Comets Chapter index in this window — — Chapter index in separate window Video lecture for this page This material (including images) is copyrighted!. See my copyright notice for fair use practices. Select the photographs to display the original source in another window. The passage of the comet Hale-Bopp through our part of the solar system created spectacular displays in the spring of 1997. Every newspaper, television and radio station carried some report with photos of the comet. We had a foretaste of the Hale-Bopp show when Comet Hyakutake passed close to the Earth in the spring of 1996. Hale-Bopp was one of the brightest comets to grace our skies this century, coming close to the displays put on by Comet West in 1976 and Halley's Comet in 1910. Many people were justifiably interested in Hale-Bopp---it was a gorgeous site! Many people also tuned into the news and astronomy web sites in the summer of 1994 when the comet Shoemaker-Levy 9 smashed into the planet Jupiter. Predictions of Jupiter's demise were, of course, greatly exaggerated---Jupiter took the hits in stride. One view of comets as destroyers of worlds in 1857. (Original source unknown) King Harold and nation cower in fear at the close passage of Halley's Comet (picture from part of the Bayeux Tapestry; more pictures at The Bayeux Tapestry Gallery). Our favorable view of comets is a big change from the dread and fear people held of comets even less than a century ago. Comets were usually thought to be omens of bad events to occur on the Earth. King Harold of England took the passage of Halley's Comet to be a sign of his defeat in 1066. However, William the Conqueror took it as a good sign and led the Normans to victory over King Harold's army. As recently as 1910 people thought the end of the world was near when it was discovered the Earth would pass through the tail of Halley's Comet. Astronomers had discovered the presence of cyanogen molecules in the tail, so the popular media spread tales of cyanide poisoning of the Earth. Even with great effort the astronomers were not able to convince many people that we faced no danger---a comet's tail is extremely diffuse so the minute amounts making it through the atmosphere and being breathed by helpless human beings was much, much less than the noxious stuff they breathed everyday from industrial pollution. The tragedy of the Heaven's Gate cult shows that despite our current knowledge of comets, there are still those who view comets with great superstition or as something much more than the icy bodies they are from the outer limits of the solar system. Comets are small "potato-shaped" objects a few hundred meters to about 20 kilometers across. They are made of dust grains embedded in frozen volatiles (materials that vaporize at low temperatures) like water, methane, ammonia, and carbon dioxide (they are like "icy dirtballs" ). They are primitive objects which means they are unchanged since they first solidified from the solar nebula about 4.6 billion years ago. Comets are frozen relics of the early solar system holding valuable information about the formation of the planets. When a comet gets close enough to the Sun, it changes into something more spectacular. The picture above shows the parts of a comet that form when the cold "icy dirt ball" is warmed up by the Sun. This picture is courtesy of David Doody at JPL and is part of the Basics of Space Flight manual for all operations personnel. The comets that have exhausted their supply of ices from repeated passages near the Sun or the remaining ices become insulated enough by the dusty or rocky material to no longer vaporize are reclassified as asteroids. Some comet researchers even call the icy bodies in the Kuiper Belt and Oort Cloud (that never get near the Sun) asteroids because the ices do not vaporize. Therefore, the line between what is called a "comet" and what is an "asteroid" is a bit blurry. Nucleus All the material comes from the nucleus. This is the "icy dirtball" . Comet nuclei are 0.5 to 20 kilometers in size and are potato-shaped conglomerate of dust (silicates and carbonaceous) embedded in ice (frozen water, carbon dioxide, carbon monoxide, methane, and ammonia). They have a mass of only 1014 to 1015 kilograms (the Earth has a mass of almost 6 × 1024 kilograms---tens of billions to hundreds of billions of times larger than a comet). It is less than the size of a period on the scale of the comet drawing above. Because of their small size, comet nuclei have too little gravity to crush the material into a sphere. When a comet nears the Sun around the Jupiter-Saturn distance, it warms up. The ices sublime---they change from solid to gas without going through a liquid phase (like the white mist coming from a block of frozen carbon dioxide, "dry ice"). Jets of material will shoot out from the nucleus. These jets can alter the comet's orbit (remember Newton's third law of motion?) Since the orbit of Halley's Comet is known so well, spacecr...