The explosion was witnessed by millions on live TV

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The American space shuttle, Challenger, has exploded killing all seven astronauts on board.

The five men and two women - including the first civilian 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. School teacher Christa McAuliffe, 37, married with two children, was to be the first civilian 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.

He has called for an immediate inquiry into the disaster but he said the space programme would go on - in honour to the dead astronauts. Vice-President George Bush has been sent to Cape Canaveral to visit the victims' families.

This evening, the president went on national television to pay tribute to the courage and bravery of the seven astronauts.

He said: "We will never forget them, nor the last time we saw them this morning as they prepared for their journey and waved goodbye and slipped the surly bonds of earth to touch the face of God."

The bomber was carrying hydrogen bombs

Investigators are searching the area eight miles west of Thule for radioactive debris. The accident happened a week ago when the plane caught fire and the crew bailed out before the plane crashed through the ice.

The United States defence department says parts of the bombs have been found. But it is thought the radioactivedetonatorsare still missing.

A team of 47 men with dog sleigh teams have been brought in to clear the wreckage. The sea surrounding the crash site has since re-frozen.

One of the scientists involved in the operation said all the wreckage was emitting low level radiation but there was no evidence of radiation on the snow.

The risk of contamination is said to be slight - except to those working on the spot who are equipped with protective clothing.

Two years ago, there was a similar accident involving a B-52 over the sea off Palomares in south-east Spain. The plane dropped its bombs over the Spanish coast.

It took nearly 80 days to recover the last of the four bombs on board that plane. The Spanish subsequently banned flights carrying nuclear weapons over their territory.

Plutonium specialist Dr Wright Langham, who is serving as a consultant to the recovery operation at Thule, said preliminary indications of the radiation levels showed two of the four weapons had broken.

He said: "One point to make is that since the count level is comparable to what we saw in Spain we can equate what we have here to what we had in Spain."

Dr Langham has insisted radiation is not a hazard at Thule. Most of the crash site has now been cleared of radioactive debris.

detonator: a mechanical or electrical explosive device or a small amount of explosive; can be used to initiate the reaction of a disrupting explosive（雷管；炸藥）