Planetary scientists are coloring in the family portrait of our solar system as close-up photographs and observations stream back from Pluto, a world 3 billion miles away with towering mountains of ice, vast smooth plains and many mysteries yet to be revealed.

The fly-by of Pluto last week by NASA’s New Horizons spacecraft is rightly celebrated as a triumph of human ingenuity, the capstone of a mission that unfolded nearly flawlessly.

Yet it almost did not happen, which would have left Pluto as just a hazy dot of light.

New Horizons overcame skeptical NASA officials, repeated threats to its funding, laboratory troubles that constricted the amount of plutonium available to power the spacecraft and an unforgiving deadline set by the clockwork of the planets. Though none of the obstacles packed the drama of space-exploration crises like the Apollo 13 mission, their number and magnitude seemed unbelievable.

"If you wrote a novel about it, I don’t think people would buy it," said S. Alan Stern, New Horizons’ principal investigator.

The story of New Horizons, the little spacecraft that could, and did, visit a small planet that is now considered too small to be a planet, started 15 years ago when NASA called it quits on Pluto.

For a decade, concepts for sending a mission there had been studied but never done. In 2000, the price tag for the latest incarnation, called Pluto-Kuiper Express, appeared to be getting out of control.

"When it was canceled," Stern said, "the associate administrator at the time, Ed Weiler, held a news conference and said: ‘We’re out of the Pluto business. It’s over. It’s dead. It’s dead. It’s dead.’ He repeated himself three times."

Many planetary scientists and Pluto fans reacted in dismay, especially as it seemed to be a case of then or never.

Pluto had reached the closest point of its orbit to the sun in 1989 and was on the outbound trek, turning colder. Scientists worried that Pluto’s tenuous atmosphere would turn to ice and fall to the ground, making Pluto a much less interesting place to study until it neared the sun again – two centuries later, when they would be long gone.

There was a second orbital consideration. The quickest way to Pluto is to take a left turn at Jupiter, using the giant planet’s gravity for acceleration, which cuts the travel time by four years. But a launch after January 2006 would mean Jupiter would be too far out of alignment to provide a boost.

Stamatios Krimigis, then the head of the space department at the Johns Hopkins Applied Physics Laboratory in Maryland and a member of a committee that advised NASA on missions to the outer planets, recalled Weiler’s asking him in the fall of 2000 whether it would be possible to do a low-cost Pluto mission similar to the Near Earth Asteroid Rendezvous spacecraft that the laboratory had built and operated for NASA a few years earlier.

"I said, ‘Well, we can look at that,’" Krimigis said. He was intrigued but uncertain.

Krimigis pulled together a small group who worked over the Thanksgiving holiday to come up with a cost estimate: $500 million including the rocket. That quick study sketched out a basic design that would turn into New Horizons.

A few months later, NASA put out a call for proposals, a competition to design a new Pluto mission that would arrive by 2015 and cost less than $500 million.

In November 2001, NASA chose New Horizons.

"We busted our butts, and we won it," Stern said.

That started a four-year, two-month sprint to design, build and test the spacecraft and get it to the launching pad – but almost immediately there was an obstacle.

"Two months later, the Bush administration canceled it," Stern said, laughing.

The president’s budget proposal for 2003 included no money for Pluto, the second year in a row that the administration had tried to kill such a mission. But Congress, persuaded by Sen. Barbara Mikulski of Maryland, inserted earmarks in the spending bills to keep the Pluto mission on track.

"Every year Congress had to keep us on life support," said Glen Fountain, New Horizons’ project manager.

In 2002, the National Academy of Sciences named Pluto a top priority for NASA’s planetary science missions.

"At that point, you could feel things change," Fountain said.

Managers of spacecraft missions often talk about the trade-offs between cost, schedule and risk. Too quick and too cheap greatly raise the chance of failure.

"We don’t believe in that," Krimigis said.

With just seven instruments, the craft was about the size of a grand piano. Fountain said the philosophy at the Johns Hopkins laboratory is to stick to proven technologies and keep the design to the essentials, which reduce cost and avoid delays without increasing risk. The one compromise, he said, was a digital radio receiver that would consume less power. The Johns Hopkins laboratory had already started working on the technology in a separate project.

"We didn’t think it was a huge risk," Fountain said.

Development continued without any showstoppers, although the cost rose to $722 million.

Then, in August 2004, the Department of Energy informed the New Horizons team that it could not provide the plutonium power source. At the far reaches of the solar system, the sun is too dim for a spacecraft to rely on solar panels or batteries. Instead, a chunk of radioactive plutonium generates heat that is converted to electricity.

Security lapses and safety issues at the Los Alamos National Laboratory in New Mexico had shut down the production of plutonium dioxide pellets for New Horizons’ power generator. Not enough had been made to provide the 220 watts called for in the design.

"We said, ‘How much power could you deliver?’" Fountain said. The reply: 180 watts.

Because of design decisions like the digital radio receiver, Fountain thought the reduced power would be sufficient. In the end, the Department of Energy was able to build a power generator that put out 200 watts during the fly-by.

New Horizons launched on top of an Atlas 5 rocket on Jan. 19, 2006, making the fastest trip out of Earth’s neighborhood.

Thirteen months later, the craft was at Jupiter already, and the mission team put its instruments to the test. New Horizons captured a volcanic eruption on Io, one of Jupiter’s four big moons. That was the first observed from a volcano not on Earth.

Just after the Jupiter fly-by, New Horizons suffered its first computer glitch. For spacecraft outside Earth’s protective atmosphere, high-energy cosmic rays occasionally zip through computer memory, causing a crash and restart. Calculations indicated that there would be one such crash during the 9 1/2-year trip to Pluto.

Instead, they occurred almost once a year. But none caused lasting damage, and they proved good learning experiences.

"It was just eventful enough to keep us alert," said Christopher Hersman, the missions systems engineer. "It actually helped."

The rest of the long cruise was mostly uneventful. Flinging a spacecraft to a rendezvous at the edge of the solar system is indeed rocket science, but not groundbreaking rocket science. The equations – the basic laws of Isaac Newton – are the same ones that were used decades ago.

Still, the engineers were careful with their calculations – tiny errors can grow calamitous – and periodic checkups made sure everything was working smoothly on the spacecraft.

Then, on July 4, 10 days before the Pluto fly-by, the spacecraft suddenly fell silent. Alice Bowman, the mission operations manager, said years of experience had given her a sense when a problem might be the fault of the receiving stations and when it might be a problem with a spacecraft.

"I pretty much knew it was something on the spacecraft," she said.

It turned out the spacecraft’s computer had overloaded trying to do two things at once – receive instructions for the fly-by while compressing images in its memory banks. By design, the main computer entered what engineers call "safe" mode to avoid damage to the spacecraft, and the backup computer kicked in.

An hour and a half later, the ground stations detected the signal from the backup computer.

"Then I knew we could do it," Bowman said. "The question was, could we do it in time?"

A nine-day sequence of commands to guide New Horizons through the fly-by was set to begin July 7. Bowman spent two nights at the office, taking only short naps.

"You would be amazed how much that can do," she said. "I can’t say I slept."

With hours to spare, the craft was back in operation. Then the fly-by directions kicked in, and New Horizons did everything it was told to do.

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