Watch out: Spoilers ahead for "Greenland."

If a threatening comet was bearing down on Earth with no hope of deflection, what would happen next?

The Hollywood film "Greenland," which will premiere on video on demand on Dec. 18, looks at the effects of the fictional yet convincing Comet Clarke, a large comet that has split into still-hefty pieces. In the movie, the comet takes scientists by surprise because it came from another solar system, making its orbit more difficult to predict echoing the emerging science of interstellar objects we've spotted near Earth.

Scientifically speaking, the movie does a decent job at discussing real-life comets even without the benefit of a science advisor.

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Newscasters in the film draw direct parallels to the Tunguska event that saw a small-body explosion flatten trees in Siberia in 1908. The shockwaves from Clarke's impact echo eyewitness reports from a small asteroid breakup in Chelyabinsk, Russia, in 2012. The movie also shows fireballs from Clarke's fragments hitting the atmosphere, which is plausible given other cometary reports.

Sadly, there seems to be an absence of scientific effort in the film to track the comet's specific path, or to explain how comets and asteroids are monitored in general. While part of that can be explained away by saying it was too late to save Earth from Comet Clarke, it would have been nice to mention NASA's Planetary Defense Coordination Office, which in real life studies scenarios for deflecting asteroids and comets or for informing the evacuation of affected populations in case of a coming impact. NASA and its partners track the sky regularly for threats and happily, they haven't found anything imminently worrying yet.

Starring Gerard Butler ("300," "How to Train Your Dragon"), Morena Baccarin ("Firefly," "Deadpool") and Roger Dale Floyd ("The Walking Dead"), "Greenland" follows the problems of a family that at first, appears to be among the most fortunate on the planet.

Just prior to and then in the middle of a house party, John Garrity (Butler) receives automated messages on his phone and television advising him that the Department of Homeland Security wants him to evacuate Atlanta. He and his family have been selected to ride out the comet's impact in an isolated bunker, thanks to his specialized work as a structural engineer. But he needs to drive to Robins Air Force Base, roughly two hours south, first to catch the plane out to the bunker.

Trouble is, Garrity's neighbors don't get the same alert and they realize that, in the words of Garrity, "Something weird is going on with this comet" and they weren't invited to the bunker. The news is still cheerily talking about how you can see the comet in daylight when the first shockwaves hit the house mid-party. Before long, panic strikes.

It quickly becomes clear that this comet is no mere flyby event. Rather, it's a civilization-killer, with one of the fragments estimated at nine miles (15 kilometers) in diameter, about half as big as the asteroid that killed the dinosaurs 66 million years ago, and likely enough to wipe out cities around the world after it collides with the Earth, which in the movie is predicted to occur somewhere in Europe.

With two days to go before the big hit, Garrity, his estranged wife Allison (Baccarin) and diabetic son Nathan (Dale Floyd) race to an air force base, where they are stranded in a bureaucratic mess that echoes what many of us have experienced during international travel. Lost medication, family separation and a pressing takeoff time start the events of the movie really rolling. Everything becomes very sad and difficult to watch only 30 minutes after the two-hour film begins.

While older viewers may recall the (slightly) more lighthearted and "America will save the world" attitudes depicted in the 1990s films "Deep Impact" and "Armageddon" that also covered cosmic collisions with Earth, "Greenland" features no wisecracking Bruce Willis situation. You quickly see the looting, violence and general chaos that takes hold when society breaks down.

Presidential alerts flash constantly on cell phones, the news shows harrowing scenes of destruction, and local grocery stores quickly empty of essential goods. To be honest, you may find "Greenland" a tough watch after 2020 and the effects of the pandemic, but then again, escapist disaster films may be just the antidote you need after a long year.

Happily, within this melee comes some moments of hope. You see the military personnel who, like today's medical workers, willingly put their lives on the line to help. Dale Floyd manages to overcome the sad, sick child trope for a memorable performance as he attempts to sew his family back together. Also watch for Scott Glenn ("The Right Stuff" movie from 1983), who makes a brief appearance late in the film; his performance is a tear-jerker, showing that even at age 81, Glenn sure can hold anyone's attention.

"Greenland" is less a tale about overcoming a comet collision and more a documentary-style discussion of what happens to people living on a planet under such peril. The film is not an easy watch, but the story will stick with you for the science, the memorable performances and the ambiguous ending.

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom and on Facebook.

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'Greenland' offers grim take on how humanity will respond to a comet impact - Space.com