Glenn, a 77-year-old senator poised to return to space 36 years after becoming the first American in orbit, was to be in the passenger seat Wednesday on one of five T-38 jets the astronauts use for training and transportation.
Glenn and his crewmates tried on their space suits one last time before the launch so NASA engineers could iron out any wrinkles before flight, reports CBS News Correspondent Eric Engberg. Experts say the weather is going to be clear and beautiful for an expected launch Thursday at 2 p.m. EST.
CBS.com will carry a live WEBCAST of the launch Thursday.
Hurricane Mitch, which NASA weather observers had been following closely, veered westward Tuesday, away from Florida. It was not expected to have any effect on the space mission.
Wednesday night, Glenn and the six other astronauts were to stand at the base of the launch pad, with the brilliantly floodlit Discovery looming above them, and spend a final pre-mission hour with their families.
"They'll stand at the fence and look at the ship that will take them into space," said Bruce Buckingham, a NASA spokesman. "It's an awesome thing."
The family members will stand with the astronauts, talking in the shadows thrown by powerful xenon bulbs that flood the launch pad and the spacecraft with white light. It is a bittersweet moment, usually filled with embraces, whispered farewells, and hopes for a safe journey.
"It is the last chance for them to spend some time with their families," Buckingham said.
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The launch can be extended until 4:30 p.m. Thursday. After that, the mission would be rescheduled for Friday, which also has a favorable forecast.
The mission is scheuled to last eight days, 20 hours, with a landing back at the Kennedy Space Center.
Glenn will become the oldest person ever launched into space, breaking the record held by Story Musgrave, who was 61 when he made his last shuttle flight two years ago.
There are 83 experiments on board, with Glenn participating in 10. Nearly all of his experiments will explore the effects of weightlessness on the human body. These effects are similar to the symptoms of aging that many people suffer on Earth. Scientists hope the research will lead to new treatments.