cIn 1577, the Dutch astronomer Tycho Brahe began making the first real observational studies of comets by observing a bright comet that appeared in Earth’s skies. Brahe concluded that the comet had to exist outside of the Earth’s atmosphere due to the fact that the position of the comet in the night sky appeared not to change in relation to an observer’s location on the Earth. This information suggested that the comet had to be very far away.
Sir Isaac Newton became curious about comets and began to apply his laws of gravitational motion comets. His conclusions stated that comets were accelerated by gravity towards the Sun and that they move in orbits about the Sun that are either parabolic or highly elliptical in nature. We now know today that this is relatively accurate; cometary orbits are periodic in nature, with periods as long as millions of years or as short as that of Comet Encke, which has an orbital period of 3.3 years.
Figure Two: Comparison of Planetary and Comet Orbits
Edmund Halley (1656 – 1742) extended Newton’s study of comets. He took interest in a series of very bright comets that appeared in 1531, 1607, and 1682; Halley noted that the orbits of these comets were very similar and that all three could actually be the same comet with an orbital period of 76 years. His prediction proved correct when he accurately predicted the return of the comet in 1758. In his honor, the comet was named Halley’s Comet. The last appearance of Halley’s Comet took place in 1986, and it is not scheduled for a return visit until the year 2062.
The exploration of comets has, in recent decades, taken stellar leaps, with specific missions to visit comets. This has been accomplished through the use of interplanetary probes that are launched from the Earth and rendezvous with a comet. The first such encounter took place in 1985 when the U.S. satellite International Sun-Explorer was moved from its set orbit so that it could perform a flyby of the comet Giacobini-Zinner. The spacecraft, now named International Comet Explorer (ICE) passed about 7800 kilometers behind the comet, traveling through the tail on September 11, 1985.
When Comet Halley passed close to the Earth in 1986, six probes were set to gather information about the comet. Two craft, the U.S. ICE probe and a Japanese craft called Sakagaki observed the comet from a distance of many million kilometers while a second Japanese probe, Suisei, passed to within 1 million kilometers of the comet. The other three spacecraft were targeted to gather data on the nucleus of the comet. Two Soviet probes, VEGA 1 & 2, encountered the cometary nucleus on March 6 and 9, 1986, respectively; they both passed into the comet’s atmosphere and passed to within 8000 kilometers of the nucleus. The final spacecraft, a European probe named Giotto, made an even closer rendezvous with the nucleus on March 14, 1986, passing to within 605 kilometers of the nucleus. At this close proximity, Giotto was actually able to beam back images of Halleys nucleus…