Senator Glenn, 77, has swallowed a radio transmitter and a temperature sensor contained in a capsule the size of a large vitamin pill.

The transmitter monitors changes in his body temperature and sends the information to a sensor he is wearing on his wrist.

During a break from work, he told mission control in Houston that he was enjoying the weightless environment but said it was bringing its own problems.

John Glenn: Flying food

"You have a lot of problems like eating food that has to be controlled or if it gets away it gets on everybody.

"This morning I had oatmeal and raisins. I guess with old folks you normally think it goes down an old man's necktie but this time it wound up on my glasses, so you have to think of things like that."

He was restrained by straps when he became the first American to orbit the earth in 1962.

Senator Glenn has once again seen the Australian city of Perth lit up 340 miles (547 km) below him.

BBC Science Correspondent James Wilkinson: John Glenn has become an American legend

In 1962, residents of the western Australian city turned on their lights to show him a glimpse of humanity at a time when there were doubts about whether he would return to earth safely.

The view was recreated at around 0050 local time on Saturday, when lights at major stadiums, skyscrapers and thousands of homes were turned on to provide a bright contrast with the darkness of the surrounding desert.

"The lights show up very well," Senator Glenn said from Discovery. "Please thank everybody for turning them on, will you?"

John Glenn: "This is beautiful!"

There had been fears that clouds or the crafts more northerly flight path would obscure the view.

The 77-year-old senator has said he was having a wonderful time on his trip.

"This is beautiful. The best part of it is, to do a trite old statement, zero-G and I feel fine."

John Glenn (centre) during takeoff

His commander said the senator had been grinning from ear to ear and "we haven't been able to remove it yet."

As he took off Senator Glenn gave the thumbs up sign to the onboard video camera and shook hands with some of his six fellow astronauts.

On Friday he was due to start medical tests - during his nine-day voyage, he is to be the subject of numerous experiments to find out about the ageing process and to see how older people fare in space.

Discovery blasted off successfully on Thursday, but lost an aluminium panel two seconds into the flight.

However, mission control said the mishap should not affect the voyage or its landing.

A hero's return in 1962 for John Glenn

The panel protects the shuttle's drag parachute, sometimes used during landing. Mission control said the craft could be landed without the chute if necessary.

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

President Bill Clinton and a host of Hollywood stars were among the 250,000 spectators who watched the takeoff.

Discovery is a luxurious ship compared to the cramped rocket Glenn used in 1962.

The crew of seven will perform more than 80 experiments and release a satellite for two days of solar studies.