Comets, Jupiter's Family of

a group of short-period comets whose aphelia are situated near the orbit of Jupiter. Jupiter is a planet of great mass, and its gravitational attraction greatly affects cometary motion when the comets periodically approach the planet. Thus, it has been calculated that Lexell’s Comet and Comet Brooks II formerly traveled along orbits of considerable dimensions but became comets of short period after approaching Jupiter. This phenomenon is called capture. The opposite process, the escape of comets from Jupiter’s family by the action of the planet’s gravitational attraction, is also observed; this occurred later with Lexell’s Comet. Comets that belong to Jupiter’s family often approach the sun and therefore rapidly disintegrate. The loss of comets through disintegration and escape is compensated by new captures. In the last two centuries, 60 comets in Jupiter’s family with periods of less than 10 years have been observed.

There are 25 comets with periods of revolution between 10 and 100 years whose distances from the sun at aphelion are close to those of Saturn, Uranus, and Neptune. Thus, for some time it was thought that these planets also had their own families of comets. However, the orbits of these comets are strongly inclined relative to the corresponding planetary orbits, making it impossible for the comets ever to approach the planets. As the American astronomer H. Russell demonstrated in 1920, the gravitational attraction of Jupiter is the principal factor in the motion of these comets even though they do not belong to Jupiter’s family.