12. Asteroids and meteorites
The orbits of asteroids
The main belt
Billions of asteroids are confined within a wasteland between the orbits of Mars and Jupiter, like so much rubble left over from the creation of the solar system. Most of them occupy a great ring, known as the asteroid belt, at mean distances of 2.2 to 3.3 AU from the Sun and with orbital periods of 3 to 6 years. For comparison, the mean distance between the Earth and the Sun is roughly 1 AU, or one astronomical unit, about 150 billion, or 1.5 x 1011, meters. Not all asteroids lie in this belt, but those that do are said to belong to the main belt.
Not all asteroids are found in the main belt. An especially interesting type is further away from the Sun than the asteroid belt. They move along Jupiterís orbit, keeping pace with the giant planet. The first known one, 588 Achilles, was discovered photographically by the Heidelberg astronomer Max Wolf (1863-1932) in 1906. Hundreds of them are now known, travelling on both sides of Jupiter in two clouds, one preceding the giant planet and one following it. As with Achilles, they are all named after heroes of the Trojan War and they are therefore collectively known as the Trojan asteroids.
The Trojan asteroids are held captive by the gravity of both Jupiter and the Sun. They are found near two of the five Lagrangian points, named after the Italian-born French mathematician Joseph Louis Lagrange (1736-1813) who predicted their existence 134 years before the discovery of 588 Achilles. At these points, the gravitational force of Jupiter is equal to that of the Sun, which is much more massive than Jupiter but also a lot further away from the asteroids. These Lagrangian points lie in the corners of equilateral triangles that have Jupiter and the Sun at the other corners.
Although the vast majority of asteroids travel in the main belt lying between the orbits of Mars and Jupiter, there are some notable exceptions that reside within the inner solar system. Known as the near-Earth asteroids, they move inward toward our planet as they travel around the Sun.
There are three populations of near-Earth asteroids, called the Atens, Apollos and Amors. Both the Aten and Apollo asteroids move on eccentric orbits that can cross the Earthís path in space. The Atens are always close to the Sun, never moving out as far as the orbit of Mars. The elongated orbits of the Apollo objects loop in from the main belt to within the Earthís orbit. The Amors travel around the Sun between the orbits of Mars and the Earth, and often cross the orbit of Mars.
Why do some asteroids move near the Earth, while most of them stay in the asteroid belt? Gaps of missing asteroids in the main belt provide some clues to these wandering interlopers. These are the Kirkwood gaps, discovered long ago by the American astronomer Daniel Kirkwood (1814-1895).
The locations of these clearings correspond to orbital resonance with Jupiter, in which the orbital periods are exact fractions of the giant planetís period. Any object that orbits the Sun at the 3:1 resonance, for example, would have exactly one third, or 1/3, the orbital period of Jupiter, and it would complete three circuits around the Sun for every one that Jupiter completes. Such an asteroid would revolve around our star at a distance of 2.5 AU with a period of 3.95 years, compared with Jupiterís orbital distance of 5.2 AU and orbital period of 11.86 years. An asteroid that happened to stray into this resonance would come close to Jupiter at almost the same part of the asteroidís orbit at regular 11.86-year intervals and the accumulated gravitational interaction with Jupiter could dislodge the asteroid form its orbit.
A satisfactory explanation of the Kirkwood gaps was not achieved until the 1980s when powerful computers were used to study Jupiterís influence on the motion of asteroids. The computer simulations showed that Jupiter induces a chaotic zone in the vicinity of an orbital resonance, and that an asteroid that moves into the resonant orbit will eventually be tossed out of it. An asteroid in the chaotic zone can spend tens of thousands of years in a well-behaved, near-circular orbit. But that ordered, placid behavior can be unexpectedly interrupted after 100 thousand years or so, when the orbit is suddenly stretched and elongated in a chaotic way.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University