One man's "space junk" is another man's space souvenir, according to Sen. John Glenn (D-Ohio) - who ought to know.

Long before the odyssey of Skylab, the former NASA astronaut and first American to orbit the earth was giving away pieces of his fallen rocket booster as mementos of his 1962 Friendship 7 mission.

"Part of my booster fell on a farm in Rhodesia and was picked up by a farmer," the senator said yesterday. Glenn keeps the square-foot chunk of steel at home and has cut about 15 to 20 "little pieces" from it over the years to hand out as souvenirs.

Although he was monitoring the descent of Skylab along with a good many other people yesterday, Glenn said he was somewhat amused by this latest space event.

"People have been concerned about space junk and reentry for a long time, and although Skylab is the biggest so far there have been several hundred of these objects to come down," Glenn said.

To prove him point, Glenn said, he recently brought his own sample of space junk into his office. The government of Rhodesia had returned it to NASA, which made tests on the material before giving it to him.

"It survived all the reentry, and I've kept it ever since," Glenn said, adding that he stopped cutting off pieces of it several years ago because it was getting smaller and smaller.

Glenn's space exploit attracted the attention of millions, most of whom got up in the middle of the night on Feb. 20, 1962, to watch the live countdown and actual liftoff on television.

"We went through the most expensive part back then," Glenn said. He emphasized that the Skylab venture "is just the payoff" in preparation for a planned space shuttle and more scientific research ahead. CAPTION: Picture, SEN. JOHN GLENN. . . with part of space rocket