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What is the biggest known comet?

It all depends on what you mean by "biggest" -- brightest, largest, or most massive...

Brightest: the apparent brightness of a comet is a function of its orbit: how close to the Sun does it get and how close to the Earth. Even when a comet's orbital elements are well-known, you cannot reliably predict how bright it will be as seen from the Earth: some of us may remember the ballyhoo about Comet Kohoutek back in 1978(?), and Halley's 1986 apparition was much dimmer than expected (I remember having trouble even finding it with binoculars).

Largest: not much is known about the actual size of comets because all we can see is the light emitted and reflected from the comet's coma and tail except when comets pass across the face of the Sun: then, we never see a silhouette, which means that comets must have radii less 50 km. Heaviest: again not much is known except an upper limit based on the fact that comets do not measurably affect the orbits of planets they closely approach. This upper limit is 1/1,000,000 times the mass of the Earth. Ask me more! I know a bit about comets having been an official observer at the Joint Observatory for Cometary Research run by NASA and N. M. Tech for several years.


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