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Faulkner observes that [[Comet Halley]], a short-period comet, is highly inclined and in a retrograde orbit. Comet Halley is, therefore, an important exception to the distinction. Faulkner ''also'' observes that short-period comets ''do not'' differ from long-period comets in composition.<ref name=Faulkner1/>

Faulkner observes that [[Comet Halley]], a short-period comet, is highly inclined and in a retrograde orbit. Comet Halley is, therefore, an important exception to the distinction. Faulkner ''also'' observes that short-period comets ''do not'' differ from long-period comets in composition.<ref name=Faulkner1/>

Cometary orbits are highly elliptical<ref name=Faulkner1/>, and some of these orbits are almost parabolic.<ref name=Weisstein/> ''Hyperbolic'' cometary orbits have never been observed.<ref name=Faulkner1/> This last observation is credited to Jan Oort himself.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/kboc.html The Kuiper Belt and the Oort Cloud]." ''The <s>Nine</s> 8 Planets'', March 18, 2007. Accessed June 2, 2008.</ref>

Cometary orbits are highly elliptical,<ref name=Faulkner1/> and some of these orbits are almost parabolic.<ref name=Weisstein/> ''Hyperbolic'' cometary orbits have never been observed.<ref name=Faulkner1/> This last observation is credited to Jan Oort himself.<ref name=Arnett>Arnett, Bill. "[http://www.nineplanets.org/kboc.html The Kuiper Belt and the Oort Cloud]." ''The <s>Nine</s> 8 Planets'', March 18, 2007. Accessed June 2, 2008.</ref>

The more important point is that comets have a limited life span. After a few hundred perihelion passes at the Sun, a comet loses all of its tail-forming substance.<ref name=Arnett2>Arnett, Bill. "[http://www.nineplanets.org/comets.html Comets]." ''The <s>Nine</s> 8 Planets'', May 1, 2003. Accessed June 2, 2008.</ref> In addition, comets often destroy themselves in collision with planets and other bodies, or are ejected from the solar system entirely after making close flybys of [[Jupiter]] and other [[gas giant]]s.<ref name=Arnett2/><ref name=Faulkner1/><ref name=Faulkner2>[[Danny Faulkner|Faulkner, Danny]]. "[http://www.creationontheweb.com/content/view/1830 More problems for the 'Oort comet cloud']." ''Journal of Creation'' 15(2):11, August 2001. Accessed June 2, 2008.</ref>

The more important point is that comets have a limited life span. After a few hundred perihelion passes at the Sun, a comet loses all of its tail-forming substance.<ref name=Arnett2>Arnett, Bill. "[http://www.nineplanets.org/comets.html Comets]." ''The <s>Nine</s> 8 Planets'', May 1, 2003. Accessed June 2, 2008.</ref> In addition, comets often destroy themselves in collision with planets and other bodies, or are ejected from the solar system entirely after making close flybys of [[Jupiter]] and other [[gas giant]]s.<ref name=Faulkner1/><ref name=Arnett2/><ref name=Faulkner2>[[Danny Faulkner|Faulkner, Danny]]. "[http://www.creationontheweb.com/content/view/1830 More problems for the 'Oort comet cloud']." ''Journal of Creation'' 15(2):11, August 2001. Accessed June 2, 2008.</ref>

For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovulcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.<ref name=Faulkner1/>

For these reasons, astronomers realized that the supply of comets would require replenishment over the supposed great age of the solar system. Comets cannot be interstellar, else they would have hyperbolic orbits. Some astronomers have proposed cryovulcanism as the source of comets, but comets are not sufficiently dissimilar in composition to support this.<ref name=Faulkner1/>