IT WAS live, but only vaguely interesting. The world had, after all, witnessed countless other space shuttle launches. Challenger itself had blasted off nine previous times and that same world was becoming blas.

But this was live because it was slightly different. One of the seven astronauts was a civilian, the first "teacher in space", Christa McAuliffe.

It was a nice public relations stunt, one of the biggest, designed to lift a space programme which was slipping into the doldrums. If it had gone to plan, in this fickle world of ours, it's unlikely it would have even been remembered.

But just 73 seconds after take-off, the unimaginable happened. Challenger exploded. It was live and now they were all dead.

The gleaming shuttle, the rust red fuel tank, the twin rocket boosters, all there in crystal clarity. The gargantuan orange flame against an azure backdrop as Challenger flew sky-high. A beautiful spectacle, a typical take-off, until it exploded, like a huge firework. Where the shuttle had been, only a strangely beguiling question mark remained, a white plume of vapour, which seemed to ask the question why.

It's one of those images never to be forgotten. It marked a point in history witnessed live by millions around the globe. People can tell you exactly where they were at the time. It was 5.41pm in England; it was January 28, 1986; I was in Sheffield, where I was training to be a journalist; I was at my friend's house eating a plate of chicken supreme and rice after an uneventful day at college. I was shocked.

Thinking back, it's the expression on spectators' faces that comes flooding back. Ronald Reagan had wanted the Cape Canaveral launch to be a spectacle so the astronauts' families and friends were given front row seats at the Kennedy Space Center, including Christa's son Scott and his classmates.

As Challenger took off, their faces beamed with pride and excitement. Within seconds it was ten miles high and the crowd whooped its approval. Then, as it blew up, some people oohed and arrghed like they would at a fireworks display, before the smiles on their faces froze, became perplexed, and finally turned to expressions of horror. Incredulity battled with anger and ended in despair. People ran in all directions as the debris plunged into the Atlantic. At a moment like this they needed to do something even though nothing could be done.

Fifteen years on, the memories are as strong as yesterday of an event which set back the American space programme. The disaster, the causes and the less-well know aftermath will be chronicled tonight in The Challenger (BBC2, 9pm), which looks back at how the tragedy could have been prevented. There is expert testimony from Jud Lovingood, Senior Manager at NASA in 1986; interviews with Roger Boisjoly, then a senior engineer at Morton-Thiokol, the contractors who built the solid rocket boosters for the space shuttle; other technicians and engineers; witnesses who were at the scene on that fatal day and a moving interview with Christa's mother Grace Corrigan.

Challenger has a place in history more than any other shuttle - but for all the wrong reasons. It should have been a momentous PR exercise, the first time a teacher was sent into space. Instead she was sent to her death along with her six colleagues.

The fallout couldn't have been worse. At least two of the astronauts might not have been killed instantly by the blast. Instead, they may have died of injuries sustained as their crew compartment hit the sea at 200mph.

The explosion should not have happened at all. The launch was being rushed. Challenger was using parts cannibalised from another shuttle. The engineers were unhappy. The weather was wrong. It was too cold, unusually so for Florida. There were icicles forming on the shuttle and a critically important O-ring, a rubber seal on a rocket booster, was known to malfunction in the cold. It did in spectacular fashion. It leaked, flames burning like a blowtorch through the huge external tank of liquid hydrogen.

NASA was to blame, according to Christa's mother Grace. "It was their fault," she says. "It was an accident but it was preventable. Any of the other times they'd lifted up, it probably wouldn't have happened. It was the cold. You could see icicles on the shuttle. It just didn't seem right.

"We didn't have any say in it. But the engineers did say that they shouldn't have taken off. We didn't know that at the time. We knew that afterwards. But we knew it was so cold, it just didn't seem right. We were in Florida. You don't get that weather in Florida."

Christa had been fearless. The flight was something she wanted to do, ever since her husband Steve had heard about Reagan's bid to find "the first citizen passenger in our space programme, one of America's finest, a teacher", and encouraged her to apply.

Many teachers couldn't be bothered to fill in the 48-page application form. About 11,000 did, including Christa, who was selected as one of 113 semi-finalists. She won through to a short-list of ten and the history teacher from Concord, New Hampshire, was chosen for the fateful voyage.

The training took a year and Christa became a international celebrity, even dining with the President himself.

While the training went well, the engineers were constantly under the cosh to be ready for a PR stunt designed to recapture the magic of space flight. They worked dangerously long hours, had to take parts from another shuttle and had strong misgivings about the launch. Bad weather led to one postponement and there was also a problem with the hatch locking mechanism, causing another aborted attempted. Then the temperature plummeted - but the launch was to go ahead regardless. One engineer is reputed to have told his wife: "They're going to launch tomorrow and kill the astronauts. But outside of that it was a great day."

On the anniversary of the disaster, on Sunday, Christa's mother will dedicate a mural at Framingham State University, New Hampshire, depicting her daughter's life.

"It doesn't feel like 15 years since it happened, in some respects," says Grace, who had hoped to see her daughter President one day. Christa's children, Caroline and Scott, would agree, the pain is still part of their everyday lives.

To the rest of the world, the catastrophe is simply part of history, albeit a poignant moment in time, which still makes people pause and think.

l The Challenger, BBC2 9pm, tonigh