Small asteroids hit the Earth on a daily basis. Most of these disintegrate soon after coming into contact with our atmosphere.

Larger asteroids can, of course, be more dangerous. So much so that NASA chief Jim Bridenstine recently warned of the importance of the space agency's planetary defense system against the space rocks.

However, new NASA research, based on a mysterious asteroid impact from as far back as 1908 suggests that dangerous mid-size impacts are far less frequent than previously estimated.

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A once-in-a-millennia occurrence

The research, published in a special issue of the journal Icarus, shows that relatively small mid-size asteroid impacts happen, on average, once a millennium.

This, according to NASA, is much less than the previous estimate of once every century.

What's more, the findings have also shed light on the way that large asteroids break upon entering the Earth's atmosphere.

Asteroid OSIRIS-REx making its first pass over Bennu’s north pole. Source: NASA/Goddard/University of Arizona

The new research was sponsored by a workshop held at NASA’s Ames Research Center in Silicon Valley called, Reexamining the astronomical cold case of the 1908 Tunguska impact event.

Inspired by Tunguska

Due to the fact that the Tunguska event happened in 1908, much of the details surrounding the asteroid impact are shrouded in mystery.

Unlike today, there weren't a huge amount of cameras and sensors to draw data from, in real time as the impact happened. In fact, at first, scientists didn't reach the site of the impact, near the Tunguska River in Russia. That happened more than a decade after the event.

Trees flattened by the intense shock wave created in the atmosphere as an asteroid exploded above Tunguska on June 30, 1908. Source: Public Domain/Wikimedia Commons

Fast forward to 2013 and a new meteor impact in Russia happened at a time when hundreds of people were able to film it and sensors and computer modeling were in place to take measurements and analyze the effects.

The meteor entered the atmosphere near Chelyabinsk, Russia, and though it was smaller than the Tunguska asteroid, it was still the size of a five-story building before it started to break up in the atmosphere.

Readings and research after the impact of the 2013 meteor have now provided insight into the mystery of Tunguska.

Asteroids that could wipe out cities

The shockwave that emanated from the Chelyabinsk meteor blew out roughly a million windows and injured more than a thousand people. And yet, it was relatively small compared to the Tunguska asteroid.

The Chelyabinsk meteor via CNN

Using computer resources and records from surveys of the impact area of Tunguska, researchers were able to predict the likelihood of impact rates using a much wider array of factors.

Modelers were able to perform a statistical study of over 50 million combinations of asteroids and entry properties that could have a similar effect to the Tunguska asteroid.

These approaches, aided by the validation of data from the Chelyabinsk impact, led to a revision on existing estimates of what might have happened in 1908 at Tunguska.

"Because there are so few observed cases, a lot of uncertainty remains about how large asteroids break up in the atmosphere and how much damage they could cause on the ground," states Lorien Wheeler, a researcher working on NASA’s Asteroid Threat Assessment Project.

"However, recent advancements in computational models, along with analyses of the Chelyabinsk and other meteor events, are helping to improve our understanding of these factors so that we can better evaluate potential asteroid threats in the future."

By combining the new data with the most up-to-date population estimates for asteroids, the researchers came to the conclusion that impacts as big as Tunguska — an asteroid large enough to wipe out a city — are likely to happen once in a millennium, not once in a century, as was previously thought.