Celestial Sentry: Earth May Have Secret Defense Against 'Planet Killer' Asteroids, Study Finds

The possibility of an asteroid striking Planet Earth has been a scary proposition for ages, but a new study could provide some relief.

The study, led by a professor at Sweden's Luleå University of Technology, suggests that Earth has a built-in defense system — its intense gravitational forces — that blows up asteroids before they can hurtle toward Earth.

Dozens of asteroids reach close proximity to Earth and the moon each year, but catastrophic hits are rare.

The forces that break up the asteroids before they reach Earth's surface are called tidal forces. They are the same forces that come from the moon to cause tide changes on our planet.

Those forces can be so strong that objects entering Earth's orbit break up.

According to LiveScience.com, one such phenomenon happened in 1994, when tidal forces disrupted pieces of the comet Shoemaker-Levy 9, torn apart two years earlier, while it was en route to Jupiter.

Still, at the time, scientists didn't have evidence to explain why the comet broke apart.

Now, it's believed that near-Earth asteroids are often broken up by tidal forces when they surge toward planets.

"Some 10 years ago we looked for families of NEAs that would have formed in such tidal disruptions, but didn't find any," Mikael Granvik, first author of the new study and a planetary scientist at Sweden's Luleå University of Technology, told LiveScience.com in an email.

However, a follow-up study provided answers: Fragments formed this way would "mix with the background so quickly" that identifying the grouped pieces would be impossible, Granvik said.

But in 2016, Granvik created a model that calculated the asteroids' trajectories to determine their numbers at different distances from the sun.

Granvik and his team compared the model's results with seven years' worth of asteroid observations made by the Catalina Sky Survey, a NASA-funded Arizona telescope-based program that detects NEAs.

The team found that there were far more asteroids than they expected. However, many were much smaller. That's when Granvik realized that they were most likely tidally disrupted fragments from larger asteroids.

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With that idea in place, Granvik collaborated with co-author Kevin Walsh, a researcher at the Southwest Research Institute in Colorado. They found that in such situations the asteroids lost between 50 percent and 90 percent of their mass.

In such a scenario, it explained the missing number of asteroids in Granvik's model.

"While individual families are hard to find, the combination of multiple families will produce a signature that we can identify," Granvik said.

Other simulations showed that fragments would hang around for an average of 9 million years before hitting the sun or a planet or being booted from the solar system.

The tidal force disruption can cause issues, creating more rocks that could fling into Earth at some point. However, it's still unlikely that Earth will encounter a rock large enough to cause extinction because the fragments are smaller than one-sixth of a mile in diameter.

Earth's tidal forces could be tested as early as 2029. An asteroid called Apophis, named after the Egyptian "God of Chaos," is expected to make its closest passing to Earth that year.

However, NASA expects the asteroid to miss Earth by approximately 20,000 miles — if it's not broken into pieces first.