The other week I went to see a science-fiction thriller called Pandorum which opens – rather memorably – with a scene in which a befuddled Dennis Quaid falls out of a space-pod dressed only in his underpants. The film that follows amounts to a prolonged bout of paranoid hysterics. People scream and run about and get eaten. There is a dark and rusting spaceship, a gaggle of barely glimpsed monsters and a sexy, effortlessly confident warrior woman who puts her bungling male counterparts to shame. It is, if you appreciate this sort of thing, a perfectly serviceable motion picture.

But Pandorum gives us something else as well. Every scene – every frame – comes tainted with the nagging, lingering whiff of deja vu; a sense that there is an altogether better movie nestled deep inside, waiting to burst forth. Afterwards it strikes me what that movie is, and I reel out of the cinema like a suitor at the end of a misbegotten date. The only reason I like Pandorum, I realise, is because I am still in love with Alien.

It is now 30 years since Ridley Scott's breakthrough picture was unleashed upon an unsuspecting public, although the exact birthday remains a mystery. It made its debut at London's Odeon Leicester Square in September 1979, but was not rolled out to the other UK cinemas until January of the following year. Since then, its influence has bloomed and mutated. Alien was the film that set the visual template (grungy and industrial) for any director keen to shoot a picture about monsters in outer space. It was the film that contained a grisly, chest-bursting centrepiece that tapped into the fears of the age. And yet ultimately it all comes back to Ripley. In the figure of the coltish, resolute Sigourney Weaver, Alien may just be the film that overhauled the old, unreconstructed horror genre and dared to put a woman centre-stage.

Because make no mistake: a horror movie is what Alien is. "It's basically a haunted house film," explains the critic David Thomson. "The only difference is that the old dark house just happens to be a spaceship." Thomson has examined the film and its three sequels in his book The Alien Quartet, and feels that this quality is what separates Scott's outing from the ones that followed. "It is a very slow-building film that gives the sense of some great unnamed terror to come. That's a quality that has much more to do with horror than it does with science fiction."

This gave rise to Ridley Scott's joke that nothing actually happens for the first 45 minutes. In its opening sections, Alien rattles, seemingly aimlessly, around a utilitarian space freighter (the Nostromo) and introduces us to its bickering seven-member crew, John Hurt and Ian Holm among them. If 20th Century Fox was hoping for another Star Wars (and the evidence suggests it was), the studio had another think coming. These characters are not Jedi warriors or gung-ho buccaneers; just average working Joes, bored from too long in each other's company and chain-smoking in front of antique computer screens.

Then, boom! The film bursts into hideous life with one of cinema's most notorious setpieces. Hurt's character, impregnated by an extra-terrestrial, abruptly goes into labour at the breakfast table. His chest explodes and the beast is loosed.

Watching the scene now, at a 30-year lag, you find yourself drawn as much to the reactions of the other actors as to the creature itself. Scott famously shot the film in one take with four cameras, and purposely kept the actors in the dark as to what, exactly, they were about to witness. It is safe to assume that none of them were as startled as Veronica Cartwright (playing the Nostromo's navigator), who is shown recoiling in genuine horror from a spray of blood. "What you saw on camera was the real response," recalls co-star Tom Skerritt. "She had no idea what the hell happened. All of a sudden this thing just came up."

Cartwright's shock would be mirrored in cinemas around the world. "Everybody remembers the moment when the creature comes out, because it was such a staggering event; totally beyond prediction," says Thomson. "I remember seeing the film at the time with my wife and she was so horrified that she stood up and walked right out of the theatre. Afterwards she admitted that it was a very well-made film and all of that. But she could not take it; could not live with that possibility. It was as though she thought: if that can happen, anything can."

One could argue that it already had. After speaking to Thomson, I sound out Mark Jancovich, professor of film studies at the University of East Anglia and an expert on the horror genre. Jancovich explains that changes in the 1970s censorship opened the door to more gory, body-centred special effects. "But there is an argument that these films actually reflected wider concerns. There was a growing sense in the 70s that a range of environmental factors – be it pollution, pesticides, food additives, man-made cancers – meant that we were no longer in control of our bodies. That our sense of self can change, mutate and become monstrous. You see that in the early films of someone like David Cronenberg, and obviously in Alien: this sense that all that was familiar and secure starts breaking down. And in the year after Alien you saw this burst of other big-budget 'body horrors', from The Howling, An American Werewolf In London, Altered States through to John Carpenter's remake of The Thing."

In Scott's film, however, the horror came garnished with sexual politics. Take another look at the creature that hatches from John Hurt's chest. It was designed by the Swiss artist HR Giger, who borrowed freely from the images in Francis Bacon's 1944 painting Three Studies For Figures at the Base of a Crucifixion, which in turn took its lead from the Greek myth of the Furies. Scott's film was initially pitched as "Jaws in space" and Giger's alien features the requisite razor-blade teeth and unreadable, implacable air. Sometimes it is limpid and wet, fashioned on the set out of oysters and clams brought in from a local fishmongers. Sometimes it is hard and blunt. Not to put too fine a point on it, the alien in Alien comes in two guises: vaginal and phallic.

"Alien is a rape movie with male victims," explains David McIntee, author of the Alien study Beautiful Monsters. "And it also shows the consequences of that rape: the pregnancy and birth. It is a film that plays, very deliberately, with male fears of female reproduction."

Does this make Alien a conservative film or a radical one? Over the years the debate has been teased out in either direction. In the opinion of the cultural critic Barbara Creed, for instance, Scott's film epitomised what she refers to as "the monstrous feminine". It trades in classic Freudian imagery (penis-shaped monsters; dark, womb-like interiors) and shudders at the bloody spectacle of childbirth. Here is a horror film made by men that exploits a particularly male fear of all that is female.

Others beg to differ. Ripley, they argue, is the game-changer; the character that sends Alien (and its sequels) off in a bold new direction. "Ripley is pretty revolutionary," insists McIntee. "All of a sudden you have a horror film that has a younger female character who is a survivor and a heroine as opposed to a victim."

Originally conceived as a male character, Ripley would go on to form the heart and soul of the Alien pictures. As played by Weaver (then 29 and a relative unknown), she initially seems set up to play the conventional role of the sexy scream queen. But after a while, we realise that not only is she not screaming, she also appears to have no romantic interest whatsoever. Instead, she furrows her brow and tackles tasks with a steely determination. It is Ripley who makes the right decision in refusing to allow the alien aboard the ship, only to be overruled by her male colleagues. For good measure, it is Ripley whose misplaced maternal instincts (hurrying off in search of the ship's cat) enable her to avoid the monster's attack.

Only at the end does the film let her down, slapping us with that oddly gratuitous sequence in which Ripley escapes in a space-pod and proceeds to strip down to her undies. It is as though the makers were so alarmed by what they had unleashed that they tried to rein her back at the last minute. "No one would deny that Sigourney Weaver looks fetching in her underwear," chuckles David Thomson. "But that's really a case of Ridley Scott playing to a very old-fashioned gallery."

Ripley, the evil stepmother

James Cameron has often been accused of playing to that same gallery. And yet his sequel, Aliens, which came out seven years later, proved even more radical; taking what had been a subtext and pushing it to the foreground. "The series became feminist," says Thomson. "Maybe that was because Weaver was given a bigger voice and a producer's credit. Maybe the guys wouldn't have gone in that direction out of choice. But they couldn't make the films without her."

Not that Weaver has ever claimed to have much in common with the character she plays. "I'm no Ripley," she says. "I had doubts that I could play her as strongly as she needed to be played. But I must say that it was fun exploring that side of myself. Women don't get to do that very often."

Released in 1986, Aliens was as much about Ripley as it was about the aliens. Cameron's film gave her a surrogate daughter to protect, and a new band of swaggering men to lock antlers with. Most intriguing of all, it explored the growing affinity between its heroine and the alien "queen" that she battles. "The thing about Ripley is that she is not especially sympathetic to the human beings in her world," says Thomson. "In the second film you do get the feeling that she and the beast have a kind of understanding. There is a scene at the end where Ripley threatens to kill off its offspring and the beast backs off. That shows a certain level of kinship. They are both mothers, after all."

McIntee takes it further still. "There's a strange twist at the end of Aliens, in that Weaver almost becomes the villain. The queen just wants to do what's best for her offspring. Ripley is like the evil stepmother who wants to kill the children."

It is the first rule of Hollywood that every good idea must eventually be milked until it dries out. So it was with Alien, which begat (the arguably superior) Aliens and then led on to the diminishing returns of Alien 3 and Alien: Resurrection. "Actually the third film is underrated," McIntee points out. "With those first three movies you have the sense that they are playing, probably unintentionally, with the three classic female archetypes from folklore. Ripley goes from being maiden [in Alien], to mother [Aliens], to crone [Alien 3]. That's where they went wrong with Alien: Resurrection. If there is no fourth archetype, there's nowhere to go."

So where, exactly, does that leave Ripley? One could argue that Alien's heroine remains an anomaly; a rare bird in a genre that traditionally serves women up as poultry. Conversely, you might even claim that she wasn't so revolutionary to begin with. "I'd agree that she's a great character; a great archetype," says Mark Jancovich. "But this is actually an archetype that goes right back to [gothic novelist] Ann Radcliffe in the late 18th century. Ripley is cut from the same cloth as all those other gothic heroines. She's like your classic three-chord pop song. It's just that some versions are catchier than others."

Others see it differently, they see Ripley breaking the mould. "She advanced the movie heroine pretty far and pretty fast," McIntee says. "I think the entertainment industry would be very different without her. Without Ripley there would be no Leela in Futurama and no Buffy either. She's probably also responsible for toughening up the Bond girls. If you look at the first Bond movie to be released after Alien, you suddenly have the sight of Carole Bouquet running around with a crossbow."

Before Ripley, the horror movie was a more ordered place. Here was a happy hunting ground in which young, sexually active women were there to be punished and the abiding image of motherhood was provided by mummified Mrs Bates in Alfred Hitchcock's Psycho. Alien took a tired landscape and shook it up. Maybe it truly did open the doors to a brave new era, reconciling the genre's in-built fear of femaleness by casting the alien in our midst, the unknowable sex in the role of hero. Almost certainly, it paved the way for movies such as Pandorum: a dunder-headed romp that is nonetheless smart enough to put a powerful woman at centre-stage while leaving the men to stagger about in their underwear, unsure what's going on.

Over in Europe, Ridley Scott's 1979 picture was originally released under the extended title "Alien: The Eighth Passenger". Now this, I'm guessing, refers to HR Giger's unpleasant extra-terrestrial. Yet it might just as easily refer to Ripley: the horror movie heroine who came in from the cold. Three decades on, she looks far wilder and more dangerous than some mocked-up "Jaws in space".