In the Hollywood film "Don't Look Up", which is currently streaming on Netflix, scientists played by Leonardo DiCaprio and Jennifer Lawrence try to warn the world of an impending comet impact. Without spoiling anything, at least this much can be revealed: it isn't all that easy to convince the world of the impending doom.
Even if the comet in the film is more of a metaphor for long-term threats to humanity such as climate change, the question still arises: how likely is a comet impact? After all, impacts of objects from space have shaped the history of the earth. The Chicxulub crater, a 66-million-year-old asteroid impact crater about 180 km across in Mexico, has even been linked to the extinction of the dinosaurs.
However, there is no need to worry too much about a "planet killer" at the moment: "A person is more likely to win the lottery than to experience a large impact in their lifetime," says Christian Möstl from the Space Research Institute of the Austrian Academy of Sciences (OeAW) in Graz. In an interview, he explains why that is so.
One in a hundred million
In the Netflix film "Don't Look Up", a comet serves as a metaphor for climate change, which is often ignored. Why are comet and asteroid impacts popular scenarios in Hollywood?
Christian Möstl: In principle, every disaster film begins with a scientist who is ignored. However, long-term threat scenarios such as climate change happen on very long timescales. An impact, on the other hand, causes immediate devastation and is perfect for a movie. We also know that such impacts have happened several times in Earth's history.
Per year, the chance that a large comet or asteroid more than ten kilometers in diameter will hit is about one in a hundred million.
As a scientist, are you satisfied with the Hollywood version?
Möstl: The portrayals in the films "Armageddon" and "Deep Impact" were generally realistic. Consultants from NASA were even used for "Don't Look Up".
So, should we be afraid of an impact in real life as well?
Möstl: The probability of a real impact is very small. A person is more likely to win the lottery than to experience a large impact in their lifetime. Per year, the chance that a large comet or asteroid more than ten kilometers in diameter will hit is about one in a hundred million. People are afraid because they are bad at assessing improbable risks. Uncertainty also arises when the media artificially exaggerates harmless objects. The tabloids produce a planet killer pretty much annually.
Hardly any danger
So, the end of the world won't come from an impact?
Möstl: A major impact would certainly be a global catastrophe. The last such event was the asteroid impact in Yucatán, Mexico, which wiped out the dinosaurs 66 million years ago. These timescales extend so far beyond a lifetime that the odds approach zero. Statistically, there is one impact of an object with a diameter of about 50 meters every 100 years. Anything less than 20 meters is stopped by the atmosphere, like a wall.
Why aren't we seeing more evidence of large impacts?
Möstl: Time plays a large role. An impact billions of years ago can hardly be traced after such a long time. The comparatively young crater of the Yucatán asteroid, on the other hand, can still be clearly seen today. We know from statistical models that impacts must have happened again and again. But over a very long period of time, the geological traces are missing because the craters weather over long timescales or disappear due to plate tectonics or volcanism.
Unexpected objects with a diameter of more than ten kilometers could only come from the Oort cloud. In such a case, we would only have a few months' lead time.
So, comets and asteroids are not a threat?
Möstl: Asteroids and comets should be divided into two populations for risk assessment. In the inner solar system, there is the asteroid belt between Mars and Jupiter and some objects that travel with Jupiter as Trojans. Here we know all objects that are larger than ten kilometers, as well as their orbits. However, there is a group of objects that are between 100 and 500 meters in diameter that are very difficult to detect. These asteroids are dangerous. They could cause regional devastation if they hit inhabited areas. Statistically, such an impact occurs once every few thousand years. Unexpected objects with a diameter of more than ten kilometers could only come from the Oort cloud, which shrouds our solar system and is too large and too distant to catalogue. In such a case, we would only have a few months' lead time.
That sounds threatening.
Möstl: The probability that a comet is deflected in the direction of Earth is negligible. The solar system is so large that a collision is very unlikely. And viewed from the outside, the earth is very sparsely populated and mostly covered with water, which further reduces the risk for humans.
Deflection instead of collision
What could we do in the event of an impending collision?
Möstl: If we saw such an asteroid early enough, we could theoretically deflect it. But that depends heavily on the composition. Even a boulder with a diameter of a few hundred meters could be blown up if it had a porous consistency.
If we saw an asteroid soon enough, we could theoretically deflect it.
What possible countermeasures are there?
Möstl: In principle there are three options. If there is not much time, the only option would be to use nuclear weapons to blow up the object. Given more time, as heavy a probe as possible could be packed onto a rocket to fire at and deflect the object as with a projectile. A more hypothetical idea that would take much more time is the so-called gravity tractor, whereby a probe orbits the comet or asteroid for a long time and, over years, deflects the object from its trajectory using the tiny impulse of its thrust.
Is that actually feasible?
Möstl: The space organizations in Europe and the USA are cooperating on the DART and Hera missions to shoot down an asteroid with a probe for the first time. DART launched in November 2021 and is being steered into the smaller partner of a binary asteroid system. Hera will be launched in 2024 to document the consequences. This will give us realistic data for the first time to clarify whether an object can be deflected in this way. According to the physics it should work. We should therefore put as many resources as possible into surveying the sky and learning more about the nature of asteroids and comets.
Solar storms offer a much more realistic catastrophe scenario.
How good is the surveillance of dangerous objects?
Möstl: Every night the starry sky is combed by a network of telescopes for potentially dangerous objects. The goal is to find all objects with a size of 100 to 500 meters. We currently know about 8,000 objects of more than 140 meters in diameter that come close to Earth. That is about a third of the estimated total. A lot has happened in the past ten years: we have identified several thousand objects that measure more than 100 meters.
What dangers from space are the experts worried about?
Möstl: Solar storms are my specialty. They offer a much more realistic catastrophe scenario, even if they look less spectacular. An event like the Carrington Event would be much more threatening today than it was in 1859 because our civilization uses much more electromagnetically sensitive technology. Statistically speaking, every 50 to 100 years there is a major solar storm that hits the earth and leads to network failures regionally or perhaps even more extensively.