COMET. — The typical form of a comet (from Gr. κομήτης) consists of a head or coma, which surrounds a brighter nucleus, and a tail extending in the direction opposite to the Sun. The apparent size of the head can vary from that of a small star to having the angular dimension of the Moon; the nucleus is generally very small and bright, though sometimes quite faint and barely visible; the tail can at times extend for many degrees, or it may even be absent, especially in faint comets. Occasionally, comets are so bright that they can be observed in broad daylight, but most are not visible to the naked eye. The coma is so transparent that it is possible to observe stars through it without any dimming of their brightness.
It is believed that cometary heads are composed of meteoritic stones separated by empty spaces. While comets are still at a great distance from the Sun, they shine with reflected sunlight. As they approach the Sun, their own light becomes increasingly prominent, caused by the growing intensity of solar radiation. Streams of matter, gas, or vapor emerge from the nucleus and take the form of luminous arcs or crescents. A repulsive force drives this material away from the nucleus in the direction opposite to the Sun, forming the tail. The orbit of this material and the shape of the tail depend on the force with which the gases are expelled from the nucleus and on the ratio between the repulsive pressure of solar radiation and the Sun’s attractive force.
Due to the diffusion of the head, the determination of its diameter is highly uncertain. On average, it is estimated to be ten or fifteen times the diameter of the Earth, though values ten times larger are also recorded. The size of the tails is also very uncertain. Cases are not rare in which their length reaches one astronomical unit (150 million km) or more.
Little is known about the mass of comets. Since comets passing through the system of Jupiter’s satellites cause no perturbation in their orbits, it is concluded that the mass is extremely small. The tails are so tenuous that the matter in a thousand cubic kilometers is less than that in a single cubic centimeter of ordinary air.
In the spectrum of the head, the appearance of bands of cyanogen and carbon is characteristic. Near the Sun, lines of sodium also appear. Recently, in addition to the lines of C, H, O, N, molecular spectra of C₂, CN, CH, N₂, NH, and OH have also been observed. Many of these gases are poisonous, but since each molecule is separated from its nearest neighbor by a distance of several kilometers, there is nothing to fear from passing through a comet’s tail.
The orbits of comets are mostly parabolic or nearly parabolic, with periods of thousands of years. Of the comets whose return has been observed, 25 with periods of less than 10 years form the so-called Jupiter family, whose orbits lie near that of Jupiter. Their originally parabolic orbits were transformed into elliptical ones by the perturbing force of the giant planet. Of the other comets, with periods between 10 and 76 years, two belong to the Saturn family, one to the Uranus family, and four to the Neptune family.
The fact that no comet has ever been found with a strictly hyperbolic orbit excludes the possibility that comets came directly from outside the solar system; it must be admitted that they originally belonged to the system. The Sun is surrounded by a cloud of comets extending to at least 150,000 astronomical units. Prof. Oort estimates the number of comets at 50 billion, with a total mass of 1/1000 that of the Earth.
He conjectures that comets, along with asteroids, were the fragments of a planet between Mars and Jupiter that broke apart. However, this hypothesis must be regarded as purely speculative, since no plausible cause for the disintegration of a planet is yet known.