Astronomers first observed Halley’s Comet as far back as 200 BCE. The comet's periodicity was first determined in 1705 by English astronomer Edmond Halley, who it was eventually named after. Halley's Comet last appeared in the inner Solar System in 1986 and won’t appear again until mid-2061. Although the nucleus of the comet itself is not that large, the coma can extend to a very large size.
As the gas molecules in the coma are ionized by the solar ultraviolet radiation pressure from the solar wind, a stream of particles emitted by the Sun, pulls the coma's ions out into a long tail, which may extend more than 100 million kilometers into space. The nucleus itself is only 15 kilometers long, 8 kilometers wide and 8 kilometers thick. In comparison, the earth has a diameter of 12,756 km and the sun has a diameter of 1,392,000 km. This means that Halley’s Comet is only about 4% the size of earth but because of it’s coma, it appears much larger to the naked eye. While the planets orbit around the sun, Halley’s Comet orbits towards and away from the sun, all the way out to Jupiter, perpendicular to the planet’s orbits.
Halley is classified as a periodic or short-period comet, one with an orbit lasting 200 years or less. This contrasts it with long-period comets, whose orbits last for thousands of years. Most short-period comets, those with orbital periods shorter than 20 years and inclinations of 20–30 degrees or less, are called Jupiter family comets. Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley type comets.
Only 54 Halley-type comets have been observed, compared with almost 400 identified Jupiter family comets. The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were altered by the gravity of the giant planets and directed into the...