The explosion spread itself across the sky, pinned against the Earth’s atmosphere like a giant butterfly. A moment earlier, the Challenger shuttle had been a white streak propelled upon a tongue of flame, building to its ferocious full-throttle speed of 17,000 mph, and then, suddenly, frozen in time. Forever.

The image will still be there Friday, indelible, hanging over Cape Canaveral like a cloud, when NASA marks the 25th anniversary with a somber day of remembrance.

For a quarter-century before that, American astronauts had climbed aboard giant rockets that hurled them into space with such remarkable reliability that, by the morning of the second shuttle launch of 1986, the liftoffs had gone from harrowing to humdrum. NASA had shot 55 bullets at the sky, and each time seen its crews return successfully to Earth.

Seeking to reignite the American imagination about space travel, NASA had chosen Christa McAuliffe — a bubbly 37-year-old high school social studies teacher from Concord, N.H. — to be the first “ordinary citizen” in space. As McAuliffe boarded the Challenger shuttle, a launchpad technician handed an apple to the teacher.

This was the moment that every child had been waiting for who ever dreamed of joining Buck Rogers or Luke Skywalker as they raced across the heavens. The possibilities of space were — by definition — infinite, limitless. In a way that it rarely does anymore, the nation stopped what it was doing to watch the schoolteacher in space.

America was at the exact midpoint of its manned space program on that frigid morning 25 years ago today. Just as NASA had done when it replaced chimps in its capsules with human astronauts, the space agency had calculated that interest — and funding — would increase if the country saw recognizable faces in the astronaut corps. As McAuliffe and her six crewmates waved a final goodbye, no one realized that it was also the beginning of the end of the shuttle’s ascendancy.

Just 73 seconds

The Everyman era of space travel was to last just 73 seconds, ending in a fireball and two sickening spirals of white smoke, as the Challenger’s booster rockets continued firing even after the explosion. Millions of schoolchildren watched the launch on television. McAuliffe’s parents and her 9-year-old son, Scott, had flown to Cape Canaveral, where they looked on — first in confusion, then mounting horror.

Dan Barstow had just finished telling his third-grade class in Hartford, Conn., that “one of us was going into space,” when the announcement came that something had gone terribly wrong. As the teacher tried to make sense of what had happened for the 8-year-olds in his care, he refused to believe it himself.

“I remember hoping that they had survived, that somehow the space shuttle had broken off and made it into the water,” said Barstow, now president of the national Challenger Center for Space Science Education, established just months after the tragedy by families of the crew. In fact, the compartment holding the astronauts remained intact after the explosion and flew upward another three miles before it began its two-minute free fall.

Today’s official remembrance at Florida’s Kennedy Space Center also serves as a grim prelude to the 10th anniversary of Sept. 11, another day of blue skies and broken hearts. A decade after the attacks, the scuttling of NASA’s shuttle program will be complete, and the last of the space agency’s fleet of “space trucks” will finally be what critics always claimed the giant winged craft was: a white elephant.

“The thing I fear is that we don’t have a really coherent plan for NASA to explore beyond Earth,” said Scott Parazynski, a Stanford graduate who flew five shuttle missions and walked in space seven times. “The long-range plans for NASA remain unclear.”

Parazynski is chairman of the board of the Challenger Center, which reaches 400,000 schoolchildren a year through its Learning Center (www.challenger.org) — including one at the Chabot Space and Science Center in Oakland. It is one of McAuliffe’s lasting legacies, and the center even provides the lessons that she prepared but never got to teach during what McAuliffe had described as “the ultimate field trip.”

Inspired to change

The loss of the Challenger crew reverberated far beyond the wrenching emotions of that day. President Ronald Reagan — paraphrasing a sonnet by John Gillespie Magee, a young American airman killed in World War II — recalled the astronauts waving goodbye before they “slipped the surly bonds of earth to touch the face of God.” But their commitment to a cause greater than themselves transformed the life of then-10-year-old Kim Reed in San Jose.

The daughter of Mayor Chuck Reed was in middle school, “just sort of an ordinary student, active in things but without any serious direction,” the mayor recalled this week. “Shortly after that, she announced she was going to the Air Force Academy to become a pilot. She did not ask. It was a statement.” Now an Air Force pilot, Kim Nicole Campbell returned from her final deployment to Iraq in October, and is stationed at Davis-Monthan Air Force Base in Tucson, Ariz.

A quarter-century after the accident, most of the students at Christa McAuliffe School in Saratoga have little idea who the first educator-astronaut was, and no memory at all of the horrible event that inspired the school to change its name. School administrators plan no observance of the Challenger tragedy. Instead, they have decided to celebrate McAuliffe’s birthday in September. “It’s much more child-development appropriate for our kinders and our first- and second-graders,” said Principal Louise Ostrov.

The tragedy set in motion the space program’s long slide into irrelevance, with no manned flights from American soil planned after the shuttle is retired. An investigation into the explosion laid the blame on faulty O-rings on the shuttle’s booster rockets.

But Parazynski believes McAuliffe’s brief flight ushered in a new era, in which American space startups such as SpaceX, Virgin Galactic, Orbital and Armadillo provide the common man — and woman — with a ticket to ride.

“I still believe that space is for everyone,” he said. “There are a lot of doomsayers who suggest that human spaceflight is dying, but there will be opportunities for many more people to experience the world from high above. We live in exciting times.”

Mercury News staff writer David E. Early and The Associated Press contributed to this report. Contact Bruce Newman at 408-920-5004.