Can a comet cease to exist?
When a comet nears the Sun, a part of its material gets vaporized. After many orbits a comet may lose its mass completely and cease to exist.
Can a comet fail to return?
Sometimes, a comet may be deflected by the gravity of Jupiter so that the shape of the comet’s orbit is changed from an elliptical trajectory to a hyperbolic one resulting in the comet being ejected from the Solar System into deep space never to return.
What is unusual about the shape of the orbit of the comets as compared to those of planets?
Comets move in elliptical orbits as in the case of planets. But while orbits of planets are nearly circular, in the case of comets the ellipses are highly elongated. In some extreme cases, a comet’s orbit may be very nearly parabolic. It is because of this reason that at aphelion a comet may be extremely far away from the Sun, often about 100,000 times farther away from the Sun as the Earth is again, for the same reason, the period of same reason, the period of some comets is very long, sometimes thousands or even millions of Earth years. Comet Kohoutec which appeared in 1974 will repair after ten million years!
What is a comet family?
A group of comets, with each member having a similar aphelion distance, all under the gravitational influence of a large planet, is called a comet family. An example is the Jupiter family of comets. Saturn, Uranus and Neptune also have smaller comet families.