By Amanda Barnett and Ben Brumfield CNN
LAUREL, Maryland (CNN) -- The first human footprint on the moon. The landing of the Curiosity rover on Mars. NASA scientists put the significance of the New Horizons spacecraft flyby of Pluto on Tuesday right up there with those milestones.
The most beautiful part may come Wednesday, when the images that the craft took and transmitted back to Earth are released.
They will be from the closest point the probe came to the dwarf planet as it made its milestone in space exploration history.
The Pluto mission completes the reconnaissance of the classical solar system, and it makes the United States the first nation to send a space probe to every planet from Mercury to Pluto.
The flyby came exactly 50 years after the Mariner 4 probe accomplished the first flyby of Mars, which sent back the first ever photos of a planet taken from space.
"I think it's fitting that on that 50th anniversary, we complete the initial reconnaissance of the planets with the exploration of Pluto," said Alan Stern, the mission's principal investigator.
With Wednesday's photos, New Horizons is also sending a banquet of knowledge to scientists. "Quite a waterfall of data for us tomorrow morning," Stern said late Tuesday.
In the weeks that New Horizons was homing in on Pluto, NASA and its partner, the Johns Hopkins University Applied Physics Laboratory, published a barrage of images. First, they showed blurry spots that grew larger.
By Tuesday, clear photos revealed the contours of the planet's surface and of that of its largest moon, Charon. Wednesday's potentially even more spectacular images and information are just the beginning of 16 months' worth of data the probe will send back to Earth.
The world will be very lucky to see the new images. It wasn't a sure bet that New Horizons would survive to send them, after passing by projectiles racing through Pluto's gravitational field.
Collisions with dusts particles could have ripped through the probe like shotgun blasts, since the spacecraft moves about 8.5 miles per second. That's about 40 times as fast as a bullet exiting a gun barrel.
New Horizons went incommunicado for 12 hours as it passed Pluto, and mission managers packed into mission control held their breaths. The radio silence was planned, while the craft collected data on Pluto and its five moons, but it was during that most dangerous phase.
Then, New Horizons ended its silence with a signal back to Earth, and the room burst into applause. The probe had survived its historic pass.
At 7:49 a.m. ET, the unmanned piano-size spacecraft went over Pluto about 7,750 miles from its surface, NASA said.
"I have to pinch myself," said Alice Bowman, mission operations manager. "Look what we accomplished."
Renowned astrophysicist Stephen Hawking sent his congratulations via a YouTube video. "We explore because we are human, and we want to know. I hope that Pluto will help us on that journey," he said.
At its lightning speed, New Horizons took nine years to make it to Pluto. And it took a shorter route, which is a stroke of luck, too.
Had the cosmic timing been different, Pluto's vast, elliptical orbit could have put it much farther out in the solar system than it is now. Since it takes the dwarf planet 248 years to circle the sun once, decades may have had to pass for Pluto to come within reach.
Even with that relatively close distance, the probe traveled more than 3 billion miles to reach Pluto on a complex flight path.
The New Horizons probe used the massive gravitational pull of planets in our solar system to help propel it, as it curved past them, and that may not have worked had Pluto been at another point in its orbit.
The dwarf planet already is dazzling mission managers. If Pluto had been covered in clouds, they wouldn't have seen much.
Instead, they're seeing a world with "various kinds of brightness, very dark regions near the equator, very bright regions just to the north of that and a broad intermediate zone over the pole," mission investigator Stern said.
And there's snow.
"Pluto has strong atmospheric cycles. It snows on the surface," Stern said.
"Pluto has turned out to be an extraordinarily complex, interesting world," said John Grunsfeld, a NASA associate administrator.
Replacing previous ideas of what the planet may have looked like with the reality was worth the trip.
On Monday, New Horizons settled one debate about Pluto -- its size. Information gathered by the probe indicates Pluto is 1,473 miles (2,371 kilometers) in diameter. That's somewhat bigger than earlier estimates, and it means Pluto is larger than all other known solar system objects beyond the orbit of Neptune.
And now, beyond Pluto
New Horizons looks like a gold foil-covered grand piano. It is 27 inches (0.7 meters) tall, 83 inches (2.1 meters) long and 108 inches (2.7 meters) wide. It weighed 1,054 pounds (478 kilograms) at launch. It has seven instruments on board to map the surfaces of Pluto and its largest moon, Charon. It also is studying their atmospheres.
The spacecraft was launched on January 19, 2006, before the big debate started over Pluto's status as a planet. In August of that year, the International Astronomical Union reclassified Pluto as a dwarf planet.
But Stern disagrees with the IAU's decision.
"We're just learning that a lot of planets are small planets, and we didn't know that before," he said. "Fact is, in planetary science, objects such as Pluto and the other dwarf planets in the Kuiper Belt are considered planets and called planets in everyday discourse in scientific meetings."
New Horizons will keep flying, heading deeper into the Kuiper Belt, a region that scientists think is filled with thousands of icy objects.
NASA and mission managers will decide later this year whether to extend the mission to study another small world out there.
CNN's Amanda Barnett reported and wrote this story from the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland. CNN's Ben Brumfield wrote and reported from Atlanta.
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