—PIERRE SIMON, MARQUIS DE LAPLACE,
SYSTEM OF THE WORLD, PART 1, CHAPTER 6, 1796
Comets may act as the creators, the preservers, and the destroyers of life on Earth. A surviving dinosaur might have reason to mistrust them, but humans might more appropriately consider the comets in a favorable light—as bringers of the stuff of life to Earth, as ocean-builders, as the agency that removed the competition and made possible the success of our mammalian ancestors, as possible future outposts of our species, and as providers of a timely reminder about large explosions and the climate of the Earth.
Thirty-Two Perihelion Passages of Halley’s Comet
239 B.C. March 30
163 October 5
86 August 2
11 B.C. October 5
66 January 26
141 March 20
218 May 17
295 April 20
374 February 16
451 June 24
530 September 25
607 March 13
684 September 28
760 May 22
837 February 27
912 July 9
989 September 9
1066 March 23
1145 April 22
1222 October 1
1301 October 23
1378 November 9
1456 June 9
1531 August 25
1607 October 27
1682 September 15
1759 March 13
1835 November 16
1910 April 20
1986 February 9
2061 July 28
2134 March 27
Every apparition listed (except, of course, the last two) was recorded by the astronomers of Earth.
A comet is also a visitor from the frigid interstellar night that constitutes by far the greatest part of the known universe. And a comet is, further, a great clock, ticking out decades or geological ages once each perihelion passage, reminding us of the beauty and harmony of the Newtonian universe, and of the daunting insignificance of our place in space and time. If, by chance, the period of a bright comet happens to be the same as a human lifetime, we invest it with a more personal significance. It reminds us of our mortality:
Like hundreds of other little boys of the new century, I was held up in my father’s arms under the cottonwoods of a cold and leaf-less spring to see the hurtling emissary of the void. My father told me something then that is one of my earliest and most cherished memories. “If you live to be an old man,” he said carefully, fixing my eyes on the midnight spectacle, “you will see it again. It will come back in 75 years. Remember,” he whispered in my ear, “I will be gone, but you will see it. All that time it will be traveling in the dark, but somewhere, far out there”—he swept a hand toward the blue horizon of the plains—“it will turn back. It is running glittering through millions of miles.”
I tightened my hold on my father’s neck and stared uncomprehendingly at the heavens. Once more he spoke against my ear and for us two alone. “Remember, all you have to do is be careful and wait. You will be seventy-eight or seventy-nine years old. I think you will live to see it—for me,” he whispered a little sadly with the foreknowledge that was part of his nature.*
Comet Halley is unique in our epoch—a bright, periodic, sometimes spectacular comet that sews the generations together, stitching back through history and forward into epochs to come, awakening us from the delusion that we exist separate from our past and our future, time-binding the human species. Your grandparents perhaps looked up in 1910, your grandchildren may well look up in 2061, and perhaps you yourself looked up back in 1986 to see Halley’s Comet—the same comet* that Edmond Halley saw the summer after he and Mary Tooke were first wed. This is the comet all but one of whose apparitions back to the year 239 B.C. were carefully noted by Chinese astronomers. It has been described on tablets, silk, bamboo paper, parchment, newsprint and computer disks. This is the comet that cheered the !Kung hunter-gatherers and frightened nearly everyone else in all those cultures far-flung over the surface of the Earth and back at least millennia in time. We share this comet with many others.
Late Babylonian tablet, with an account in cuneiform writing of the apparition of Halley’s Comet in 164 B.C. This is a portion of a systematic Babylonian astronomical and astrological textbook, and reads in part, “The comet which previously had appeared in the east in the path of Anu in the area of the Pleiades and Taurus, to the west … passed along in the path of Ea.” Courtesy The British Museum.
When will a comet sweep the sky
Seizing and binding these evil leaders?
—FROM AUTUMN DAYS IN THE STATE OF K’UEI, BY TU FU (712–770 A.D.)
TRANSLATED BY HEATHER SMITH AND XIE YONG
An apparition of Halley’s Comet occurred in 760, and was recorded in the Chinese chronicles.
A million years ago or so, as our hominid ancestors were hunting game and figuring out how to build a house, a passing star sent a gravitational ripple through the cloud of comets that envelops the Sun, and a pristine small world of ice went careening in toward the Sun. Ten or twenty thousand years ago, as our ancestors were dealing with the Wisconsin Ice Age, the comet finally arrived in the planetary part of the solar system, made a close encounter with one of the major planets, and began the conversion into its present orbit—that now takes it, once each 76 years, closer to the Sun than Venus is, and farther from the Sun than Neptune. Every 76 years, more or less, for the last ten thousand, this comet has swept by, captivating the inhabitants of the Earth and then vanishing into the night.
It would have been a great satisfaction to know that everyone who saw this wonderful object did so with the same feeling of elation and wonder—one would almost say veneration—with which the average astronomer regarded this beautiful and mysterious object stretching its wonderful stream of light across the sky.
—E. E. BARNARD, THE PREEMINENT OBSERVATIONAL ASTRONOMER OF THE TIME, ON THE 1910 APPARITION OF COMET HALLEY
Seventy-six years is a few generations or less, and so Halley’s Comet is a kind of metronome beating out the rhythm of human progress or decline. Its 1910 apparition was the first since the invention of the airplane and the last before nuclear weapons. We have lately arranged the means of our self-destruction, and there is a real question of how many humans will be left the next time Halley’s Comet comes by the Earth, in July 2061.
The dangers that we face are part of the tortuous process, now well under way, of the unification of the planet—in language, culture, science, and commerce. They are both driven by the identical technological advances—this critical and delicate time coincides with the widespread availability of nuclear weapons. At the present rate of change, it seems likely that—the end of the Cold War notwithstanding—in the period between now and 2061, the turning point for the human species will have been reached.
If we survive until then, our passage to the next apparition of Halley’s Comet should be comparatively easy. That perihelion passage will be in March 2134, when the comet will make an unusually close encounter with the Earth. It will come within 0.09 Astronomical Units or 14 million kilometers, less than half the distance of the 1910 encounter. It will then be brighter than the brightest star. If there are those to do the commemorating, the years 2061 and 2134 should be celebrated for the courage, intelligence, and common purpose of a species forced by urgent necessity to come to its senses.
As everyone knows, contending inclinations war within each of us—toward understanding, creativity, and growth, and toward chauvinism, violence, and fear. This battle, on which the fate of the world truly rests, can be illustrated even by a mote of dust: Consider the history of a certain speck of matter dancing in the air before you—formed, it may be, from material ejected billions of years ago by a star on the other side of the Milky Way Galaxy; wandering for eons in the interstellar dark; gathered up into the forming solar nebula some five billion years ago; adhering to other similar grains in a lump of cosmic matter that eventual
ly formed one of trillions of comets, probably in the vicinity of Uranus and Neptune; ejected out to the Oort Cloud; perturbed into the inner solar system, arriving a few thousand years ago; spewed off into the cometary tail, still encased in a particle of ice; wandering as a microplanet in interplanetary space; accidentally intercepted by the Earth a few years ago; gently settling through the Earth’s atmosphere; and now bobbing in the air, illuminated by a sunbeam, and giving no hint of its epic cosmic voyage.
Now consider another mote of dust, a little larger but with a similar history: It streaks into the Earth’s atmosphere and burns up, ionizing the surrounding air, while it itself is reduced to molecules and atoms. That bit of matter makes a meteor trail, glowing brightly for a moment, perhaps to the accompaniment of expressions of delight by onlookers below. But if you think hard enough, you can find a use even for a shooting star. The trail of ions that a meteor briefly leaves will reflect very high frequency (VHF) radio waves. Since at any given moment there are enormous numbers of meteor trails in the atmosphere—most too faint to be seen with the naked eye—they collectively provide a kind of reflecting surface surrounding the Earth, off which radio waves of the proper frequencies can be bounced. Since the duration of an individual ion trail is less than a second, the message must be sent very fast. This has led to a new field of technology called Meteor Burst Communications.
And why would anyone go to such lengths, when there are perfectly adequate means of communication at hand? Because, if anti-satellite weaponry gets going, communications satellites would be among the early casualties in a nuclear war. Meteor Burst Communications has been developed so that a nuclear war can be fought. The comets have been enlisted. For the first time since they were thought to be warnings sent by an angry God, comets have practical value.
Every encounter of Halley’s Comet has been an occasion to express hopes and fears. So has the apparition of bright non-periodic comets. Almost ritually, it has been a time to pray. So for the next apparition of Halley’s Comet, and for Hale-Bopp, we offer a prayer of our own: We live on a fragile planet, whose thoughtful preservation is essential if our children are to have a future. We are only custodians for a moment of a world that is itself no more than a mote of dust in a universe incomprehensively vast and old. May we therefore learn to act, before all else, for the species and the planet.
Then, for innumerable nights to come, there will be humans to witness the grandeur of the comets that grace the skies of Earth.
[The Comet of 1680] was observed with exquisite skill by Flamsteed and Cassini: And the mathematical science of Bernoulli, Newton, and Halley investigated the laws of its revolutions. At [its next return], in the year two thousand three hundred and fifty-five, their calculations may perhaps be verified by the astronomers of some future capital in the Siberian or American wilderness.
—EDWARD GIBBON, THE DECLINE AND FALL
OF THE ROMAN EMPIRE, CHAPTER 43, 1777
That Gibbon, writing on the eve of the American Revolution, would foresee that the present territories of the United States and Russia would one day be the centers of astronomical discovery is almost as impressive as calculating the periodic return of comets, which he so clearly admires. But by the twenty-fourth century, the new centers of astronomical learning may be on Mars—or even on the comets themselves.
Comets approach the Sun, flicker a few hundred times, and die like moths around a flame. But a vast repository of them waits at the periphery of the solar system. When the present configuration of continents is unrecognizably altered, when the Earth is engulfed by the expanding Sun, when, in its dotage, our star feebly illuminates the charred remains of this planet—then, even then, the skies will still be brightened as young comets, newly arrived from the interstellar dark, make their wild perihelion passages. When the rest of the solar system is dead, and the descendants of humans long ago emigrated or extinct, the comets will still be here.
The Comet as a global cultural event. Courtesy Ruth S. Freitag, Library of Congress.
The Comet of 1812
It was clear and frosty. Above the dirty, ill-lit streets, above the black roofs, stretched the dark starry sky. Only looking up at the sky did Pierre cease to feel how sordid and humiliating were all mundane things compared with the heights to which his soul had just been raised. At the entrance to the Arbat Square an immense expanse of dark starry sky presented itself to his eyes. Almost in the center of it, above the Prechistenka Boulevard, surrounded and sprinkled on all sides by stars but distinguished from them all by its nearness to the earth, its white light, and its long uplifted tail, shone the enormous and brilliant comet of 1812—the comet which was said to portend all kinds of woes and the end of the world. In Pierre, however, that comet with its long luminous tail aroused no feeling of fear. On the contrary he gazed joyfully, his eyes moist with tears, at this bright comet which, having traveled in its orbit with inconceivable velocity through immeasurable space, seemed suddenly—like an arrow piercing the earth—to remain fixed in a chosen spot, vigorously holding its tail erect, shining and displaying its white light amid countless other scintillating stars. It seemed to Pierre that this comet fully responded to what was passing in his own softened and uplifted soul, now blossoming into a new life.
—LEO TOLSTOY, WAR AND PEACE, VIII, 22
This is actually a reference to the Great Comet of 1811. It could still be seen with the naked eye in early 1812, but just barely. It was, however, a splendid object in the late fall of 1811.
*Loren Eiseley, The Invisible Pyramid (New York, 1970). We wish that Loren Eiseley had lived to see it.
*Or very nearly the same comet: the nucleus has lost many meters of ice since the perihelion passage of 1682.
APPENDICES
Cometary Orbits and Meteor Showers
APPENDIX 1
Orbits of Selected Long-Period Comets
Comet Name Perihelion Distance (A. U.)
1811 I Great Comet 1.035
1844 III Great Comet 0.251
1858 VI Donati 0.578
1861 II Great Comet 0.822
1881 III Great Comet 0.735
1882 II Great Comet 0.008
1908 III Morehouse 0.945
1910 I Great Comet 0.129
1937 IV Whipple 1.734
1943 I Whipple-Fedtke-Tevzadze 1.354
1957 III Arend-Roland 0.316
1957 V Mrkos 0.355
1962 VIII Humason 2.133
1965 VIII Ikeya-Seki 0.008
1969 IX Tago-Sato-Kosaka 0.473
1973 XII Kohoutek 0.142
1976 VI West 3.277
1980b Bowell 3.364
Note that perihelion passage for these long-period comets varies from very close to the Sun (0.008 A.U.) to the middle of the asteroid belt, between the orbits of Jupiter and Mars (1.4 to 5 A.U.) There must be many long-period comets with more distant perihelia that are unknown to observers on Earth.
SOURCE: B. Marsden and E. Roemer, in Comets, L. Wilkening, ed., University of Arizona Press, 1982.
APPENDIX 2
Orbits of Selected Short-Period Comets
The inclinations of the comets shown here vary from less than 2° (almost exactly in the ecliptic plane which includes the Earth and the planets), to 162° for Comet Halley; since a 90° inclination would mean a comet orbiting the Sun perpendicular to the ecliptic, a 162° inclination means a comet whose orbit is inclined some 180 − 162 = 18° from the ecliptic, but traveling in the opposite sense to the direction in which the planets revolve about the Sun. The eccentricity of the cometary orbits shown varies from about 0.04 (very close to a circle) to 0.97 for Comet Halley, which is extremely elongated.
SOURCE: Marsden and Roemer, 1982.
APPENDIX 3
Major Naked-Eye Meteor Showers, Late Twentieth Century
APPENDIX 4
Further Information
In addition to works listed in the Bibliography, further information about comets can be obtained from:
The Planetary So
ciety The Astronomical Society of the Pacific
65 North Catalina Avenue 1290 24th Avenue
Pasadena, CA 91106 San Francisco, CA 94122
(818) 9 WORLDS (415) 661-8660
Other resources for those who wish to dig more deeply include local planetariums and popular science publications such as Sky and Telescope or Astronomy. Periodic news on cometary findings is provided by Comet News Service: A Quarterly Review and Irregular Bulletin, P.O. Box TDR #92, Truckee, CA 95734.
Bibliography
Books on Comets Mainly for General Audiences
Asimov, Isaac. Asimov’s Guide to Halley’s Comet. Walker, New York, 1985.
Brandt, John C., ed. Comets: Readings from Scientific American. W. H. Freeman, San Francisco, 1981.
Brandt, John C. Introduction to Comets. Cambridge University Press, Cambridge, 1981.
Calder, Nigel. The Comet Is Coming! Viking Press, New York, 1980.
Chapman, Robert D., and John C. Brandt. The Comet Book: A Guide for the Return of Halley’s Comet. Jones and Bartlett, Boston, 1984.
Comets: Career Oriented Modules to Explore through Pictures in Science. National Science Teachers Association, 1984.
Dahlquist, Raf, and Theresa Dahlquist. Mr. Halley and His Comet. Polestar, Canoga Park, CA, 1985. A charmingly illustrated children’s book in rhyme.
Flaste, Richard, Holcomb Noble, Walter Sullivan, and John Noble Wilford. The New York Times Guide to the Return of Halley’s Comet. Times Books, New York, 1985.
“Halley’s Comet.” The Planetary Report, 5, 3, May/June, 1985.
Halley Watch Amateur Observers’ Manual for Scientific Comet Studies. Hillside, NJ: Enslow Publishers, 1983.