Located at the outer extremes of the solar system is a cloud of material, probably left over from the formation of the solar system itself. This cloud, known as the Oort Cloud, is believed to be the reservoir from which comets emanate. At such vast distances from the Sun, small bodies such as comets preserve some of the original material from which the Sun and planets were formed. Thus a study of comets is important for understanding the history of the solar system.
Every now and then part of the material may break away from the cloud. Under the influence of gravity, it accelerates towards the Sun as a comet. These comets, travelling in parabolic orbits, are known as long-period comets and by definition have orbital periods greater than 200 years, though the actual periods are generally a few thousand years or more. A second source of comets, probably accounting for many of the short-period comets, is the Kuiper Belt, an icy ring of material with its inner boundary just outside the orbit of Neptune and probably extending to about 50 AU.
Occasionally the orbits of comets travelling in the same plane as the planets may be perturbed by the gravitational effects of the major planets, mainly Jupiter, into elliptical orbits. These comets have shorter periods, by definition less than 200 years, and since their orbits are known fairly precisely, their returns can be predicted with some accuracy.
In the cold depths of space, comets are no more than chunks of frozen gases, ices and dust. However, in the vicinity of the Sun the constituents of the nucleus vaporise and the gases and dust form a coma around the nucleus. Under the influence of the solar wind the gas and dust in the coma are swept away to form the tail, which always points away from the Sun. Some very bright comets may also have an ion tail, oriented at a slight angle to the dust tail and bluish in colour.
Comets are brightest when near the Sun and are often visible only in twilight. Every 10 years or so a bright comet with a prominent tail makes its appearance. Comets move at about the same speed as planets and do not shoot across the night sky, contrary to popular belief. Instead, they rise and set along with the stars. Only close observation reveals their slow motion relative to the starry background.
Table 12 lists comets predicted to appear during 2015 and which are predicted to become brighter than about magnitude 12. The table does not, of course, include any new comets which might possibly be discovered during the year. Novice observers should note that comets are notoriously unpredictable and that the predicted brightness in Table 12 is given as a guide only. The magnitude given is the total magnitude of the coma and the brightness is spread out across the whole diameter of the comet. For this reason the comet will appear much fainter than a star of the same magnitude. As a guide, a comet of 10th magnitude would appear about as bright as a star of the 12th magnitude.
For observations of comets to be of scientific value the observer should concentrate on the following: estimates of the total visual magnitude of the comet (preferably made over the entire apparition to allow construction of a light curve), the diameter of the coma, the degree of condensation, estimates of the length and position angle of the tail, and detailed visual descriptions, sketches and photographs. A guide to observing comets can be found on the ASSA website and observations should be submitted to the Comet group of the Shallow Sky Observing Section (p 113).
Fact sheet: Comets
nuclear diameter: 1-40 km (Halley:16 x 8 x 7 km) nuclear mass: 1014-1019g (Halley: 1017-1018g) nuclear mass loss per apparition: ~1 % coma radius: 104-105 km hydrogen cloud radius: 107 km dust tail length: 106-107 km dust tail particle size: 0.1-100 microns dust tail direction: antisolar, becoming curved as dust particles follow independent orbits ion tail length: 106-108 km ion tail direction: antisolar short-period comets: period < 200 years long-period comets: period > 200 years comet discovery rate: ~12 per year number of comets discovered to date: ~4 000 average number of apparitions per year: ~17
For the record
The first recorded comet observation from southern Africa was made on 1652 December 17 by Jan van Riebeeck (see p 104). The first observation of Halley's Comet from southern Africa was made in 1682 September 08 by Governor Simon van der Stel.
Comets up close
Seven comets have been targets of spacecraft missions. Comet 1P/Halley has been studied most often, by Sakigake (1985 Jan 07), Giotto (1986 Mar 14), ICE (1986 Mar 28), Vega 1 (1986 Mar 06), Suisei (1986 Mar 08) and Vega 2 (1986 Mar 09). Comet 9P/Tempel was visited in 2005 January 12 by the Deep Impact mission, and in 2011 February 14 by Stardust. 19P/Borrelly was studied by Deep Space 1 (2001 Sep 22), 21P/Giacobini-Zinner by ICE (1985 Sep 11), 26P/Grigg-Skjellerup by Giotto (1992 Jul 10), 81P/Wild by Stardust (2004 Jan 02), and 103P/Hartley by EPOXI (2010 Nov 04). As of this writing, the Rosetta mission (launched March 2004) is underway to comet 67P/Churyumov-Gerasimenko, carrying the Philae lander. Rosetta is scheduled to enter an orbit around the comet in May 2014 and the lander, with its compliment of 10 science instruments, will be deployed in November.
Table 12. Comets at perihTable 12. Comets at perihelion Comet Date V 15P Finlay 2014 Dec 8 C/2012 K1 PanSTARRS 2014 Aug 9 88P Howell 2015 Apr 6 9 67P Churyumov-Gerasimenko 2015 Aug 13 9 141P Machholz 2015 Aug 25 11 22P Kopff 2015 Oct 25 11 10P Tempel 2015 Nov 14 11 2013 US10 Catalina 2015 Nov 16 6 Key: Date: Perihelion date. V: Estimate of peak magnitude. Source: Tim Cooper, Jonathan Shanklin (BAA Comet Section).