David Bowie’s Major Tom, sitting in his tin can. Elton John’s Rocket Man, missing Earth and his wife. Matt Damon in “The Martian,” left behind to starve. Matthew McConaughey in “Interstellar,” weeping as he watches his kids age without him.

Much art has been made of the infinite loneliness of space travel. And how could it not? Loneliness may be a universal human condition, but what could be lonelier than being removed from the human race entirely?

And so when Adam Sandler, as moody Czech astronaut Jakub in Johan Renck’s “Spaceman,” is asked by a young girl during a broadcast to Earth if he’s lonely, he answers in platitudes but his eyes betray the truth. Yes, he is lonely. Very lonely.

At one point in Sandler’s career, the idea of the actor in a spacesuit as an anxious astronaut heading to the outskirts of Jupiter could only have signaled comedy. But we’ve seen enough great work at this point from Sandler in dramatic roles to know what he’s capable of when the stars are aligned, and he gives a hugely empathetic performance here.

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If there’s a flaw in “Spaceman,” despite its tantalizing promise, it’s not a lack of acting heft but strangely, a lack of story heft. Adapted from the novel “Spaceman of Bohemia” by Jaroslav Kalfar, it paints a world that should be fascinating but is often reduced to seductive yet ultimately frustrating dream sequences. They’re lovely, but we’d like to actually know more about Jakub and his past on Earth, not to mention his relationship with wife Lenka (the always-wonderful Carey Mulligan) beyond seeing her run through yellow fields of flowers.

We begin halfway through Jakub’s mission. That’s 189 days since he left Lenka and Earth for a solo trip to explore the Chopra cloud near Jupiter, glistening and purple and mysterious, beating the Koreans to the punch.

What year are we in? The spaceship looks late-20th century, not 2024 and certainly not futuristic. The production design is terrific here, evoking what such an environment might look like when a guy’s been living in it for six months — rather like what a studio apartment might look like after six months without a cleaning. There are half-consumed bottles of space food. The toilet is constantly malfunctioning, but ground control cares more about fixing the cameras than the plumbing.

Fact is, Jakub, like John’s Rocket Man, misses his wife. They’ve been sending each other video messages, but hers have become spotty. She is pregnant, and angry at having been abandoned for a year. In fact, Lenka records a message telling Jakub she’s deeply unhappy and wants to leave him.

All of which creates a huge Houston-we-have-a-problem moment for the mission, which needs a focused astronaut. The head of the Euro Space program, Commissioner Tuma (Isabella Rossellini) decides Jakub won’t get to see Lenka’s message. But he senses something is wrong.

And then one night, Jakub awakens with a spider crawling out from his mouth.

Phew — it’s only a dream. But soon enough, the spider appears for real, an actual, six-eyed, life-sized alien. Well, we think for real. The spider could be a dream, and we’re surely meant to consider that possibility. (In fact, maybe the whole mission is a dream and Jakub is a guy in a studio apartment, but let’s not go there.)

At first, Jakub thinks he’s going nuts. He puts on his suit and tries to kill the alien with exterminating gas. But the spider helpfully explains that this won’t hurt him. He is, he recounts, on his way from his own planet, traveling through space and time. Oh, and he may have been around since the beginning of the universe. Also: he’s voiced by Paul Dano, in a gentle tone arguably reminiscent of HAL in “2001: A Space Odyssey.”

If HAL were, perhaps, a therapist — because that’s essentially what the spider becomes, trying to ease Jakub’s loneliness but also very curious about his life on Earth. He calls Jakub “skinny human,” and absolutely loves the hazelnut spread “skinny human” eats, calling it “rich and creamy.”

But mainly they discuss Jakub’s marriage. Jakub is defensive. “Why do you resist exploration?” asks the spider, whom Jakub names Hanuš. At another point, Hanuš asks: “You have many boundaries, skinny human, perhaps they are the cause of your loneliness?”

This interplay is toggled with scenes of Lenka back home, as well as flashbacks of the genesis of the couple’s love, memories that the spider forces Jakub to explore — along with that whole genesis-of-the-universe thing.

And so the spaceship approaches the mysterious purple cloud, a place that represents both the beginning and perhaps the end, while Jakub approaches an understanding of his love for Lenka and where that all fits in the universe, too.

These late scenes are both beautiful visually and somewhat lacking. Is the message simply that one must travel across space and time (and past Jupiter) to realize what love means? Questions arise but are not explored. For example, we learn briefly that Jakub’s father back in Czechoslovakia had been an informer under Soviet rule, but little time is taken to detail how this affects Jakub.

Still, it’s a pleasant and occasionally mesmerizing ride, thanks in no small measure to Sandler’s skillful empathy and yet another absorbing turn by Mulligan, who never disappoints. In the constellation that is Hollywood, her star continues to be one of the brightest.

“Spaceman,” a Netflix release, has been rated R by the Motion Picture Association “for language.” Running time: 107 minutes. Two and a half stars out of four.