A HISTORY OF COMETS - PART 1 FROM HARBINGERS OF DOOM TO CELESTIAL WANDERERS As celestial phenomena go, comets are impressive: bright blotches of light with long, beautiful tails, suddenly appearing to blaze across the sky before fading away just as unexpectedly as they arrived. Although we now understand quite a bit about these cosmic passers-by and welcome their arrival to our skies, to ancient astronomers peering at the stars just a few thousand years ago, comets were something entirely alien. Halleys Comet of 1066 represented in the Bayeux Tapestry Credit: By Myrabella (Own work) [Public domain or CC0], via Wikimedia Commons Comets are not one-time visitors. Originating in the icy depths of our Solar System, they travel on elliptical orbits that swing around our Sun and return back, in most cases beyond the orbit of Neptune. These incredibly elongated orbits mean that it can take years – from a few to hundreds or even thousands – for a comet to reappear in our Solar System, throwing up difficult observational hurdles. Ancient stargazers had little information to work with, having only seen a particular comet once in a human lifetime. The seemingly transient and unpredictable behaviour of comets rendered them completely mysterious, as they did not seem to fit in with our ancestors growing understanding of celestial bodies and their motions. Rather than being viewed as beautiful and intriguing, comets were instead regarded as omens of impending doom, a reputation that took many centuries – and the efforts of many astronomers – to overcome. FROM SKY TO STONE Despite their chequered early reputation, humans have been captivated by comets for much of history and have probably been gazing at them longer than any archaeological record can testify. Some archaeologists suggest that prehistoric rock paintings, found in several sites across the globe, may portray comets. Early rock carvings resembling comets have been found in Scotland dating back to the second millennium BC, and a comet-like shape found on rock carvings in Val Camonica, Italy, dates back to the late Iron Age. The first systematic observations of the sky are credited to the Chaldeans, who lived in the region of ancient Babylon, or modern day Iraq. They started practicing astronomy during the third millennium BC, the time of the Bronze Age, and left detailed records on a vast number of clay tablets. While explicit references to comets have to date only been found on Chaldean tablets from the last few centuries BC, indirect sources, such as the Roman philosopher Seneca, report that Chaldean astronomers had long had a keen interest in these unpredictable objects, most likely developing some quite sophisticated ideas about their nature. Chinese astronomers were also fascinated by comets. They kept the most complete body of ancient observations of comets ever found, dating back to the 11th century BC. A Chinese book known as The Mawangdui Silk Texts, an almanac transcribed on silk in the fourth century BC, is the first known illustrated catalogue of comets. This book documents a variety of comets in great detail, commenting on the appearance, path, and peculiar properties of each comet, including the number of tails, for example. But this almanac also included an additional record that has accompanied comet-gazing for a long stretch of history: listed were details of the catastrophe or disaster thought to be associated with each comet, illustrating the dread and fear once caused by these cosmic harbingers of doom. ANCIENT VIEWS Ancient Greek philosophers put forward a number of different ideas to explain the physical nature of comets. To some, comets were celestial bodies just like planets. To others, they were linked to celestial mechanics such as planetary conjunctions, and to yet others they were burning clouds or optical phenomena in the Earths atmosphere. Although far from correct, the latter view was adopted by Aristotle in the fourth century BC, and prevailed for some 2000 years. According to Aristotle, Earth was at the centre of the Universe, with the Moon, Sun, and the other Solar System planets known at the time orbiting it. This view left no room for comets, so Aristotle suggested that they were windy exhalations from the Earth that reached out into our atmosphere. Viewing comets as atmospheric phenomena made it easier to connect their appearance to earthly – and inauspicious – events, and so Aristotles theory provided fertile ground for the fear and superstition that plagued comets for centuries. The only voice that dared contradict this view was that of Seneca, in the first century BC. According to Senecas Naturales Quaestiones, various interpretations of comets as either atmospheric phenomena or celestial bodies had already been developed by Chaldean astronomers. Seneca himself believed that comets were more like planets than Aristotles windy exhalations. He also recognised that their sporadic behaviour would make studying them quite complex, but hoped that future observations and investigations would shed light on their nature. BECOMING MODERN Aristotles view of the heavens dominated almost undisputed throughout the Middle Ages. This situation started to change, however, with the dawn of the Renaissance in Europe, which reignited a scientific interest in studying the natural and physical world. New ideas emerged as astronomers assembled an increasingly larger body of data. In the fifteenth century, Italian scientist Paolo dal Pozzo Toscanelli conducted the first systematic observations of several comets. He recognised the importance of measuring both the orientation of the comets coma – the nebulous envelope surrounding the comets nucleus – and the position of the nucleus. In the sixteenth century, German astronomer Peter Apian and the Italian scientist Girolamo Fracastoro realised that a comets tail always points away from the Sun – a revelation that would be key to correctly interpreting these objects and their physics. Our understanding of comets experienced a dramatic leap forward with the appearance of a Great Comet in 1577. Great Comets are rare and exceptionally bright – so bright that they can be easily spotted by someone just glancing at the sky. The Great Comet of 1577 was as bright as Venus and boasted a tail that stretched over a patch of sky some forty times the diameter of the full Moon. This offered Danish astronomer Tycho Brahe a unique opportunity. STARRY SHIFTS The path of the comet Tycho Brahe saw in 1577, in his hybrid geo/heliocentric model. Image courtesy History of Science Collections, University of Oklahoma Libraries. (available online) [Public domain or CC0], via Wikimedia Commons Brahe, the last great astronomer before the invention of the telescope, decided to try and estimate the distance to this comet by measuring its parallax – a concept related to perspective. One of the methods used to measure distances to landmarks on the Earths surface involves looking at the landmark from two separate locations. Its observed position shifts against even more distant objects in the background, due to a difference in perspective – similar to the apparent movement you see if you hold your finger at arms length and view it alternately through each eye. This shift, called parallax, is larger for closer objects than for more distant ones. Observing the Great Comet of 1577 from Denmark, Brahe noticed that the comets position in the sky differed very little from similar measurements performed by other astronomers across Europe. The difference should have been much larger had the comet been relatively close to Earth, either at or within the orbit of the Moon. From Brahes calculations, the comet appeared to be at least four times farther than the Moon, so he deduced that it must belong to the heavens, not to our atmosphere. This was reflected in the revision of Brahes model of the Universe - a hybrid between the classical geocentric model, and the heliocentric one that had been proposed in 1543 by Polish astronomer Nicolaus Copernicus – to include comets. Finally, the great debate about the nature of comets – atmospheric phenomena or celestial bodies – was settled. But this was only the beginning: a new era of inquiry about comets, planets, and the fundamental physics that governs their motions was about to commence. A HISTORY OF COMETS - PART 2 TESTING GRAVITY: HOW COMETS HELPED TO PROVE NEWTON RIGHT In the seventeenth century, science was thriving across Europe. The concept of a heliocentric Solar System was slowly spreading, bringing with it a reignited curiosity for astronomy and a lessened fear of previously mysterious celestial objects, such as comets. Cometary science was to take many great steps forward in the coming centuries – but first, comets had a vital part to play in developing one of the most fundamental theories in all of physics: Newtons law of universal gravitation. The Copernican system, by Andreas Cellarius. In public domain, via Wikimedia Commons The idea of a heliocentric system, proposed in 1543 by Polish astronomer Nicolaus Copernicus, was supported by the studies and observations of Italian astronomer Galileo Galilei, who first introduced the telescope to the practice of astronomy in 1609, and German astronomer Johannes Kepler, who formulated the laws of planetary motion between 1609 and 1619. This model firmly dismissed many older notions about the heavens; at the same time, the idea of comets being optical phenomena in Earths atmosphere was also abandoned. Although comets had always been viewed as unwelcome omens connected to earthly events and catastrophes, firmly categorising them as celestial bodies unrelated to our planet weakened their reputation as portents of doom, slowly bringing them back into the realm of scientific investigation. ON OPEN-ENDED ORBITS Kepler observed several comets throughout his life, and tried to study their motion. However, he believed comets to be interstellar objects moving along straight lines, and so never fully grasped their dynamics. Some progress on understanding cometary orbits came from Polish astronomer Johannes Hevelius, who observed nine comets and described them in his Cometographia treatise, published in 1668. Hevelius suggested that comets moved on open-ended parabolic orbits, meaning that they would be seen once and never again. This turned out to be partly true. Hevelius was on the right track, but a robust understanding of the orbital motion of comets would emerge only a couple of decades later, courtesy of English scientist Isaac Newton. The Great Comet of 1680 over the Dutch town of Alkmaar, in a painting by Lambert Doomer. Rijksmuseum, Amsterdam. In public domain Newton had been building on the work of Kepler and Galileo since the 1660s, finally announcing the results of his investigation into planetary orbits in 1687. In fact, it was thanks to the encouragement – and economic support – of a friend and colleague that Newton eventually published his Philosophiae Naturalis Principia Mathematica, the pioneering text in which he detailed a theory of universal gravitation. This friend of Newtons was Edmond Halley - an English astronomer whose name is indelibly linked to comets. With his new theory at hand, Newton calculated the orbit of the Great Comet of 1680. His results were in excellent agreement with observations. Shortly after, Halley applied Newtons theory to the study of comets. This work would produce two major results: a striking confirmation of Newtons theory of universal gravitation, and a revolutionary change in our understanding of comets. The Great Comet of 1680, first comet orbit calculated using the theory of universal gravitation, by Sir Isaac Newton. In public domain, via Wikimedia Commons A PREDICTED RETURN Halley decided to use Newtons theory to calculate the orbits for several comets – some of which he had observed during his lifetime, and many more that he had retrieved from ancient records. In the process he noticed something strange: some comets seemed to have remarkably similar orbits. He suspected that one comet in particular, which he saw in 1682, had already made an appearance once before, and was in fact the same one observed by Kepler in 1607 and by Apian in 1531. With this, Halley was the first to suggest that comets may be periodic, moving along very elongated ellipses rather than parabolic paths. He went so far as to forecast the comets return. With an estimated period of about 76 years, Halley expected the comet to be visible again between the years of 1758 and 1759. Halley died before he could view it with his own eyes, but others were able to test and confirm his conjecture and, with it, Newtons theory of gravity. Interest in Halleys prediction spread to continental Europe. French mathematicians Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute spent several months performing detailed calculations on how the comets path would be perturbed by various planets, working out its expected date of return and position in the sky. Astronomers across Europe began looking for it as early as 1757, avidly stargazing until the long-sought-after comet was finally spotted again on 25 December 1758 by Johannes Palitzsch, a farmer from Germany. The sighting of this comet, which has since carried Halleys name, was a proof of Newtons theory of gravity, demonstrating that it can be applied not only to planets but to other celestial bodies too. It also marked one of the greatest triumphs of science: finally we understood how comets slotted into the cosmos, just as Seneca had hoped for many centuries earlier. Comets were no longer so unpredictable. As such, they started to be viewed as harmless, and slowly but surely popular prejudices and superstitions linked to comets began to fade away. IMPACTING SCIENCE Although the nature of comets had been established, the investigation of these objects raised many new questions and opened up new disciplines of study. One avenue of investigation was the role of comets in Earths history. As increasing numbers of fossils were excavated from the depths of our planet, some scientists tried to explain their variety as having been caused by a series of short-lived, catastrophic events such as floods, volcanic eruptions, and cometary impacts. Others advocated a more uniform process, with gradual changes slowly taking place over millennia. Both interpretations eventually converged to form the currently dominating view of Earths history as a slow, gradual process punctuated by occasional natural catastrophic events. Another field of study had its origins in an astronomical catalogue compiled by French astronomer Charles Messier in the 18th century. While scanning the sky in search of comets – and discovering thirteen new ones – Messier made an inventory of over a hundred blurry objects that he dubbed nebulae. Originally intended as an aid for comet hunters such as himself, Messiers catalogue later became a very valuable database for astronomers with diverse research interests – his nebulae turned out to be stellar clusters, star-forming clouds, remnants of exploding stars, and even galaxies beyond our own. Although it seemed like our understanding of comets was becoming more complete, one vital piece was missing from this puzzle – we may have understood more about what comets are, but where do they come from? What prompts these celestial bodies to appear in our skies? These questions were to be answered in the coming centuries by astronomers who revealed much about not only comets, but also about the Solar Systems place in our Galaxy. A HISTORY OF COMETS - PART 3 ON THE ORIGIN OF COMETS In the eighteenth century, cometary science was tackling big questions. Previous years had brought a greater understanding of the properties and nature of comets, and a consensus that these mysterious objects were celestial bodies that, like planets, travelled on very elliptical orbits around the Sun. Now, the debate shifted from nature to origin: where do comets come from, and how did they – and our Solar System – form? A major step forward in the understanding of how comets formed came from German philosopher Immanuel Kant, who wrote his General History of Nature and Theory of the Heavens in 1755, before he turned his attention to mainly philosophical issues. In this early work, Kant suggested that the Sun and its planets formed from an extended, diffuse nebula and that comets, too, originated from this cloud. Kants nebular hypothesis was embraced and developed by French scientist Pierre-Simon Laplace. In 1805 he published his Celestial Mechanics, a seminal work about the Solar System where he also described its formation from the gravitational collapse of a primordial cloud of gas. However, Laplace could not find a place in this work for the highly eccentric orbits of comets, nor the almost random directions from which they seem to appear in the sky. He therefore argued for an interstellar origin, with comets streaking inwards from far beyond our Solar System. This view prevailed until the second half of the nineteenth century, when astronomers discovered that the Sun – and by extension our whole Solar System – moves through our Galaxy. If comets were interstellar objects, we would see an excess of comets coming from the direction of the Suns motion as it moves through interstellar space... but we do not. This spurred Italian astronomer Giovanni Schiaparelli to suggest that comets belong to the Solar System and surround the Sun in an almost uniform cloud – a view that became generally accepted only some decades after it was proposed. SHOWERS IN THE SKY The nineteenth century saw the discovery of many more periodic comets like the famous Halleys Comet. In 1819, German astronomer Johannes Encke realised that four comets that had been observed in the previous few decades might be the same object returning on a periodic orbit. He computed the comets orbit, estimated its period to be 3.3 years, and correctly predicted its return in 1822. Astronomers have referred to it as Enckes Comet ever since. Shortly after, in 1826, Austrian infantry captain Wilhelm von Biela discovered another such comet. With an estimated period of almost seven years, Bielas Comet came back right on schedule in 1832. The comet was observed to split into two pieces during its subsequent return in 1846, and both pieces were observed again for a final time in 1852. The two fragments of Bielas comet observed in 1846, in a drawing by Otto Struve. Credit: University of Cambridge, Institute of Astronomy. The 1852 visit may have been the swansong for Bielas Comet, but this disintegrating body put on a different celestial show some years later. When a meteor shower was observed in 1872, while Earth was passing very close to this comets orbit, astronomers realised that the shooting stars were nothing but debris produced by the comets disintegration. Although a causal link between comets and meteor showers had been suggested a few decades earlier to explain the spectacular Leonid and Perseid meteor showers, the observation of Bielas Comet and its relics provided the first robust proof. As for the physical properties of these wandering bodies, new clues were unveiled during the 1835 return of Halleys Comet, when German astronomer Friedrich Bessel observed streams of vapour emanating from the comets nucleus. In his Physical Theory of Comets, Bessel argued that the force exerted by these jets must modify the orbit of the comet and cause its period to shorten from one orbit to the next – something that had been observed in Enckes Comet a few years earlier. SNAPPING THE SKIES While astronomers were making great progress using telescopes, the arrival of photography in the mid-nineteenth century opened up a new way to study our skies. Perhaps unsurprisingly given their photogenic nature, astronomers quickly applied this new technique to comets. The first comet to be photographed was the Great Comet of 1858, also known as Donatis Comet after Italian astronomer Giovanni Donati who discovered it. A few years later, Donati was also the first astronomer to use spectroscopy to study the composition of a comet. By splitting the light from celestial bodies into its constituent colours through a prism, spectroscopy allows astronomers to investigate the chemical composition of distant and otherwise inaccessible objects. Donati recorded the spectrum of a comet, now known as Comet C/1864 N1, that had been discovered by German astronomer Ernst Wilhelm Tempel in 1864. The spectrum contained three features that are now known to be produced by molecules of diatomic carbon (C2) in the comets coma. Reproduction of one of the original plates of Comet Halley taken on 25 May 1910 at Helwan, Egypt. Courtesy of D. A. Klinglesmith and J. Rahe Further observations around the turn of the twentieth century uncovered more about the chemical makeup of cometary comas, identifying sodium ions and a variety of carbon-, oxygen- and nitrogen-based molecules. The presence of one of these compounds, the highly toxic cyanide, caused widespread panic across the world in the lead-up to the 1910 return of Halleys Comet. Mainstream media announced that Earth would pass through the comets poisonous tail, causing many to fear that the end of the world was approaching. Fortunately, this was one of very few episodes from recent history in which comets were viewed with suspicion or terror. TOWARDS A COMPLETE PICTURE In the first half of the twentieth century astronomers were collecting and studying more high-quality astronomical data than ever before, building up an impressive database. This allowed them to delve into the physical nature and origin of comets in great detail. In 1950, American astronomer Fred Whipple proposed a new model to describe comets. Rather than a loose collection of dust and debris kept together by ice, he suggested that comets have an icy nucleus, consisting primarily of frozen volatiles like water, carbon dioxide, methane, and ammonia, and containing only traces of dust and rock. Whipples dirty snowball model was later confirmed by ground- and space-based observations, although with minor corrections, as the nuclei of comets turned out to be both larger and darker than he had envisioned. Another major leap forward was made in 1950 by Dutch astronomer Johannes Oort, who suggested that the Solar System is surrounded by a huge cloud of dormant comets extending over a thousand times farther than the orbits of Neptune and Pluto. This cloud is gravitationally bound to the Solar System and originated from the same primordial nebula that gave birth to the Sun and the planets. Oort demonstrated that as the Sun moves across the Galaxy, stars passing near the outer boundaries of this cloud may perturb the motion of some of these sleeping comets just enough to modify their orbits and kick them into the inner Solar System. Closer to the Suns heat, ices in the comet nuclei sublimate, creating the spectacular tails that give these objects their distinctive appearance. Although the Oort cloud has never been directly observed, astronomers are quite certain of its existence. It is likely that comets did not form there, though; they probably took shape closer to the Sun and were later expelled outwards as a consequence of repeated interactions with the giant planets. Illustration showing the two main reservoirs of comets in the Solar System: the Kuiper Belt, at a distance of 30 to 50 AU from the Sun, and the Oort Cloud, extending up to 10 000 AU from the Sun. Credit: ESA This cloud is not the only reservoir of comets in the Solar System; as suggested by Dutch astronomer Gerard Kuiper in 1951, most comets with a relatively short period are located in a flattened, ring-like distribution that begins just outside Neptunes orbit. This Kuiper Belt (also known as the Edgeworth-Kuiper Belt) was first observed in 1992, and over a thousand Kuiper Belt Objects have been found there since. With the idea of reservoirs of dormant comets in the distant, mysterious outer reaches of our Solar System, our knowledge of our cosmic neighbourhood was becoming more complete. However, we still had much to learn about the icy bodies lurking within these reservoirs – something that would all change with the arrival of the space age.
Posted on: Mon, 19 Jan 2015 22:39:24 +0000
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