Comets, Asteroids, and Meteors Help (page 2)

By — McGraw-Hill Professional
Updated on Sep 17, 2011

The Asteroid Belt

In 1772, a German mathematician named Johann D. Titius noticed that the orbits of the planets seemed to have radii, or average distances from the Sun, that fell into a neat mathematical progression. At that time, Saturn was the outermost known planet. Titius noticed that the radii of the planetary orbits, in astronomical units (AU), could be found by applying the progression. Later, the astronomer Johann E. Bode expounded on this number sequence, which has become known as Bode’s law .

The progression is defined as follows. Start with the number 3, and double each number, getting the sequence 0, 3, 6, 12, 24, 48, 96, and so on. Then add 4 to each of these numbers so that you get 4, 7, 10, 16, 28, 52, 100, and so on. Finally, divide each of these numbers by 10. This gives the following sequence:

S = 0.4, 0.7, 1.0, 1.6, 2.8, 5.2, 10.0, . . .

These numbers, with the conspicuous exception of 2.8, correspond almost exactly to the orbital radii of the planets Mercury (0.4 AU), Venus (0.7 AU), Earth (1.0 AU), Mars (1.6 AU), Jupiter (5.2 AU), and Saturn (9.5 AU). The question naturally arose among scientists: Is this significant? Did the planets form at these distances from the Sun for some physical reason? Today, most astronomers doubt that Bode’s law is anything more than a coincidence. Back in the eighteenth century, however, it was suspected that there was some modus operandi to this correspondence and that an undiscovered planet must lie 2.8 AU from the Sun. Why had it escaped notice?

A search was begun for the “missing planet.” In 1801, the Italian astronomer Giuseppe Piazzi found an object orbiting the Sun at the correct distance. However, it was small, certainly not large enough to be a planet. It appeared starlike, a mere point of light, but its motion relative to the distant stars gave it away as a resident of the Solar System. It was called an asteroid (meaning “starlike”) and was given the name Ceres . More asteroids were soon found, also orbiting the Sun at distances of approximately 2.8 AU. Ultimately, thousands were discovered, and they all orbit in or near the zone corresponding to the missing slot in the Titius-Bode sequence. This zone has become known as the asteroid belt .

Most astronomers in Piazzi’s time concluded that the asteroids, also called planetoids , were objects that had failed to congeal into a planet or else were the remnants of a planet that was shattered by a cosmic catastrophe. Today, the prevailing theory holds that these objects are part of the original cloud of boulders, rocks, and dust that surrounded the Sun in the earliest part of the Solar System’s lifespan and that the gravitation of Jupiter prevented their getting well enough organized to condense into a planet.

Most of the asteroids follow nearly circular paths around the Sun and orbit between Mars and Jupiter. However, some known asteroids follow orbits that take them inside the orbit of Mars, and others wander outside the orbit of Jupiter. A few maverick asteroids cross the orbit of the Earth and, every few million years, pass even closer to our planet than the Moon. There is plenty of evidence, in the form of visible craters, that asteroids have crashed into the Moon. Such craters are erased in time by erosion when they are formed on Earth, but a few craters have been found on our planet that strongly suggest that we, along with every other object in solar orbit, are on the “asteroid visitation list.”

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