Museum of Freemasonry - Masonic Library
This is another chapter in the great book of Astronomy.

The solar system is a collection of planets, asteroids, comets and assorted dust gasses and rocks, orbiting the Sun, all in much the same plane. It seems likely that most of those gasses and dust are the left-over remnants that did not accrete to grow into planets, asteroids or comets.

Comets have always been items of note and usually of astrological significance in the early days, because they were so little understood. Until the 16th century they were regarded as atmospheric phenomena. In more modern times they have become items of astronomical interest and still not fully understood.

Although comets which are visible to the naked eye are not common, it is thought that out beyond the orbit of Pluto, myriads of comets orbit the Sun in complete anonymity, never coming near enough to be visible. It is even postulated that there is a great cloud of them, known as the Oort Cloud, surrounding the visible portions of the solar system.

The appearance of a great comet is undoubtedly one of the most striking celestial phenomena which can be seen by the naked eye. The fact that the occurrence of comets is generally unpredictable, and that they may suddenly appear as large and prominent objects, only serves to make them more remarkable. Study of their frequency and their orbits suggests that comets in the solar system number many millions.

The vast majority of comets which have been fully studied have closed elliptical orbits and are thus true members of the solar system. Some orbits are so eccentric that they may be treated as parabolic. Calculation shows that those comets with open hyperbolic paths which are escaping from the sun’s influence have been perturbed by the planets (especially Jupiter). Despite the fact that comets have been lost by the solar system, none have been observed to enter it from interstellar space along hyperbolic paths.

The closest point of an orbit to the Sun is called perihelion and the furthest point is called aphelion, terms which will be used often in this talk.

At great distances from the Sun, comets are small, faint indistinct objects, but as they approach volatile materials begin to be vaporized to produce the main head or coma. At times this may become exceptionally large as in the great comet of 1811 for example, where the diameter seemed to be one and a half times the diameter of the Sun. The tail may begin to develop at a considerable distance, as happened with comet Schuster 1976C, which has the greatest known perihelion to date (6.882 A.U., that is, beyond Jupiter), but which nevertheless had a moderate tail. Tails may not only be highly conspicuous, but also exceptionally long; that of the great comet of 1843 (a Sun grazer) had a length of about 3,200,000,000 km.

At times a small point-like nucleus, may be seen in the centre of the comet. Occasionally, multiple nuclei develop. A feature which can only be observed from spacecraft, is a vast hydrogen halo surrounding the visible portions of the comet.

Various spacecraft investigated comet Halley in 1986, and provided one unexpected result; the density was remarkably low, perhaps one quarter that of water ice. Despite the apparent enormous size of the tail and coma, the nucleus itself must be mainly empty space and it has been graphically described as a dirty snowball, consisting of a loose aggregate of ice and dust particles that must have been formed under very cold conditions at a great distance from the Sun. The nucleus is irregular being roughly 15 km by 8 km. That still amounts to nearly one thousand cubic kilometres of snow and grit. It probably gets smaller each pass by the Sun and it has exceptionally low albedo (or reflectivity). Planet Venus, by comparison, has 76% and comet Halley 0.05%, indicating that the surface of the latter is covered in a carbon-rich dust or crust. The dark layer was broached in several places by major jets in which the gas and dust were released to form part of the tail.

Comet Halley’s ices are apparently composed of water, carbon dioxide, carbon monoxide with a little methane. Other elements found were carbon, nitrogen, oxygen, silicon, sulphur, and magnesium, all at solar abundances. The ices give rise to one of the two components seen in the cometary tails, the ionized gas trail whose behaviour is controlled by the solar wind. The solid particles form the generally featureless dust trail.

Cometary particles are known to be responsible for many meteor showers, when the Earth passes through the tail of a comet as it does regularly in its own orbit around the Sun (see separate talk on meteors) but the density of a cometary tail is usually so low, that it is not likely to produce any observable effects.

On very rare occasions collision with the main body of a comet may be expected, and this is almost certainly the explanation for the brilliant fireball and immense explosion which occurred on 30th June, 1908 in the Tunguska area of Siberia. Trees were uprooted forty kilometres away and the pressure wave recorded as far away as the British Isles. Yet no major fragments have ever been found, only microscopic iron and silica particles having been recovered from the soil. These results are consistent with an encounter with the head of a comet largely composed of ices and small solid particles and which exploded in the atmosphere.

The astronomer Edmund Halley wrote a book about comets in 1705 which provided the proof that the laws of astronomical motion detailed in the Principia by Newton, were in fact correct.

A brilliant comet with a tail stretching half-way, or more, across the sky is a glorious spectacle. Not unnaturally the ancients found it frightening and an indication that the gods were angry, so that disasters of all kinds were likely to follow. A bright comet was seen just before the Battle of Hastings in 1066 and is depicted in the famous Bayeaux Tapestry. The real mystery was that nobody knew what comets were or how they moved. They could not be predicted, they would appear without warning, and remain prominent for a few days, weeks or months, before gradually fading away. They did not flash across the sky like a meteor, but their shifts against the starry background were very noticeable from night to night.

Halley was extremely interested in the subject and collected all the recorded observations that had been seen between the years 1337 and 1698. One of these comets was that of 1682, which he had himself, observed. He calculated the various orbits using Newton’s principles and realised that the three comets of; 1531, 1607 and 1682, had moved in almost identical paths. Could it be that they were merely returns of one and the same comet, moving around the Sun in a period of roughly 76 years? Halley believed so and predicted that the comet would again be seen in 1758, after he was dead. His forecast was fulfilled and the comet reappeared on Christmas Day, 1758, when a German amateur spotted it. During 1759 it made a brave display and became known as Halley’s Comet.

Comets may be divided into two types. There are the so-called periodical comets that move around the Sun in elliptical paths; some of these have periods of only a few years, but Halleys is the only bright periodical comet with a period of less than five centuries. The remaining ‘great’ comets that attract general attention move in much more eccentric orbits, so that they only come close to the Sun and Earth after a long interval, a thousand, ten thousand, even a hundred thousand years. This is why you simply cannot know when to expect them.

A comet is not basically self-luminous (although it does reflect a certain amount of light when close to the Sun), and can only be seen when reasonably near to perihelion. Halley was the first to explain the way in which comets move and he provided a splendid confirmation of Newton’s theories.

Kohoutek’s comet of June 1974 was a visual disappointment, as it never became brilliant but it was scientifically valuable being the first examined from space (Skylab). Its period is approximately 75,000 years.

The greatest comet-hunter was Charles Messier (1730-1817) who found 21, himself and also studied those found by other astronomers. Another finder of note was Jean Louis Pons who, in his career, found 36 comets. One of them proved to be of particular interest because, when its orbit was computed, it was found to move around the Sun in only 3.3 years. Working backwards, much as Halley had done, Encke (who had computed the orbit), decided that the comet was identical with those observed in 1786, 1792 and 1805. In fact Pons discovered the same comet, twice. Encke predicted that the comet would return in 1822 and it did so. Since then it has been seen regularly every 3.3 years and the re-appearance in 1964 was the 47th recorded.

When a comet is remote from the Sun its tail does not develop. As the comet nears perihelion, the tail may become very long, and it points, more or less, away from the Sun. When the comet has passed perihelion, it is seen receding from the Sun, tail first. A comet’s tail always points away from the Sun.

The coma is the roughly spherical, temporary atmosphere of gas and dust expelled from a comet’s nucleus as a result of solar heating when the comet is sufficiently close to the Sun. The nucleus and coma are together, known as the comet’s head. The coma, which is typically drawn into a teardrop shape by the solar-wind, contains neutral and ionized gas molecules that shine by fluorescence and reflected sunlight. The coma typically develops at 3.4 A.U. from the Sun (an Astronomical Unit is 93 million miles, the distance from Earth to Sun), and can grow to between 100,000 and 1,000,000 km, reaching its maximum extent just after the comets perihelion passage. The main tails of comets point away from the Sun. Occasionally a short, anti-tail can be seen appearing to be pointing towards the Sun, but it is an effect of perspective. The light of the tail is mainly sunlight reflecting on the dust liberated from the nucleus. The dust in comet tails undergoes a repulsive force from the Sun, identified as solar radiation pressure. The tails have a curve – the locus of the dust particles let loose from the comet as it proceeds in its orbit and pushed back radically from the Sun according to their size and mass. Comets also have a second, straight tail that points at an angle from the Sun. It is not reflected sunlight but has a spectrum of various ions. The Sun has an out-flowing solar wind, composed of high-velocity ions that follow the spiral trajectory of its magnetic field. The solar wind catches ionic material from the comet to make the second, ionic tail.

In the early 1950’s Fred Whipple proposed the ‘dirty snowball’ model for comets, which suggests that the nucleus is made up of solid material (dust, pebbles, boulders) cemented together with ices, such as water, ammonia and methane. In the vacuum of space, the ices sublimate to gas directly from the solid state (by-passing the liquid stage) and are released from the comet as it is warmed on approaching the Sun. The vaporization of the ices lets loose the dust grains and these are propelled from the nucleus by the escaping gasses which expand rapidly at about one AU from the Sun.

With the release of the grains the comet develops its reflective coma and increases in brightness, often in bursts as puffs of dust are released.

Sometimes a crust of dust insulates the ices beneath making sublimation patchy and sometimes the nucleus may fragment as with comet Biela, or the surface evolves into a thick and solid crust, causing the comet’s activity to decrease from one return, to the next, perhaps becoming an extinct comet as Chiron (the satellite, or moon of Pluto), may be.

Comets deviate from the simply computed orbits due to perturbations caused by the giant planets (especially Jupiter that has 318 Earth masses). Other deviations are due to non-gravitational forces such as the rocket effect that occurs when matter is ejected by the expanding gasses, from the comet’s nucleus, in a preferential direction. These perturbations may lead to a slight, or a spectacularly chaotic change in the orbit. The comet may even be captured into temporary (or even permanent) orbit around a planet and make a few revolutions before being ejected into a completely new orbit. In extreme cases it may even be disrupted by a close approach to Jupiter, or may collide with it, as with comet Shoemaker-Levy 9 in 1994.

The source of comets remains speculative. Jan Oort showed in 1950 that many long-period comets have their aphelion more than 20,000 AU from the Sun. This spherical reservoir is now known as the Oort Cloud and is supposed to extend between 20,000 and 200,000 AU. Gerard Kuiper proposed that short-period comets come from another reservoir, a disk of comets beyond Neptune, now known as the Edgeworth-Kuiper belt.

The solar system resulted from the collapse of an inter-stellar cloud, forming a proto-sun, surrounded by a disk of gasses and solids that had not been pulled in by gravity. Comets, like asteroids, were formed as planetisimals from the accretion of gas and dust in this protosolar nebula, and they retain primordial matter, which has remained essentially unprocessed since that time.

The inner four planets, Mercury, Venus, Earth and Mars together with the asteroids are solid and formed closer to the Sun, the gas giants; Jupiter, Saturn, Uranus and Neptune plus the comets were formed at a greater distance from the Sun, out of the lighter materials that was pushed further away from the newly-formed Sun, by solar radiation and the solar wind.

It is likely that cometary collisions with the Earth were much more frequent early in the history of the solar system and played an important part in the history of our planet. Part, though likely not all, of the ocean water could have been brought by comets. As well as water, complex organic molecules could also have acted as seeds for the development of life on Earth.

As William Preston once asked “what is Freemasonry?” and then answered his own question with “the study of science, and the practice of morality.” I hope that you have found this scientific talk on comets interesting and stimulating as there is so much, that we take for granted, to be learned about astronomy.


VW Bro Robert Taylor