A Comet Is Blessing And Threat in One Ball of Light
+ Von Del Chamberlain +
The light of a comet is a beguiling light.
It is a soft, some say is beautiful,
yet others have sheltered their eyes from it in fear.
Like the red star that from his flaming hair
Shakes down disease, pestilence and war.
Homer, describing Achilles' helmet, c. 900 B.C.
When beggars die there are no comets seen.
The heavens themselves blaze forth the death of Princes.
Shakespeare's Julius Caesar
Actually what we see when a comet passes our way is sunlight that has been diverted through a body that carries with it ancient secrets, plunging through our region of space, just for a moment tantalizing us with the challenge to wrestle from it the information it carries.
I suppose that by now all of you have read or heard about the comet that is plunging through our part of space. I hope you have actually seen it during late March when it passed less than 10 million miles away. Between the middle and end of March it dashed from early morning visibility in the south clear up past the North Star where it was in the sky all night long, and now it is toward the northwest after evening twilight. It was bright between during the last half of March and easiest to see before the Moon flooded the sky with light. If you have been watching this comet, you will surely want to continue as it changes in brightness and moves now in our evening sky.
The factors that make a comet spectacular in the sky have to do with its composition and the relative positions of Earth, comet and Sun as this frozen "dirty snowball" comes plummeting past the Sun. When it is near the Sun it feels the solar heat and responds by emitting a cloud of dust and gases that form a huge halo around the solid nucleus, hundreds of miles in diameter. It is this cloud, called the "coma," that reflects sunlight making the comet much brighter than it would otherwise be. Light and the pressure from atomic particles flowing out from the sun push the coma away from the Sun's direction, producing the tail of the comet that streams out millions of miles. Now, if Earth is in a favorable position with respect to the comet and Sun so that we have a good viewing aspect, we see one of the grandest spectacles of all. Such has been the case for Comet Hyakutake. Such will continue to be the case for a few more days. Your opportunity to see it is now or never, for it will not come this way again for thousands of years.
Like so many other things we know of, comets seem to have come out of Pandora's box. Throughout much of recorded history they were thought of as symbols of evil and cataclysm.. Consider, for example, these quotations:
A fearful star is the comet, and not easily appeased, as appeared in the late civil troubles when Octavius was Consul; a second time by theM-^Ewar of Pompey and Caesar; andM-^Ewhen Claudius Caesar having been poisoned, the empire was left to Domitian, in whose reign there appeared a blazing comet.
Pliny the Elder
A Blazing Star,
Threatens the World with Famine, Plague and War;
To Princes, death; to Kingdoms many crosses;
To all Estates, inevitable Losses;
To Herdmen, Rot; to Ploughmen, hapless Seasons;
To Sailors, Storms; to Cities, civil Treasons.
Du Bartas, La Semaine
The thick smoke of human sins, rising every day, every hour, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge.
Andreas Celichius, Lutheran bishop of Altmark, 1578
With discoveries of methods to learn about the physical universe from light emanating out of objects in space, we put together a different picture of what comets represent. Now we think they were formed at the edge of the solar system some 4.5 billion years ago as the Sun and planetscame into existence out of a nebulous cloud of unorganized material. They have stored away this primordial material, and the text book of earlier times imbedded within the ices, particles of dust and lumps of rocky material. This is likely the oldest matter existing in relatively unchanged form that can reveal to us some of the story of how and when the solar system came into existence. Evidence suggests that large numbers of comets orbit way beyond the planets. Occasionally nudged by the gravity of some passing object, these natural encyclopedias of creation are cast inward where they pass the orbits of planets, one after the other. If they come close to a planet, their orbits are substantially changed. The space nomads plummet on courses that bring them closer to the star that had previously carried them out where feeble solar energy caused them little grief.
However, as a comet approaches the Sun it undergoes the most rapid change it has ever experienced. Sunlight heats the ices, releasing atoms of gases and particles of dust, resulting in a growing cloud, the "coma," surrounding the tiny frozen "nucleus." The pressure of sunlight and wind of atomic particles blowing outward from the Sun elongate the coma into the "tail," always flowing in a direction away from the Sun.
All this leads to opportunity for those who would understand by looking around with every capability that will yield knowledge. Telescopes of every kind, on earth and in space, turn to harvest the light of the speeding time capsule. Sunlight bouncing off the particles of dust can tell us something about the sizes of the particles. Sunlight absorbed by atoms of gases and re-emitted can give us a partial chemical analysis of the comet. In many ways we can learn about the early solar system as well as about the interaction of solar energy with cosmic material.
When Comet Halley last visited our vicinity, we had a space probe ready to pass near it and give us first images of a comet nucleus. Now, the Hubble Space Telescope is used to study Comet Hyakutake. Someday we will be able to retrieve samples of a comet for careful study in our finest scientific laboratories.
One of the questions I am nearly always asked in an interview about comets is, "how close have they come to Earth?" Comet Hyakutake is one of the closer comets we have observed with modern instruments, yet it certainly is not the closest of all. Without a doubt comets have hit Earth in times past, and they probably will again. In 1908 an object struck a remote region of Siberia, leveling a huge section of forest. The best guess we have is that this was a tiny comet.
Every object we study in the solar system that does not have an atmosphere to erode the surface, has vast numbers of craters. Some of these impact pits were caused by asteroidal material, but many of them must have been caused by comets. During the summer of 1995 we watched as fragments of a comet plummeted into the atmosphere of the giant gas planet Jupiter. On Earth, we have learned to recognize both small and large craters caused by impact. Indeed, one of the theories relating to the demise of the dinosaurs is that a comet impact created a "nuclear winter" that so severely changed Earth's climate that many living things were exterminated. Not to worry, these large bodies do not strike very often, and when they do they can cause so much damage that those lingering for a while would be far too busy to worry, so why worry about it.
Comets, then, are missiles of knowledge that have represented lots of contradicting things to various people over time. They can inform us about the fringes of the planetary system we reside in and about the distant past. On the other hand, they are one of the greatest threats we have ever known, and we must learn all we can about them in hopes of being able to lessen that threat. One of these bombs of knowledge is near us now. Your final and perhaps most favorable (depending on weather and just how the comet behaves) opportunity to enjoy Comet Hyakutake should be between now and April 25. Since the Moon is new on the 17th, you should begin your comet watching at once and continue every clear evening until moonlight and twilight interferes too much. Comet Hyakutake is headed for a close encounter with the Sun: it will pass only 21.5 million miles from it at perihelion, rounding the corner and heading back out into frigid space. It could break up in the intense radiation of the Sun.
I hope you will go out and watch Comet Hyakutake at every available opportunity, enjoying the play of star light warming this natural space probe for the brief moment it visits us. As you watch, please consider how people of the past have felt about these wanderers, about the knowledge we harvest from each one of them, and about how exciting it is to add such a rosette- phenomenon to your personal collection of experiences. Stay tuned, for this might be act one of a two part drama. Comet Hale-Bop is coming in, where it might put on an even greater show about this time next year.