Update 6:15 p.m. It's official: NASA's New Horizons spacecraft has gotten humanity's first up-close look at Pluto.
The spacecraft sent word of its triumph Tuesday across 3 billion miles to scientists waiting breathlessly back home.
Confirmation of mission success came 13 hours after the actual flyby.
Early indications had been encouraging, and a cheering, flag-waving celebration swept over the mission operations center in Maryland at the time of closest approach Tuesday morning. But until New Horizons phoned home Tuesday night, there was no guarantee the spacecraft had buzzed the little, icy world.
The unprecedented encounter was the last stop on NASA's grand tour over the past half-century of the planets in our solar system. New Horizons arrived at Pluto after an epic journey that began 9½ years ago, back when Pluto was still considered a full-fledged planet.
According to NASA, the spacecraft swept to within 7,700 miles of Pluto at 31,000 mph. The pictures from closest approach should be available starting Wednesday.
6:48 a.m. NASA zooms in on Pluto, for closest views yet
New images of Pluto have arrived from a NASA space probe, and they're already allowing scientists to update what we know about the dwarf planet – such as its size. NASA's New Horizons probe has traveled more than 3 billion miles to send photos and data about Pluto back to Earth.
NASA is set to release more images and data gleaned from New Horizon's closest approach to Pluto, which was achieved just before 5 a.m. PT Tuesday.
A trove of information is expected to be released Tuesday — particularly tonight, after NASA reconnects with the New Horizons craft that's been focused on gathering information about Pluto.
In addition to a delay of more than 4 hours (due to the probe's distance from Earth), information will trickle back to NASA at a rate that would frustrate many Internet users.
Alice Bowman, the New Horizons mission operations manager, says the data rate is around 1,000 bits per second, with a maximum of around 4,0000. That's just a fraction of the traditional 56K speed of U.S. dial-up accounts.
A key revelation that's already come out about Pluto concerns its size — NASA says its diameter is 1,473 miles, or 2,370 kilometers, ending a debate that has raged since the planet's discovery in 1930.
"Pluto's newly estimated size means that its density is slightly lower than previously thought, and the fraction of ice in its interior is slightly higher," NASA says. "Also, the lowest layer of Pluto's atmosphere, called the troposphere, is shallower than previously believed."
More details about Pluto's unusual characteristics will no doubt add to a debate among scientists over how to categorize it because of its small size and other factors.
"Pluto also orbits at a funny angle compared to the other planets," as NPR's Geoff Brumfiels has reported. "And there are a whole lot of other Pluto-like things cluttering up the outer reaches of the solar system."
— Bill Chappell/NPR