COMETA. — The typical form of a c. (from the Greek κομήτης) consists of a head or coma, surrounding a brighter nucleus, and a tail extending in the direction opposite the sun. The apparent size of the head may range from that of a small star to the angular dimensions of the moon; the nucleus is generally very small and bright, though at times also very faint and barely visible; the tail may sometimes extend for many degrees, but may also be absent, especially in faint c. At times 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 it is possible to observe the stars through it without any diminution of their brightness.
It is supposed that cometary heads consist of meteoric stones separated from one another by empty spaces. While c. are still at a great distance from the sun, they shine with reflected sunlight. As they approach the sun, the development of their own light, caused by the increasing intensity of solar radiation, becomes ever more pronounced. Streams of matter, gas, or vapour emerge from the nucleus and assume the form of luminous arcs or crescents. A repulsive force compels this matter to move away from the nucleus in the direction opposite the sun, forming the tail. The orbit of this matter and the form 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.
Because of the diffusion of the head, the determination of its 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 very uncertain. Cases are not rare in which their length reaches one astronomical unit (150 million km.) or more.
As for the mass of c., we are groping in the dark. When c. passed through Jupiter’s satellite system, they caused no perturbation of the satellites’ orbits, from which it is concluded that their mass is extremely small. The tenuity of the tails is such that the matter contained in a thousand cubic kilometres is less than that contained in one cubic centimetre of ordinary air.
In the spectrum of the head, the appearance of bands of cyanogen and carbon is characteristic. Near the sun, the sodium lines also appear. Recently, in addition to the lines of C,H,O,N, the molecular spectra of C₂, CN, CH, N₂, NH, and OH have also been observed. Several of these gases are poisonous, but since each molecule is separated from its nearest neighbour by a distance of several kilometres, there is nothing to fear from passing 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 form, 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 no c. with a strictly hyperbolic orbit has ever been found rules out the possibility that c. arrived directly from outside the solar system; it must be admitted that they originally formed part of the system. The sun is surrounded by a cloud of c. extending to at least 150,000 astronomical units. Prof. Oort estimates their number at 50 billion, with a total mass equal to 1/1000 of the mass of the earth.
He conjectures that c., together with the asteroids, were fragments of a planet between Mars and Jupiter that had broken apart. This hypothesis, however, must be regarded as purely speculative, since no plausible cause for the disintegration of a planet is yet known.