During the 19th century it was shown that the radiant (i.e., spatial direction) of the spectacular meteor showers of 1866, 1872, and 1885 coincided well with three known cometary orbits that happened by chance to cross Earth's orbit at the dates of the observed showers. The apparent relationship between comets and meteor showers was interpreted by assuming that the cometary nucleus was an aggregate of dust or sand grains without any cohesion, through a concept known as the "sandbank" model. Meteor showers were explained by the spontaneous scattering of the dust grains along a comet's orbit, and the cometary nucleus began to be regarded only as the densest part of a meteor stream.
At the end of the 19th and the beginning of the 20th century, spectroscopy revealed that the reflection of sunlight by the dust was not the only source of light in the tail, showing the discontinuous emission that constitutes the signature of gaseous compounds. More specifically, it revealed the existence in the coma of several radicals—molecular fragments such as cyanogen (CN) and the carbon forms C and C , which are
chemically unstable in the laboratory because they are very reactive in molecular collisions. Spectroscopy also enabled investigators to detect the existence of a plasma component in the cometary tail by the presence of molecular ions, as, for example, those of carbon monoxide (CO+), nitrogen (N2+), and carbon dioxide (CO2+). The radicals and ions are built up by the three light elements carbon (C), nitrogen (N), and oxygen (O). Hydrogen (H) was added when the radical CH was discovered belatedly on spectrograms of Comet Halley taken in 1910. The identification of CH was proposed by the American astronomer Nicholas Bobrovnikoff in 1931 and confirmed in 1938 by Marcel Nicolet of Belgium. In 1941 another Belgian astronomer, Pol Swings, and his coworkers identified three new ions: CH+, OH+, and CO2+. The emissions of the light elements hydrogen, carbon, oxygen, and sulfur and of carbon monoxide were finally detected when the far ultraviolet spectrum (which is absorbed by Earth's atmosphere) was explored during the 1970s with the help of rockets and satellites. This included the very large halo (107 kilometres [62 million miles]) of atomic hydrogen (the Lyman-alpha emission line) first observed in Comets Tago-Sato-Kosaka 1969 IX and Bennett 1970 II.
Although the sandbank model was still seriously considered until the 1960s and '70s by a small minority (most notably the British astronomer Raymond A. Lyttleton), the presence of large amounts of gaseous fragments of volatile molecules in the coma suggested to Bobrovnikoff the release by the nucleus of a bulk of unobserved "parent" molecules such as H O, CO , and NH
(ammonia). In 1948, Swings proposed that these molecules should be present in the nucleus in the solid state as ices.
In a fundamental paper, the American astronomer Fred L. Whipple set forth in 1950 the so-called "dirty snowball" model, according to which the nucleus is a lumpy piece of icy conglomerate wherein dust is cemented by a large amount of ices—not only water ice but also ices of more volatile molecules. This amount must be substantial enough to sustain the vaporizations for a large number of revolutions. Whipple noted that the nuclei of some comets at least are solid enough to graze the Sun without experiencing total destruction, since they apparently survive unharmed. (Some but not all Sun-grazing nuclei split under solar tidal forces.)
Finally, argued Whipple, the asymmetric vaporization of the nuclear ices sunward produces a jet action opposite to the Sun on the solid cometary nucleus. When the nucleus is rotating, the jet action is not exactly radial. This explained the theretofore mysterious nongravita-tional force identified as acting on cometary orbits. In particular, the orbital period of 2P/Encke mysteriously decreased by one to three hours per revolution (of 3.3 years), whereas that of 1P/ Halley increased by some three days per revolution (of 76 years). For Whipple, a prograde rotation of the nucleus of 2P/ Encke and a retrograde rotation of that of 1P/Halley could explain these observations. In each case, a similar amount of some 0.5 to 0.25 percent of the ices had to be lost per revolution to explain the amount of the nongravitational force. Thus, all comets decay in a matter of a few hundred revolutions. This duration is only at most a few centuries for Encke and a few millennia for Halley. At any rate, it is millions of times shorter than the age of the solar system. However, comets are constantly replenished from the Kuiper Belt and the Oort cloud.
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