Watch the Discovery lift off

Mr Glenn, 77, returns to the final frontier as the world's oldest astronaut - 36 years after he became the first American to orbit the earth.

Discovery, a luxurious ship compared to the cramped rocket he used in 1962, successfully blasted off at 1919GMT (1419EST) from Kennedy Space Centre.

After Nasa launch commentator Lisa Malone completed her countdown she added: "Lift-off of Discovery with a crew of six astronaut heroes and one American legend."

Launch was delayed slightly by an aeroplane entering airspace above the craft.

James Wilkinson: Glenn - America's latest hero

At a post-launch press conference Nasa said a panel protecting the shuttle's parachute, which is used to slow the spacecraft down during landing, had fallen off two seconds into the flight.

They added the mishap would not affect the mission.

President Bill Clinton and a host of Hollywood stars watched the launch from the VIP boxes.

US Correspondent Palab Ghosh: "Making history ... again"

And about 250,000 spectators caught a fleeting glimpse of the take-off from the surrounding area.

Mr Clinton said: "It's a great day for America and a great day for our senior citizens. I hope that all Americans share the exuberance that I feel."

Veteran observers said public interest in the launch was similar to those for the first missions to the moon and Mr Glenn, now a US senator, was the sole reason.

Studying the ageing process

During his nine days in space, Mr Glenn will be a medical guinea pig. Scientists will study his bones, heart and immune system to see how his body is reacting to the stresses of launch and the comparative comfort of zero gravity.

They hope to use the data to find out more about the ageing process and to see how older people fare in space.

A hero's return in 1962 for John Glenn

On Wednesday, Mr Glenn, along with his crewmates, had a last chance to sharpen his flying skills by roaring over the Kennedy Space Centre in the passenger seat on one of five T-38 jets that the astronauts use for training and transportation.

A Nasa test director, Steve Altemus, says flying the jets the day before launch is a tradition for shuttle astronauts.

"It keeps their skills honed to a peak right before launch," said Mr Altemus.

The publicity surrounding the flight has generated interest from all over the world but there has been some criticism of the science.

Some researchers have argued that you can learn very little from studying only one person.

Solar experiments

But for the scientists, John Glenn is not the only interest on the flight. The crew of seven will perform more than 80 experiments and release a satellite for two days of solar studies.

Boarding Discovery

Apart from the US crew, Discovery will also be carrying a Japanese researcher and a European Space Agency astronaut from Spain.

Forecasters said the weather promised to be "100% favourable" for launch at Cape Canaveral.