We've made it to Pluto by NASA's calculations, the last stop on a planetary tour of the solar system a half-century in the making.
The moment of closest approach for the New Horizons spacecraft came at 7:49 a.m. EDT Tuesday, culminating a journey from planet Earth that spanned an incredible 3 billion miles and 9½ years.
Based on everything NASA knows, New Horizons was straight on course for the historic encounter, sweeping within 7,800 miles of Pluto at 31,000 mph. But official confirmation won't come until Tuesday night, 13 nerve-racking hours later. That's because NASA wants New Horizons taking pictures of Pluto, its jumbo moon Charon and its four little moons during this critical time, not gabbing to Earth.
NASA marked the moment live on TV, broadcasting from flight operations in Maryland.
"This is truly a hallmark in human history," said John Grunsfeld, NASA's science mission chief.
"It's a moment of celebration," added principal scientist Alan Stern from Johns Hopkins University's Applied Physics Laboratory, the spacecraft's developer and manager. "We've just done the anchor leg, we have completed the initial reconnaissance of the solar system, an endeavor started under President Kennedy more than 50 years ago."
The United States is now the only nation to visit every single planet in the solar system. Pluto was No. 9 in the lineup when New Horizons departed Cape Canaveral, Florida, on Jan. 19, 2006, but was demoted seven months later to dwarf status. Scientists in charge of the $720 million mission, as well as NASA brass, hope the new observations will restore Pluto's honor.
"It's a huge morning, a huge day not just for NASA but for the United States," NASA Administrator Charles Bolden said from NASA headquarters in Washington.
Inside "countdown central" at Johns Hopkins in Laurel, Maryland, hundreds jammed together to share in the remaining final minutes, including the two children of the American astronomer who discovered Pluto in 1930, Clyde Tombaugh. The actual flight control room was empty save for a worker sweeping up; the spacecraft was preprogrammed for the flyby and there was nothing anyone could do at this point but join in the celebration.
Stern led the festivity, joined on stage by his team and Tombaugh's two children.
The crowd waved U.S. flags and counted down from nine seconds, screaming, cheering and applauding. Chants of "USA!" broke out.
It takes 4½ hours for signals to travel one-way between New Horizons and flight controllers, the speed of light. The last time controllers heard from the spacecraft was Monday night, according to plan, and everything looked good.
New Horizons already has beamed back the best-ever images of Pluto and big moon Charon. Pluto also has four little moons, all of which were expected to come under New Horizons' scrutiny. The pictures are "mind-boggling to put it mildly," Bolden said.
As Stern told reporters Monday, "The Pluto system is enchanting in its strangeness, its alien beauty."
The newest pictures, from the actual flyby, won't be transmitted until well afterward so the seven science instruments can take full advantage of the encounter. In fact, it will take more than a year to get back all the data.
On the eve of the flyby, NASA announced that Pluto is actually bigger than anyone imagined, thanks to measurements made by the spacecraft, a baby grand piano-size affair. It's about 50 miles bigger than estimated, for a grand total of 1,473 miles in diameter.
Pluto is now confirmed to be the largest object in the so-called Kuiper Belt, considered the third zone of the solar system after the inner rocky planets and outer gaseous ones. This unknown territory is a shooting gallery of comets and other small bodies.
If a mission extension is granted, New Horizons will seek out another Kuiper Belt object before heading out of the solar system — for good.