Long-period comets have orbital periods longer than 200 years. In addition, their orbits are often highly inclined to the ecliptic suggesting that they, like the short-period Halley-type comets originate in the spherical shell of icy bodies known as the Oort Cloud.
Long-period comets may be perturbed from their resting place in the Oort cloud by a passing star or giant molecular cloud, or even through tidal forces generated by the bulge and disk of our Galaxy. Such gravitational influences may send these icy bodies on a path towards the centre of the Solar System in highly elliptical orbits. Originating from a spherical distribution, the high inclinations of the orbits arise since they can enter the inner Solar System from any angle. Long-period comets tend to be the most spectacular comets we see in the night sky, with the two most recent ‘great’ comets – comet Hale-Bopp (1997) and comet Hyakutake (1996) – having predicted orbital periods of several thousand years. Their brilliance is due to the fact that they have not made many (if any) passes through the inner Solar System, and so still retain a large percentage of their volatiles. Since it is the sublimation of these volatiles from the nucleus of the comet as it nears the Sun that gives rise to the coma and highly-visible tails, long-period comets have more material with which to put on a show.