Glenn undergoes medical checkups ahead of his shuttle mission

(CNN) -- Sen. John Glenn was removed in August from one of the main age-related experiments to be performed during his return to space next week because he failed to meet the medical criteria, NASA officials said Wednesday.

The purpose of the experiment was to test the hypnotic effects of a natural hormone, melatonin, on people in orbit.

The experiment was considered important because it might help identify factors contributing to sleep problems during space flight, which mirror sleep problems experienced by the elderly, according to NASA.

Scientists also had hoped to evaluate the effectiveness of melatonin as a sleep aid during space flight.

Dr. Charles Czeisler of Brigham and Women's Hospital in Boston, the study's lead researcher, refused to elaborate, citing NASA's rule forbidding disclosure of medical information without the subject's permission.

Glenn will still participate in two other planned experiments on aging -- monitoring sleep and use of protein.

Glenn, who 36 years ago made history when he became the first American to orbit the Earth, will be making his second flight.

Glenn and Clinton during a visit to Johnson Space Center in April 1998

On Tuesday, the White House announced that President Clinton will attend the October 29 launch.

But NASA officials said that despite the high publicity and Clinton's presence, they would feel no extra pressure to launch the shuttle on schedule.

A senior member of NASA's Aerospace Safety Advisory Panel said the space agency wouldn't dare launch Discovery just because Clinton was in the viewing stands.

"No way, no way" Seymour Himmel said. "No, it's going to be within the boundaries that are allowed or it ain't going to go."

The 1986 Challenger disaster is still too fresh in too many minds, Himmel said.

The Associated Press contributed to this report.