Nucleus, of comet

Grigg-Skjellerup was visited by the Giotto spacecraft in 1989. NASA s demonstration spacecraft DS1 imaged the nucleus of Comet Borrelly in 2001. 

In 2004, the NASA Stardust spacecraft passed through the dust cloud surrounding the nucleus of comet Wild2 and captured more than 10 000 particles ranging from 1 to 300 pm in size (Brownlee et al., 2006). These particles were returned to Earth for study in terrestrial laboratories in 2006. Initial results are described in the December 15, 2006, issue of Science. 

Data from the laser-induced fluorescence of C2 radicals obtained in the laboratory during 193 nm photolysis of C2H2 have been used to explain band profiles of C2 in the nucleus of comet Hyakutake, observed by the Hubble Space Telescope. In conjunction with ab initio computations, the data have led to the proposal that photolysis of C2H2, in the laboratory and in comets, proceeds by a sequential mechanism, first producing C2H and then C2. Two excited electronic states of C2H have been identified and 2 11) through which photodissociation in the second step occurs. Measurements of the kinetics and translational energy release in the near-UV photodissociation of the allyl radical have indicated that allene formation is the dominant H-loss reaction channel. ... 

Fig. S3 Nucleus of comet HaUey, observed from the GIOTTO mission in 1986. Courtesy ESA, MPAe, Lindau...

Heat is transported through the layers of the ice into the nucleus of the comet. The temperature and rates of heat conduction are controlled by the coefficients of thermal conductivity. 

Image of comet Halley s nucleus, showing jets of dust and volatilized ices. This picture was taken by the Giotto spacecraft as it raced by in 1986. Image courtesy of ESA. 

Zolensky, M. E. plus 74 coauthors (2006) Mineralogy and petrology of comet 81 P/Wild2 nucleus samples. Science, 314, 1735-1739. 

In the 1950 s, three major concepts have brought d isive advances to the physics of comets Oort s model of a distant cometary cloud, Biermann s solar wind theory of tail formation and dynamics, and Whipple s icy conglomerate model of the cometary nucleus. The main ideas of these concepts shall be outlined briefly. 

The size of a cometary nucleus cannot be measured directly, since even in the largest telescopes it remains an unresolved point of light. Photometric brightness measurements of comets still far away from the Sun before a radiating halo has formed, together with a phase law and a plausible value for the albedo, yield diameters of the order of 1-20 km (Roemer ). Periodic comets are, on the average, smaller than new ones, since they lose about 0.1 % of their masses per revolution. 

Two stable molecules which can be regarded as primary constituents of the nucleus were identified in the microwave spectrum of Comet Kohoutek, namely HCN at 3.4 mm (Huebner et al. and CHjCN at 2.7 mm (Ulich and Conclin Up to now these identifications could not be repeated in other comets. Production rates are estimated to be some 10 mol/s at 1 a. u., in the range of the visual radicals. The search for these molecules and also for CO which had been detected in the UV spectrum of Comet West was unsuccessful in Comet Bradfield 1978 VII, probably because the production rates of this comet were lower (F. P. Schioerb et al. Upper limits for the column density of HCN and CH3CN were less than those derived for Kohoutek, while the upper limit for the CO production was comparable to that inferred from Comet West. Also the very important detection of the 1.35 cm line of HjO in Comet Bradfield 1974 III by Jackson et al. has not yet been confirmed in other comets. 

The nucleus of a comet is a conglomerate of ices and dust particles of low density ( lg/cm ). Dimensions of nuclei range roughly from 1 to 20 km. Periodic comets are generally smaller than new ones. 

HannerM. S., Aitken D. K., Knacke R. F., McCorkle S., Roche P. F., andTokunaga A. T. (1985) Infi-ared spectrophotometry of comet IRAS-Araki-Alcock (1983d) a bare nucleus revealed Icarus 62, 97—109. 

The second of the two tails, the dust tail, consists of tiny particles of matter expelled from the coma and nucleus by the Sun s radiative pressure. This tail is visible only because the particles of which it is composed reflect sunlight. The composition of cometary dust is thought to be similar to that of interstellar dust, but much remains to be learned about this portion of the comet s structure. In April 2003, NASA launched an experiment to obtain better data about the composition and structure of cometary dust. High-altitude research airplanes were flown through the upper atmosphere toward the tail of Comet Grigg-Skjellerup. Special collectors in the planes scooped up samples of the cometary dust. Analysis may take years. 

Based on information from these sources, scientists estimate that the nucleus of a comet consists of about 42 percent volatile compounds (about 80 percent of which is water) and 58 percent solid particles, generally described as "dust." About 45 percent of this nuclear dust is similar in chemical composition to the carbonaceous chondrites found in meteorites that is, they are primarily silicate in composition. (Carbonaceous chondrites are discussed at greater length in the next section.) Another 40 percent of the nuclear dust is organic in nature, that is, composed of carbon compounds of varying degrees of complexity. The final 15 percent of nuclear dust is composed of very small particles with masses of only a few atto-grams (10 l8g). 

CNO cycle A series of nuclear reactions with carbon as a catalyst by which hydrogen is converted to helium, coma A mass of glowing gas and dust that surrounds the nucleus of a comet, produced by the vaporization and sublimation of matter from within the nucleus. 

The first in situ polarimetrie observations of comets P/Halley and P/Grigg-Skjellerup by Levasseur-Regourd et al. [23,24]. Evidence of dust particles of different physical nature near the nucleus. 

A polarization decrease was detected in the innermost coma in comets Halley [6,67] and Hale-Bopp [28,29] at distances < 1000 km from the nucleus, which led to the hypothesis that this is due to fresh dust recently ejected from nucleus [6], However, we believe that a decrease of the polarization in the vicinity of the nucleus also may be due to multiple scattering. According to Dollfus and Suchail [6] the optical thickness of comet Halley s atmosphere was about 0.9 at the cometocentric distance 500 km. Ferndndez [68] also indicated a significant optical thickness >1 within roughly 100 km of the nucleus s center of comet Hale-Bopp. 

The remaining solar system members include comets and centaurs. Comets consist of a nucleus of dust and ice a kilometre or two across surrounded by a gaseous coma and with a long tail that appears when the comet nears the sun. Centaurs are more like asteroids in size, but some of them develop comet-like comas. Their orbits are unstable because of the gravitational influence of the giant outer planets. 

The international Rosetta comet rendezvous mission is designed to perform a detailed investigation of a comet in our solar system. As part of the core payload for this mission, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will determine the elemental, isotopic, and molecular composition of the atmospheres and ionospheres of comets as well as the temperature and bulk velocity of the gas and ions and the homogenous and inhomogenous reactions of gas and ions in the dusty cometary atmosphere and ionosphere [78]. More specifically, the global molecular, elemental, and isotopic composition and the physical, chemical and morphological character of the cometary nucleus will be determined. In addition, Rosetta will elucidate the processes by which the dusty cometary atmosphere and ionosphere are formed and characterize their dynamics as a function of time, heliocentric, and cometocentric positions. 

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