“It’s inevitable that a comet will hit, but when and how big it is isn’t clear.”

Reel Science

The best sci-fi disaster movie on HBO Max reveals a possible apocalypse

"Greenland" and its planet-killing comet doesn't get the science of cosmic disasters entirely wrong. A planetary scientists explains why.

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Picture this: It’s an ordinary day — save for that chunk of comet fast approaching Earth. But it’s no big deal. Scientists say it’s going to land somewhere far off in the Atlantic Ocean.

But when you gather with friends to watch the once-in-a-lifetime event on TV, wonder quickly transforms into horror. The fragment unexpectedly changes trajectory, incinerating central Florida and sending shockwaves 1,500 miles away.

If that weren’t bad enough, scientists just realized an even bigger space rock will wipe out most of humanity in two days. It’s the stuff of nightmares — and also the premise behind the thriller Greenland, now streaming on HBO Max.

Greenland stars Gerard Butler as John Garrity, an engineer who must race against the clock to get his family to a military bunker in Greenland before the largest comet fragment causes an extinction-level event.

Part of the movie’s terror comes from its seemingly plausible scenario: How could we possibly keep ourselves — and our family — safe if a planet-killing comet actually hit Earth?

In Greenland, a family races toward sanctuary.

Should we really be worried about a planet-killing comet hitting Earth in our lifetime? Is this kind of comet a real threat that NASA is seriously considering?

According to Paul Byrne, an associate professor of planetary sciences at Washington University in St. Louis, we don’t need to build our bunkers just yet — even if humanity isn’t entirely in the clear.

“Some of the bits that are happening are not all that unrealistic,” Byrne tells Inverse. “But is the scenario itself likely to play out in the near term? No.”

What is a “planet killer” comet?

The movie’s tagline describes the fragments of the comet, known as Clarke, as a “planet killer.” It appeared suddenly and from a “different Solar System.”

That’s pretty unrealistic, Byrne says. Astronomers are pretty good at tracking comets and asteroids — they wouldn’t be surprised by the sudden appearance of one. For example, earlier this year, NASA scientists concluded it’s unlikely asteroid Bennu will hit Earth within the next several hundred years.

However, Byrne says the way the comet falls in fragments to Earth in Greenland is reasonably realistic.

“Comments can hit in one piece, or they could fragment, it just really depends on the nature of the comet,” Byrne says.

In Greenland, humans have to contend with a “planet-killing” event.


But the two-day lag period between the smaller fragment and the larger planet-killing fragment is not likely to occur, he adds. Instead, it seems more like a plot device to add dramatic tension to the movie.

“Is what's happening in the movie realistic? No, it's not,” Byrne says. “It's not clear why those bits over several days hit Earth.” The only way this kind of prolonged impact would occur would be if the fragments were actually orbiting Earth instead of just passing through its atmosphere.

An object — like a comet — entering Earth’s atmosphere would have to travel a minimum of 11 kilometers (6.8 miles) per second in order to surpass Earth’s escape velocity and strike the planet, Byrne says.

Other factors, like the comet’s energy and size, likely matter more than speed when determining whether an object will be a “planet killer.”

“You could have an object traveling at 11 kilometers a second, and it could do far more damage than an object going 20 kilometers per second — if the slower object is much more massive,” Byrne says.

While the initial fragment that wipes out Tampa in Greenland is about the size of a football field, Byrne says factors beyond largeness matter. The asteroid that helped kill off most dinosaurs was so devastating not just because of its size — 6 miles in diameter — but because it hit a specific area of the Yucatan Peninsula, releasing a huge amount of carbon dioxide into the atmosphere.

Can a comet hit the Earth — and would we survive it?

Inevitably, a comet will almost certainly strike the Earth. Whether it will happen in our lifetime is uncertain.

“It’s inevitable that a comet will hit, but when and how big it is isn’t clear,” Byrne says.

Over the time scale of perhaps a million years, it gets more likely that a comet will strike Earth. But since that’s beyond the scope of our lifetime, Byrne says anyone reading this article shouldn’t worry about a planet-killing comet anytime soon.

“The likelihood of something that we need to worry about hitting us — that would wipe out the planet in the next 100 years — is extremely low,” Byrne says.

But let’s assume this unlikely scenario occurs and a massive comet does hit the Earth.

In Greenland chunks of a comet devastate most of Earth.


In that case, using a bunker in some remote area to avoid the immediate impact of the comet is a pretty good plan, says Byrne — though he says it’s unclear why the movie set the bunker in Greenland as opposed to, say, Siberia.

If you don’t have access to a bunker — and if the comet’s impact is confined to just one part of the planet — it’s technically possible you could also survive by getting on a plane and flying somewhere safe.

“If it were small enough to only kill a city, you wouldn't even need to go anywhere, and you wouldn't even feel it,” Byrne says.

A comet as big as the asteroid that likely killed the dinosaurs might not kill all of humanity today, but “you'd set civilization back a long time,” Byrne says, not to mention widespread famine possibly killing billions and making functioning society impossible to maintain.

The reality is that most people won’t get access to a military bunker, like John Garrity, nor will we be rich enough to fly away on a private jet. As is always the case even in more normal natural disasters, such as hurricanes or fires, the poorest among us will be the most at risk.

Can we stop a comet from hitting Earth?

Absolutely, says Byrne, but we need enough advance notice to anticipate the arrival of the comet.

With enough time, scientists can predict its general path and approach— certainly much earlier than the scientists in Greenland. Even observations of a few weeks can help inform an estimate of a fragment’s path, Byrne explains.

The Double Asteroid Redirection Test (DART) mission is intended to prevent a hazardous asteroid from harming Earth.


Congress also requires NASA to define every near-Earth object 140 meters in diameter or larger above Earth, which helps with tracking near-Earth objects that could potentially be dangerous.

NASA has taken this precaution on step further with the development of the Double Asteroid Redirection Test (DART). NASA’s website describes the program as a “planetary defense-driven test of technologies for preventing an impact of Earth by a hazardous asteroid.” The DART spacecraft is expected to launch sometime after November 24, 2021.

Meanwhile, blowing up a comet fragment or asteroid using nukes — the plot of another sci-fi epic, Armageddon — is “nonsense” according to Byrne, “but there are possible technological approaches we could use to redirect the trajectory” of these space objects.

These technologies include:

  • Using the “gravitational pull” of a small satellite orbiting the comet
  • Spray painting the side of a comet or asteroid with white paint, which would reflect sunlight and produce a thrust that would alter the comet’s trajectory

But the timespan in the movie before Clarke’s planet-killing fragment hits — 48 hours — is nowhere near enough time to prevent a comet’s destruction. If we were really caught off guard by a comet to that degree, we wouldn’t have a fighting chance.

“What's crucial is that we have to see this stuff coming,” Byrne says.

Reel Science is an Inverse series that reveals the real (and exposes the fake) science behind your favorite movies and TV.

Greenland is streaming now on HBO Max.

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