Sam Durrance has some helpful hints for comet-chasers. "You have to have binoculars," advises the Johns Hopkins University astrophysicist. "It's a little fuzzy thing, just past the Pleiades."

Durrance himself can expect a somewhat better view of Comet Halley next March when it comes within 66 million miles of Earth in its orbit around the sun. As a passenger on the space shuttle Columbia, he says, "I'll have the best seat in the house."

Just now, he is perfectly positioned in his own house, a cozy bungalow on a quiet street, to enjoy the Durrance family tableau vivant in all its cacophonous glory. As Becky, his wife of seven years, pops popcorn over the stove, Molly, the poodle-terrier, barks; Sue Tuggle, the mother-in-law, laughs; Susan, the baby, squeals; and Benjamin, the 3-year-old, scampers into the living room, where his dad is busily simplifying the daunting basics of cometology, cosmology, ultraviolet astronomy and such.

"You're so dumb!" Benjamin gleefully shrieks at a reporter.

"It's like this all the time," Benjamin's dad explains with a shrug.

Durrance has seen little enough of his household since he was picked last March to be a "payload specialist" on the shuttle flight, the first to employ his specialty, ultraviolet astronomy. This is a relatively little-used technique that can be applied only outside the atmosphere, which blocks out ultraviolet rays -- though enough of these seem to have slipped through to account for Durrance's healthy tan.

He is 42, 5 feet 11, with the economical build of a jogger. Fixing his inquisitor with his own inquisitive gaze, he speaks in deliberate sentences, punctuated by a perky laugh. He's described by fellow Hopkins physicist Knox Long, whom Durrance beat out in the competition for the shuttle flight, as "a hard person to get angry at."

"He's solid," Long adds. "Not overly excitable."

"Very easy to talk to," says Becky Durrance.

Durrance has donned for the occasion his blue NASA jumpsuit, graced by zippers and cunning pockets, and Ben is wearing similar garb. "You can't convince him that he's not going up, too," the father says of the son. The space agency has also supplied Durrance with zero-gravity prescription glasses, a finely calibrated Seiko watch and even special underwear -- all of which he is duty bound to return at the end of his tour. He hopes to go up four times in all.

"Then maybe they'll put Sam's underwear on display in the Smithsonian," Becky muses.

She describes the last several months without her husband -- who has been doing a lot of preshuttle shuttling between NASA outposts in Houston, Cape Canaveral and Huntsville, Ala. -- as "in two words, 'constant stress.'

"I've gotten used to being the single parent," she says, voicing the standard lament of the astronaut's spouse.

As for the astronaut himself, "I wouldn't say it's constant stress, but it's constant work. I know there's a big reward at the end, so it keeps me going."

He grew up in Tampa, Fla., developed an early liking for things mechanical, designed and built competitive racing cars from high school on. He ended up as a scientist specializing in ultraviolet astronomy, although "I was never particularly studious as a child."

"He was always so interested in space travel," says his wife of their first dates in Boulder, Colo., where he was doing research and she was studying speech therapy at the University of Colorado. "He talked a lot about wanting to be an astronaut. He was different."

Durrance will get his wish by operating the shuttle's ultraviolet observatory, "Astro," a collection of high-tech instruments -- including the Hopkins Ultraviolet Telescope that he helped design -- in order to shed light on the universe.

During the seven-day flight, he will be responsible for aiming the Hopkins telescope (along with the Ultraviolet Imaging Telescope and the Wisconsin Ultraviolet Photopolarimetry Experiment) at "a potpourri of astrophysical targets" including quasars, galaxies, supernova remnants, diffuse nebulae and the auroras of Uranus and Saturn.

"All the hot topics of astrophysics that we can make a contribution to through ultraviolet astronomy" is how he summarizes Columbia's mission.

Observations of the 4 1/2-billion-year-old comet, meanwhile, should produce "a key piece of information about the constituents of the solar system when it was formed," he says. By taking some first-ever ultraviolet measurements of otherwise invisible elements like neon, argon and helium in their primordial quantities, the flight should also test standard cosmological theories about the amount of matter in the universe, the Big Bang and how it will all end. Perhaps, Durrance says, the data will even confirm predictions of a heretofore undetected, superhot substance called the "intergalactic medium."

But not everything about being an astronaut is intergalactic.

"You have to take what they call a 'Class 3 flight physical,' " Durrance says. "It's a day and a half long, which is done at the clinic in Houston. They measure just about everything you can think of measuring in your body. They take a lot of blood. In fact, the first morning, they take seven or eight vials of blood.

"Then there's a psychiatric interview. It's interesting what they do -- asking you the same question over and over in several different ways. They don't really reject you for personality -- they're probably looking for psychosis or something like that. It's a funny clinic, because it's a clinic for healthy people. It's well staffed with bright doctors and nurses, and they tell you what they're doing right along.

"They have a chair, like a dentist's chair, that spins around, and they blindfold you. And while you're spinning around, you have to rock your head forward, sideways, back, left, right, and they give you a cadence to do that. And it's to try and induce motion sickness.

"They don't make you sick really, but they want to see how you respond, I guess. It's a sensation that I'd never felt before. You're spinning around, and when they stop the chair, your perception is that you're going the other way, because all of the fluids in your semicircular canal are moving at a constant rate. And when they stop, what you really perceive is acceleration.

"I also felt various symptoms. You get what they call 'Stomach Awareness I,' a tingling sensation in your hands, sort of tingling sensations in your legs, and then you begin to feel pallor, sort of. That's 'Stomach Awareness II.' But they stop you before you get sick. They watch you and they seem to know and ascertain at a certain point -- and then they stop it."

Whatever NASA throws at him Durrance seems ready to handle.

"I'm sure that when I walk out to the launching tower with the thing fully loaded with fuel, I'll be a little nervous," he says.

As for the flight's biggest thrill, it won't be the hard science.

"It'll be the view out the window, definitely," Durrance says without the least hesitation.