10.7 Comets
Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (or atmosphere) and/or a tail. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across. Comets have a variety of different orbital periods, ranging from a few years, to 50 or 100 years, to hundreds of thousands of years, while some are believed to pass through the inner Solar System only once before being thrown out into interstellar space.

- Hale-Bopp (C/1995 O1): It was probably the most widely observed comet of the twentieth century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months. Hale-Bopp was discovered on 23 July 1995 at a very large distance from the Sun, raising expectations that the comet could become very bright when it passed close to the Sun. Hale-Bopp met or exceeded most predictions for its brightness when it passed perihelion on April 1 1997. The passage of Hale-Bopp was notable also for inciting a degree of panic.

- Halley's Comet: It can be seen every 75-76 years. It is the most famous of all periodic comets. Although in every century many long-period comets appear brighter and more spectacular, Halley is the only short-period comet that is clearly visible to the naked eye, and thus, the only naked-eye comet certain to return within a human lifetime. Halley's Comet last appeared in the inner Solar System in 1986, and will next appear in mid 2061.

- Long-period comets: They have highly eccentric (elongated) orbits and periods ranging from 200 years to thousands or even millions of years. However, by definition they remain gravitationally bound to the Sun; those comets that are ejected from the solar system due to close passes by major planets are no longer properly considered as having "periods". Their orbits take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic.

- Oort cloud: It is a postulated spherical cloud of comets situated about 50,000 AU from the Sun. This is approximately 1000 times the distance from the Sun to Pluto or nearly a light year. The outer extent of the Oort cloud places the boundary of our Solar System at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun.

- Periodic comets: They are comets having orbital periods of less than 200 years (also known as "short-period comets") or which have been observed during more than a single perihelion passage.

- Short-period comets: They are generally defined as having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets at aphelion.

- Single-apparition comets: They are similar to long-period comets, but have parabolic or hyperbolic trajectories which will cause them to permanently exit the solar system after passing the Sun once.