THE WANDERING ASTRONOMER

Thursday, December 14, 2023

Here It Comes!

And there on the dark corner of a domestic field that will forever not be England gathered every proton and neutron of that quintessential nuclear family: father, mother and 2.73 children. I, being considerably younger, measurably smaller than now, and cutting as imposing a figure as a dust-sullied papier mache ballerina figurine, represented the 0.73. Like millions of others who were seeking dark meadows far displaced from obscuring light, we were answering the divine summons to behold, admire and be dazzled into a speechless stupor by Halley's Comet. Mind you, we were hardly intrepid pilgrims braving the assailing gales and scorching deserts to reach a mythical shrine. Father grumbled despairingly. Mother sighed resignedly. Brother thought it all perfectly pointless--an interesting attitude from one who collected Bee-Gees paraphernalia. Sister went along but reserved the right to later file a grievance. Only the little one - 16- was nudged and tugged by wonderment.

As nautical twilight slowly dissolved on that negligibly warm evening, I stood awe-struck beneath the unbounded firmament, immersed in that mystical flavor of silence one can only experience when in the company of family: "Gee willikers, it's cold!" (Not verbatim) "Watch your language!" Soon, however, the bickering subsided as aurora's veil swept aside to reveal the panoramic viewfield onto the infinitude. Like opera aficionados anxiously awaiting the arrival of the grumbling Leporello, we (I) held our (my) collective breaths and beheld well, uh, not entirely sure. Was it there? Where was it? Heavens above, had it left already? After a few moments of frantic searching, we found it in all its glorious obscurity: a minuscule teardrop one could occult with an extended thumb and scarcely brighter than Polaris, the 48th brightest star in the sky. After five whole seconds of reverential quiet, I heard, like the fluttering of cherubic wings, "Oh, this is stupid, can we go home?!" "Dad's right."

That couldn't be it! After 76 years! After its stunning 1910 apparition, in which it loomed so large and grand that Earth literally passed through its tail!* After it weaved itself so elegantly through human history: the 164 BCE apparition recorded on a clay, cuneiform Babylonian tablet; the 12 BCE appearance that had loomed large over Rome and had portended the death of Marcus Agrippa. In AD 451, the comet was seen as having caused the defeat of Attila the Hun at the Battle of Chalons. In 1066, it swooped in to cause Harold II's death at the Battle of Hastings. (The comet even became emblazoned on the Bayeux Tapestry that chronicled that war of Norman aggression). The 1222 apparition was said to have induced Genghis Khan to invade and improve Europe. The 1301 appearance that Florentine artist Giotto di Bondone captured in his Adoration of the Magi. Over and over again, it had appeared like some mischievous sprite as capable of wreaking havoc as it was of inspiring masterworks of art.

Halley's Comet as depicted on the Bayeux Tapestry

Of course, nobody knew that this strange 'hairy star' was the same body returning periodically until the eminent British astronomer Edmond Halley (1656-1742), having studied a comet that appeared in 1682 and noting observations of a comet reported by German astronomer Peter Appian (1496-1552) in 1531 and another by none other than Johannes Kepler (1571-1630) in 1607, concluded that this mysterious comet was following a nearly 76-year orbit around the Sun. In his 1705 publication "A Synopsis of the History of Comets," Halley predicted that the comet would return in 1758. Even though Halley did not live to see the comet when it did return toward the end of 1758, he did become its namesake.

It should have been far grander. The experience of beholding it should have been a transcendental experience. It was like preparing to see a monarch and expecting all the while to experience that sacred moment when he clasped your hand and with a subtle wink and nod would suggest that you might someday ascend to his exalted height. Instead, you merely snatched a glimpse of his greying head bobbing amidst a bustling throng like an aging and far-too-distant dandelion in a field of edelweiss.

It turned out that the 1986 apparition of the most famous comet was largely a dud around the world, even in the Southern Hemisphere, where, by a quirk of celestial mechanics and residues of the black magic that still linger about, viewing had been most favorable. Millions had sought it out and millions minus perhaps 100 had found the experience to be a crashing bore. However, Dr. Brian Marsden, from the Harvard-Smithsonian Center for Astrophysics, in an April 13, 1986 article published by the New York Times, tried to offer the crestfallen skywatchers a dollop of consolation by assuring us that the May 2134 apparition of Halley's Comet would be stunning! Yippie... Sort of like trying to cheer up a condemned French aristocrat en route to the guillotine by telling him that at least the tumbrils had new seat warmers.

Lamentably, Halley's Comet didn't remain visible to the naked eye for long. Nor does it ever. Its fleeting apparitions are all a result of Kepler's Second Law of Planetary motion, which states, in effect, that an orbiting body's velocity is related to the distance from the body it orbits. The closer it approaches, the faster it moves.** Halley's comet is furiously fast when near the Sun. During its Feb 1986 perihelion passage, Comet Halley hurtled along at 112,654 kilometers an hour! Yet, when it lumbers along the most distant part of its orbit, its speed diminishes down to the bewildered Sunday motorist rate of 3,270 kilometers an hour. Think of an actor who conceals himself backstage for days and days and days, only to finally skimper along stage long enough to sing "Yes, we have no bananas" before retreating into the back for a prolonged--but in that case much appreciated--hiatus.

That brings us neatly, or clumsily, to the matter at hand. On December 9th, Halley's Comet reached aphelion, its point of greatest distance from the Sun, at about 35.14 Astronomical Units, or slightly more than 35 times farther from the Sun than Earth (3.27 billion miles). Last Saturday Comet Halley lethargically and unceremoniously passed aphelion and just as quietly turned around. Now, Halley's hairy star is moving toward the Sun again after having spent almost four decades traveling away from it. It is gliding slowly but inexorably back toward perihelion, which it is due to attain on July 28, 2061.

Until then, we wait. Presently, it shines at an estimated magnitude of 25.5: 63 million times fainter than the dimmest naked eye star, well beyond the range of almost every telescope. If it is any consolation, there is very little to see: a 9.5 mile x 5 mile agglomeration of pitch dark, dusty, fissure-strewn, pockmarked, cracked ice. As an analogy, think of the time you and your friends dumped out every ice cube into a bowl, added powdered sugar, coffee grounds and milk and let it partially melt before shoving it forcefully back into the freezer in order to create the ultimate iced coffee confection that would earn you millions while cementing your status as an hitherto unheralded genius. The comet nucleus would be similar, sans the sweetness. An astronaut strolling along its surface now would see a midnight black icescape illuminated by a Sun scarcely larger than a star, but 400 times brighter than the full moon! With an albedo of 0.03, Halley's Comet allows only 3% of that incidental light back into space, hence its dimness. That astronaut would notice dips, valleys, tiny summits and could even peer over the edge to see the star-adorned firmament below, or above, depending on your mood. Our courageous space traveler should be wary however. The surface gravity of Halley's Comet is about one 50 millionth that of Earth. He/she could launch into space with an impolitely loud sneeze.

The astronaut wouldn't have to worry about the tails, however. Those will only form when Halley's Comet ventures close enough to the Sun: when the solar winds repel the charged particles to form the straight blue ion tail while the dust liberated from the sublimating ices form the curving white dust tail. Only then would it scorch the receiving heavens with its flaming incandescence and traverse the skies like the chariot of Helios Hyperion. Only then will its tail extend more than a million miles into space and so briefly become larger in breadth than the star it orbits. Halley's Comet will only assume that aspect in 2061. Now, it is merely a week into its 38-year odyssey toward the inner solar system.

How it will be greeted remains the greatest unknown. During its 1986 visitation, Halley's Comet was met by what is now colloquially known as "Halley's Armada": the Soviet Union's Vega 1 and Vega 2 probes; Japan's Suisei and Sakigake craft and the European Space Agency's Giotto (curiously, NASA did not contribute a craft***).

How much grander will be the fleet that welcomes it on its next go-round? How much brighter, more spectacular, more awe-inspiring and soul-elevating its 2061 apparition will prove to be is also uncertain. What isn't uncertain is that the jagged jumble of sooty ice that has been flying around the same orbit for nearly 16,000 years will return in 2061 to the cheers, exultations and delight of billions. Innumerable humans who, for that brief span, will sense in that cometary apparition the spiritual procession of Halley, Kepler, Gengis, Agrippa and Attila: a brief connection to the ever expanding breadth of human history. Of course, as always, we're the ones investing the majesty and mysticism into the celestial sphere. Halley's Comet is merely obeying the laws of physics with nary a care for the elegance of its motions.

We'll be there, either in spirit or corporeal form to join the fray, to celebrate another passage, to watch the robotic craft descend locust-like onto the approaching comet and take our moment under its shadow. But for now, we can simply peer into the depthless hollows of Hydra, where Halley's Comet remains in full flight, but out of sight and say: Here it comes.

*We might smugly delude ourselves into thinking that we are far removed in time from the type of superstitious hysteria that plagued the ancients. Well, in 1910, when astronomers realized that our home world would be slicing along Halley's Comet's tail, panic ensued. (Recall that throughout history comets were often regarded as harbingers of war, plague, famine, disease, pestilence, misery, mayhem, upheaval, dread, death, and despair. And, since war, plague, famine, disease, pestilence, misery, mayhem, upheaval, dread, death and despair were lamentably commonplace, any comet that happened to emerge from the inky black was seen as being the cause of what would have happened, anyway.) In fact, French astronomer Camille (1842-1925) was widely reported as claiming that the cyanogen gas and other toxic substances contained within the tail would "snuff out all life on Earth." (He made no such claim and he strove earnestly, but ultimately in vain, to distance himself from the preposterous notion that had mistakenly been attributed to him.) While most scientists tried to assure the public that no danger would ensue, there was panic buying of gas masks, protective umbrellas and even 'anti-comet' pills which were purported to ward off 'the evil effects' of Halley's Comet. Whether or not these pills, which consisted of sugar, were marketed as just a silly novelty or as genuine protective medicine is unclear. It is clear that some unscrupulous hucksters, as opposed to the morally upstanding hucksters, took full advantage of the widespread anxiety Halley's 1910 apparition engendered in the public.

**Kepler's Second Law is also a consequence of the Conservation of Angular Momentum. Think of how a skater spins more rapidly when she draws her arms in or how the coins turning around one of those inverted vortices revolve at a more rapid rate as they approach the central hole.

***We should remember that the tragic Challenger disaster occurred in January of that year.