Comet Hale-Bopp

Comet Hale-Bopp, designated C/1995 O1 (Hale-Bopp), is named after Alan Hale and Thomas Bopp, two amateur astronomers of Cloudcroft, New Mexico, and Glendale, Arizona, who independently discovered it on the night of 22 July 1995 whilst observing the globular cluster M70, in Sagittarius, which happened to be close on the sky. It was immediately realized that the comet was an unusual, possibly exceptional object.

Discovered more than 1,000,000,000 kilometres away, it was further from the Sun than any previous cometary discovery, even further than the planet Jupiter, and its brightness was more than a hundred times that of Comet Halley when at the same distance. At the time of its discovery, its head or coma was more than a million kilometres across (larger than the Sun), and first estimates of its size indicated that the diameter of the nucleus probably exceeded 100 km, more than 10 times that of Comet Halley. The initial orbit even indicated that Hale-Bopp might pass close to the Earth in 1997. Astronomers' eyes bulged!

In the event, although the comet's orbit passes close to that of the Earth in January, the timing of its perihelion passage (the point in its orbit closest to the Sun), is not quite right for a really close approach. When the comet is at its brightest, the Earth is nearly 200 million kilometres away. Nevertheless, the behaviour of Comet Hale-Bopp continues to live up to expectations, and astronomers hope that we may be about to see one of the Great Comets of the century.

Path of Comet Hale-Bopp (from March 1996 to May 1997).

Full size image of orbit

The comet moves in a nearly parabolic orbit inclined almost perpendicular to the plane of the planetary system. It passed close to Jupiter in March/April 1996 and makes its closest approach to the Earth on 22 March 1997 at a distance of 200 million kilometres. Its closest approach to the Sun, known as perihelion, takes place on 1 April 1997 at a distance of 137 million kilometres. Soon after the end of April the comet again crosses the ecliptic plane now heading south, and soon disappears into the glare of the Sun. Meteors from Comet Hale-Bopp may cross the Earth's orbit on 4 January.

The orbit is unusual in passing close to the orbits of both Jupiter and the Earth, and in being inclined at almost 90 degrees to the plane of the planetary system. The comet passed close to Jupiter in March/April 1996, and possibly strong gravitational perturbations by Jupiter during each perihelion passage means that its dynamical evolution is particularly rapid. Despite its relatively short period (it is 4,600 years since its last close approach to the Sun), Comet Hale-Bopp may have experienced only a dozen perihelion passages since it was captured from an initial orbit in the Oort cloud with a period of revolution measured in tens of millions of years.

The Oort cloud is a nearly spherical swarm of comets that surrounds the solar system stretching halfway to the nearest star. Calculations of the comet's long-term evolution indicate that owing to its high inclination Comett Hale-Bopp may finally fall into the Sun, although it will probably not meet this fate for at least another half-million years.

Star Charts

Full size chart for Feb. 1997

Full size chart for 2-Mar. 1997

Full size chart for 30-Mar. 1997

The three star charts show the path of the comet against the stars during January, February, March and April. They have been drawn to show the view from Armagh at a particular time (GMT), but if you are observing from another site, or at a slightly different time, the view will be similar except that the aspect will be slightly different. For example, the view from London is similar to that shown, except that it occurs about half an hour earlier.

The first two charts show the view looking towards the north-east before dawn at 05.00 and 04.00 at the beginning of February and March respectively. These show that the comet lies close to the horizon below the constellation Cygnus, the Swan, sometimes known as the northern cross. Notice how close the comet lies to the horizon, although as February gives way to March it gradually gets brighter and becomes easier to see.

The third chart shows the view looking towards the north-west after sunset during the last week of March and early April. This is the best time to view the comet: it is at its brightest; it is a reasonably social hour; and there will be no significant contamination due to moonlight.

In order to see the comet at its best it almost goes without saying that it is necessary to have a dark, clear sky. Clouds are astronomers' biggest bug-bear, and unless the sky is clear right down to the horizon the view is bound to be limited. A dark sky is an absolute necessity in order to see the full beauty of the cometary head and tail.

The advice is to take whatever chance the weather provides, and to make observing Comet Hale-Bopp a priority whenever clear skies are forecast. Plan ahead, and take the trouble to locate a site with a clear view to the north-east (January to mid-March) or north-west (mid-March to end April). It is well worth driving a short distance to avoid city lights and domestic light pollution, and if you have access to a pair of binocularsá— - use them. Binoculars greatly improve the view compared with the naked eye, and make it easier to 'star-hop' to the comet if it should turn out to be fainter than expected or if there is mist or cloud near the horizon. Allow at least 5-10 minutes for your eyes to become properly dark adapted. For those with cameras, loading it with a fast film, setting it on a firm mount and using an exposure of 10 to 30 seconds on full aperture should be sufficient to take a reasonable picture of the comet.

Further Reading

The Armagh Planetarium (01861-524725) has an excellent range of books on cometary astronomy, and provides a fast mail order service. Details of other suppliers can be found in astronomy magazines such as Astronomy Now, available from newsagents. The following books, which should be available from most libraries, give a good introduction to the field.

Bailey, M.E., Clube, S.V.M. and Napier, W.M. The Origin of Comets. Oxford: Pergamon Press, 1990. Comprehensive history of comets describing all theories of cometary origin and the development of Man's concerns about the cosmos.

Brandt, J.C. and Chapman, R.D. Rendezvous in Space: The Science of Comets. New York: W.H. Freeman & Co., 1992. Popular introduction to comets and the results of the 1986 apparition of Halley's comet.

Gehrels, T. Hazards due to Comets and Asteroids. Tucson: University of Arizona Press, 1994. Comprehensive collection of technical articles covering most aspects of comets and their likely interaction with the Earth.

Niven, L. and Pournell, J. Lucifer's Hammer. London: Futura Publications, 1978. Classic work describing a cometary collision with the Earth.

Spencer, J.R. and Mitton, J. The Great Comet Crash. Cambridge: Cambridge University Press, 1995. Illustrated articles on the collision of Comet Shoemaker-Levy 9 with Jupiter in July 1994.

Yeomans, D. Comets. New York: Wiley Science, 1991. Chronological history of the science and folklore surrounding comets.

Links

Mark E. Bailey
Armagh Observatory
December 1996

Last Revised: 2010 February 22nd