WHEN DID science fiction become respectable? Was it in 1932, when Aldous Huxley published Brave New World? Was it not until the 1960s, when a whole constellation of major writers appeared, such as Ursula Le Guin and Walter Miller (he wrote A Canticle for Leibowitz) and, in Russia, Boris and Arkady Strugatsky? Was it only in the late '70s, when the once-lowly sf story began to appear in such august settings as the New Yorker?

Or has it not happened even yet?

I think it has not happened even yet. Science fiction thrives -- millions read it -- but it has not become fully respectable. There are two quite different reasons. One is that most English professors continue to stand aloof, and it is they who in the long run decide what counts as literature and what doesn't. Reviewers are but the crickets of a summer.

The other and much more important reason is that science fiction, when it is most itself, disdains respectability. That is, it prefers ideas to technique, practically anything to "art." If it keeps an eye on the future, this is not for its own sake (will I still be read in a hundred years? every poet asks anxiously) but out of curiosity about the universe, concern for the world.

The work of James Tiptree, Jr. is a case in point. Tiptree is a major science fiction writer. She looks more major all the time. (She? You bet. James Tiptree is one pseudonym -- another is Raccoona Sheldon -- of an extraordinary woman who lives in northern Virginia. I'll keep calling her Tiptree here.) Her work is odd and brilliant and occasionally sentimental; at no time is it safely respectable.

Take the first work of hers I ever encountered. That was a novella called "A Momentary Taste of Being," back in 1975. It occurs on a spaceship called Centaur. For 11 years Centaur has been in flight, looking for a habitable planet to which some of the 20 billion people on earth can emigrate. Naturally there are tensions on board: personal, political (the United States, the Soviet Union, and China all have teams on board), even physical. I found it utterly gripping. I believed in that voyage, as I don't in the usual space odyssey. Yes, I kept saying; yes, there would be psychosis like this; yes, these are real people -- and Tiptree's PhD in psychology shows in her incredible understanding of them, I would have added if I'd known then that she had one. Yes, only a master writer handles language so unerringly.

And then I came to the ending, and was shocked almost out of my mind. The voyage of Centaur turned out to have a purpose unknown either to the governments on earth which had sent it, or to the 60-person crew. Put simply, the 60 human beings are giant sperms, rushing to meet equally large ova on the planet they have found. Crazy! Definitely not a respectable idea. Bellow wouldn't use it, or Joyce Carol Oates, either. And yet . . . one puts the novella down, not feeling that here is a trick ending, but that here is an unusually daring metaphor for the life force, or the evolutionary drive, or whatever it is that uses human beings, that causes there to be so many billions of us, so eagerly pushing outward.

Brightness Falls From the Air is Tiptree's second novel and sixth book. It is just as odd as "A Momentary Taste of Being," and nearly as successful. Why only nearly? Chiefly, I think, because Tiptree tends to do her best work in the middle range: long short stories and short novels. Brightness runs to almost 400 pages. That provides room for a few more convolutions than some readers will prefer.

The scene opens on the planet Damiem just as a ship is arriving. The time is many thousands of years in the future. ("Momentary Taste" occurs, of course, barely a century ahead. We may easily be 20 billion people by 2075.)

Damiem is a planet with a terrible history. It is inhabited by a winged race called the Dameii. They are not human or humanoid in ancestry -- like most life forms on Damiem they evolved from insects. But they look somewhat like human beings with two sets of wings, and they are an intelligent and gentle race, loving as parents, skilled as artisans.

ONE OF THEIR peculiarities -- from our point of view -- is that emotion stimulates a set of glands on their backs. For each emotion they secrete a slightly different exudate, much as for each emotion we mostly assume a different facial expression.

Long ago, an early exploration party discovered that if a human being tastes the tiniest amount of this exudate, it produces sensations of bliss. They did not tell their government that, or even that they'd found a new planet; they simply went into business. Over thars they discovered that the sensation of pain gave the best variety of the liqueur which was now famous on many planets under the name Stars Tears. They and their successors devised wonderfully ingenious tortures for the Dameii, eventually discovering that agony mitigated with just a trace of joy produced the finest vintage of all.

Now wings happen to be Tiptree's own symbol for whatever is best about life, and pain for what is worst. In an early and wonderful story called "Angel Fix," an alien visitor to earth defines a good person as one "to whom pain felt by others is real." Tiptree in her own person, explaining why she never says unkind things about her fellow science fiction writers, says, "they are the nearest to winged people that we have, and I would shut up forever rather than hurt one of them. Dead or alive."

The production of Stars Tears is thus about as criminal as something can be. When the Dameii are nearly extinct, a small warship happens to touch down, and its crew discovers what is going on. Most of them are killed in an impulsive rush to stop it -- but reinforcements are sent, and the trade is stopped. That was perhaps 50 years before the time of the book.

Now the planet is a protected place, with a human administrator, a handsome middle-aged woman named Cory Estreel. The Dameii have made a come-back. Limited tourism is permitted. The ship that arrives in the first chapter brings 13 wildly assorted visitors, and launches five or six different plots. One, you can be sure, is an attempt to resume production of Stars Tears. One involves racial revenge. The third is Cory Estreel's own bizarre story, and so on. To my taste there is at least one plot too many.

And yet . . . Tiptree writes with so sure a hand that the book still succeeds. She can show you the human in the alien, and the alien in the human, and make both utterly real. By sheer narrative power she can force you to suspend disbelief and stay up half the night reading, as I did. And yet there are a thousand subtle touches, beginning with the triple play in the book's title, taken from a 17th-century poem. (Tiptree shares with that great and mysterious science fiction writer Samuel R. Delany the ability to find titles that keep unfolding new meanings as you read. Delany's novella "We, in some Strange Power's Employ, Move on a Rigorous Line" has perhaps the aptest title in all science fiction -- but some of Tiptree's come close.)

Brightness is not the book I'd advise a newcomer to commence Tiptree with. It's too big a mouthful. I'd start a beginner with her early collection of stories Ten Thousand Light Years From Home or perhaps with Warm Worlds and Otherwise. But then I'd unhesitatingly send him or her on to Brightness, as one might have sent an early European visitor on to Niagara Falls. Me, I finished reading it at 3:30 on a night when I had to be up at 7 the next morning. There are not many books I'd do that for.