Comet Hale Bopp
Comet Hale Bopp
Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria.

Comet Hale Bopp

The Comet Hale Bopp, pho­tographed on 4 April 1997 from the Jo­hannes Ke­pler Ob­ser­va­to­ry in Linz (Aus­tria). The low pres­sure of the so­lar pho­tons trans­ports the dust par­ti­cles in the co­ma away from the nu­cle­us to form the dust tail.

Comets are a rare celestial spectacle. They only appear in the sky a few times over the course of a human life. And yet anyone who has seen a comet will likely never forget such an astronomical event.

The myth of comets

Comets are called hairy stars, from the Ancient Greek word κόμη (kómē), meaning a head of hair. They are rare, yet impressive, occasionally fascinating phenomena in the sky. Unlike the familiar Moon and the planets passing in front of the band of the Milky Way, comets are characterised by a diffuse point of light from which the tail they are famous for emanates.

So it was without a doubt that a greater significance would be placed on comets. In the history of humanity, they have generally been associated with approaching calamity. Their unpredictable appearances embodied disruption in the divine order of the world. So it is no wonder that our ancestors saw something mystical in comets – harbingers of war and catastrophe, plagues, famine and revolution. They symbolised unpredictability and sinisterness.

This applied to almost every culture, even in the oldest significant document on a comet in Western civilisation, dating back to the twelfth century BC, when Nebuchadnezzar I reigned in Babylon: "When a comet reaches the path of the Sun, Gan-ba will be diminished; an uproar will happen twice." But even in the Age of Enlightenment, the comet’s tail was considered to be a divine rod of discipline in many places, associated with punishment and penance. Today, of course, people tend to have more positive thoughts in the event of the rare, exciting appearance of a comet. The picture that they create in the night sky is special – a sign of the diversity of our cosmic home and of its beauty.

Comets in the ancient world

Astronomy – the observation of the stars – is a science thousands of years old. The first systematic observations of the night sky were carried out by the Chaldeans, who lived in Ancient Babylon in the third millennium BC. They recorded their measurements in cuneiform script on clay tablets. The Roman philosopher Seneca reported that the Chaldean astronomers were very interested in comets – the phenomena that appear unpredictably in the sky.

The Bayeux Tapestry - oldest depiction of Halley’s Comet
The Bayeux Tapestry - oldest depiction of Halley’s Comet.
The tapestry portrays 58 scenes that document the conquest of England by the Normans in 1066. It is likely the oldest depiction of Halley’s Comet, which passed its perihelion at the time of the Battle of Hastings on 14 October. In the following scene, King Harold, vanquished in battle, asks whether he should consider the comet a harbinger of doom.
Credit: ESA / D. Pazos - CC-BY-SA IGO 3.0.

Chinese astronomers were fascinated by comets as well. From observations that date back to the eleventh century BC, they compiled the fi rst almanac of comets, with detailed illustrations, extensive descriptions and amazingly precise measurements.

The question that academics asked over 2000 years ago was primarily: where do comets belong? Are they an atmospheric phenomenon, burning clouds, and so attributable to the Earth? Or are they part of the Cosmos, the domain of the stars and planets? Aristotle (384 - 322 BC) saw no place in the celestial sphere for comets in his geocentric world view, which placed the Earth at the centre of the Universe. He believed that comets were formed by gases that rose from the Earth into the upper atmosphere, where they caught fire due to the Sun’s heat. It was a view that, by virtue of his philosophical authority, endured until the Middle Ages, even though Democritus (c. 460 – c. 370 BC) had been on the right track one century earlier – he thought that comets originated from the domain of the planets.

A new world view

On the Revolutions of the Heavenly Spheres – this book, which Nicolaus Copernicus published in 1543 a few weeks before his death, changed the world. In it, the Canon of Frombork (Frauenburg) in Masuria placed the Sun at the centre of our planetary system as a result of his mathematical calculations, thus providing the base for the heliocentric world view. Since antiquity, and for almost 1800 years, Earth had been considered the 'centre of the Universe'.

This had implications for our view of comets. Tycho Brahe (astronomer from Denmark, 1546 –1601) estimated the distance of the Great Comet of 1577 to the Earth – still without the benefi t of the telescope, but using observational data from across Europe (and so from various angles) instead. The comet and its tail, which extended over a distance of 40 Moon widths in the night sky, had to be at least four times farther from Earth than the Moon. Comets were now unambiguously astronomical objects. Aristotle’s model, in which comets are burning gases emitted by the Earth, became obsolete. Brahe, Kepler, Galileo, Hevelius – the major astronomers of the Renaissance were not just concerned with the new heliocentric view of the world and its signifi cance for planetary orbits; they were no less fascinated by comets. Their paths around the Sun were of a different nature – elongated ellipses, sometimes even open parabolas.

The next scientifi development of fundamental signifi cance came with the invention of the telescope in 1609. On the basis of ever more precise orbital calculations of the planets and, above all, a few comets, Isaac Newton formulated his Law of Gravity in 1687.

Halley's comet

Following the publication of the Theory of Gravity in 1687 – stating that all bodies mutually attract each other – Isaac Newton attempted to calculate the orbit of the Great Comet of 1680. The results met the records precisely. This caused a friend of Newton's, the polymath Edmond Halley (1656–1742), to apply the new Theory of Everything to the orbital data of other comets as well. In doing so, Halley used measurements of a comet he had himself observed in 1682, as well as observations of other comets of the time and records from antiquity. Some of these comets had strikingly similar orbits. He soon had a thought: might the comet that Johannes Kepler saw in 1607 and the one that Bavarian astronomer Peter Apian described in 1531 be one and the same? Everything suggested so.

The Kuiper Airborne Observatory gazed out at Halley's Comet
The Kuiper Airborne Observatory gazed out at Halley's Comet during its last visit to the inner Solar System in 1986.
Several spacecraft were dispatched to take a good look at the visitor. The Kuiper Airborne Observatory gazed out at Halley's Comet from the stratosphere, set against the Milky Way.
Credit: Kuiper Airborne Observatory

This made Edmond Halley the fi rst person to demonstrate that comets return periodically. They come from the depths of the Solar System into the proximity of the Sun and the four inner planets, go around the Sun, and disappear again beyond the orbits of Jupiter and Saturn. Their paths mark out more or less elongated ellipses, although hyperbolas are also possible.

Halley was so convinced by his calculations that he made the assertion that the comet of 1682 would be visible again in either 1758 or 1759. He was right. Halley's Comet – 'mankind's favourite comet' as the famous astronomer Carl Sagan said – appears in the vicinity of the Earth roughly every 76 years, displays its impressive coma, and disappears into the vastness of space again.

The comet's nucleus

Just a few kilometres in size, comet nuclei are relics of the era in which planets formed in the outer regions of the Solar System. They only display their magnificent tails – and therefore become visible – when they approach the Sun. In 1950, Fred Whipple was the first person to hypothesise that a solid nucleus at the front of the comet causes this phenomenon. While he assumed it to be a 'dirty snowball', we now know that dust and organic compounds – carbon and hydrocarbon compounds – are the dominant substances.

Fred Whipple, astronomer at Harvard University
Fred Whipple, astronomer at Harvard University.
At lecturers held during the 1950s, Fred Whipple, an astronomer at Harvard University, passed on to students his impression of comets as 'dirty snowballs', which was certainly a common perception at that time, as researchers believed that comets were conglomerates of ice and carbon compounds.
Credit: Jonathan Blair/Corbis via Getty Images.

Comets contain few frozen volatile constituents, but these are key to the comet's activity. The nucleus generally has numerous pores and cavities, meaning comets have a very low density, comparable to that of cork, for example. Comet nuclei are among the darkest celestial objects that we know, similar to charcoal. They cannot be directly observed from Earth, as they are too small. Space exploration missions have provided most of the information about comets that we have today.

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