Comets are true long distance nomads. They come from the outback of the Solar System, from distances so immense it spans the empty space between our Sun, and the closest star, Proxima Centauri. Comets are chunks of dirty snow and rock that, under the right circumstances, can put on a show of astronomical proportions. This month we are treated to Comet Hyakutake which will be visible all night the week of March 25th.
These ghostlike icy wanderers are all bound to the solar system by the gravitational attraction of the Sun. Most orbit the solar system beyond the orbit of Pluto in what is known as the Oort cloud. Gravitational disturbances from passing stars, or collisions among comets may alter the orbit enough to send the comet in towards the Sun. More than a dozen new comets are discovered each year, but seldom do they become bright enough to be seen by the unaided eye.
Like all unexpected intruders into the orderly and predictable heavens, comets have been viewed throughout history with suspicion and alarm. They have been represented as swords and daggers and even decapitated heads. They have been blamed for battles lost, the assassination of Julius Caesar, and the eruption of Mount Vesuvius. Even as recent as the 1910 return of Comet Halley brought alarm. One enterprising physician even prescribed "comet pills." They sold as fast as they arrived at the pharmacy, especially after it was announced that the Earth would pass through the tail of the comet. They were to be taken every hour until the comet was gone. Fortunately we now know far more about comets and can appreciate them as a rare and beautiful celestial treat.
Like asteroids, comets seem to be the leftover debris from the formation of the solar system. Analysis of the composition of comets has found frozen water, ammonia, methane, carbon dioxide, hydrogen cyanide and other compounds embedded in and around a rocky core. The core, or nucleus of the comet is typically a few miles across.
Far from the Sun the comet does not have a characteristic whispy tail. It remains a solid chunk of rock and ice, quite dark and invisible to the Earth. When the comet gets closer to the Sun and begins to heat up, things begin to change. As the comet is heated, gasses expand and ices begin to evaporate. The gas and dust that surround the comet’s nucleus forms the coma which can be thousands of miles in diameter. As the comet moves closer still, the gas and dust form the tail that can stretch outwards for millions of miles.
The comet swings around the Sun and begins the long journey back to it’s cold and dark outlands. Comets are classified as either long or short period comets. A short period comet (such as Halley) has an orbit of less than 200 years. A long period comet is greater than 200 years. Some comets have such large orbits that it is impossible to predict when they will return.
Comet Hyakutake was discovered in January 1996 by the Japanese comet hunter Yuji Hyakutake. The comet will pass within 0.10 astronomical units of Earth. It is estimated that the comet may reach a magnitude of 1, however this will be spread out over an area about twice the diameter of the full moon, and therefore will not appear as brightas a star of that magnitude. Still, Hyakutake should be easily visible to the unaided eye the last week of March. In April the comet is receding from Earth as it nears the Sun, and should dim a little, then become lost in the glare of the Sun.
Use the star map to track Hyakutake. The best places to view the comet is away from city lights, the darker the skies, the better the view,and the more detail you will see. Binoculars or a small telescope will reveal more detail. Hyakutake moves rapidly across the sky from night to night, it’s position shifts nearly 18 degrees each night. Once it reaches the north sky it is visible all night.
If you do want to see comet Hyakutake, you will have to catch it as it passes Earth. Once it swings around the Sun, it will only be visible in the predawn sky to observers in the southern hemisphere.
Links to more Comet Hyakutake information
If you have comments, email me at kmiles@ccil.org
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