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Gravity in the Solar System

Newton’s law of gravity allows the planets and other objects to take only certain types of orbit.  These are circular, elliptical, parabolic or hyperbolic orbits.  This list is in order of increasing energy for a given distance of closest approach to the Sun (perihelion).  Both parabolic and hyperbolic orbits are open.

Objects on parabolic orbits start out more or less at rest at a great distance from the Sun and are typical of comets from the Oort Cloud which pass only once through the inner solar system.  No comet has been seen coming into the solar system on a significantly hyperbolic orbit, but some comets come close to planets and are given hyperbolic orbits, leaving the solar system forever.

At the age of 26, Edmund Halley was inspired by the great comet of 1682 and used Newton’s new theory to calculate the orbits of 24 comets.  He realised that the comets of 1531, 1607 and 1682 were the same object and predicted its return in 1759.  In fact, it reappeared on 25th December 1758.  This was a major triumph for Newton’s theory and the comet was named after Halley.  Halley’s Comet will make its 31st recorded appearance in 2061.

shoemaker_levy.jpgJupiter is the largest planet in the solar system and can have dramatic effects on the orbit of comets.  On 8 July 1992 a comet, later known as Shoemaker-Levy 9, passed within 20 000km of Jupiter.  The tidal stress of Jupiter acting on the comet caused it to break up into 21 large fragments, each of which then continued on its own orbit.  Two years later, the comet fragments had formed into a long line and between 16 and 22nd July 1994 they plunged into Jupiter.

Comets and asteroids occasionally hit the Earth.  One famous impact occurred 65 million years ago, probably ending the reign of the dinosaurs and opening the era of the mammals.  The impact crater associated with this catastrophic event has been identified at Chicxulub on the Yucatan peninsula in the Gulf of Mexico.
A montage (not to scale) showing the fragments of the comet Shoemaker-Levy 9 approaching Jupiter © HST