CAPE CANAVERAL, Fla. (AP) _ John Glenn boarded the shuttle Discovery today for a mission of science and sentiment, a return with six crewmates to the high frontier he pioneered alone for America 36 years ago.

Glenn, who will become the world's oldest space traveler, rode with his fellow astronauts on a launch pad elevator 195 feet up to the boarding platform outside Discovery's hatch.

All the crew wore their baggy orange space suits and tight white caps. Glenn looked up several times and grinned broadly as technicians fussed over adjustment of his suit.

One by one, with Glenn going next-to-last, the astronauts crawled on hands and knees into the space cabin, where the technicians helped to strap them into the launch couches _ on their backs with their feet in the air.

Glenn grasped a handhold and powerfully pulled himself into his position on the lower deck with two others. Twice when instructed, he pulled his body upward so a parachute on his back could be adjusted.

During a routine communications check from Mission Control in Houston, Glenn replied vigorously, ``PS2, loud and clear,'' at his turn. PS2 stands for payload specialist 2, his rank on the mission.

The countdown continued flawlessly toward a 2 p.m. launch, and a cloudless sky promised perfect weather for the start of the mission.

Earlier, launch workers filled Discovery's huge external propellant tank with more than 500,000 gallons of supercold liquid hydrogen and liquid oxygen. The chemicals energize the shuttle's main rocket engines at launch, joining the thrust of two solid rocket motors. Together they provide the lift needed to vault the 4.5-million-pound shuttle into orbit.

President Clinton, many members of Congress and an estimated quarter-million people were expected to jam the Kennedy Space Center area for a fleeting glimpse of history roaring into space. Veteran observers said the launch frenzy rivaled that of missions to the moon and far exceeded most recent shuttle flights.

Glenn, 77, was the first American in orbit in 1962, on the third U.S. manned mission. He returns to space on the nation's 123rd manned mission.

The astronauts were awakened today at 8:30 a.m. and quickly sat down to a traditional pre-launch breakfast of steak and eggs. A silent television scene showed them seated at a table, wearing matching dark blue shirts and smiling and gesturing. In the center of the table was a cake frosted with the colorful crew patch that identifies their mission. This is one of the rituals of pre-launch.

A weather forecaster was to brief the astronauts, but NASA officials said there was little to report. The weather remained 100 percent favorable for launch.

Even though he is a U.S. senator and an American space hero, Glenn returns to orbit as the lowest-ranking of Discovery's crew.

The other astronauts are Curtis L. Brown, the commander; Steven W. Lindsey, pilot; mission specialists Stephen K. Robinson, Scott E. Parazynski, and Pedro Duque, and Payload Specialist 1, Chiaki Mukai.

Glenn's return to space results from a combination of political clout, persistence, good health and his heroic reputation. Glenn forcefully lobbied NASA for months to put him on a space shuttle crew.

He argued that he could play a unique role in scientific studies of aging. Glenn got his wish and will serve as a medical guinea pig in 10 experiments measuring the effects of weightlessness on the human body. Many of these changes are similar to those suffered by the elderly on Earth.

Doctors examining Glenn said he had the body, strength and stamina of a much younger man.

For NASA, flying a national icon brought both risks and rewards.

The sentimental space journey has brought new international media attention to the agency. More than 3,500 journalist registered to cover the mission, and the launch was being televised live by major broadcasters.

At the same time, Glenn's presence meant that if anything went wrong, the glare of public scrutiny would be all the harsher.

``Would it be any different from any mission? Sure,'' said Joe Rothenberg, NASA's chief of spaceflight. ``Because like having a teacher on board, it really has got very high visibility and an awful lot of people would look at it as something happening to an American hero and we were part of the process. There's no question about that.''

Glenn first captured national acclaim by orbiting the Earth three times in a one-man Mercury capsule he called Friendship 7. The accomplishment prompted ticker tape parades, a speech before Congress and a visit to the White House.

Ironically, his fame may have barred him from later missions. Glenn said recently that he learned President Kennedy had ordered him grounded to avoid the risk of a national hero dying in space.

Glenn grew up in New Concord, Ohio, the son of a plumber and a school teacher. When World War II started he abandoned dreams of becoming a doctor to earn the wings of a Marine aviator. He flew a total of 149 combat missions in World War II and Korea and is credited with downing three MiGS.

He continued as a Marine Corps flier after the wars and in 1958 was selected as one of the original seven American astronauts. After Alan B. Shepard and Virgil I. Grissom flew suborbital Mercury missions, Glenn was selected for the first orbital flight. His mission lasted only 4 hours, 55 minutes, but it boosted America's drive to the moon.

When he failed to be tapped for another space mission, Glenn resigned from the Marines and NASA in 1964 and returned to Ohio. He was elected a U.S. Senator in 1974, on his third try. He is retiring in January after four terms.

The three other surviving men of Mercury, Walter Schirra, Gordon Cooper and Scott Carpenter _ all of them at the Cape to do TV commentary _ wished Glenn well.

Carpenter, at a news conference Wednesday, hinted that other old astronauts also should go into orbit, noting, ``One 77-year-old man in space does not a data base make.''

The others of the original seven, Shepard, Grissom and Donald K. Slayton, are dead.

In addition to the medical studies involving Glenn, there are 73 other scientific experiments that will be conducted on the mission. Discovery is scheduled to return to the Kennedy Space Center on Nov. 7.