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Just What Are They?
The asteroid belt is a region consisting of millions of asteroids that exist between the orbits of Jupiter and Mars. The asteroids orbit the Sun and occasionally some asteroids are flung towards the inner Solar System. In 1857, a professor by the name of Daniel Kirkwood at Jefferson College in Pennsylvania noticed an interesting phenomenon occurring in this region. He noticed that certain regions in the asteroid belt had been cleared of asteroids. This phenomenon was named after him. Professor Kirkwood correctly explained this occurrence as an effect of Jupiter's gravity on the asteroids.
The orbits of the asteroids around the Sun are elliptical. Some of these asteroids have orbits that are much more elliptica than others. When the asteroid's average distance to the Sun is plotted on a graph, gaps appear. An explanation for this phenomenon was the orbital resonances with Jupiter's orbital period.
Orbital resonance is simply when two orbital bodies exert a gravitational influence on each other, which can be expressed as a ratio of integers. For example, an asteroid with an orbital resonance of 3:1 orbits the Sun three times for each one orbit of Jupiter. Every three orbits Jupiter and the asteroid would be relatively in the same position, as a result, the asteroid would experience a gravitational force or what is known as the perturbing effects of Jupiter. Over time, this force would cause a change in the orbit of the asteroid, which would result in even more elliptical orbits of the asteroid and a gap at that distance.
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What We Know Now
With modern computers, scientists have been able to determine that the effects of Jupiter on the asteroids is not just to pull them in and out of orbit. In fact, we now know that Jupiter repeatedly tugs on the asteroids in the same part of the orbit over and over again, which permutes the orbit of the asteroids. The orbit may eventually get so perturbed that the asteroid may pass too close to a planet and get hurled out into a new orbit about the Sun. In some cases the asteroids can directly collide with a planet.
The process does not happen immediately, but once it gets started it can quickly progress. Asteroids can, however, remain in a resonant orbit with Jupiter for millions, even billions of years before their orbits get shifted. The gaps are thus not emptied at once. The nature of the orbital resonance can determine how likely it is for the orbits to shift. Asteroids with orbits near those of the Kirkwood gaps can enter the gaps and eventually get flung out into the Solar System.
The gaps are most prominent at orbital resonances of 4:1, 3:1, 5:2, 7:3 and 2:1. Weaker gaps have also been found at various distances and orbital resonances. Asteroids with high eccentricity orbits have recently been found which lie within the gaps. The Griqua family and the Alinda family are good examples of such asteroids. Over millions of years, these asteroids will increase the eccentricity of their orbits until they come close enough to a planet and eventually break free.
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Do Asteroids in These Gaps Pose a Threat to Earth?
We now know that bombardments of the inner Solar System by asteroids is linked to the orbits of these asteroids in the Kirkwood gaps. In fact, some of the asteroids presently traveling through the Solar System could have possibly been in these gaps and over time, due to perturbations from Jupiter broken free and are now journeying through the Solar System. However, the process takes millions, sometimes even billions of years. Any threat to Earth at the present time might be from near Earth asteroids that are already present in the inner Solar System. So It seems highly unlikely that an asteroid in these gaps might break free of its orbit any time in the near future or for that matter for many millions of years. Therefore, we can remain certain of at least one thing: the asteroids in these gaps do not pose any immediate threat to Earth, however there are other asteroids that we should still keep a close eye on.
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