Most disaster movies are about going big. But “Greenland,” in which Gerard Butler runs around trying to save his family as a comet gets ready to hit the earth, is a movie that takes pains to make the end of life as we know it look like something that could actually happen. In its relatively small-scale, often rather plodding B-movie way, it wants to do for apocalypse thrillers what “Contagion” did for outbreak movies. And there are moments when it does. The comet that’s heading toward Earth is actually a collection of rock fragments, and the TV news coverage of its impending arrival has the slightly bland quotidian steadiness you’d expect to hear from CNN. Early on, Butler, playing an Atlanta structural engineer — with a different actor he would have been an architect, but someone must have decided that was too wussy for Butler — named John Garrity, takes his 7-year-old son out to a supermarket to pick up some wine and beer for a barbecue they’re hosting, and the signs of doom that surround them have a chilly plausibility: the row of military planes in the sky, the comet glimpsed from a distance. No one knows what’s going to happen, because this hasn’t happened before. If the sensation of impending crisis accidentally overlaps with what a lot of us were feeling back in March, when the pandemic was just taking hold, that coincidence works for the movie. Back at the house, John and his wife, Allison (Morena Baccarin), and their friends sit around staring at the TV as the first fragments begin to hit; everyone’s still hoping that it won’t be too bad. Then they see news footage of a medium-size fragment hitting Tampa, singing the city with a blanket of fire, and the reaction is, “Uh-oh.” The relatively low-key, yep-this-is-what-it-would-feel-like quality of “Greenland” is the best thing about it. In this case, however, there’s a downside to the relative lack of spectacle. John and his family have received a Presidential Alert (just like an Amber Alert) from the Department of Homeland Security instructing them to drive to a nearby air base. It seems that they’re part of a select group of civilians who’ve been chosen to get on military planes and fly to shelter at a classified location. The reason for all this is revealed early on: Of the many comet fragments that are pelting the planet, one, set to hit near Western Europe in 48 hours, is a nine-mile-wide chunk large enough to cause an extinction-level event. Can you say dinosaur? No, I’m not referring to Gerard Butler and what’s left of his movie stardom. He’s an actor who I used to find stolid as a rock and have come to develop a certain fondness for, because I’ve seen him achieve a fascinating intensity in a thriller like 2018’s “Den of Thieves.” (Actually, that was about the only one, but there’s always hope.) According to his IMDb page, Butler has no less than eight movies in pre- or post-production; that’s a sign that as he fell off the A-list, he has found a home as a gritty second-tier actor churning out semi-anonymous mid-budget pulp. The trouble with Butler’s movies is that they tend to be as stolid as he is. John and Allison have been teetering toward a separation, and as they arrive at the military base, where crowds of desperate people are trying to press their way in, like teenagers without tickets at a Harry Styles concert, everything starts to go wrong. Their son, Nathan (Roger Dale Floyd), is a diabetic, and they accidentally leave his insulin in the car. John has to go back and retrieve it, at which point they literally get separated. Then it turns out that Nathan can’t go on the plane; no one with a condition that requires meds can. For a while, “Greenland” turns into a road movie in which the two parents, picked up by different fleeing heartland citizens, attempt to reunite without the benefit of cell phone service, and a bit too much of this can drive you batty. A thriller isn’t supposed to be a cakewalk; if it were, it wouldn’t thrill. But the twists, complications, and bumps in the road have to engage the audience. We need to think, “Wow, what’s he going to do now?” rather than, “Jesus, if this turns out any worse, I’m going to have to take an aspirin.” For much of its distended two-hour running time, “Greenland” is an aspirin thriller. Eventually, the family members do find one another, and that’s the message: that those prone to separate should think about what it means to come together. This has been a theme of other Butler films, as if he were trying to be the Jason Statham of couples’ therapy. And it comes to the fore in “Greenland,” since most of the movie isn’t about Butler acting like a bruiser (though he does face off against one goon who tries to steal his special-selected-family wristband, silencing the guy with a hammer). Scott Glenn, as Allison’s father, makes his shaggy presence felt; the actor is in his early 80s but still spry. And by the time John and his family make it to Greenland — spoiler alert (if you can call a movie’s title a spoiler)! It’s where those military safety bunkers are — we’re more than ready for that giant fragment to hit. But it’s amazing how shallow its deep impact proves to be.