The Great Comet of 1965

by Owen Gingerich

OF ALL the memorable comets that have excited astronomers and stirred men’s imaginations, not one had more impact on our concepts of the universe than the Great Comet of 1577. Discovered in November of that year, the comet stood like a bent red flame in the western sky just after sunset. The celebrated Danish astronomer Tycho Brahe was among the early observers: he caught sight of the brilliant nucleus while he was fishing, even before the sun had set. As darkness fell, a splendid twenty-two-degree tail revealed itself. Tycho’s precise observations over the ten-week span before the comet faded away were to deal the deathblow to ancient cosmogonies and pave the way for modern astronomy.

In the sixteenth century nearly everyone accepted Aristotle’s idea that comets were meteorological phenomena, fiery condensations in the upper atmosphere. Or, if not that, they were burning impurities on the lower fringe of the celestial ether, far below the orbit of the moon. In 1577 most astronomers still subscribed to the ancient belief that the moon and planets were carried around the earth on concentric shells of purest ether. Tycho, by comparing his careful measurements of the comet’s position with data from distant observers, proved that it sped through space far beyond the moon. The Comet of 1577 completely shattered the immutable crystalline spheres, thereby contributing to the breakdown of Aristotelian physics and the acceptance of the Copernican system.

But the most renowned and most thoroughly studied of all comets is the one associated with Edmund Halley. It was the first to have a periodic orbit assigned, thus securing for comets their place as members of the solar system. Halley had matched the Comet of 1682, which he had observed, with those of 1531 and 1607. Assuming these to be different appearances of the same celestial object, he predicted another return in 1758. Although he was ridiculed for setting the date beyond his expected lifetime, the comet indeed returned, and Halley’s name has been linked with it ever since.

On its latest return, in 1910, Halley’s comet put on a magnificent display, reaching its climax several weeks after perihelion passage in mid-April. During the early part of May it increased until the brilliance of its head equaled the brightest stars and its tail extended sixty degrees across the sky. Later in May, the earth grazed the edge of the tail. The thin vacuous tail caused no observable effect on earth, except for such human aberrations as the spirited sale of asbestos suits. That no terrestrial consequences were detected is not surprising when we learn that 2000 cubic miles of the tail contained less material than a single cubic inch of ordinary air.

IF PRIZES were offered for cometary distinctions, then last year’s Comet Ikeya-Seki would win a medal as the most photographed of all time, and it might win again for the range of astrophysical observations carried out. As it swung around the sun, its brilliancy outshone that of the full moon, and within ten days its tail extended almost as far as the distance from the earth to the sun. The behavior of the comet was neatly explained by the “dirty snowball” theory. According to this widely accepted picture, a comet’s nucleus is a huge block of frozen gases generously sprinkled with dark earthy materials. Occasionally the gravitational attraction of nearby passing stars can perturb a comet from its cosmic deep freeze in the distant fringes of the planetary system beyond Neptune; the comet then can penetrate the inner circles of the solar system, where it develops a shining gaseous shroud as its surface vaporizes under the sun’s warming rays. Hence, the closer a comet approaches the sun, the more it vaporizes and the larger and brighter it becomes. Comet Ikeya-Seki passed unusually close to the sun, becoming possibly the brightest comet of the century; the resulting tail was the fourth longest ever recorded.

Today I look back with a wry smile to the Sunday morning last September when I decoded the telegram bringing the first word of the new comet. Early that morning in Ben ten Jima, Japan, a youthful comet hunter, Kaoru Ikeya, had discovered a fuzzy glow not charted on his sky maps. At the same time, another young amateur 250 miles away, Tsutomu Seki, had independently detected the new celestial visitor. Both men had used simple, homemade telescopes for their discovery, and both had sent urgent messages of their find to the Tokyo Astronomical Observatory.

News of the comet’s appearance was quickly relayed from Tokyo to my office at the Smithsonian Astrophysical Observatory. Here the name “Comet Ikeya-Seki” was officially assigned, as well as the astronomical designation 1965 f. Throughout that day, September 19, the communications center at Smithsonian alerted observatories and astronomical groups all over the world — Flagstaff, Rio de Janeiro, Johannesburg, Prague, Peking, Canberra — in all, more than 120. Included were the twelve astrophysical observing stations of the Smithsonian Observatory, whose specially designed satellitetracking cameras are ideal for comet photography. Within hours a confirmation of Ikeya-Seki arrived from the Woomera, Australia, station.

By Tuesday afternoon, half a dozen approximate positions were in hand, more than enough for us to try for a crude preliminary solution of the comet’s orbit. Unfortunately, the positions from the observing stations were only approximate “eyeball” measurements obtained by laying the film onto a standard star chart with marked coordinates. Furthermore, the observatory’s computer program had not been fully checked out. When the rough observations were used in different combinations, the computer produced two orbits in wild disagreement. Nevertheless, Professor Fred L. Whipple, director of the Smithsonian Astrophysical Observatory and author of the “dirty snowball” comet theory, noted that the second of the preliminary orbits closely resembled the path of a famous family of sun-grazing comets. The agreement was too close to be coincidence, he reasoned, and therefore the second solution must be correct.

Professor Whipple’s astute suggestion provided the first hint of the excitement that was to come. Several of the previous sun-grazers had been spectacular objects. Notable among them was the Great Comet of 1843, whose seventy-degree tail stretched 200 million miles into space, setting an all-time record, and whose brilliance induced the citizenry of Cambridge to build a fifteen-inch telescope for Harvard equal to the largest in the world. And the second comet of 1882 achieved such brilliancy as it rounded the sun that it could be seen in broad daylight with the naked eye.

In the few days following the first computer solutions three “precise” positions were reported to the Central Telegram Bureau, one from Steward Observatory in Tucson, Arizona, and two from the Skalnate Pleso Observatory in Czechoslovakia. When these new positions were fed by themselves into the computer, the result indicated an ordinary comet, and not a sun-grazer at all. But our programmers noticed that something was seriously wrong. When positions from the satellite-tracking cameras were included in the calculations, the computer gave different answers. Among them was the interesting possibility that Comet IkeyaSeki might die by fire, plunging directly into the sun.

Then, suddenly, the mystery vanished. Six accurate positions from veteran comet observer Elizabeth Roemer at the Flagstaff, Arizona, station of the U.S. Naval Observatory established the path with great precision. One of the earlier “precise” observations had been faulty, and with its elimination, the others fell into place. Comet Ikeya-Seki was accelerating along a course that would carry it within a solar radius of the sun’s surface. And since a comet’s brightness depends on its closeness to the sun, there was every indication that Comet Ikeya-Seki would become a brilliant object.

Armed with predictions of Comet Ikeya-Seki’s sun-grazing path, the Smithsonian staff set out to forewarn space scientists and radio astronomers whose attention does not normally encompass comets. We called a press conference to describe the magnificent view hoped for as the comet swung around perihelion, its nearest approach to the sun. First discovered in the morning sky, the comet would cross into the evening sky for only a few hours on October 21. If a tail of this comet were to appear in the evening, it would sweep across the western sky after sunset on that evening. Afterward it would reappear in the morning twilight. Such a prediction was hazardous, because although the comet’s trajectory was well established, its brightness and tail length resisted astronomical forecasting since no one knew just how much material would be activated as it sped past the sun.

Had we examined more carefully the historical records of Comet 1882 II, we might have been more cautious in telling the public to look for the tail of Comet Ikeya-Seki sweeping across the western sky after sunset on October 21. Each new observation of the 1965 comet confirmed that it was a virtual twin of the Great Comet of 1882; thus, by looking at the observations from the last century, we should have guessed that the comet’s enormous velocity as it rounded the sun — one million miles per hour — would dissipate the tail so widely that it could not be seen in the dark sky. On the other hand, we hardly dared publicize what the computer’s brightness predictions showed: that Comet Ikeya-Seki would be visible in full daylight within a few degrees of the sun!

AND thus it happened that thousands of would-be observers in the eastern United States maintained a cold and fruitless search in the early morning hours of October 21. Thousands of others, especially in the American Southwest, had the view of a lifetime — a bright comet with its short silvery tail visible next to the sun in broad daylight. Simply by holding up their hands to block out the sunlight, they could glimpse the comet shining with the brilliance of the full moon. Hazy, milky skies blocked the naked-eye view for observers in the eastern United States and much of the rest of the world; even in New England, however, telescopes revealed the comet with a sharp edge facing the sun and the beginnings of a fuzzy tail on the other side. Professional astronomers were excited by the opportunity to photograph the object at high noon. For the first time, the daylight brilliance of a comet permitted analysis from solar coronagraphs. Airborne and rocket-borne ultraviolet detectors examined features never before studied in comets.

The spectrum observations ended eight decades of controversy. In most comets, the reflected spectrum of sunlight is seen, combined with the more interesting bright molecular spectrum from carbon and carbon compounds. The molecules are excited by the ultraviolet light from the sun, and glow in much the same way that certain minerals fluoresce under an ultraviolet lamp. But back in 1882, when spectroscopy was in its infancy, the great sun-grazing comet yielded an entirely different spectrum. Scientists at the Dunecht Observatory in Scotland thought they saw emission lines from metal atoms such as iron, titanium, or calcium, but a similar spectrum was never found in subsequent comets. Some observers expressed their disbelief in this unique record.

Astronomers did not get another chance to examine a comet so close to the sun until October 20, 1965. On that morning at the Radcliffe Observatory in South Africa, Dr. A. D. Thackeray obtained spectrograms of the nucleus of Comet Ikeya-Seki, then only 8 million miles from the sun. These showed bright lines of both iron and calcium. The telegraphic announcement, again relayed by the Central Bureau, set other spectroscopists into action. Within days, there were reports of nickel, chromium, sodium, and copper.

Though fully expected from a theoretical point of view, these observations confirmed that the impurities in comets had a chemical composition similar to that of meteors. The connection is not fortuitous; for many years astronomers recognized that those ephemeral streaks of light in the night sky, the meteors, were fragile cometary debris plunging through the earth’s atmosphere. As the gases boil out of a cometary nucleus, myriads of dirty, dusty fragments are lost in space. In time, they can be distributed throughout a comet’s entire orbit, and if that path comes close to the earth’s own trajectory, a meteor shower results.

The Leonid meteors are a splendid example of “falling stars” closely related to a comet. A meteor swarm follows close to Comet TempelTuttle. Every thirty-three years, as the comet nears the earth’s orbit, a particularly good display of Leonids appears around November 16. The recovery of this same comet in 1965 was followed by a November shower in which hundreds of brilliant meteors flashed through the sky within a period of a few hours. Nonetheless, the 1965 Leonids provided a sparse show compared with the hundreds of thousands seen in 1833 and 1866. In 1899, astronomers predicted yet another fireworks spectacular. The prognostication proved to be a great fiasco, for gravitational attraction from the planet Jupiter had slightly shifted the orbit of the comet and its associated meteor swarm. Ever since, astronomers have been wary of alerting the public to meteors or comets. Our enthusiasm in predicting the greatness of Comet Ikeya-Seki on October 21 was indeed risky.

Nevertheless, the daylight apparition of Comet Ikeya-Seki was but a prelude to a more spectacular show. Its surface thoroughly heated by its passage through the solar corona, the comet developed a surrounding coma of gas and dust some thousands of miles in diameter as it left the sun. As it slowed its course and receded from the hearth of our planetary system, the solar wind drove particles from that coma into a long stream preceding the comet.

As soon as Comet Ikeya-Seki could once again be seen in the early morning sky, its long twisted tail caused a sensation. Standing like a wispy searchlight beam above the eastern horizon, the tail could be traced for at least twenty-five degrees. Its maximum length corresponded to 70 million miles, ranking it as the fourth longest ever recorded. Only the great comets of 1843, 1680, and 1811 had tails stretching farther through space. (Quite a few comets have spanned greater arcs of the sky because they were much closer to the earth. Their actual lengths in space could not compare with that of the Great Comet of 1965.) At its peak brightness, Comet Ikeya-Seki was about equal to the sun-grazers of 1843 and 1882. Even after it receded from the sun, its nucleus shone brilliantly through the morning twilight. By all accounts, Comet Ikeya-Seki compared favorably with the great comets of the past. Those portentous sights, compared to giant swords by many a bygone observer, had little competition from city lights, smog, and horizon-blocking apartment buildings.

Comet Ikeya-Seki surprised most astronomers by developing a strikingly brilliant tail on its outward path from the sun, especially when compared with the poor show on its incoming trajectory. Had they looked in Book III of Newton’s Principia, however, they would have seen another sungrazing comet neatly diagrammed with a short, stubby tail before perihelion passage and the great flowing streamlike tail afterward. Newton spent many pages describing that Great Comet of 1680. Especially interesting to American readers is the generous sprinkling of observations reported from New England and “at the river Patuxent, near Hunting Creek, in Maryland, in the confines of Virginia.”

In the new world not only astronomers were interested in the comet. From the Massachusetts pulpit of Increase Mather came the warning,

As for the SIGN in Heaven now appearing, what Calamityes may be portended thereby? . . . As Vespasian the Emperour, when There was a long hairy Comet seen, he did but deride at it, and make a Joke of it, saying, That it concerned the Parthians that wore long hair, and not him, who was bald: but within a Year, Vespasian himself (and not the Parthian) dyed. There is no doubt to be made of it, but that God by this Blazing-star is speaking to other Places, and not to New England onely. And it may be, He is declaring to the generation of hairy Scalps, who go on still in their Trespasses, that the day of Calamity is at hand.

Superstitions concerning comets reached their highest development and received their sharpest attacks at this time. For centuries comets had been considered fearsome omens of bloody catastrophe, and Increase Mather must have been among the great majority who considered the Comet of 1680 as a symbol fraught with dark meanings. The terrors of the superstitious were compounded when a report came that a hen had laid an egg marked with a comet. Pamphlets were circulated in France and Germany with wood blocks of the comet, the hen, and the egg. Even the French Academy of Sciences felt obliged to comment:

Last Monday night, about eight o’clock, a hen which had never before laid an egg, after having cackled in an extraordinarily loud manner, laid an egg of an uncommon size. It was not marked with a comet as many have believed, but with several stars as our engraving indicates.

In a further analysis of this comet, Newton’s Principia reported that a remarkable comet had appeared four times at equal intervals of 575 years beginning with the month of September in the year Julius Caesar was killed. Newton and his colleague Halley believed that the Great Comet of 1680 had been the same one as seen in 1106, 531, and in 44 B.C. This conclusion was in fact false, and the Great Comet of 1680 had a much longer period. Within a few years, however, Halley correctly analyzed the periodicity of the famous comet that now bears his name.

Is Comet Ikeya-Seki periodic like Halley’s? If so, can it be identified with any of the previous sun-grazers? The resemblance of Comet IkeyaSeki to Comet 1882 II has led many people to suppose that these objects were identical. The orbits of both of these comets take the form of greatly elongated ellipses, extending away from the sun in virtually identical directions. Nevertheless, even the earliest orbit calculations scuttled the possibility that the comets were one and the same, since at least several hundred years must have passed since Comet Ikeya-Seki made a previous appearance in the inner realms of the solar system. On the other hand, it is unlikely that Comet Ikeya-Seki, Comet 1882 II, and a half dozen others would share the same celestial traffic pattern and remain unrelated. The only reasonable explanation is to suppose that some single giant comet must have fissioned into many parts hundreds of years ago.

Indeed, the Great Comet of 1882 did just that. Before perihelion passage, it showed a single nucleus; a few weeks afterward, astronomers detected four parts, which gradually separated along the line of the orbit. The periods for the individual pieces are calculated as 671, 772, 875, and 955 years. Consequently, this comet will return as four great comets, about a century apart.

It was, therefore, not at all unexpected when the Central Bureau was able to relay the message on November 5 that Comet Ikeya-Seki had likewise broken into pieces. The first report suggested the possibility of three fragments, but later observers were able to pinpoint only two. One of these was almost starlike, the other fuzzy and diffuse. Though first observed two weeks after perihelion passage, the breakup was probably caused by unequal heating of the icy comet as it neared the sun.

If the Great Comet of 1965 was itself merely a fragment, what a superb sight the original sungrazer must have been. Appearances of comets with known orbits total 870, beginning with Halley’s in 240 B.C., but the earliest known sun-grazer of this family is the Comet of 1668. In medieval chronicles and Chinese annals, and on cuneiform tablets, hundreds of other comets have been recorded, but the observations are inadequate for orbit determinations. Undoubtedly, that original superspectacular sun-grazer was observed, but whether it was recorded and whether such records can be found and interpreted are at present unanswerable questions.

A similar search of historical records, which holds more promise of success, is now under way at the Smithsonian Astrophysical Observatory. The comet with the shortest known period, Encke, cycles around the sun every three and a third years. Inexorably, each close approach to the sun further erodes Comet Encke. The size of its snowball has never been directly observed, but a shrewd guess based on the known excrescence of gaseous material places it in the order of a few miles. By calculating ahead, Professor Whipple has predicted the final demise of Comet Encke in the last decade of this century. By calculating backward in time, he has concluded that it might once have been a brilliant object. Its three-and-athird-year period would bring a close approach to the earth every third revolution, so that a spectacular comet might appear in the records at ten-year intervals. In the centuries before Christ, the Chinese and Babylonian records show remarkable agreement, but the register is too sketchy, and so far, Comet Encke’s appearances in antiquity have not been identified.

In addition to Encke there are nearly 100 comets whose periods are less than 200 years. Like Comet Encke, they face a slow death, giving up more of their substance on each perihelion passage. On an astronomical time scale, the solar system’s corps of short-period comets would be rapidly depleted if a fresh supply were unavailable. On the other hand, there is apparently an unlimited abundance of long-period comets that spend most of their lifetime far beyond the planetary system. Astronomers now envision an extensive cloud of hundreds of thousands of comets encircling the sun at distances well beyond Pluto. Originally there may only have been a ring of cometary material lying in the same plane as the earth’s orbit — the leftover flotsam from the solar system’s primordial times. Perhaps the density of material was insufficient to coalesce into planetary objects, or perhaps at those great distances from the sun the snowballs were too cold to stick together easily.

Gravitational attractions from passing stars presumably threw many of the comets out of their original orbits into the present cometary cloud. These gravitational perturbations still continue, and a few comets from the cloud reach the earth’s orbit every year. Their appearances are entirely unexpected, and their discoveries are fair game for professional and amateur alike. But since most professional astronomers are busily engaged in more reliable pursuits, persistent amateurs manage to catch the majority of bright long-period comets. Devotees such as Ikeya and Seki have spent literally hundreds of hours sweeping the sky with their telescopes in the hope of catching a small nebulous wisp that might be a new comet. The great sun-grazer was the third cometary find for each man. Within a week of its discovery, a British schoolteacher, G. E. D. Alcock, also found a new comet — his fourth. Alcock started his comet-finding career in 1959 by uncovering two new comets within a few days.

How does an amateur, or a professional, recognize a new comet when he finds one? Most newfound comets are as diffuse and formless as a squashed star, completely devoid of any tail. In this respect they resemble hundreds of faint nebulae that speckle the sky, with this difference: nebulae are fixed, but a comet will inevitably move. Consequently, a second observation made a few hours later will generally reveal a motion if the nebulous wisp is indeed a comet. However, most comet hunters compare the position of their suspected comet with a sky map that charts faint nebulae and clusters. Then the discovery is quickly reported to a nearby observatory or directly to the Central Bureau.

Today the chief reward for a comet find lies in the tradition of attaching the discoverer’s name to the object, but in times past there have been other compensations. Jean Louis Pons, who discovered thirty-seven comets during the first quarter of the nineteenth century, rose from observatory doorkeeper to observatory director largely as a result of his international reputation for comet finding. And the Tennessee astronomer E. E. Barnard paid for his Nashville house with cash awards offered by a wealthy patron of astronomy for comet discoveries in the 1880s. Barnard has recorded a remarkable incident relating to the great sun-grazing comet of 1882:

My thoughts must have run strongly on comets during that time, for one night when thoroughly worn out I set my alarm clock and lay down for a short sleep. Possibly it was the noise of the clock that set my wits to work, or perhaps it was the presence of that wonderful comet which was then gracing the morning skies, or perhaps, it was the worry over the mortgage in the hopes of finding another comet or two to wipe it out. Whatever the cause, I had a most wonderful dream. I thought I was looking at the sky which was filled with comets, long-tailed and short-tailed and with no tails at all. It was a marvelous sight, and I had just begun to gather in the crop when the alarm clock went off and the blessed vision of comets vanished. I took my telescope out in the yard and began sweeping the heavens to the southwest of the Great Comet in the search for comets. Presently I ran upon a very cometary-looking object where there was no known nebula. Looking more carefully I saw several others in the field of view. Moving the telescope about I found that there must have been ten or fifteen comets at this point within the space of a few degrees. Before dawn killed them out I located six or eight of them.

Undoubtedly Barnard’s observations referred to ephemeral fragments disrupted from the Comet 1882 II then in view.

A great majority of the comets reaching the earth’s orbit go back to the vast comet cloud, never to be identified again. Occasionally, however, a comet swings so close to the great planet Jupiter that its orbit is bent, and it is “captured” into a much shorter period. A “Jupiter capture” has never been directly observed, because most comets are still too faint when they reach Jupiter’s orbit. Nevertheless, about a year ago, astronomers came almost as close as they ever will to witnessing the aftermath of this remarkable phenomenon.

In January, 1965, the press reported the discovery of two new comets by the Chinese, a rather unexpected claim inasmuch as it has been centuries since the Chinese discovered even one comet, not to mention two. To everyone’s astonishment a pair of telegrams eventually reached our Central Bureau via England, confirming the existence of the objects. At the same time, the Chinese managed to flout the centuries-old tradition of naming comets after their discoverer. In the absence of the discoverer’s name, our bureau assigned to both comets the label Tsuchinshan, which translated means “Purple Mountain Observatory.”

Tsuchinshan 1 and Tsuchinshan 2 have remarkably similar orbits, whose greatest distances from the sun fall near the orbit of Jupiter. As these faint comets swung around that distant point in 1961, Jupiter was passing in close proximity. Quite possibly the gravitational attraction from Jupiter secured the capture of a long-period comet in that year, simultaneously disrupting it into the two Tsuchinshan fragments. However, it is more likely that the capture occurred at a somewhat earlier pass, a point that will eventually be established by a computer investigation. In any event, the observation of a comet pair with such a close approach to Jupiter is without precedence in the annals of comet history.

The complete roster of comets for 1965 included not only the Tsuchinshan pair, Comet Alcock, and the once-in-thirty-three-years visit of TempelTuttle, but the recoveries of four other faint periodic comets and another new one, Comet Klemola, which was accidently picked up during a search for faint satellites of Saturn. Of this rich harvest, Comet Ikeya-Seki received more attention than all the others combined. Day after day, the Smithsonian observing stations around the world kept a continual photographic watch as the long twisted tail developed and faded. These thousands of frames — an all-time pictorial record — may eventually be combined in a film to illustrate in motion the details of cometary tail formation.

By now the Great Comet of 1965 has faded beyond the range of either Ikeya’s or Seki’s small telescope, and has apparently vanished from the larger instruments of professional astronomers as well. Perhaps in a millennium hence an unsuspecting amateur, never imagining that he has caught a sun-grazer, will find it on its next return.

“When discovered, the comet was only a white spot in the moonlit sky,” Seki recently wrote to us. “I did not even dream that it would later come so close to the sun and become so famous.”