The Making of Kubrick’s 2001 is a 1970 book edited by Jerome Agel. A rare artifact in and of itself, the book finds the Marshall McLuhan collaborator and paperback pioneer spending 368 packed pages chronicling contemporary attempts not just to make, but to understand the transcendent science fiction film. The final page features a lone quote from the film: “Its origin and purpose still a total mystery.”

In fact, as Agel’s book documents, origin and purpose were and remain the least mysterious things about 2001: A Space Odyssey. The film reached the screen 50 years ago today, the culmination of a five-year effort to create what Kubrick called, in his letter of inquiry to collaborator Arthur C. Clarke, “the proverbial ‘really good’ science fiction film.”

While initial audiences struggled to discern the movie’s meaning, they understood what made it different. It was a science fiction film that took its science seriously, an approach that proved to open more doors than it closed. This was despite the best efforts of Kubrick advisors like Fredrick Ordway, an astronomer who told the director it was absolutely “essential” to add a didactic voiceover narrative, lest all the filmmaker’s efforts go to waste when viewers were left confounded.

While today we think about the film’s brain-bending final trip as its most striking sequence, audiences seeing 2001 in 1968 were most impressed by the astoundingly convincing effects dramatizing life in space.

“2001: A Space Odyssey cannot be easily judged if only because of its dazzling technical perfection,” wrote Tim Hunter in the longest movie review the Harvard Crimson had published to date. “To be able to see beyond that may take a few years.”

Viewers saw Gary Lockwood jogging around the Discovery perimeter: a set the size of a ferris wheel, built at the expense of $750,000, or over $5 million in today’s dollars. They saw Keir Dullea erupting into an airlock, an effect achieved by dropping him towards the camera on a bungee cord. They saw a moon-flight attendant seeming to walk up a wall and across the ceiling, an illusion that required an entire room to rotate.

What made this all particularly striking was the matter-of-fact way in which it was handled. Effects that would have served as the thrilling climax of lesser films were deployed for a sequence that came early in 2001, with virtually no direct relevance to the plot. Kubrick could have cut straight from the apes to the Clavius briefing, and the film would still have been unforgettable.

It’s hard to overstate just how much research Kubrick and Clarke put into 2001. Every detail, from the date of human evolution to the plausibility of survival in a vacuum, was checked. Kubrick even filmed conversations with several scientists and a rabbi, with the idea that the feature would open with a ten-minute interview reel.

That was abandoned. So was the idea of voiceover narration, and an original score composed by Alex North. As sprawling as 2001 is, its uniquely eerie effect is the product as much of what was deleted as what was put in.

As Clarke’s accompanying novel proves in exhausting detail, there was indeed an explanation for everything that appeared onscreen in 2001. Kubrick left much of that ambiguous in the final film, though, forcing viewers to stretch their minds in search of answers. Why did the monoliths appear? Why did HAL malfunction? What happens to Bowman in the final sequence? The film contains tantalizing hints, but no absolute answers.

2001 had an immediate and transformative impact on science fiction films, providing the creative fuel that turned speculative fiction into a core pillar of popular entertainment.

When George Lucas set out to make his space opera a decade later, he knew his challenge was to marry the whiz-bang fun of action serials with the technical sophistication and cinematic scope of 2001. Kubrick’s influence, and Lucas’s liberties, are manifest in the very first shot of Star Wars, as a Star Destroyer flies in from behind the camera using precisely the technique that gave the Discovery model its convincingly awesome scale. This time, though, it’s an action scene.

One of the most surprising revelations in Agel’s book is that Kubrick actually considered depicting the aliens that Bowman ultimately encounters.

“Original script idea was for extraterrestrial resembling Giacometti sculpture,” Agel wrote. “Actor was dressed in skin-tight, all-white costume and photographed using 2:1 anamorphic lens, which made him appear extremely thin when projected without the anamorphic.” Test shots make the actors look like licks of white flame against a black background.

In the end, this approach was abandoned; the closest thing to any kind of literal depiction of the aliens in 2001 comes in the form of the chattering voices of György Ligti’s Adventures. (The composer was not thrilled that Kubrick altered his composition electronically, without permission.)

Kubrick’s decision not to show the aliens took nerve, but it’s the decision that’s been most often vindicated by failed experiments in lesser films. Time and again, films inspired by 2001 have teased aliens until an inevitably disappointing climax. Annihilation and Arrival are just two recent examples; also see (or don’t see) Interstellar, in which Christopher Nolan foolishly thinks he can beat Kubrick at his own game.

If you haven’t seen Alien lately, you might forget just how indebted Ridley Scott was to Kubrick. While the bumptious crew could have been regulars at the Mos Eisley cantina, the silently ominous space ambience — complete with lovingly slow exterior pans of the Nostromo against the stars — come straight out of 2001.

Alien actually gains effect because audiences were no longer assuming that an alien movie would necessarily be a monster movie. It’s easy to empathize with the curious scientists who go poking around on an unknown planetoid, perhaps hoping to get the Star Child treatment. Not this time, chaps.

Perhaps the best-case scenario for a climactic alien reveal is Steven Spielberg’s moving Close Encounters of the Third Kind, which succeeds as the anti-2001. Instead of being profoundly strange, Spielberg’s aliens are just like us…more or less. Knowing he had to choose between the close encounter and the space odyssey, Spielberg initially stopped short of bringing us inside the alien ship. The studio later pressured him to do so for a re-release, but he deleted the sequence for later director’s cuts.

In Close Encounters, we’re left to imagine what Richard Dreyfuss might encounter up there among the stars. Spielberg must have known that he couldn’t convey the experience of voyaging “beyond the infinite” any more convincingly than Kubrick did.

What remains extraordinary about 2001, half a century later, is that the film retains its irreducible strangeness. It’s as beautiful as ever, and as poignant as ever — the end of HAL remains perhaps the most indelible death scene in all of cinema — but it continues to challenge us as well.

– Jay Gabler