If comets are boiling away, why are there any left?

How do comets survive, when so much of thier mass is outgassed when they come close to the sun?

They don't. Comets which orbit the inner solar system are very short-lived, lasting no more than a few tens of thousands of years on average. We still see comets this late in the solar system's lifetime because new comets occasionally fall in toward the sun from the Oort cloud, a spherical cloud of comets about 50,000 times as far from the sun as the earth (about 2000 times as far away as Pluto).

March 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer), Ryan Anderson (more by Ryan Anderson) (Like this Answer)

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