Brightness, comet

Whipple Mass losses of several up to 100 tons per second, depending on the comet s size and distance from the Sun, are necessary to explain the observed changes of the periods, and these numbers agree with the gas and dust production rates derived from observations of the spectra and of the brightness distribution in the comas and tails of some bright comets. 

Though much effort has been made during the last 10 years to detect primary molecules in comets by radio techniques only a few positive results could be achieved (for a review of this very complex subject and for a table of negative results see Snyder It should be emphasized that these observations are very difficult and that the signals are only marginal in most cases. Furthermore, it is not always easy to coordinate the cooperation with radio astronomers in due time when a bright comet appears. With respect to the importance of these observations, all efforts should however be made to continue the search for molecules in the microwave and radio spectra of future comets. 

Water ice seems to be the major constituent. Carbon-containing molecules (COj, CO) are of comparable, though lower abundances (up to 30%), It is not yet clear whether CO or CO or both are parent molecules of the nucleus. Average production rates for water in bright comets are 1(P -10 mol/s in 1 a. u. solar distance. 

Halley s comet A bright comet with a period of 75-76 years, its last visit to the inner solar system was in i986. This comet was the first short-period comet to be recognized, its orbit was first calculated in i705 by Edmund Haliey, after whom it is named. The comet moves around the sun in the opposite direction to the planets and is associated with two meteor showers, the EtaAquarids (May) and the Orionids (October). 

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. 

It must be kept in mind that there are certainly differences in the chemical composition and in the sizes of the comets which add to the picture. For instance, the two comets, Morehouse 1908 III and Humason 1962 VIII, had exceptionally bright CO -tails which point to a large content of CO or its jarent mol ules in the nuclei. Comet Humason was exceptional also because its perihelion distance was as large as... 

Qualitative abundance ratios of HjO /CO have been derived from surface brightnesses of ionized gas tails by a comparison of red sensitive to blue sensitive plates for 13 comets by Miller . The comets could be grouped into three classes, one containing 5 comets with relatively high HjO brightness, one containing 4 comets with moderate H O brightness and a third class with 4 comets which had a very weak red tail. However, there was no noticeable dependence on the comets orbital parameters or heliocentric distances. 

There is additional astronomical information on cometary silicates that provides far more information than simply the presence of olivine. High-resolution and good signal-to-noise ratio IR spectra show additional fine strucffire on the 10 p,m silicate feature of bright LP comets. A small feature at 11.9 p.m is also due to olivine and a slope change at 9.2 pm and 9.3 pm is attributed to pyroxene and amorphous silicate with pyroxene composition (Manner and Bradley, 2003) (Figure 11). 

Figure 1.25 Comet tailing generated by polishing on specimen surface (a) bright-held image and (b) Nomarski contrast image. (Reproduced with permission of Struers A/S.)...

The "bright new star" was actually a comet, known today as Halley s comet. Halley s comet reappears on a regular basis every 76 years. Each of its appearances has caused fear and amazement among people around the world. Indeed, comets are among the most dramatic of all astronomical events. 

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