Laurel won't learn whether the flyby was successful until more than 12 hours later.

Excitement at the Hopkins lab is building as that moment nears.

"Pluto is sort of a capstone of our solar system exploration and sort of opening up this new realm," said John Grunsfeld, associate administrator of NASA's science mission directorate. "Pluto is a member of our solar system and the last unexplored territory. We'll open up new ground."

There have been many estimates of Pluto's size over the years, with scientists recently converging on a radius of between 1,160 and 1,175 kilometers. NASA fact sheets list it at 1,195 kilometers, but scientists nonetheless said New Horizons' measurements were a surprise.

The suggestion that Pluto is less dense than previously thought could have implications for scientists' understanding of how it and its large moon Charon formed and began to orbit each other, Stern said.

The finding also suggests that Pluto's troposphere, the lowest layer in its thin atmosphere, is shallower than was predicted, Stern said. Scientists believe gas from Pluto's atmosphere is streaming into space, but they did not expect to detect it until a day or two away from Tuesday's encounter. Instead, they detected it five days out.

Scientists are gearing up to receive much more information in the coming days, though it will take more than a year to download all of the data to be collected during the flyby. The spacecraft was just over 600,000 miles from Pluto as of late Monday morning.

"Fasten your seatbelts;" Stern said, "New Horizons has arrived at the Pluto system."

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