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

The flyby 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.