Comets

Comets

Although no longer regarded as omens of death and destruction, comets have a fascination all of their own. They appear to come from nowhere, usually move across parts of the sky well removed from the zodiac, and then gradually fade from view. Yet despite this strange behavior most comets, if not all, seem to be regular members of the solar system. They move in elliptical path with the sun at one focus, and therefore accompany the sun and its family of planets on their journey through space.

Comets differ not only in size in appearance, but also in the time they take to orbit the sun. Some have comparatively short periods of revolution. Comet Encke, for example, a small comet discovered in 1818 by the German astronomer Johann F. Encke, orbits the sun once every 3 ½ years. Comet Halley, last seen in 1986, has a much longer period of about 77 years, while comet Mrkos 1957 is thought to travel 28 times farther from the sun than Pluto and to have a period of 13,000 years.

Composition of a comet

A comet is made mostly of ice – water H2o ice, with a little methane (CH4) ice, and some ammonia (NH3) ice. Striking the Earth’s atmosphere, a modest cometary fragment would produce a great radiant fireball and a mighty blast wave, which would burn trees, level forests and be heard around the world. But it might not make much of the crater in the ground. The ices would all be melted during entry. There would be few recognizable pieces of the comet left, perhaps only a smattering of small-grains from the non-icy parts of the cometary nucleus. The Soviet scientist E. Sobotovich has identified a large number of tiny diamonds strewn over the Tunguska site. Such diamonds are already known to exist in meteorites that have survived impact, and that may originate ultimately from comets.

On many a clear night, if you look patiently up at the sky, you will see a solitary meteor blazing briefly overhead. On some nights you can see a shower of meteors, always on the same few days of every year – a natural fireworks display, an entertainment in the heavens. These meteors are made by tiny grains, smaller than a mustard seed. They are less shooting stars than falling fluff. Momentarily brilliant as they enter the Earth’s atmosphere, they are heated and destroyed by friction at a height of about 100 kilometers.

Meteors are the remnants of comets. That meteors and meteorites are connected with the comets was first proposed by Alexander von Humboldt in his broad-gauge popularisation of all of science, published in the years 1845 to 1862, a work called Kosmos. It was reading Humboldt’s earlier work that fired the young Charles Darwin to embark on a career combining geographical exploration and natural history. Shortly thereafter he accepted a position as naturalist aboard the ship H.M.S. Beagle, the event that led to The Origin of Species.

Old comets, heated by repeated passages near the Sun, break up, evaporate and disintegrate. The debris spreads to fill the full cometary orbit. Where that orbit intersects the orbit of the Earth, there is a swarm of meteors waiting for us. Some part of the swarm is always at the same position in the Earth’s orbit, so the meteor shower is always observed on the same day of every year. June 30,, 1908 was the day of the Beta Taurid meteor shower, connected with the orbit of Comet Encke. The Tunguska Event seems to have been caused by a chunk of Comet Encke, a piece substantially larger than the tine fragments that cause those glittering, harmless meteor showers.

Comet tails

Comets now are being discovered at the rate to five or six a year, but most of them are faint, telescopic objects. A really large and bright one is a comparatively rare but most spectacular addition to the night sky. As it moves towards the sun it groves from a faint, fuzzy patch into an object of great glory, throwing out one or most tails that stretch far across the sky as if in honour of the sun. As its swings away from the sun the tail or tails lead the way but shorten, and eventually the entire object fades in obscurity. A comet’s tail can stream to a distance of millions of miles from the brightest part of “head” but always points more or less directly away from the sun.

A large comet has been described as a “bag full of nothing”. The aptness of this was well shown in 1910 and 1986, when the earth passed through the tail of comet Halley. Nothing unusual appeared in the sky, stars shone undimmed through the comet’s tail, and when the head passed between the earth and the sun, not a trace of it could be seen on the sun’s disk.

Halley’s Comet. Photo: Space.com

How the tails are formed

A comet’s trail consists of extremely thin gases (mostly compounds of carbon) and fine dust. It shines by luminescence induced by solar radiation energizing the gases, and also by sunlight reflected by the dust. The material is released by the nucleus, presumably under the heating action of the sun. It then is driven away in the form of a trail, partly by the pressure of the sun’s radiation and more forcibly by the combined action of the “solar wind” and the interplanetary magnetic field. Only the nucleus seems to be at all substantial, and even this, according to one theory, is no more than a collection of metallic and stony-iron particles embedded in the spongy mass of frozen gases.

Since comets are such light affairs they can easily be deflected from their paths by the planets. Massive Jupiter and Saturn are most effective in this respect, and in the past they have caused the period of Comet Halley to vary between 75.5 years and 79.4 years. Preliminary investigations indicate that it should again pass close to the Sun in 2063.

Comet Biela and the Bielids

Some comets actually have been seen to break up: in 1846, for instance, comet Biela broke into two distinct comets which then gradually separated. The twins returned in 1852, but did not reappear on subsequent occasions. Instead, in 1872, when the earth passed through the track of the lost comet, a fine shower of meteors occured. This was repeated at later returns, indicating that the comet had disintegrated. The shower, known as the Andromedes or Bielids, is now an annual event, but the hourly number of meteors has been consistently small for many years.

Comets have always evoked fear and awe and superstition. Image: Black Dark Blue Mosaic Art by © Megan Jorgensen (Elena)