(originally run May 9, 1997)
For the past several months Earthbound skywatchers, at least those of us in the northern hemisphere, have been treated to a celestial show that must certainly rank among the top such displays that we will ever see in our lifetime. Comet Hale-Bopp, discovered by this author on July 23, 1995 and independently by Arizona amateur astronomer Thomas Bopp that same night, has essentially lived up to its expectations and has given us a show that we will never forget.
Hale-Bopp has dazzled casual viewers from around the globe, with even skywatchers in light-polluted major metropolitan areas being able to catch sight of the comet's brilliant glow. From rural locations Hale-Bopp was nothing less than magnificent, appearing as bright as the brightest stars and displaying both a brilliant curving dust tail and a ghostly second tail made up of gases that have been ionized and consequently are fluorescing like the gases in a neon sign.
From the scientists' perspective Hale-Bopp has been an equally impressive object, becoming perhaps the most intensely-studied comet in history. It has been observed in almost every part of the spectrum, from x-rays to radio waves, and new results are being reported on almost a daily basis. Space will permit only a few of these findings to be recounted here.
Even small telescopes were sufficient to reveal a striking pattern of "hoods," or "ripples," in the comet's inner regions. Careful study of these features has revealed that they are made up of streams of dust grains ejected from the rotating nucleus, and that are subsequently spiraling outward like the water coming out of a rotating lawn sprinkler. This has, among other things, allowed scientists to determine that the comet's nucleus rotates every 11 1/2 hours.
Various types of molecules have been identified within Hale-Bopp, including some substances never before seen in comets. In addition to "expected" molecules such as water, carbon monoxide, and methane, the list of substances detected in Hale-Bopp includes sulfur dioxide, formic acid, methyl cyanide, hydrogen cyanide, and methanol. The detection of these and many other substances can provide us with clues about the formation of the Earth and possibly even about the origins of life itself.
In addition to the two main tails that could be seen by casual viewers, in mid-April a team of astronomers in Europe announced that in specially filtered photographs taken with a telescope in the Canary Islands they had detected a faint third tail composed of sodium atoms. The source of this sodium is still not completely understood, but some scientists believe that it comes from sodium compounds released from dust grains that have been ejected from the nucleus.
Perhaps one of the most fascinating science results from Hale-Bopp came from data obtained by Roland Meier and his colleagues at Mauna Kea Observatory in Hawaii. Meier's observations indicate that the abundance of deuterium (an isotope of hydrogen which contains a neutron within its nucleus) relative to ordinary hydrogen is the same in the comet as that in Earth's seawater. This offers strong evidence for the idea that the Earth has obtained at least some of its water from cometary impacts throughout its history.
It's been said that "all good things must come to an end," and Hale-Bopp is no exception. Now receding from both the Earth and the sun, the comet has faded noticeably from its peak display a month ago, although it is still a fairly dramatic sight in our evening sky. It is traveling southward through the inner solar system -- having crossed the plane of the Earth's orbit on May 6 -- and is now rapidly moving southward in our sky as well. Now visible low in the west after dusk, it sets earlier each evening, and these next few nights will provide our last decent view of it. By early next week moonlight will start to interfere with viewing, and by the time we once again have a dark evening sky (after full moon on the 22nd), the comet will be too deep in twilight to be seen easily. Our show will essentially be over then.
The situation is somewhat different for skywatchers in the southern hemisphere. From their perspective Hale-Bopp has been invisible since late last year, until just a couple of weeks ago when it began to be visible low in their northwestern evening sky. Now, with the comet's heading southward our friends in Australia and South America will be treated to a fairly decent display over the next month or so. While their show will not be as dramatic as ours has been, they have the consolation of being able to watch the comet as it recedes into the outer solar system. Meanwhile, the northern hemisphere does get one more brief view of Hale-Bopp during the late summer and early fall of this year, but it will be low in the south during the pre-dawn hours, and by then will probably have faded below naked-eye visibility.
Large telescopes in the southern hemisphere should be able to follow Hale-Bopp for several more years, although the time will eventually come when it is no longer visible with any instruments we have. It will continue to travel outward, though, until it reaches the farthest point in its orbit, some nine times the distance of Pluto from the sun. Then, it will begin the long journey inward, until about 2380 years from now, when it returns once again to light up the night skies for our distant descendants.
Meanwhile, here on Earth life goes on. What kind of world will greet Hale-Bopp when it makes its next return? Will our society be free of the hate, the crime, the poverty, the superstitions, that so permeate it today? As we watch Hale-Bopp recede into the darkness, let us keep in mind that the world of the 44th Century begins with us, here, in 1997.
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