Guest Author - Lea Terry

Though known since at least 240 BC, Halley’s Comet wasn’t understood until 1705, when Edmund Halley predicted the comet’s movement using Newton’s newly formulated laws of motion. Though this provided some insight, legend still surrounded the comet, which was believed to precede disaster.

Though Halley predicted the comet’s next appearance, he didn’t live to see it, dying in 1742. Others in the scientific community ridiculed Halley’s prediction, but the comet next appeared on Christmas Eve 1758, just as he had said. The comet was eventually named after Halley. By this time, the comet’s legend was already growing. There has long been a myth that the comet was excommunicated by Pope Callistus III in 1456 C.E; this story has been disputed for several years. By 1910, the myths surrounding the comet were fully cemented. During this appearance, the earth passed through the comet’s tail, causing widespread panic. Cyanide gas had been discovered in the comet’s tail, and people all over the world feared death--some clever entrepreneurs even sold “comet pills” that promised protection. Of course, the comet passed by without catastrophe. (Except for the death of Mark Twain, who was born during the comet‘s previous appearance in 1835.)

By the comet’s appearance in 1986, it was famous around the world. In 1984 and 1985, the USSR, Japan, and Europe launched five spacecraft to rendezvous with the comet, and NASA redirected one of its deep space satellites to monitor the solar wind. Halley’s Comet is one of only four comets to be studied by spacecraft.

Halley’s Comet is one of the best known comets, but is probably not typical. Unlike many comets, which are either very faint or appear infrequently, Halley’s Comet is large and passes by Earth regularly. Though large by comet standards, it is still smaller than most celestial objects, with a nucleus measuring just 10 x 5 x 5 miles. Halley’s Comet looks bright in the night sky, but is one of the darkest objects in the solar system, with an albedo of only 0.3. It also has a low density, about 0.1 gm/cm3, and is probably very porous. Halley has a retrograde orbit, which is inclined 18 degrees to the ecliptic. It also has a highly eccentric orbit, like other comets.

The comet’s next trip to the inner solar system will be in the year 2061.