Comet

We have learned a great deal about comets in the intervening centuries, but there still remain some unanswered questions. 

Comets, like planetoids and meteorites, belong to the group of small heavenly bodies. According to the nature of their orbits, we distinguish two groups  

Long-period comets their extended ellipsoidal orbits reach far outside our solar system (up to half the distance to the next fixed star). This group includes the comet Kohoutek, discovered in the 1970s, which requires about 75,000 years for a single orbit. 

Short-period comets these display a strong tendency for their farthest point from the sun (aphelia) to coincide with a giant planet s orbital radius, so that we can distinguish so-called comet families . The Jupiter family of comets is the largest and numbers around 70 comets. The shortest orbital period known is that of the short-period comet Encke—about 3.3 years. 

According to Delsemme (1998), the two groups of comets originated as follows  

Besides the planets and their satellites, the Solar System harbors a large number of smaller objects, ranging from hundreds of kilometers in size down to dust particles. If they consist of solid material and have at least the size of small boulders, they are called asteroids. If they enter the atmosphere of Earth and reach the surface, they are named meteorites. If they are very small and bum up on entry, they are referred to as meteors. If the bodies contain a substantial fraction of ices and develop tails as they come closer to the Sun, they qualify as comets. However, the classifications are not very consistent for example, remnants of the dust tails of comets cause meteor showers, and older comets, once they have expended most of their volatile matter during many passes near the Sun, may not be distinguishable from asteroids. This section is devoted to comets and the next section (7.3) deals with asteroids. 

Comets have been observed since antiquity. Commonly, they are defined as heavenly bodies with a luminous head and one, sometimes two, tails. Sir Edmund Halley recognized that the comet of 1682 was the same as that seen in 1607 and before in 1531. He predicted the next appearance of that comet, now called comet Halley, for the year 1759. Unfortunately, his death in 1742 prevented him from witnessing the realization of his prediction. By applying Kepler s laws and Newton s theory to the observations, it was clear to astronomers since Halley s time that many comets orbit the Sun in elongated, elliptical orbits, while others appear to have nearly parabolic ones. 

At the same Liege conference, Oort (1950) proposed the existence of a large reservoir of nuclei of comets at a distance of, according to most recent estimates, about 30000 to 50000 AU. This assemblage of nuclei is now known as the Oort cloud. Oort s prediction was based on a study of 19 highly elliptical orbits of new comets - that is, comets that enter the inner Solar System presumably for the first 

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The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

For remote sensing, spectroscopy at THz frequencies holds the key to our ability to remotely sense enviromnents as diverse as primaeval galaxies, star and planet-fonuing molecular cloud cores, comets and planetary atmospheres. 

Latin carbo, charcoal) Carbon, an element of prehistoric discovery, is very widely distributed in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites. 

Alkanes have the general molecular formula C H2 +2 The srmplest one methane (CH4) rs also the most abundant Large amounts are present rn our atmosphere rn the ground and rn the oceans Methane has been found on Juprter Saturn Uranus Neptune and Pluto and even on Halley s Comet... 

What a storyi Fullerenes formed during the ex plosion of a star travel through interstellar space as passengers on a comet or asteroid that eventually smashes into Earth Some of the fullerenes carry pas sengers themselves—atoms of helium and argon from the dying star The fullerenes and the noble gas atoms silently wait for 251 million years to tell us where they came from and what happened when they got here... 

In addition to these laboratory-based experiments it is interesting to note that the Swan bands of C2 are important in astrophysics. They have been observed in the emission spectra of comets and also in the absorption spectra of stellar atmospheres, including that of the sun, in which the interior of the star acts as the continuum source. 

Formaldehyde cyanohydrin was detected in Halley s comet by the Vega I space probe (27). 

When very high velocities are encountered, metal loss from erosion-corrosion can be general. T ically, however, erosion-corrosion produces localized metal loss in immediate proximity to the disrupted flow. Smooth, rolling, wavelike surface contours are often produced, or distinct, horseshoe-shaped depressions (Fig. 11.1) or comet tails... 

Figure 11.2 Comet-tail erosion patterns on copper. (Magnification 15x.)...

Figure 11.5 Erosion-corrosion at elbow of a brass tube. Note also the borseshoe-shaped depressions and comet tails aligned with flow direction in the straight section.

The classic signature of erosion-corrosion is the formation of horseshoeshaped depressions, comet tads, grooves, or sand dunelike surface contours oriented along the direction of fluid flow (Figs. 11.1,11.2,11.3,11.5, and 11.8). Occasionally, erosion-corrosion will produce smooth, almost featureless, surface contours (Fig. 11.15), although even in this case oriented metal loss often exists around the perimeter of the affected region. If erosion-corrosion has been recently active, affected surfaces will be free of accumulated deposits and corrosion products. 

Figure 11.26 shows a component removed from an air-conditioning compressor. Pinhole perforations in this component had allowed cooling water to leak into the freon. Many failures of this type had occurred previously. Examinations of the internal surfaces of both the canister and the tubes entering it revealed evidence of metal loss. Tiny perforations at the bases of deep grooves were noted in the tubes. Deep, general, smooth metal loss surrounded irregular islands of intact surface (Fig. 11.27). The canister walls displayed a similar metal loss and comet-tail-shaped depressions (Fig. 11.2).

Silver, L.T., and Schultz, P. (Eds.) Geological Implications of Impacts of Large Asteroids and Comets on the Earth, pp. 1-528, Special Paper 190, The Geological Society of America, Boulder, Colorado, 1982. 

Dale, D., Pacult, R., and Reinhard, R., The Joint NASA/ESA Cometary Mission to Comets Halley and Tempel 2, European Space Agency Report No. ESA SP-153, Paris Cedex, France, pp. 3-5, October 1979. 

Redux was soon extended to other aircraft including the DeHavilland civilian airliner Dove and the jetliner Comet [202]. In the Fokkcr F27/F50 Friendship , it accounted for about 70% of stmctural bonds (about 550 parts). Over 1000 F27/F50 aircraft were constructed and were in service for over 30 years, indicating high fatigue resistance and durability ([198], pp. 80-81). When attempts were used to substitute low-temperature curing epoxies for PVF, joint corrosion due to salt spray exposure required expensive repairs. 

It is probable that the solvents given in the individual reagent monographs are not suitable for all the substances with which the reagent will react. This point should be taken into account especially for quantitative work and the user should make appropriate modifications. In particular, there must be no loss of substance or reaction product by dissolution (formation of comet tails by the chromatographic zones). 

Haar-scharfe,/. extreme precision, -schwefel, m. capillary sulfur, -schweif, m. coma (of a comet), -seite,/. hair side, grain side, -sil-ber, n. capillary silver, -spalte,/. hair crack, fine fissure, -sprung, m. hair crack, -stern, m, comet, -strang, m. Boi. Ac Pharm.) peucedanum hair cord, -vertilgungsmittel. 

Scbwankungsgrenze, /. limit of variation. Scbwanz, m. tail train, -bahn, m. stopcock with an outlet thru the end of the key. -kiigelcben, n. bulb drawn out to a narrow opening, -pfeffer, m. cubebs. -stem, m. comet. 

Book II investigates the dynamical conditions of fluid motion. Book III displays the law of gi avitatioii at work in the solar system. It is demonstrated from the revolutions of the six known planets, including Earth, and their satellites, though Newton could never quite perfect the difficult theory of the Moon s motion. It is also demonstrated from the motions of comets. The gravitational forces of the heavenly bodies are used to calculate their relative masses. The tidal ebb and flow and the precession of the equinoxes is explained m terms of the forces exerted by the Sun and Moon. These demonstrations are carried out with precise calculations. 

Test flight of the De Ha /illand Comet 1 jet transport prototype. (Corbis-Bettmann)... 

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