ࡱ> 02/U@ "bjbj ",2222222 $DDDDDDDDTVVVVVV$ R z2DDDDDz22DDD2D2DTDTD,22,D8 pr,H 0,  ,FT62222 2,DDDDDDDzz The views expressed then have changed in details. in the paper we see the first study of the cometary mantle.. comparison between todays view of the nucleus and Whipples The origins connection was to bring a vast investment in cometary missions and an explosion of cometary research in the 1980s Commentary on A Comet Model. I. The Acceleration of Comet Encke by Fred L. Whipple (Astrophys. J., 111, 375-394, 1950) Michael J.S. Belton National Optical Astronomy Observatories, Tucson, AZ 85719. The publication, in 1950, of Whipples paper A Comet Model. I. The acceleration of Comet Encke is universally recognized as a fundamental turning point in the study of the physical nature of comets. By demonstrating that evaporation from the surface of a small monolithic nucleus made up of a conglomerate mixture of water and other ices in a weak matrix of particulate meteoric material might explain the known physical and chemical attributes of comets, including mysterious changes observed in some of their orbital motions, Whipple created a good part of the foundation of our current appreciation of the significance of cometary nuclei. The paper was the first of a sequence of three that he published as sole author in the Astrophysical Journal between 1950 and 1955 and in which he laid out a theory of the icy conglomerate cometary nucleus. These papers provided a comprehensive account of not only the comets themselves, but made direct physical connections to meteoric phenomena, the zodiacal light, and indicated how their properties might lead to constraints on the origin of the solar system. The early 1950s were momentous years for cometary studies. In addition to Whipples work, Oort (1950) demonstrated that an immense cloud of some 1011 comets must presently exist, weakly bound to the Sun, to supply the comets that are observed in the inner solar system; meanwhile Kuiper (1951) published his ideas on the origin of the solar system in which he identified the snowballs that condensed and accumulated at low temperature in his model solar nebula outside the distance of Neptune as the probable source population for the comets in Oorts cloud or comet trap. Most importantly Kuiper also noted that remnants of this circular comet ring, now known as the Kuiper belt, probably are still present . These works were not conceived in a vacuum of ideas. There were important precursors: For example, Opik (1932) had shown that the orbits of near parabolic comets and meteors out to aphelion distances of 106 au were sufficiently stable to perturbations of passing stars to remain part of the suns family over the age of the solar system, and van Woerkom (1949) had shown that the long period comets appeared to be a viable source of the short period comets via capture by Jupiter. The idea of ices in comets had already been suggested by Vsekhsviatsky (1948), and the concept of a monolithic cometary nucleus was moving towards ascendancy as eloquently outlined in the Astrophysical Journal by Vorontsov-Velyaminov (1949). In a recent conversation with Whipple (he continues to work in his office engaged in research making much of his some ninety-two years of experience) about the origins of his ideas about his 1950 paper, I learned of the strong influence of the work of Opik, then at Harvard, and of Whipples early conviction that the then popular gravel-bank concept of comets was impossible. Whipple was working on meteor theory when on a big day in my life he realized that aspects of meteor physics could be applied to comets. As a result of both observational and theoretical advances much has changed in our understanding of comets in the fifty years since Whipples paper appeared. The first discovery of an object (other than Pluto) beyond Neptune was achieved by Jewitt and Luu in 19xx. Today, with xx such trans-Neptunian objects known, the existence of the Kuiper belt cannot be doubted and this region of the solar system is the focus of intense observational exploration (xxxx) and theoretical experimentation (Moribidelli, 1998) to understand its physical structure and dynamical properties. While the relatively large bodies that have been detected so far may not qualify as typical cometary nuclei, a thought that needs testing, they must surely co-exist with a large number of Kuiperian snowballs that do. Modern computing power helped Quin and Tremains (19xx) demonstrate concept of an inward dynamical diffusion of such objects from the belt through the realm of the giant planets to provide an ample source of short period comets in the inner solar system. Oorts ideas have similarly fared well. The Oort cloud is firmly established as the primary source region for new and long period comets and some of the short period comets, although, as xxx has shown, the effects of galactic tides seem to dominate the stability of the cloud rather than the, still important, perturbations due to passing stars. Modern computing power in the hands of xxx (xxx) and others has shown how the cloud can be populated and that the location of the primary source region is in the Uranus-Neptune region of the primitive nebula, i.e., interior to the Kuiper belt. Even the likely internal structure of the cloud is beginning to emerge from these calculations. Most significantly for Whipples ideas are the observational advances. An epic, 17-year, observational project led by AHearn (1995) has revealed that while the compositions of comets are largely similar, they do show chemical differences depending on their likely place of origin. Comets dynamically related to the Kuiper belt tend to be depleted in carbon-chain molecules relative those that are dynamical related to a place of origin through the Oort cloud interior to the Kuiper belt. The discovery of objects (Centaurs) in transitional orbits like Chiron (Kowal 19xx) that was later observed by Meech and Belton (19xx) to acquire cometary properties on approach to the sun provides hard facts bolsters the concept of inward dynamical diffusion from the Kuiper belt. Observations of water molecules with extremely high spectral resolution in Comets Halley and Wilson from the Kuiper Airborne Observatory by Mumma and colleagues (1988) have yielded ortho-to-para ratios that potentially relate to the temperature of formation of the water molecules. These, as well as other observations to many to mention, provide firm support for Whipples original suggestion that study of the properties of cometary nuclei might place limits on primitive conditions in the solar nebula at the time when the planets were formed. AHearn, M. F. et al., (1995). "The ensemble properties of comets: Results from narrowband photometry of 85 comets, 1976-1992," Icarus 118, 223-270.  !" 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