WASHINGTON - NASA enthusiastically tapped John Glenn for a return to orbit, but the 76-year-old senator must first complete a vigorous training program before he can become the oldest person to fly into space.The space agency also said Friday that an Idaho woman, Barbara Morgan, who backed up Christa McAuliffe, the teacher who died in the Challenger explosion 12 years ago, will train as a new astronaut."I see this as another adventure into the unknown," said Glenn, one of America's original space heroes and the first American to orbit the Earth. He noted that his plan to become the oldest human in space fulfills "an unquenchable questing spirit" that is typical of the nation."I am ready," Glenn said Friday at a news conference. "I guarantee you I will give it very best I have."Describing Glenn as "one of the greatest heroes of the 20th century," NASA administrator Daniel Goldin said the pioneering astronaut will have "a long-awaited second flight" as crew member in a space shuttle mission in October.Training for the mission must start within a matter of weeks, officials said, if Glenn is to be ready by October.Glenn, an Ohio Democrat, said the full training program has not been worked out, but he said he would try to complete it during breaks from the Senate.A NASA official said Glenn faces some space shuttle emergency training that requires "some real strength and agility" unusual for a 76-year-old man.In 1962, Glenn became the first American to orbit the Earth, circling the globe three times aboard the Mercury spacecraft Friendship 7. He was greeted with ticker tape parades and enduring fame, but was never offered another space ride _ until now.Goldin also said that Morgan, who was runner-up to Christa McAuliffe in NASA's Teacher in Space program, will be among the astronaut selectees who will report this summer to the Johnson Space Center. Morgan will undergo a long training period before becoming eligible to fly on the space shuttle.Officials said Morgan was given the chance because NASA "had a long-standing obligation" to the Idaho school teacher who was supposed to have followed McAuliffe into orbit in the Teacher in Space program.Selection of Morgan, said John Lawrence, a Johnson Space Center spokesman, does not mean that NASA is resuming its policy of flying civilians on the space shuttle. That program, which was to include teachers and journalists, was abandoned after the Challenger explosion in January 1986 killed McAuliffe and six crew members.The decision to send Glenn back into space struck was a popular idea in official Washington."That's great," President Clinton said through his spokesman, Mike McCurry. "I'm thoroughly delighted. I've always encouraged him to think about this, and I'm pleased NASA thinks it's the right decision."Former Utah Sen. Jake Garn, who is the only senator to have ridden a space shuttle, said: "I'm tremendously excited for John to have this opportunity, and I confess to a bit of jealousy.""He is still one of my heroes, and I plan to be there during the launch."Even Rep. Tim Roemer, D-Ind., a longtime critic of NASA's space efforts, said the decision to send Glenn back into space "is the right stuff.""He may ... rekindle some of the magic and glory that NASA has lost in the last few years," said Roemer.At his news conference, Glenn noted, "I always wanted to go back up again."He said he first made a formal proposal to return to space two years ago after he learned that scientists thought sending an older person into orbit could help do research into aging."So I thought, If I can pass a physical, why not me?" said Glenn.He approach Goldin with the idea, but the administrator said he first wanted to be sure that Glenn was healthy enough for the mission and that there was useful science to be done."Until they satisfied me, I wasn't ready to decide," said Goldin.Glenn twice went through vigorous physical examinations. The results were added to all of medical data collected on Glenn over four decades."I've been looked at physically more closely than any astronaut who has ever gone into space," said Glenn.Goldin said he made his decision this week and denied that it was influenced by the White House or anything but "rational thought."The astronaut-senator will conduct two experiments on his mission dealing with aging. Glenn said the physical decline caused by normal aging on Earth is similar, in some respects, to the rapid changes that occur among astronauts living in the weightlessness of space.NASA spokesman Lawrence said that to prepare for the mission Glenn will have to learn emergency procedures for getting out of a crippled space shuttle. These include sliding down a pole and then dropping into a parachute training pit, a technique used to bail out of the craft.Glenn also will have to be able to climb through a top hatch of the shuttle and then rappel about 20 feet down a rope. This simulates what space shuttle crew members would do if the craft lands on water.Glenn, described by Goldin as "NASA's newest and oldest astronaut," said he was prepared to do whatever the experts ordered him to do to get ready for launch."I expect to go to Houston for training not as Senator Glenn, but as John Glenn, mission specialist," he said.