The words you never hear from NASA: “This is actually really hard to do.”

Or: “We’re not sure this is gonna work.”

There is a code among engineers: Express confidence, exude competence. But space is hard, as we keep pointing out lately. Things blow up. Computers reboot themselves at inopportune moments. A planet turns out to be in a slightly different location, and even a different size, than you’d previously thought.

Uncertainty about success is implicit in aerospace engineering, particularly when you are trying to send a robotic probe to the edge of the solar system and persuade it to perform a 600-maneuver dance — hydrazine thrusters firing like mad — as it conducts 433 scientific observations of Pluto and its moons in a one-shot flyby. And by the way, you’ve already had a last-minute computer glitch, one of the 249 “contingencies” that you’d dreamed up that might ruin the mission.

At this point everyone knows that the New Horizons mission was a screaming success, but I’m not sure everyone grasps how difficult this was to pull off, and how dicey in the home stretch, and how the team based at the Johns Hopkins University Applied Physics Laboratory had to make some tough calls when things were down to the wire.

If you look at the first big close-up photo of Pluto, you’ll notice the shadow line, the terminator. The spacecraft wasn’t supposed to aim right there — it was off a little bit, slightly closer to the terminator than they had planned, Alan Stern, the principal investigator, told me last week. But therein lies a huge engineering lesson from New Horizons: You don’t have to be perfect. Good enough is great enough when you’re getting the first pictures of Pluto. That image was fantastic.

The New Horizons team was aiming for a rectangular box in space 90 kilometers by 60 kilometers. They hit their box, but came in a little bit high, which translated to arriving about 72 seconds early and 40 miles closer to Pluto’s surface. This wasn’t a mistake, because perfection isn’t possible when you’re dealing with a spacecraft and astronomical objects that are all moving, and when you have an imperfect knowledge of the position of the main object in question. Yes: They never knew exactly where Pluto was.

Remember, it’s 3 billion miles away and headed away from the Earth at this point in its orbit. Astronomers have only seen about a third of its orbit around the sun (a Plutonian year is 248 Earth years). I asked Project Manager Glen Fountain if it was possible the spacecraft would turn in the wrong direction and take photographs of empty space, and he said no — not unless the ephemeris data was way off.  On Encounter Day, a couple of astronomers from Chile showed up and were greeted warmly by Stern. The Chilean astronomers had helped refine the orbit of Pluto; until they pitched in, the Pluto orbit carried a margin of error of 10,000 kilometers, Stern told me.

Fountain, the project manager, gave the very first media briefing on the first media day, Sunday, and was pressed by a reporter from Irish television about the fact that the spacecraft wasn’t perfectly centered on the target box. Fountain said, in effect, it was darn good enough.

When the reporter suggested that the team would have a difficult meeting to decide what to do, Fountain shot back: “I think it’s going to be a very easy meeting.” He’d decided: “At this point we feel we should leave well enough alone.”

Fountain is one of the heroes of the Pluto mission. Another is Alice Bowman, the mission operations manager. She emerged as something of a cult figure as the flyby date of July 14 approached. Our front-page story in The Washington Post on July 11 told how she slept on the floor of her office the nights of July 4 and 5 after New Horizons suddenly went into “safe” mode, with a 5-rpm controlled spin that, if not corrected, and fixed  very precisely and quickly, would have ruined a mission that had been planned over decades and which cost the taxpayers $720 million. Beyond being a highly competent engineer, she’s also self-effacing and eager to share credit with her team. “She’s so genuine,” one of her colleagues told me when we discussed Bowman’s emerging celebrity.

Bowman spent Tuesday “just sort of stressed,” she said later. That’s because she didn’t know if New Horizons would ever be heard from again. It had said goodbye Monday night before entering its most intensive stretch of observations. Bowman spent Tuesday double-checking her data. By the team’s calculation, it would detect a brief bulletin from New Horizons at 8:53 p.m., a kind of “I survived” message with some data about system operations.

At 8:52:37 the NASA Deep Space Network picked up the signal.

“Okay, we’re in lock with carrier. Stand by for telemetry,” Bowman announced in the control room while hundreds of us watched from a nearby auditorium.

Then: “Okay, we’re in lock with telemetry with the spacecraft.” Big cheer in Mission Operations and in the auditorium. Another cheer went up in Mission Operations when an engineer in charge of “autonomy” said: “Autonomy is very happy to report nominal status. No rules have fired.” That meant the computer glitch of July 4, in which the autonomy system shut down the main computer, hadn’t been repeated — which meant that the spacecraft had almost certainly succeeded in conducting all its scientific observations as planned. Bowman broke into a huge smile. That was when she knew that the mission had worked.

In the auditorium later, as the team entered and the leaders were introduced, people chanted “Alice! Alice! Alice!”

During the press conference, Bowman said: “I can’t express how I’m feeling to have achieved a childhood dream of space exploration. I’m pretty overwhelmed at this… . Tell your children and everyone out there, do what you’re passionate about. Don’t do something because it’s easy. Do something because you want to do it. Give yourself that challenge, and you will not be sorry for it.”

There were many other heroes of the Pluto mission, and I won’t try to list all of them, but there’s no question that the person who made it happen, and who ultimately will always be associated with this rediscovery of Pluto, is Alan Stern.

Stern is a character. You can pick that up in my story that ran last month that includes Stern’s tart comment about astronomers demoting Pluto to “dwarf planet” status. Stern is a planetary scientist at the Southwest Research Institute in Boulder, Colo., and as principal investigator is the leader of the mission, as well as its best advocate. More than a year ago I heard Stern give a presentation about New Horizons at the big space conference in Colorado Springs, and he promised that it was going to be one of the greatest NASA missions in decades. Big talker, one might have concluded. But he backed it up.

“We did it!” he said after the flyby, to applause in the auditorium. People laughed knowingly when he said, “I have never been one to underplay the significance of this mission.” He called the mission “one small step for New Horizons and one giant leap for mankind.”

Some might call that hype, but Stern earned the right to have the last word. He did it. And he was right in his prediction about Pluto: It wasn’t going to disappoint anyone.

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