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When watching a movie or playing a video game and the asteroid belt is shown, you see hundreds of asteroids flying around and bumping into each other. It looks impossible to safely fly a ship through the asteroid belt, which adds to the excitement of the scene.
In reality, however, the asteroid belt is really quite unoccupied. You could point a rocket at a random point in the asteroid belt and the chances are excellent that it would fly right through without hitting a thing.
This is because the asteroid belt is absolutely massive. Sure, there are millions of asteroids in this belt, but given how large it is, there is still far more empty space than there is dangerous rocks.
Even within this largely empty area known as the asteroid belt, there are certain spots that are even emptier than the rest.
American astronomer Daniel Kirkwood studied the asteroid belt, and in 1866 found that certain points were largely unoccupied. Measured in astronomical units (AUs) he noted significant drops in the population of asteroids at 2.5 AU, 2.825AU, 2.96 AU, and 3.28 AU. These are now known as Kirkwood gaps.
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He went on to figure out that what these distances have in common is how frequently their orbits line up with the orbit of Jupiter. For example, the area at 2.5 AU goes around the sun three times for each time Jupiter goes around once. The 3.28AU gap does two orbits for every one of Jupiter’s.
This is known as being in resonance with Jupiter.
In the case of Kirkwood gaps, this resonance with Jupiter makes the orbit of the asteroids become chaotic, and in many cases, causes the asteroid itself to get ejected from the asteroid belt.
When this happens, it will often take off toward the inner-planets, which happens to include Earth.
The vast majority of these ejected asteroids throughout history either burned up from the sun or eventually made their way out of the solar system, but some of them crash into Earth, the Moon, or other planets.
While the Kirkwood gaps are largely empty now, some asteroids do occasionally enter them, especially after two asteroids from other places within the asteroid belt collide. That is believed to be what created Asteroid Bennu and Asteroid Ryugu, which are thought to be fragments from asteroid Polana after it got hit by another object.
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Eventually, these asteroids entered a Kirkwood gap and got ejected into the inner solar system. Asteroid Bennu is considered to be the most dangerous of all known asteroids at this point because it continues to orbit the inner solar system in a way that could eventually collide with Earth.
Fortunately, space agencies keep a close eye on this (and many other) asteroids so that steps can be taken to avoid a disaster.
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