The five men and two women - including the first teacher in space - were just over a minute into their flight from Cape Canaveral in Florida when the Challenger blew up.

The astronauts' families, at the airbase, and millions of Americans witnessed the world's worst space disaster live on TV.

The danger from falling debris prevented rescue boats reaching the scene for more than an hour.

In 25 years of space exploration seven people have died - today that total has been doubled.

President Ronald Reagan has described the tragedy as "a national loss".

The Challenger's flight, the 25th by a shuttle, had already been delayed because of bad weather. High winds, then icicles caused the launch to be postponed from 22 January.

But Nasa officials insist safety remains their top priority and there was no pressure to launch the shuttle today.

The shuttle crew was led by Commander Dick Scobee, 46. Christa McAuliffe, 37, married with two children, was to be the first school teacher in space - picked from among 10,000 entries for a competition.

Speaking before the launch, she said: "One of the things I hope to bring back into the classroom is to make that connection with the students that they too are part of history, the space programme belongs to them and to try to bring them up with the space age."

President Reagan has put off his state of the union address. He was meeting senior aides in the Oval Office when he learned of the disaster.