Comet Halley (Halley's Comet)
What is a Comet?
Comets are leftovers from the formation of the solar system, about 5000 million years ago. Near to the sun, small rocky planets such as Earth formed. Further out, large gas planets like Jupiter formed. Still further out, the masses of dust and ice didn't form planets. They stayed separate and formed a cloud of comets known as Oort's cloud.
Most comets are undetectable from Earth. Occasionally two comets may collide, and an icy piece of one of them falls toward the sun. The journey toward the sun takes several hundreds of thousand of years. Once the comet reaches the sun, it swings past it and returns to Oort's cloud, in as much as a million years since it left.
Some comets orbit the sun much more frequently than what I've just described. These short-period comets have been captured by the solar system.
When a comet travels toward the sun, it passes all the planets in the solar system. Some of the smaller planets have no effect on the comet. If the comet passes too close to a larger planet—Jupiter or Saturn—it may become trapped. The gravitational force of a planet such as Jupiter or Saturn will slow the comet, leaving it without enough energy to return to Oort's cloud.
Halley's Comet is a Short-Period Comet
A comet which has been caught in the gravitational force of one of the larger planets is doomed to spend the rest of its life orbiting the sun in much less time than the one million year round-trip between Oort's cloud and the sun. Comet Halley is a short-period comet. It orbits the sun every 76 years.
The 1986 Apparition of Comet Halley
Comet Halley's most recent apparition was in 1986. Within a three-week period in March 1986, a group of six spacecraft flew past the nucleus of Comet Halley, at distances ranging from 17.4 miles to 372.8 miles. Professional and amateur astronomers, organized into nine disciplines of the International Halley Watch, monitored Halley's Comet using ground-based instrumentation. The scope and size of the international scientific community organized to study Comet Halley was unprecedented in the history of science.
International Halley Watch (IHW)
The United States Congress appropriated funding for the formation of the International Halley Watch (IHW). In 1980, a National Aeronautics and Space Administration (NASA) science working group was formed to establish the objectives of the organization. NASA Headquarters established two lead centers for the IHW, one in Germany, and the other at the Jet Propulsion Laboratory (JPL) in Pasadena, California.
Nine observing disciplines (networks) were formed to encourage, coordinate, and archive the scientific data resulting from the observations of Comet Halley during its apparition from 1982 to 1989.
IHW Small Island Network
The International Halley Watch's Large-Scale Phenomena Team was responsible for establishing a worldwide network to observe and photograph Comet Halley. More than 90 observatories in the Northern Hemisphere had the comet in view during every hour in each 24-hour period.
The situation changed in February 1986, when Halley's Comet rounded the sun. The comet was far south of the celestial equator and either difficult or impossible to see from many northern observatories. The Southern Hemisphere is dominated by oceans. The observatories there were too far apart to continuously monitor the comet. One of the nine networks formed by the IHW's Large-Scale Phenomena Team was the Small Island Network.
The idea of the Small Island Network was to place portable Schmidt cameras on remote islands to monitor the comet. The IHW's Large-Scale Phenomena Team only had funds to purchase the equipment. One of the most difficult aspects of the program was finding volunteers to operate the cameras. The six locations chosen to be in the Small Island Network were Tahiti, Easter Island, the British Antarctic Survey's Faraday Station on the Antarctic Peninsula, the South African Astronomical and Cedarberg observatories (both on the mainland), and Reunion Island.
My observing partner and I spent a month in Tahiti observing and photographing Comet Halley for the International Halley Watch's Small Island Network!
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