What is it about movies on space travel that so often inspires filmmakers to go navelgazing instead of stargazing? From the recent “Ad Astra” to 1997’s “Contact” and beyond, many space exploration films end up closer to the mortal coil than the cosmic plane.

Maybe it’s because most screenwriters are not terribly interested in science and astronomy, or perhaps the silence of space proves too potent a metaphor for family issues, existential angst, and regretful relationships. Whatever the reason, the new Netflix movie “Spaceman” hovers in the same airless atmosphere of interplanetary solemnity and psychology. Indeed, the film is so solipsistic that it feels more like a therapy session — albeit one where an alien is the therapist.

The movie’s eponymous astronaut, Jakub, is on a spacecraft heading toward a radiant pink cosmic cloud called Chopra. Six months in space, he is definitely starting to feel the isolation of his solo mission, with matters not helped by his pregnant wife, Lenka, not answering his calls back on earth. Concurrently, his cohorts in mission control get hold of a pre-recorded video of Lenka telling Jakub she’s leaving him. While his comrades never forward the message, Jakub realizes Lenka wants out with the help of a spider-like creature who comes aboard and questions him about his relationship.

The word “comrades” is used a bit comically here because the movie’s characters and the featured space program are Czech, and because flashback scenes of Jakub as a teenager mention the country’s communist past. There’s also the matter of vaguely Slavic accents, though all the actors speak English. Even Adam Sandler as Jakub seems to attempt an accent, though he’s not consistent with it or believably European. 

What the actor is effective at playing is Jakub’s world-weariness, with Mr. Sandler sporting dark circles under his eyes and drooping jowls under a dense beard to suggest not just exhaustion but depression.

Those looking for traces of Mr. Sandler’s comedy will not find much, though there are hints of it in a jackhammering, noisy toilet and the cheesy promotions Jakub is required to do for the mission’s sponsors. The movie’s most wacky element, though, is the alien creature, whom Jakub names Hanuš. 

We are told this spidery being fled its own world after an attack by other beings. Upon orbiting our planet, Hanuš became fascinated with us earthlings, and when Jakub’s spaceship passes by on its course, it decides to continue its study of humans by befriending Jakub. This leads to many queries by Hanuš, voiced by Paul Dano as if he’s channeling Kiefer Sutherland’s halting character in “Dark City.” 

Because Jakub never informs mission control of his visitor, viewers may wonder if the character is seeing things due to insomnia or isolation-induced psychosis. Yet the filmmakers make the alien quite real in its tarantula physicality. Director Johan Renck and cinematographer Jakub Ihre also impress with the space scenes’ slowly rotating, oscillating camerawork, made to mirror zero-gravity. Scenes flashing back to Jakub’s youth and early moments in his relationship with Lenka cast a spell as well with their warped, tunnel vision perspective.

What the filmmakers cannot do, unfortunately, is convince us that there is a story here. The majority of the movie sees Hanuš ask Jakub about his despondency and this leads ultimately to its climax — if one can call it that — of personal reckoning and expiation as the spaceman finally speaks to his wife. The next scene looks to stir one’s emotions, too, though it’s more likely to have people reaching for bug spray than for tissues. 

Yet the preposterousness doesn’t end there, for in its final 15 minutes, as Jakub’s mission to collect particles from Chopra becomes the film’s focus again, the scenario dips even further into banality and sentimentality. 

Mr. Sandler and Carey Mulligan, as Lenka, do what they can with their roles, yet the script doesn’t give them much to do but mope and wonder, the latter not about the mysteries of the universe but about where their relationship went wrong. In place of wide-eyed, widescreen adventure, we get myopic, claustrophobic, low-key kvetching. Intimations of other themes, like the trauma of communism, fear of fatherhood, and the essence of creation, sparkle here and there, but these quickly burn out in favor of more discussion about Jakub’s abandonment of Lenka and vice versa. 

Take away its galactic trappings and replace its signature arachnid alien with a human therapist, and “Spaceman” could barely be called sci-fi. Maybe it’s a cosmic joke, one that Mr. Sandler is in on, but the payoff has the viewer declaring, “Houston, we have a problem.”