Watch the Johns Hopkins University Applied Physics Laboratory mission control at the moment New Horizons finally reached Pluto after a mission a 9½-year mission. (NASATV)

Crowds poured into the Johns Hopkins Applied Physics Lab early Tuesday morning, cheering, waving American flags and celebrating through teary eyes as the New Horizons spacecraft (theoretically) whizzed by Pluto, the last unexplored body in our solar system.

It's a busy day for the usually quiet lab. After about nine years and 4 billion miles of flight, scientists who worked on the mission were expressing an elated mix of sleep deprivation and overwhelming pride.

[Humankind just visited Pluto for the first time — at least we hope so]

"We are a country of explorers," said John Grunsfeld, NASA’s top official for science, just before the New Horizons team initiated a countdown marking the spacecraft's closest approach to the dwarf planet. "I'm extremely proud to be part of the country that first explored the whole solar system."

The room was filled with mission team scientists and their family members, media from around the world and, of course, bombastically excited space enthusiasts who traveled across the country to see the event  exactly 50 years to the day after the Mariner 4 first explored Mars.

"This is so cool!" cheered April Whitt, an astronomer from the Fernbank Science Center who traveled all the way from Atlanta "in a heart beat" to witness the last of the firsts in terms of exploration within our planetary neighborhood.

Final countdown!!!!! We have closest approach! #plutoflyby

A video posted by Rachel Feltman (@rafeltman) on

She was joined by a local enthusiast, "Planetary Patty" Seaton, a science teacher from the Owens Science Center in Prince George's County.

"We've been waiting for this for 10 years," Seaton said. "I've been skipping down the hallways and wondering why people aren't doing the same."

Caroline Ryschkewitsch, whose husband works in APL and was formerly a chief engineer at NASA, came to the event with her 88-year-old mother, Lorena Furman,  to celebrate the milestone as a family.

"We're terribly excited to see man go this far," Ryschkewitsch said.

"I think it's fantastic," Furman added. "When I was young, you'd see the moon and that's as close as you'd get."

All of the lead investigators for the mission took turns making remarks, thanking their colleagues and raising a round of cheers from the crowd. The biggest cheer took place when Grunsfeld thanked American taxpayers for their commitment to the project.

[This may be the second-best photo we’ll ever see of Pluto]

Also in attendance were Annette and Alden Tombaugh, the children of Clyde Tombaugh — the man who first discovered Pluto in 1930 and whose ashes are on board the spacecraft. Stamatios Krimigis also made a few remarks at the event, which is notable because he is now the only man who has built instruments that have flown to each of the solar system's planets.

"Sometimes, if you live long enough, your dreams come true," Krimigis said. "Today my dream is coming true."

Although the celebration started  at exactly 7:49 a.m. — the moment when New Horizons has been calculated to whiz past Pluto as it is collecting data and taking high-definition photos — it will be a mystery whether the spacecraft actually survived the encounter.

[New Horizons finally makes it to Pluto, sees craters and ‘great mounds’]

Because the craft is traveling at about 30,000 mph, a particle the size of a grain of rice could destroy the mission, although it was estimated by mission scientists that the risk of that happening was about  one in 10,000.

The spacecraft can't collect data and transmit it back to Earth at the same time, so takes hours for the information to zip back home. A confirmation signal that the craft is alive and well is expected to reach Earth about  9 p.m.

Read more:

Pluto: From discovery to first encounter

Sen. Mikulski talks up Pluto mission, pushes for more space funding

The latest New Horizons images show Pluto and Charon as full-fledged worlds