According to Wikipedia:
“The Oort cloud may have billions of objects with absolute magnitude
brighter than 11 (corresponding to approximately 20-kilometre diameter)”
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$\begingroup$ What do you mean by "why do we have so few comets?" $\endgroup$ – zephyr Jun 26 '17 at 14:20
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$\begingroup$ What I mean is that why do we observe so few comets? I mean, a comet with a 20 kilometer diameter, will be visible from the earth, I suppose. $\endgroup$ – amsquareb Jun 27 '17 at 8:31
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3$\begingroup$ The Oort cloud is very very far away. You may think it's a long walk to the chemist, but that's peanuts by comparison. $\endgroup$ – Carl Witthoft Jun 27 '17 at 12:22
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Most Oort cloud objects stay in the Oort cloud, and we never see them.
Comets don't last long. A comet is made of a mixture of ices and dust. A comet that enters the inner solar system has one of a few outcomes:
- it could pass out of the solar system, and either never be seen again, or enter such a long orbit that it isn't seen again for an extremely long time (like Hale Bopp)
- it could fall into the sun (or travel so close that it is completely broken up) (like comet ISON)
- it could get caught into a short period and over time have all its ices boiled away by the heat of the sun. (asteroid 2015 TB145. Halley's comet is in the process of losing its ice with each orbit it grows smaller)
- it could hit a planet. (Shoemaker-Levy 9)
In all cases, the comet is lost, destroyed or deactivated.
As comets have a short lifespan, they are rare.
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$\begingroup$ You mean like Halley's Comet? Not sure I'd call its orbital period "extremely long time." $\endgroup$ – Carl Witthoft Jun 27 '17 at 12:23
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2$\begingroup$ No, Halley's comet is one which has been caught in a short period and is in the process of having its ices boiled away. By long period I mean like Hale Bopp: thousands of years. $\endgroup$ – James K Jun 27 '17 at 15:44
