8th August 2000: The VLT images the Comet Linear breakup shower

               NEWSALERT: Friday, July 28, 2000 @ 1431 GMT
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Astronomers report that Comet LINEAR, the brightest comet of the 
year, appears to have blown apart. The comet, which at one time was 
predicted to become visible to the naked eye, dramatically faded this 

   Break Up

When NASA's two great observatories, Hubble and Chandra, recently 
observed comet LINEAR astronomers received some abrupt surprises. 
Researchers were able to catch the icy comet in a brief, violent 
outburst when it blew off a piece of its crust, like a cork popping 
off a champagne bottle.

   Spaceflight Now

Comet LINEAR (C/1999 S4)

Ephemeris Calculator for Comet Linear

Comet C/1999 S4 (LINEAR) was discovered on 27 September 1999 by the Massachusetts Institute of Technology/Lincoln Laboratory programme LINEAR (Lincoln Near Earth Asteroid Research), at the White Sands Missile Range in Socorro, New Mexico, USA. The LINEAR programme, which uses a 1m prime-focus Cassegrain telescope to search the sky for comets and asteroids passing close to the Earth, is funded by the United States Air Force. Initially, Comet LINEAR was thought to be a Near-Earth Asteroid, but observations soon showed that it had an unusual motion for an asteroid and a diffuse appearance showing both a coma and tail, typical of a comet.

Comet LINEAR was discovered at a distance of approximately 600 million kilometres from the Earth (more than four times the distance of the Earth from the Sun). Predictions of its anticipated brightness as it approached both the Earth and the Sun indicated that it could become visible with the naked-eye a few days before and after its closest approach to the Earth on 23 July at a distance of about 56 million kilometres (about 1/3 the distance of the Earth from the Sun). The comet's closest approach to the Sun (perihelion) occurs a few days later, on 26 July.

It is believed that Comet LINEAR is a first-time visitor to the inner solar system, having left its `home' in the Oort cloud, on the extremities of the solar system, many millions of years ago. The Oort cloud is a sphere of comets which surrounds the entire solar system stretching roughly half-way to the nearest star, some 15 to 20 million million kilometres from the Sun.

The star chart shows the path of the comet toward the northern horizon against the fixed stars as they will appear around midnight during July. Comet C/1999 S4 (LINEAR) will pass below the blade of the Plough at the time of maximum brightness between 20 and 27 July. It will also be circumpolar ( i.e. it will remain above the horizon all night) during most of July.

Observing Tips

In order to see any astronomical object at its best it almost goes without saying that it is necessary to have a dark, clear sky. This is especially true for faint, diffuse objects such as comets. 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 faint cometary tail.

The advice is to plan ahead and take the trouble to locate a dark viewing site with a clear view towards the northern horizon. Then take whatever opportunity is provided by the weather, whenever there is a forecast of clear skies!

Comet LINEAR is not expected to be an easy naked-eye object, and better views will be obtained with binoculars. These should provide a good view of the cometary head and tail. If the comet is fainter than expected or if there is mist or thin cloud, binoculars will also allow you to `star-hop' to the comet using the star chart above.

Finally, 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 up to about 30 seconds on full aperture may be sufficient to enable this rare visitor from the Oort cloud to be imaged. After swinging once around the Sun, Comet LINEAR is not due to return again until 32,000 AD.

Web Links


Animated Image of Linear

Orbital Elements

Daily Ephemeris


Sky and Telescope

More Images

Suggestions for Further Reading

The Armagh Planetarium (028-3752-4725) has an excellent range of books on 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 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 articles covering most aspects of comets and their likely interaction with the Earth.

Kronk, G.W. Cometography: A Catalog of Comets. Cambridge University Press, 1999. A descriptive catalogue of every comet which has been observed throughout history, from ancient times to the end of the eighteenth century.

Marsden, B.G. IAU Circular No. 7267, 1 October 1999. Announcement of discovery of Comet LINEAR.

Marsden, B.G. IAU Circular No. 7383, 17 March 2000. Predictions for the brightness of Comet LINEAR.

Napier, Bill Nemesis, Headline Press, 1999. Novel revolving around an impact hazard scenario.

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

Olsen, R.J.M. and Pasachoff, J.M. Fire in the Sky. Cambridge University Press, 1998. Paintings, photographs, drawings and other works of art concerning comets, meteors and related phenomena, illustrating the influence of science on art.

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

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

James Drysdale
John McFarland
Brian Williams
Armagh Observatory
22 June 2000

Last Revised: 2010 February 22nd