More than a decade after he would qualify in the UK for his bus pass, the US astronaut and senator John Glenn is making final preparations to be blasted back into space.

The BBC's James Wilkinson: "The concept of old age will never be quite the same again"

At 77, Mr. Glenn is the oldest man to have ventured beyond Earth, beating the record of shuttle astronaut Story Musgrave, who was 61 when he went into space.

Mr. Glenn says he is going back there to carry out scientific research into the ageing process.

He told BBC News Online that his main motivation was to investigate the similarities between ageing on Earth and what happens to an astronaut when he or she goes into space.

Glenn, "I wrote a proposal"

He said that for several years he had been interested in the similarities between ageing and spaceflight and had put a formal proposal to Nasa chief Dan Goldin to carry out ageing research in space.

"We know the "whats" of ageing but not the "whys"," he said, "Space research may be a way to find out."

Glenn on ageing and spacelight

He pointed out that when younger astronauts come back to Earth after a period in space, their body adjusts to Earth's gravity rapidly.

At his age, Mr. Glenn's body will take longer to recover from spaceflight and thus provide valuable information about the body and the effects of spaceflight.

And Mr. Glenn maintained that there is probably nobody better qualified than he is.

Glenn: "Nasa considered it"

As part of the original seven Mercury astronauts selected in 1960, he was subjected to intensive physical and mental preparation for his 1962 spaceflight, when he became the fifth man in space and the first American to orbit the Earth.

Mr Glenn before his famous mission 35 years ago

Mr. Goldin said that in order to return to space, Mr. Glenn had to prove his case scientifically and then prove that he was physically fit enough to go into space.

Following extensive medical tests, Mr. Glenn remarked that by now he must have passed more astronaut medicals than anyone in history.

He hopes that his research will provide information that will eventually help elderly people live more comfortable lives on Earth.