Should a large asteroid threaten to collide with Earth, NASA is testing a technique. How high are the odds of a dangerous space rock? You don't need to be afraid.
The Double Asteroid Redirection Test, or DART, is a test to see if hitting an asteroid with a spaceship can change the rock's trajectory to avoid a collision with the Earth. Dimorphos is a satellite of Didymos and will be targeted by the DART mission. No matter what the NASA probe does, there's no chance of the pair hitting Earth.
Didymos and Dimorphos don't pose a threat, but are asteroid impacts something most people need to worry about? It's complicated, as with many things.
What would a planetary defense mission look like?
According to NASA, a rock the size of an American football field can hit Earth every 2,000 years or so.
The question isn't whether an asteroid will hit the Earth, but whether it has happened before.
NASA says that small rocks hit the Earth's atmosphere about once a year, but that objects of that size burn up in the atmosphere and explode 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 shooting stars. The actual pieces of asteroid that are burning up are produced by meteoroids. Most of the meteorites are small fractions of an inch or a few millimeters across.
meteorites can fall to Earth's surface. One person is known to have been injured by a meteorite, and that is Ann Hodges, who was struck in the leg with a 9 pound meteorite in 1954. She got a bruise that was not life threatening.
bolide or fireballs are objects that are large enough to explode in the Earth's atmosphere. Occasionally a bolide will survive until close enough to Earth's surface that bystanders hear the explosion, and some air blasts can damage local homes like a bomb.
There are a lot more small objects around the sun than large ones, and the progression is roughly logarithmic, with only one that weighs 2 pounds for every million sand-grain-size objects.
There is a small chance of a big impact.
Scientists are trying to understand the relative risk of a serious asteroid impact.
Consider the most recent impacts. In 1908, a large asteroid or comet exploded over Siberia, causing shock waves that flattened 750 square miles of forest. The next impact, over Chelyabinsk, damaged homes as far away as 55 miles away on either side of its trajectory. The impact of Chelyabinsk caused 1,200 injuries, mostly from windows being blown in.
The events were separated by a century. There are only two events that offer a sense of how often impacts occur on this scale.
Clark Chapman at the Southwest Research Institute gathered statistics to compare the risk of dying from various causes. It could be smaller than the famous asteroid that ended the dinosaurs' reign 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 less than one in 600,000.
The odds of winning the lottery on a specific ticket are better than these.
The chances of being killed in an asteroid impact shouldn't keep you up at night. According to Chapman, an average American has a 1 in 30,000 chance of dying in a plane crash and a 1 in 60,000 chance of dying in a tornado.
Or consider COVID-19. According to the Centers for Disease Control and Prevention, 770,000 Americans have died of COVID-19 since it started. Over the past two years, approximately 1 in 427 people have died from the virus in the United States. COVID-19 is more likely to kill you than an asteroid.
It is possible to assess risk by considering how insurance companies evaluate disasters such as hurricanes, earthquakes and floods. According to the Insurance Information Institute, homeowner's insurance can cover a wide range of disasters, including asteroid impacts. It's not clear if asteroid impacts have to be a direct hit to qualify. There is some debate about this if the meteor hits some distance away.
Insurers consider the possibility because they have worked out the odds and it's within their tolerance. The math works for insurance providers because the odds of a serious impact are small.
It's not a big deal if you don't worry too much about an asteroid impact, but it's a bigger problem than you might think.
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