COMET. - The typical form of a c. (from the Greek κομήτης) consists of a head or coma, which surrounds a brighter nucleus, and of a tail extending in the direction opposite to the sun. The apparent size of the head can vary from that of a small star up to having the angular dimension of the moon; the nucleus is generally quite small and bright, at times also quite faint and barely visible; the tail can sometimes extend for many degrees, and can also be absent, especially in faint c. Sometimes c. are so bright that they can be observed in full daylight, but for the most part they are not visible to the naked eye. The coma is so transparent that through it the observation of stars is possible without any dimming of their brightness.
It is supposed that cometary heads are made up of meteoric stones separated from one another by empty spaces. While c. are still at a great distance from the sun, they shine with the reflected light of the sun. As they approach the sun, the development of their own light stands out more and more, caused by the increasing intensity of the sun's irradiation. Currents of matter, gas or vapors, issue from the nucleus and take the form of luminous arcs or half-moons. A repulsive force compels that matter to depart from the nucleus in the direction opposite to the sun, forming the tail. The orbit of this matter 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 attractive force of the sun.
Because of the diffusion of the head, the determination of the diameter is very uncertain. On average it is estimated at ten or fifteen times the diameter of the earth, but values ten times greater also occur. The size of the tails is likewise quite uncertain. Cases are not rare in which their size reaches one astronomical unit (150 million km.) or more.
Regarding the mass of c., we are groping in the dark. C. passing through the system of Jupiter's satellites caused no perturbation of their orbits, whence it is concluded that the mass is quite small. The tenuousness of the tails is so great that the matter of a thousand cubic kilometers is less than that of a cubic centimeter of ordinary air.
In the spectrum of the head, the appearance of bands of cyanogen and carbon is characteristic. Near the sun the lines of sodium also appear. Recently, in addition to the lines of C, H, O, N, the molecular spectra of C₂, CN, CH, N₂, NH, OH have also been observed. Several of those gases are poisonous, but since each molecule is separated from its nearest companion by a distance of several kilometers, there is nothing to fear from a passage through the tail of a c.
The orbits of c. are for the most part parabolic or nearly parabolic, with periods of thousands of years. Of the c. whose return has been observed, 25 with periods of less than 10 years form the so-called family of Jupiter, near whose orbit they have their aphelion. Their orbits, originally parabolic in shape, were transformed into elliptical orbits by the perturbing force of the giant planet. Of the other c., with periods between 10 and 76 years, two belong to the family of Saturn, one to that of Uranus and four to the family of Neptune.
The fact that a c. with a genuinely hyperbolic orbit has never been found excludes the possibility that c. came directly from outside the solar system; one must admit that from the beginning they were part of the system. The sun is surrounded by a cloud of c. which extends to at least 150,000 astronomical units. Prof. Oort estimates the number of c. at 50 billion, with a total mass of 1/1000 of the mass of the earth.
He conjectures that the c., together with the asteroids, were the fragments of a planet between Mars and Jupiter that broke into pieces. However, that hypothesis is to be considered as purely speculative, since a plausible cause of the crumbling of a planet is not yet known.