Eyes Up! It's Comet Time

Eyes Up! It's Comet Time

by Adam Frank for McGraw-Hill

It has spent a long lonely time in the black coldness of space. It has wandered for 300 years out at the edge of interplanetary space. Almost lost and all but utterly alone it has lived in a place where the Sun seems barely brighter than the other pinprick stars. Now it has returned. Now it is back in all its cosmic glory, passing close enough to Earth to put on a celestial show for anyone who cares to simply look up. I am talking about Ikeya-Zhang, a rare event in a human lifetime, a naked-eye comet.

As you may have learned in class already, a comet is a kind of dirty snowball the size of a mountain. Comets are a mixture of rock, water and other components. They are, essentially, remnants of the time when the solar system was assembled from its natal interstellar cloud of gas and dust. This is why astronomers love comets. Observing one with a powerful telescope gives astronomers a view back to the time when the sun and planets were still forming. The stuff of comets is so important to researchers that more than one spacecraft has been sent on the risky voyage for a close encounter. Even now there is a mission on its way to collect and return microscopic debris from a comet tail.

Comets aren't just important to astronomers. Since antiquity human beings have marveled at these bright travelers moving rapidly, night to night, against the background stars. Before city lights swallowed the night sky and turned it into a dull gray haze, most of humanity saw the full majesty of the heavens every night. Since the stars don't change at all, and the planets move along fairly regular paths, something entirely new appearing sky was not only noticed, it was also cause for some alarm. The Bayeux Tapestry, a medieval commeration of the Norman Conquest of England, shows a comet (believed to be everyone's favorite - comet Halley) appearing in the sky over the characters heads. One can even go all the way back to 240 B.C. to find Chinese records of the appearance of comet Halley. Naked eye comets, those that can be seen without binoculars or telescopes, have always been rare and spectacular apparitions.

Even in the modern world, naked eye comets can cause a stir. Across the globe there exists a dedicated cadre of amateur astronomers who regularly search the sky with telescopes for comets. On February 1, 2002, Kaoru Ikeya of Japan and Daqing Zhang of China announced their discovery that a new mountain of ice and rock was on its way past Earth toward the Sun. Soon amateur and professional astronomers around the world were watching the newly dubbed comet Ikeya-Zhang as it continued to brighten on its sunward journey. By early March, a "tail" had formed as radiation from the Sun began to heat and melt the giant ice ball (as gas from a melting comet "nucleus" escapes into space, solar radiation drives it backward creating the familiar tail shape) Ikeya-Zhang and its growing tailed continued to brighten. Within a week it became clear that telescopes would no longer be needed and the comet would soon be visible to anyone under its path.

Comet Ikeya-Zhang was special for other reasons as well. When astronomers used their data to figure out its entire orbit they found it was familiar in age to somebody's great-great-great grandparents. Three hundred and forty one years ago (in 1661) astronomers in Europe recorded the passing of comet, now known as C/1661 C1. Based on orbital calculations completed last month, a 341-year orbit matches well with the visitor currently in our skies. If this were true, Ikeya-Zhang would be the first of what are known as "long-period" comets to be identified in a subsequent trip back to the Sun. Long-period comets take more than 200 years to make a round trip. Comet Halley, with a 76-year orbit, is clearly of the short period variety. Long-period and short-period comets come from a totally different place in the solar system. Seeing a long period comet twice shows how mature we have become as an astronomical species. We have been carefully watching the skies for a long time now.

By March 18, 2002, Comet Ikeya-Zhang reached perihelion, its closest approach to the Sun, and afterwards began its long trek back into the dark and frozen wastes of space. Hopefully you had a chance to see it. If not keep your eyes up and ears open for the next time we get such as celestial visitation.

A Point to Ponder

Compare the reaction of human beings to a comet visitation 20,000, 2000, 200 and 20 years ago. How would people have reacted in each age? What does this tell you about how humanity's relationship with the sky and astronomy has changed?

Check Out These Websites

Comets
http://www.seds.org/nineplanets/nineplanets/comets.html