Mankind reached the farthest frontier of the solar system Tuesday morning when NASA's New Horizons became the first spacecraft to explore Pluto.
The probe, which is the size of a baby grand piano, revealed ancient cratered surfaces and possible signs of tectonic activity, but raised more questions to be addressed over the coming days and months.
New Horizons, which was designed, built and managed by scientists at the Johns Hopkins Applied Physics Laboratory in Laurel, sped past the dwarf planet just before 8 a.m. at a distance of less than 7,700 miles. It was the pinnacle of a 91/2-year, 3 billion-mile journey.
It arrived about 70 seconds earlier and 70 kilometers closer than scientists predicted — both within margins of error.
The visit was the first to a previously unexplored world since 1989, when NASA's Voyager 2 passed about 3,000 miles over Neptune. Scientists considered Pluto the final major objective in humanity's reconnaissance of the solar system.
New Horizons could help answer long-held questions about the formation of the solar system and universe.
"This is truly a hallmark in human history," said John Grunsfeld, chief of NASA's science mission directorate. "It's been an incredible voyage."
Images and other data collected during the probe's encounter with Pluto were not expected to be released until Wednesday.
Pictures its cameras captured Monday drew a standing ovation from project scientists and "oohs" and "ahhs" from hundreds of people who attended celebratory events Tuesday at the Laurel lab.
The photos showed a diverse landscape, with as many as five distinct varieties of terrain on one hemisphere. They include cratered surfaces that could be several billion years old, and apparently smoother areas that could be signs of geologic activity, according to Alan Stern, the mission's principal investigator and a scientist at the Southwest Research Institute in Boulder, Colorado.
One 50-mile-wide crater showed signs of erosion, while dark areas stretch along the dwarf planet's equator, scientists said.
A prominent heart-shaped bright spot on Pluto's face appeared on closer inspection to contain two distinct sections, deputy project scientist Cathy Olkin said, with a "very strong demarcation" between them.
There's ice, but also evidence of snow of Pluto's surface. The planet's pole appeared yellowish with mysterious lines cutting across it.
"We never imagined anything like this," Olkin said.
Previous images of Pluto came from the Hubble Space Telescope. After Voyager 2 visited Uranus and Neptune in the late 1980s, it passed Pluto's orbit, but the then-planet was far away in its orbit at that time.
In a ballroom on APL's Laurel campus Tuesday, hundreds of Hopkins scientists and their families and supporters counted down to the spacecraft's moment of closest approach at about 7:50 a.m., waving American flags and cheering.
The crowd included two children of Clyde Tombaugh, the late American astronomer who discovered Pluto in 1930 as a young researcher at the Lowell Observatory in Flagstaff, Ariz. Science Guy Bill Nye was also present, as were reporters from around the world.
The achievement wowed scientists and laypeople alike.
"Sometimes if you live long enough, dreams come true," said Stamatios "Tom" Krimigis, head emeritus of the Hopkins lab's space department and a 50-year veteran of planetary exploration. "My dream is coming true today."
Jenna Ledet was thrilled to get an invitation to the events from family friend Ann Harch, an engineer working on the mission.
Ledet, 21, is studying astronomy at Columbia University. She said listening to scientists discuss the mission had her hoping to one day analyze data from future planetary exploration.
"I can't wait to see what the results will be when they get more pictures and more pieces of the puzzle," Ledet said.
The intense worldwide interest in the mission caused NASA's main website to crash briefly.
Eight months after New Horizons was launched from Cape Canaveral in Florida in January 2006, Pluto was famously demoted from its status as the ninth planet in the solar system.
The International Astronomical Union voted on a three-part definition for a planet, and Pluto fell short: While it orbits the sun and has been rounded by gravity, it does not exert enough gravitational pull to clear its orbital path of other objects.
A young boy asked a panel of mission scientists Tuesday whether Pluto would be declared a planet again.
New Horizons co-investigator Fran Bagenal responded: "Dwarf people are people. Dwarf planets are planets."
The day was not without some suspense. Mission scientists waited 22 hours between their last contact with the spacecraft at 11:17 p.m. Monday and their first after the fly-by, which came just before 9 p.m. Tuesday.
A screen in the auditorium showed the mission operations center, a small room on the Laurel campus lined with monitors.
"We have a healthy spacecraft," mission operations manager Alice Bowman announced, and the auditorium erupted in applause.
As New Horizons passed Pluto, the probe made a rapid series of pre-programmed observations, including photographing the surfaces of both the planet and its moon Charon, using spectroscopy to identify what substances are present and where, and characterizing Pluto's tenuous atmosphere, thought to be eroding into space.
Successfully capturing those observations required meticulously tracking the spacecraft's trajectory and timing as it approached the Pluto system. Mark Holdridge, the manager of the encounter mission, said engineers won't know exactly how they did until they download data from the spacecraft Wednesday.
"For the real close-up stuff, it's essential we get the timing right," Holdridge said. "That's what we sold the mission on."
After nearly a decade of waiting for New Horizons to reach Pluto, the mission scientists will need more patience: It will be 16 months before all of the data New Horizons has gathered will be downloaded from the spacecraft.
In the meantime, the probe will continue into the Kuiper Belt, the region of small, icy bodies at the edge of the solar system that contains the Pluto system. Mission leaders have identified objects within the belt that the spacecraft could visit next, given NASA approval and additional funding.
The Associated Press contributed to the article.