Tuesday morning, dozens of scientists behind the New Horizons mission packed into a control room in Laurel, Maryland, for a countdown to the precise moment the spacecraft would become the first to fly by Pluto: 7:49:58 am ET.

In a nearby building, hundreds of journalists and onlookers joined in the countdown, led by the mission's principal investigator, Alan Stern. But when the count hit zero, the clapping and cheers were accompanied by a very subtle sense of anxiety. We had no live stream, no hard evidence. We weren't counting down to anything but the idea that the flyby was supposed to occur right then. We didn't know if it actually happened.

At that crucial moment, New Horizons was programmed to gather as much data as possible — rather than send any back to Earth. Consequently, it did it all on autopilot, behind a wall of radio silence. The scientists had spent a decade patiently guiding the probe 3 billion miles to Pluto, but as their work culminated in less than three minutes of flyby, they had absolutely no way of knowing how the probe was doing.

"We always talk about the spacecraft being a child, or a teenager," Alice Bowman, the mission operations manager, said in a press conference afterward. "There's nothing we could do but trust we'd prepared it well to set off on its journey on its own." Until 9 or so in the evening, they had no confirmation that the craft succeeded, rather than being destroyed by an errant piece of space debris.

Of course, we later found out that the spacecraft made it. But this is one of many contradictions experienced by the scientists running the New Horizons mission. The spacecraft is literally expanding the boundaries of human knowledge, but it's doing so with decade-old technology and along a route calculated years ago. It took years to get to Pluto, but since it isn't carrying enough fuel to slow down or enter orbit, it zipped by the dwarf planet at more than 31,000 miles per hour.

Then again, despite this speed — faster than a speeding bullet — the photos and data it collected will trickle in over the next 16 months, slowed by the incredible difficulty of transmitting signals through billions of miles of empty space. And after nine years of largely mundane flight, it executed the most complex maneuvers of its lifetime in less than an hour on autopilot: pivoting to face the planet as it passed, rapidly making hundreds of scientific measurements, and sending radio waves through Pluto's atmosphere to probe its composition.

Related 7 reasons why the New Horizons Pluto mission is a big deal

How to practice for a once-in-a-lifetime moment

If there's a saving grace, it's that Tuesday morning wasn't the first time the scientists tried to endure the extreme tension of a Pluto flyby. Starting in the fall of 2014, they gathered in Maryland three times to carry out carefully controlled flyby simulations, analyzing "data" cribbed from observations made by other spacecraft of planets and moons throughout the solar system. This sort of dry run is regularly done for crewed space missions and military operations — but it was a first for a robotic space mission.

"With the flyby, we're going to have the eyes of the world on us," Stern told me when I came to observe the final flyby simulation in April. "We'd never make it work without practice."

The third simulation took place at the Sheraton hotel in Columbia, Maryland. (The actual mission control at Johns Hopkins was occupied by other scientists overseeing the final days of the MESSENGER probe, which spent four years orbiting Mercury). By 9 am on the day I arrived, a few dozen scientists had gathered in a darkened ballroom as Stern stood at a podium detailing new set of data "sent" by New Horizons, followed by other mission scientists who detailed sets of data "collected" by each of the spacecraft's four main instruments.

"With the flyby, we're going to have the eyes of the world on us"

One scientist detailed the "new moons" discovered by New Horizons during its flyby. Another showed a photo of huge impact craters found on Pluto's surface. Occasionally, their PowerPoint slides would display a particularly recognizable feature from another real-life astronomical object (in this case, one of Saturn's moons), leading the crowd of lifelong planetary scientists to chuckle knowledgeably because they knew it definitely wasn't Pluto.

This sort of preparation was so important because, unlike NASA's previous missions to initially explore each of the solar system's planets in the '60s, '70s, and '80s, this is the first one happening in the era of social media. NASA is in a race to get new photos, data, and discoveries out to the public as quickly as they come in, which means analysis can't take place at the usual, careful pace of science.

So getting some practice at the process of quickly translating bits of data into press releases fit for public consumption is essential. Each day at the run-through, scientists would break down a batch of new data, working with press officers to write accurate press releases and create illustrations.

"This is a calisthenic exercise," said David Aguilar, who's coordinating media coverage of the mission. "It shows how choreographed this whole thing is."

The error that could have derailed the whole mission

Ten days before the flyby, the carefully orchestrated plan suddenly threatened to unravel. At 1:55 pm on July 4, mission operations manager Alice Bowman came upon a computer screen at the Johns Hopkins control center with a terrifying message: "OUT OF LOCK."

In essence, this meant that the mission control's computers no longer had any idea of where New Horizons was. For years, the probe had regularly been transmitting data on its location back to Earth, to be received by a series of 200-foot-wide telescopes in Spain, California, and Australia. But now, something had gone horribly amiss. As Joel Achenbach writes at the Washington Post, project manager Glen Fountain, Alan Stern, and other lead scientists raced to the control center to figure out what had gone wrong.

Their worst fear was that New Horizons had been destroyed by an unforeseen rock or other piece of debris. But the spacecraft was so far out in the solar system — about 32 times as far as we are from the sun — that the odds of such a collision were extremely low. Even though a pebble-size piece of debris could damage the spacecraft beyond repair, there's just not very much out there to begin with.

But even if New Horizons was unharmed, the timing of the error could potentially ruin the entire mission. Since there's no way to slow down or stop the spacecraft, if the scientists hadn't reestablished contact with it by the time of the flyby, the probe might have been unable to gather data. Decades of planning and nine years of flight would go out the window.

Decades of planning and nine years of flight would go out the window

However, the scientists had one hope: The spacecraft might have powered down its main computer and switched to a backup. If that were the case, it would have sent a different signal at a different frequency — and when the scientists used the satellite dish in Australia to search for it, they found it. At 3:11 pm, Bowman's computer showed the message "LOCKED," indicating it was back in contact with New Horizons.

It turned out that the craft's computer had become overwhelmed while trying to process a set of new commands while simultaneously compressing scientific data it'd already collected from Pluto. That had caused it to power down, turning the craft over to a backup computer and putting it in safe mode.

Over the next few days, the scientists worked to get New Horizons back into its normal mode, and it was fully on track to collect data as it passed Pluto today.

In retrospect, the whole ordeal might even sound minor. But for a few tense hours on July 4, the scientists who'd spent a decade planning a three-minute Pluto flyby had no idea if their mission would even survive.

The real scientific payoff will come months from now

These scientists have spent the past decade of their lives meticulously tracking the steady progress of this piano-size robot as it crossed the solar system. They've done this work largely outside the public eye — until recently, most Americans were unaware that NASA had a Pluto mission underway. And the vast majority of their days with New Horizons were roughly the same: On a nine-year road trip, there's sometimes just not that much to do.

Over the past few months, though, their routines have been thrown out of whack. And Tuesday, their workspace was mobbed by hundreds of space enthusiasts clutching small American flags. Compared with the slow, almost imperceptible progress they'd gotten used to, the past weeks went by in a blur, with astonishing new photos coming over and over during the last few days.

"People talk a lot about how surreal it is that we’re really here," Alan Stern said during a Monday press conference. "It feels like you’ve been walking on an escalator for almost a decade, and then you step upon a supersonic transport."

But despite the huge amount of media attention and public interest in the flyby, the real scientific payoff will come much more slowly. And it'll demand nearly as much patience as the nine-year journey to Pluto.

The first high-resolution photos of Pluto's surface came in Wednesday, beamed across billions of miles of empty space as a series of digits, to be decoded by computers on Earth. They were followed by another set of beautiful photos on Friday.

But that's still less than 2 percent of all the data New Horizons collected. Its hundreds of photos and scientific measurements will come in a slow drip, rather than a flood, because of the sheer faintness of that signal. New Horizons can send data at speeds anywhere from 1 to 4 kilobits per second — a tiny fraction of what a 56k modem from the '90s was capable of. At the low end, it'll take about 42 minutes to fully transmit one standard photo, on top of the 4.5-hour journey that data will have to make through space to reach us.

That means the scientists eager to get their hands on fresh data on a new world will have to wait 16 months to receive everything. "It's not like Voyager's encounter with Neptune, where a new image came every two or three minutes," Jeff Moore, the leader of the mission's geology researchers, told me during my April visit. "We're going to get a handful of images every day."

It'll take New Horizons 16 months to send back all the Pluto data it gathered

Over the next few weeks, NASA will continue to put out images and press releases, and they'll keep generating lots of buzz. The public will rightfully celebrate these photos of a new world.

Then most of us will forget about the New Horizons mission, and the scientists' real work will begin. Out of the public eye, they'll carefully parse the other data sent back by the spacecraft's other instruments, analyzing the surface composition and temperature of Pluto, its atmosphere, and its interactions with the solar wind.

They'll use these observations to better understand Pluto's chemical makeup, how it changes as it orbits the sun, how it formed billions of years ago, and what it all means for our understanding of the solar system — including the early history of Earth.

During the press conference that followed the jubilant flyby, Stern revealed this scientist's instinct when he was asked about a geographic feature in the photo New Horizons took yesterday, as it closed in on Pluto. "You look at that image, and if you're a scientist," he said, "you want to see all the supporting data." For that, he'll have to wait.