Why does a comet spend little time in the inner solar system?

Answer 1

A comet's orbit is very elliptical -- it is a very elongated oval. The sun is at one focus of the ellipse, and typically the other focus is somewhere beyond the orbit of Pluto, in the Oort Cloud. Kepler's Laws tell us that a body orbiting the sun will move more quickly when it is closest to the sun, in this case when it is in the inner system. So a comet will move very slowly at the outer end of its orbit, speeding up as it falls towards the Sun, whip through the inner system at a great speed as it rounds the sun, and then slow down as it heads back out to the cold. Because the orbit is so very eccentric, it takes far longer to travel through the far end of it than through the inner system; a typical case would be a comet with a 70-year period that spends only a week in the inner system each orbit.

Answer 2

Comets spend little time in the inner solar system because their highly elliptical orbits take them far away from the sun before swinging back in close. When comets do come close to the sun, the increase in solar radiation causes the volatile materials in the comet to vaporize and form a glowing coma and tail.