The flag of the venerable Explorer's Club in Manhattan flew at half-mast yesterday to honor the crew of the space shuttle Challenger. The club's membership includes many of the veterans of previous space flights, and other adventurers as well: mountain climbers, arctic explorers, volcanologists, women and men on intimate terms with the algebra of risk.

They have long grappled with the question that most Americans -- lulled to sleep by almost a decade of nearly flawless space exploration -- had forgotten: Is it worth it?

Why climb the mountain, why take the space flight, why explore the cave, why fly the test plane, why leave wife or husband, family and friends, knowing one may never return? Is it suicidal or simply selfish? Heroic or foolhardy? Is what is to be gained worth the cost?

Science fiction writer Lester Del Rey once predicted that the first man who went to the moon wouldn't come back. The space program has been luckier than that, but as the shuttle explosion made clear, danger is never far away. It just lies low, mocking confidence and precautions.

Mercury 7 astronaut Wally Schirra, for one, is incensed over what he called "the scarf and goggle" syndrome. The shuttle is not yet a passenger plane, he said. "And in fact, we're still learning how to fly that thing."

But conversations with explorers, mountaineers and others the day after the shuttle disaster revealed that they share with the astronauts a kind of explorer's creed. They are people who could no more give up their expeditions than the average person could stop traveling on commercial airliners.

Like the astronauts, the high altitude mountaineers, the climbers of the Himalayas -- where risks of dying in the attempt are one in 10, and sunset on the summit looks remarkably like the first landscape shots of the moon -- have made their peace with danger.

"If adventure has a final and all-embracing motive," wrote Wilfed Noyce, a British mountaineer who died while climbing in 1962, "it is surely this: We go out because it is in our nature to go out, to climb the mountains and sail the seas, to fly to the planets and plunge into the depths of the oceans . . . We extend our horizon, we expand our being, we revel in a mastery of ourselves which gives an impression, mainly illusory, that we are masters of our world."

BAT10 "You say there are some risks, but they are not exorbitant in terms of what you hope to get out of it. From then on, it doesn't occupy your mind," said Thomas F. Hornbein, a member of the first American expedition to reach the summit of Mount Everest, who climbed the mountain by its untried and forbidding west ridge.

Jolene Unsoeld, a Washington state representative, lost her husband and daughter to mountain climbing. Her daughter Devi was killed on an expedition with her father, as they attempted to scale the Himalayan peak Nanda Devi, after which she had been named. Willi Unsoeld, a professor of philosophy and religion who climbed Everest's west ridge with Hornbein, later died in an avalanche on Mt. Rainier. His widow speaks about the shuttle accident with difficulty.

"I think risk plays an important and valuable part in our lives," she said. "I don't think you ever appreciate life unless you take some risks. After my daughter died someone wrote to us, a little old lady, 82 years old, she scrawled out a note on the back of a paper napkin. It said, 'Death is not too high a price to pay for a life well lived.'

"So many of us go out in an auto accident . . . It's no different. And to be able to be doing something that is the most important and meaningful thing in your life then is something we can all aspire to."

Unsoeld scoffs at the idea of safety. "Safe for what? Safe to be in a cotton cocoon?"

Barry C. Bishop, vice chairman of the committee for research and exploration at the National Geographic Society, has climbed in the Himalayas more than a dozen times in the past 25 years, and was also a member of the first American Everest expedition. He lost his toes to frostbite.

"People said afterwards, 'If you'd known, would you have gone?' " My response immediately was, 'Of course.' I knew that frostbite is an inherent risk, and after that you don't even think about it. You're just prepared. To an outsider, that may appear almost suicidal, but to an individual making a commitment, nothing else matters.

"A mountaineer goes into the mountains for the sheer joy of it. That's analagous to the space explorer, too. Mountaineering is a very risky business, as is space travel."

He wouldn't have to think long if given the chance to make the shuttle: "Would I accept? Oh boy," he said, "Oh boy."

bat10 Nicholas Sullivan, 58, a Christian Brother who teaches geology at Manhattan College, knew both Judith Resnik and Dick Scobee, as fellow members of the Explorers Club. He has led countless expeditions into uncharted caves over the past three decades, including one excursion -- during Easter week of 1966, into Puerto Rico's mammoth La Ventosa cave -- that resulted in a participant's death. Cave explorers and astronauts appreciate dangers, he said, but rarely talk about them openly.

"Most people I know in exploration are quite taciturn about it. I think there's a reason for that. They just don't feel there's a need for elaborating on the obvious. You just don't sit around and say, 'Well, we're talking about a risk here folks.' I think there's a tone of professionalism -- one doesn't elaborate on such things."

If people didn't take risks, he said, "we'd never get anywhere in life . . . We would not have the knowledge of geology, biology and the paleontology of caves. If you just vegetate, you're certainly not improving yourself or your fellow human beings."

Sullivan does not think of the Challenger's crew as heroes. "Judy was a very fine scientist. She made some very fine contributions to the program. Scobee was a very fine gentleman. But they were just doing their jobs. If that's heroism, fine. But they were doing what they wanted to do."

Arlene Blum, leader of the first U.S. expedition to climb the deadly Himalayan peak Annapurna in 1978, said she found herself wondering this week whether space flight was more or less dangerous than mountain climbing. She decided it was more dangerous because the astronaut has less control over his or her fate. Climbers prepare their own equipment; they can decide to skirt an avalanche-prone slope, or cross it in the early morning when the risk is less.

Still, Blum said, "If I had a chance to be an astronaut I would probably take it -- there are just things that are important enough that you take the risk. You make your choice, and you make everything as safe as you can and then you go ahead and do it."

Said famed test pilot Chuck Yeager, "You sort of weigh the rewards against the risk. For me, there wasn't any risk too great to break the sound barrier, so you don't give it too much thought. America never could have gotten where it is today by sticking its head in the sand . . .

"Hell, the shuttle accident is nothing new. We take risks and we reap the rewards. Occasionally you get caught."

BAT10 "The prototype for man the explorer," said archeologist George Michanowsky, who has led several expeditions into the Andes and Central America, "is a Sumerian king named Gilgamesh. He is the human being who has to go and find out what is beyond the next hill. He has to know the answer to the riddle. He simply must pursue the quest. Some people have this instinct more strongly and some people have it less strongly. It's a fever in the blood.

"This is what the whole space thing is about, the whole idea of the quest to cross the ocean or climb the mountain or dive to the depths of the sea."

George Leigh Mallory, the most famous of the early Everest explorers, never returned from his third attempt at the summit. It is impossible to know whether he thought Everest worth the ultimate sacrifice, but it is possible to guess.

"What we get from this adventure is sheer joy," Mallory wrote. "And joy is, after all, the end of life. We do not live to eat and make money. We eat and make money to be able to enjoy life. That is what life means and what life is for."

Tuesday, in Beaufort, S.C., Patrick Smith, the younger brother of Challenger pilot Michael Smith, unknowingly echoed Mallory: "I'll tell you exactly how I feel," he said. "I don't have any regrets about Mike doing this. He was doing exactly what he wanted to do. There aren't too many people who've done exactly what they wanted to do with their careers. But he's done that."