SIGOURNEY WEAVER FINDS HERSELF STRANDED ON A PLANET INFESTED WITH RAPISTS, LICE, AND AN ACID-SPITTING MONSTER IN THE LATEST INSTALLMENT OF THE SCARIEST SCI-FI SERIES EVER. AND THAT WAS THE EASY PART. NOW SHE'S ABANDONING THE ROLE THAT MADE HER A POWER IN HOLLYWOOD, AS RIPLEY SIGNS OFF FOR GOOD IN ALIEN 3 The Bitch is back,'' the Alien3 trailer intones, over a shot of a terrified, stubble-skulled Sigourney Weaver, as Lieut. Ellen Ripley, vis-a-vis The Thing Itself-drooling, metallic, infinitely evil. And you wonder, if only for a second: She? Or It? ''They called me about the trailer-of course, it had already been made,'' Weaver says. ''And I said, 'Am I supposed to be the bitch?' They said, 'No no no no no-it's really like the profile of the alien against your cheek.' ''I think anyone who's seen Aliens knows that the bitch is this creature,'' she says. She sighs. ''Anyway, there was nothing I could do about it.'' We're meeting in a posh Madison Avenue hotel not far from the East Side home the 42-year-old Weaver shares with her husband, theater director Jim Simpson, 36, and their 2-year-old daughter, Charlotte. Off screen, the star seems softer and more delicate than the characters she has played: The rapt, queenly, Klimt face is small, the skin lightly freckled. There are crow's-feet at the corners of her dark eyes. She's wearing a decidedly terrestrial, even suburban, outfit: red gingham blouse with the collar flipped up, blue slacks, plain pumps. This is Ripley? But then I notice her hair. Even though it has grown back in, it's still short enough to hint that she's been through something. Something trying. Like the actors who played the inmates and wardens of the prison planet Fiorina 161, Sigourney Weaver actually had her head shaved-a risk not many female (or even male) stars would take. This risk, however, pales beside the principal one in Weaver's career these days: Alien3 (see review on page 48), by far the most troubled, difficult production in the series, is the end of the line for Ripley. Thirteen years after Alien, Weaver is stepping quite happily off the cash train she stepped onto at 29, when-an unknown, too-tall stage actress-she became, more or less by accident, the first major female action-adventure star and, eventually, a first-rank power in the movie business. Power being measured, of course, by the familiar standard: The $4 million she was reportedly paid for Alien3 (plus a share of the box office receipts) places her, for the moment at least, at the top of Hollywood's elite group of actresses earning in the multiple millions. Now the series that made her rich and famous will have to go on, if it goes on, without her. And leaving was all her idea. ''In the original (Alien3) script, the male lead sacrificed himself, and Ripley goes off, again, into space,'' she says. ''And I got to the end, and I thought, 'Oh, God.' I said, 'This is it.''' Her voice lacks Ripley's clipped, hortatory quality; instead, the broad vowels and plummy tones put one in mind of The Great Gatsby, the book from which Weaver borrowed her first name when she was 14, having been christened Susan. She grew up on Manhattan's Fifth Avenue, a child of privilege, the daughter of legendary TV executive Sylvester ''Pat'' Weaver (as president of NBC, he invented the Tonight and Today shows) and former British stage and film actress Elizabeth Inglis (The 39 Steps). The patrician package comes complete with eccentricities. My first Weaver sighting ever is in Manhattan on a winter afternoon: There she is, all six feet of her, striding down Columbus Avenue in a long coat and this hat, perhaps a foot tall, a hat that could easily be of extraterrestrial origin. She seems at once solemn, goofy, strangely beautiful. There is the piquant fact that when she was at Yale drama school, her teachers pronounced her fit only for light comedy. The Alien series, for all its otherworldliness, has put a rein on her appealing oddities, qualities she has better evinced as the possessed cellist in Ghostbusters, the obsessed naturalist in Gorillas in the Mist, and the bitch-businesswoman in Working Girl. Not since Katharine Hepburn has a star brought along such distinctive, and distinctively Eastern, baggage. Nor is this merely a matter of style. Like Hepburn, Weaver exudes an aristocratic intelligence, an air of command, that sets her apart from the run of female stars. Her endless legs and strong jaw don't hurt, either. Yet even in her creamier, dewier days, in the first Alien, her power emanated as much from mind as body: The famous BVD shots were more punctuation than point. You looked at her as she stripped down and thought, My God, all this-and lissome, too. And now she has completed some kind of cycle. ''I felt that if I put everything I could into this last one,'' Weaver says of Alien3, ''I could help make it as good as it could be.'' The shaved heads in the movie give eerie emphasis to the actors' eyes, and none are more haunting than Ripley's: This is a character who, after two films full of face-hugging and chest-bursting monsters, has arrived at an accommodation with death. When the convict Morse (played by Danny Webb) screams in her face that he'd like to shove her head through a wall, she towers over him not only physically but morally: Her utterly composed stare is one of complex resignation to the brutality of the universe. Then again, maybe Sigourney Weaver has come to feel similarly about the movie business.