Glenn suits up for the launch rehearsal

CAPE CANAVERAL, Florida (CNN) -- For the first time in 36 years, John Glenn climbed aboard a spacecraft for a launch countdown -- but Friday's ride wasn't real, only make-believe.

The 77-year-old U.S. senator from Ohio grinned as two technicians attached a parachute to the back of his orange spacesuit and made sure it was snug. Then he crawled into space shuttle Discovery, due to blast off in 20 days.

Glenn was the next-to-last of the seven astronauts to board for the launch rehearsal. Like the others, he used a rope to lower himself into his vertical seat.

TV cameras inside Discovery showed almost every move made by the first American to orbit the Earth, who's about to become the oldest person in space. Technicians snapped pictures as he wiggled into his seat and strapped in.

Discovery crew strapped in during the mock final countdown

The National Aeronautics and Space Administration had never provided televised coverage of this final phase of the practice countdown. That changed on Friday, due to large international media interest in Glenn's scheduled return to orbit on October 29.

Such rehearsals occur before every shuttle launch, and simulate every phase of pre-launch activity, from establishing sleeping patterns to practicing an emergency escape.

The 25-hour practice, which began on Thursday, was designed to end as it always does, with five seconds remaining on the countdown clock.

During the nine-day shuttle mission, Glenn will submit to a variety of medical experiments so NASA can compare the effects of aging with the effects of weightlessness.

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The American, Japanese and Spanish crew also will release and retrieve a sun-gazing satellite, test equipment destined for the Hubble Space Telescope, and conduct astronomy experiments.

On Thursday, Glenn scolded reporters for focusing too much on the human-interest part of his upcoming flight and not enough on the "cutting-edge" science he'll conduct.

"Let me castigate the press a moment here," Glenn said. "Too often you get into the human aspects of this and you don't get into the scientific stuff that gets into everybody's house all over this country."

He suggested the work being done on the mission would help researchers who are working to eliminate world hunger and to find new treatments for afflictions such as cancer and osteoporosis.

Glenn, who last rocketed away in 1962, said he's been adequately trained, even though he's had to split his time between NASA and Congress.

Glenn addresses the media Thursday

Veteran astronaut Story Musgrave, whose record as the oldest person to fly in space will be broken by Glenn, recently called the senator a "legislative passenger."

It's a description Glenn rejects, asking his critics to study mission's research program in detail before judging his part in it.

"I'm not back as a legislative passenger," he stressed. "I'm back as a science passenger."

His official title on this flight is payload specialist.

The other crew members for the Discovery launch are: Commander Curt Brown; Pilot Steven Lindsey; Mission Specialists Scott Parazynski, Stephen Robinson, and Pedro Duque; and Payload Specialist Chiaki Mukai.

Glenn essentially left the Senate, where he served for 24 years, last week. But he technically will remain a senator until January when a new Congress, including his own successor, will be sworn in.

Correspondent Miles O'Brien and The Associated Press and Reuters contributed to this report.