Space rock alert: Should you worry about an asteroid impact?

NASA is testing one technique it might use should a large asteroid threaten to collide with Earth. But just how high are the odds of such a dangerous space rock after all? You don't need to panic.

The newly-launched Double Asteroid Redirection Test, or DART, is designed to test whether hitting an asteroid with a spacecraft can alter the rock's trajectory enough to avoid a predicted collision with the Earth, given enough lead time. The DART mission will target Dimorphos, which is a satellite of a larger asteroid called Didymos. There's no chance of the pair hitting Earth no matter how the NASA probe fares, according to mission personnel.

So Didymos and Dimorphos don't pose a threat — but are asteroid impacts something most people need to worry about? The answer, as with many things, is "it's complicated."

Related: If an asteroid really threatened the Earth, what would a planetary defense mission look like?

Fortunately, a relatively big rock, one about the size of an American football field that would be able to cause severe local damage upon impact, hits Earth only every 2,000 years or so, according to NASA.

Countless tiny impacts

First off, the question isn't whether an asteroid will smash into the Earth: Not only has it happened before, but it happens pretty routinely. 

Small rocks impact all the time — something the size of a small car hits Earth's atmosphere about once a year, according to NASA, but objects of that size burn up in the atmosphere and explode well before they hit the ground.

When this occurs, no one really notices, except perhaps to think that the spectacle is really cool, since these rocks cause what we call meteors — the "shooting stars" we enjoy watching on a dark, clear night. Meteors are produced by meteoroids, which are the actual pieces of asteroid that are burning up. The vast majority of meteoroids are fractions of an inch or a few millimeters across. 

Larger objects can fall to Earth's surface as meteorites. Only one person is known to have been injured directly by a meteorite — a certain Ann Hodges, who had a 9-pound meteorite strike her in the leg in 1954. The bruise she got was not life-threatening.

Once in a while, meteoroids are large enough to get deep in the Earth's atmosphere and explode; these objects are called bolides or fireballs. Occasionally a bolide will survive until close enough to Earth's surface that bystanders hear the explosion; some airblasts can damage local homes like a bomb, as happened at the Russian town of Chelyabinsk in 2013.

In general, there are a lot more small objects orbiting the sun than large ones, and the progression is roughly logarithmic — for every million sand-grain-size objects, there will be only one that weighs 2 pounds (1 kilogram), and generally there are a trillion tiny meteors for each object in the range of 2,000 pounds (1,000 kg).

A small risk of a big impact

Scientists take a variety of approaches to try to understand the relative risk of a serious asteroid impact on Earth.

Consider the most recent larger impacts. In 1908, a large asteroid or comet exploded over Siberia, triggering shock waves that flattened some 750 square miles of forest in what is dubbed the Tunguska event. The next notable impact, over Chelyabinsk, occurred about 100 years later and damaged homes as far as 55 miles (88 kilometers) away on either side of its trajectory. The Chelyabinsk impact caused some 1,200 injuries, mostly from windows getting blown in by the shock wave. 

These events were separated by about a century. Those are only two events, but they offer some sense of how often impacts of this scale occur.

Statistics gathered by Clark Chapman at the Southwest Research Institute compare a typical American's risk of dying from various causes. Dying from a global impact, which could be smaller than that of the famous Chicxulub asteroid that ended the reign of the dinosaurs about 66 million years ago? About 1 in 75,000. The chances of dying from a regional impact that doesn't affect the entire Earth are actually smaller, about 1 in 600,000. 

These are not large odds, but they are actually better than the chances of winning your local Powerball lottery on a specific ticket. 

However, the chances of being killed in an asteroid impact shouldn't keep you up at night. Chapman, who gathered these numbers in 2007, found that an average American had 1 in 30,000 odds of dying in a plane crash and 1 in 60,000 odds of dying in a tornado; either of those fates is much more likely to be the cause of your demise than an asteroid impact.

Or consider COVID-19. Some 770,000 Americans have died of COVID-19 since the start of the pandemic, according to the Centers for Disease Control and Prevention. The population of the United States is roughly 329 million people, so over the past two years approximately 1 in 427 people have died from the virus. Clearly, COVID-19 is much more likely to kill you than an asteroid impact.

Another way to assess risk is to consider how insurance companies evaluate uncommon but disastrous events such as hurricanes, earthquakes and floods. Homeowner's insurance can cover a pretty wide range of disasters, including asteroid impacts, according to the Insurance Information Institute, which has a handy table outlining what is typically covered and what isn't. Asteroid impacts would be classified under "falling objects," but it's not clear whether it has to be a direct hit to qualify. (Generally speaking it does, but there is some debate about this if the meteor impacts some distance away). 

That insurers even consider the possibility says that they've worked out the odds, and it's within their risk tolerance. The math works for insurance providers because, while an asteroid impact could cause lots of damage, the tiny odds of a serious impact occurring reduces the risk.

The bottom line: you need not worry too much about an asteroid impact compared to many other risks, but it is a longer-term problem that humanity must deal with. 

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