"Alien" opens on a disarmingly restful note, with establishing shots of a majestic spaceship in which we first discover the seven crew members slumbering away the long voyage home in "hyper-sleep."

But the serenity soon fades as British director Ridley Scott and his collaborators build and sustain a brilliant nightmarish tension. Crew and spectators alike are kept in a state of hyper-apprehension, anticipating the sudden, deadly reappearances of a monstrous alien organism-undoubtedly one of the most bizarre and vicious creatures ever to spring from the shadows of a movie set.

Opening today in 70mm and Dolby at the Uptown, "Alien" is a stylish update on the tradition of '50s science-fiction monster thrillers like "The Thing" or "Forbidden Planet." "Alien" may seem no more ingenious or frightening than those films did at first sight, but it enhances their durable fear mechanisms with the latest refinements in special-effects artistry, space-age scenic design, sophisticated pictorial atmosphere, tantalization and dynamism. It is certain to take a respected place along the classics of cinematic suspense and horror.

The terrorized spaceship, the Nostromo, is a sort of interglactic tug, a commercial exploratory craft returning to Earth towing a gigantic complex of refineries filled with 20 million tons of fuel mined from distant planets.

The crew is awakened prematurely from hyper-sleep when the Nostromo's central computer, nicknamed Mother, detects a seeming distress signal from a neighboring planet. Since the crew is legally obliged to respond to such signals if possible, Captain Dallas (Tom Sherritt) changes course to orbit the mysterious planet. The crew descends to the surface in the detachable Nostromo.

The Swiss surrealist H. R. Giger was evidently the most influential of several famous science-fiction illustrators recruited for the production, and his specialty was this ominous, sinewy, turbulent planet on which the alien organism is discovered After touchdown, Dallas and two crewmen don spacesuits and explore the surface, a mountainous expressionistic terrain dominated by weirdly erotic outcroppings and caverns.

The signal is traced to a huge derelict spaceship. A giant skeleton reposes along one wing of the vessel. A curious crewman insists on exploring, and is lowered into a vast, ribbed chamber where he makes a fatal discovery: an exotic garden full of large egg-shaped pods. He lingers over one, and the organism inside erupts and gets a smothering grip on him.

The stricken comrade is taken back to the Nostromo, and an ordeal begins in the infirmary as the science officer (Ian Holm) attempts to remove the hideous thing that has the crewman's face in the grip of long bony fingers and his neck in a strangle-hold of a slimy tentacle.

The premise is a foolproof invitation to terror. Once the monster enters the sanctuary of the Nostromo, the only thing that matters is getting it before it gets you. The crew members struggle to hunt it down within the cavernous recesses of the spaceship, which offers the elusive creature abundant hiding places. Finally, the question becomes who, if anyone, will survive to administer the coup de grace to this dreadful intruder.

The scientist insists on trying to preserve a life form that the other characters find merely lethal. Writer Dan O'Bannon has given as dubious but meldramatically sensational twist to this old reliable cliche of the science-fiction horror genre, and the trickery allows Holm to contribute the sliest performance in the ensemble.

On the debit side, O'Bannon is overly fond of resorting to surprise double-crosses. But they're scarcely necessary where simple human folly and camaraderie are more than enough to sustain the plot. It's also regrettable that John Hurt (the executive officer) and Veronica Cartwright (the navigator), probably the most inventive and formidable actors in the company, are stuck with the most limited roles.

Nevertheless, it's an admirable cast. Hurt, Cartwright, Holm, Skerritt, Yaphet Kotto and Harry Dean Stanton (the engineers) are imaginative character actors whose work seems to get better and better. One tends to take their skill for granted until it's manifest once again in some indispensable role and you realize how those performances have begun to accumulate.

Sigourney Weaver is the only newcomer in the cast, but her movie debut may generate instant stardom. Weaver, the 29-year-old daughter of former NBC luminary Sylvester "Pat" Weaver, is an experienced theater actress, and she brings an exciting combination of intelligence, bravery and sex appeal to the portrayal of warrant officer Ripley-the most courageous and resourceful heroine seen on the screen in years.

Weaver never allows the character to degenerate into caricature. Ripley's beautiful valor in the face of danger makes a mockery of the dippy blonde "angels" and ponderous supergirls of television.

Weaver resembles a young, willowy, more intense Jane Fonda; and while they're both hot, how about a worthy romantic vehicle for her and Christopher Reeve?

The monster in "Alien" variously recalls the shark in "Jaws," the demon in "The Exorcist," the pods in "Invasion of the Body Snatchers" and even the benign extraterrestrials of "Close Encounters." But the most terrifying adaptation is the mouth: as it grows in both size and ferocity, the infernal beast, conceived by Giger and realized by several special effects designers, flashes not only jaws but jaws within jaws .

After the first shocking, snarling show of teeth, all that's necessary to provoke our self-torment is the buildup of suspense, which Scott orchestrates with masterful visual and rhythmic command-and the startling emergence of fragmentary features from some border or background of the image.

"Alien" is scary enough to create a sensation and justify taking the "R" rating seriously, especially for young children. Yet, it is graphically restrained and subtly abstracted compared to the excesses of "The Exorcist" or "Dawn of the Dead." There are two spectacular gruesome passages, but even here the horror concept is at least as frightening as the depiction . Scott has opted for the minimum effective gruesomeness, given the circumstances and current standards in trick-effect traumatization.

Obviously, fearful susceptibilities and preferences vary. I don't think any film monster has ever scared me as much as the sight of Harold Lloyd improvising a daredevil climax in a movie like "Safety Last." I left "Alien" feeling contentedly manipulated, but not in an unparalleled entertaining panic.

The monster's one blood-spattered attack will probably become the most talked-about sequence in "Alien." The climactic episodes are a rather more impressive cinematic achievement.

I felt more exhilarated leaving "Jaws," but that sensation reflected the movie's marvelous balance between terrifying and funny stimuli. The "Alien" script lacks the sustained, humorous character inter-play that enhanced "Jaws" - or, for that matter. "The Thing," whose perky, overlapping dialogue reflected screenwriter Charles Lederer's long association with Ben Hecht and Charles MacArthur. I also left "Carrie" feeling far more residual fright; but of course "Carrie" ended with a phenomenal kicker. After resolving the "Alien" nightmare, Scott lets it be, and it's a tribute to his prowess that one isn't craving a gratuitous farewell jolt.

Ridley Scott's pictorial flair gave "The Duellists" a thrilling kind of visual beauty. At a "Starwars"-sized budget of $9- $10 million, the settings and scenic opportunities in "Alien" are vastly different but still fabulous, and the new movie, filmed in England, reflects a similar vigor and intensity of vision. Impressive as they are, the images aren't just there to be admired. They're always vivid and dynamic, a story-teller's images.

Running his own camera may give Scott a special awareness of what the audience will utimately apprehend. Many other directors evolve telepathic relationships with trusted cinematographers that produce similar formal concentration and expressive range. But what matters are the results.

The impression Scott made on a small audience with "The Duellists" should now be magnified for a mass market with his streamlined, luxury model of an expressionistic science-fiction horror vehicle. CAPTION: Picture 1, Captain Dallas (Tom Skerritt) of the spaceship Nostromo discovers the source of the mysterious distress signal.; Picture 2, Nostromo's crew approaches the derelict space vehicle.