"It's an advantage up here for older folks because in zero-G you can move around much more easily," the 77-year-old Nasa pioneer said in a transmission from space.

John Glenn: steadier than after take-off

He was speaking from the deck of the shuttle Discovery, which is currently in orbit around the earth.

In brief video transmissions, Senator Glenn appeared more comfortable with weightless atmosphere than he did on Friday, when broadcasts showed him grabbing at handles.

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

Ground control in Houston said the whole crew are in good health three days into the nine-day mission, with nothing impeding the 83 planned on-board scientific experiments.

Guinea pig Glenn

Kulbir Natt: "John Glenn will be wired up to assess his breathing, snoring and eye movements"

John Glenn persuaded Nasa to let him fly on Discovery as a guinea pig for geriatric research, and is performing a series of experiments into ageing.

The first of 10 blood samples has been taken from the older astronaut and other tests involve injections of amino acids.

In the name of science, John Glenn has also swallowed a radio transmitter to monitor his body temperature.

Will the real John Glenn ...

Scientists are hoping to use the results for research into the ageing process, which is mimicked during space flight.

Like elderly people in the earth's atmosphere, astronauts both old and young lose bone and muscle mass in the weightlessness of space.

Trick or treat?

... please show himself (top left)

Before the crew began an eight-hour sleep period at 0435 GMT, they sent what they called a Hallowe'en "greeting" to earth by video.

Crowded into a control room, each astronaut held a picture of John Glenn in front of his or her face.

And as the astronauts unmasked, the real John Glenn emerged from behind one of the pictures of himself.

Robot launch

On Sunday, the crew's plans include launching a satellite to study the sun's fiery outer atmosphere.

They will use the shuttle's 15m robot arm for the deployment, a prime objective of the whole mission.

A successful launch would make up for an embarrassing failure last November, when the crew of space shuttle Columbia forgot to switch on the Spartan satellite before setting it free in space.

Realising something was wrong, the astronauts tried to retrieve the satellite, but accidentally nudged it with the robot arm so it tumbled into space.

Two spacewalking astronauts eventually recovered Spartan by hand, but its two-day mission had to be rescheduled.