Before the days of the telescope about ten comets were recorded per century. They were regarded with great superstition. No one knew where they came from or where they went or what they were. They appeared for a month or so moving slowly across the sky. Their tails looked threatening and they were associated with doom, disease, death and destruction. In 1687 Isaac Newton applied his theory of gravitation to the great comet of 1680 and calculated five of its six orbital parameters (he assumed the orbit to be parabolic). In 1705, Newton’s friend Edmond Halley published his Synopsis of the Astronomy of Comets. The heart of this work was his table of twenty four calculated cometary orbits, these dating from 1337 to 1698. To Halley’s surprise only twenty two of these were different. The comet of 1531, 1607 and 1682 had the same orbit. Halley predicted that it was periodic, a permanent member of the solar system and would come back again in 1759. When it did it was named after him. Today we have discovered about 160 short-period comets and about 900 long-period ones.
In the 1860s Sir William Huggins, the famous London amateur astronomer, was the first to look at comets through a spectroscope, a device which splits light into different wavelengths. Huggins started cometary chemistry. The same decade also saw the discovery of the relationship between comets and meteoroid streams. Comets such as 55P/Tempel-Tuttle and 109P/Swift-Tuttle have the same orbits as the famous annual meteor showers, the November Leonids and the August Perseids. Large dusty particles that break away from the nucleus slowly gain on, or fall behind, the comet, eventually forming a complete ring of meteoroid dust around the entire path of the orbit. When the Earth passes through this dust, a meteor shower occurs.
In 1950-1 the USA Harvard astronomer Fred L. Whipple introduced the dirty snowball nucleus model. This solved many problems not least of which was the source of all the gas and dust associated with bright periodic comets.
Whipple’s nucleus was rotating every few hours, so the side heated by the Sun moved round before the gas came out. This produced a rocket effect which accelerated or decelerated the comet depending on the direction of spin.
Recently we have entered the space-age of cometary history. Instead of meekly standing on Earth and waiting for comets to come to us we have rocketed off to visit them. NASA’s Stardust mission is returning to Earth (scheduled for 13 January 2006) with dust that it has captured from the vicinity of the nucleus. The Deep Impact mission in July 2005 will not just fly by a comet but will fire a projectile towards the surface. The crater that it produces should tell us much about the physical parameters of the nucleus. ESA’s Rosetta mission is on its way to comet 67P/Churyumov-Gerasimenko. In 2014 it will go into orbit around that comet when it is about 4 au from the Sun, and a small probe will land on the surface. Rosetta will stay with the comet as it travels towards the Sun monitoring the variability in activity.