Carolyns comet

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Barnard was twenty years old when he discovered his first comet in 1881; Carolyn Shoemaker was 54 when she discovered her first comet in 1983. Though she doesn't collect a 'cash prize of $200' for each of her discoveries, she has now 32 comets to her name, more than any other astronomer, living or dead (Jean-Louis Pons, a 19th-century amateur astronomer, discovered 37 comets but only 26 bear his name).

After spending 25 years as 'homemaker and mother' to her three children, Carolyn joined her better-known husband Eugene in 1980 in his search for comets and asteroids. Eugene, a geologist who is considered the father of planetary impact geology and has 29 comets to his name, was killed in a car accident in 1997 while hunting for impact craters in outback Australia.

Carolyn shares her most famous comet discovery with her husband and David Levy, an amateur astronomer who has 21 comets to his name, thirteen with Eugene and Carolyn Shoemaker. On 25 March 1993 Carolyn was as usual scanning films of the sky taken the previous night by Eugene and David from the 18-inch Schmidt telescope at Palomar Observatory in California. Unlike the observatory's main 200-inch Hale telescope which can see deep into space, the old Schmidt is an ideal telescope for surveying a wide section of sky.

There is still no 'automatic comet-seeker' to help Carolyn. She spends long hours in a dark room scrutinising pairs of photographs of the same area of sky with a stereomicroscope. The pairs of photographs are taken 45 minutes apart. As the comet will move across the sky in 45 minutes, it will be in a different position on the second film. The stereomicroscope allows Carolyn to see the two films simultaneously - one by the left eye and the other by the right eye. When viewed this way, a comet would appear to 'float' above the flat surface of the fixed stars. But it is not as simple as it sounds: a speck of dust, a satellite or a spark of light in the telescope can make comet-hunting a painstakingly slow sport (Levy considers it a 'competitive sport').

As she was slowly moving the film through the stereo-microscope she saw a blob that looked like 'a squashed comet'. When Eugene and David looked at it, they were surprised. It was the strangest thing any of them had ever seen during all their collective years of comet hunting. They had just discovered the comet of the century.

Following the tradition, the comet was named after its discoverers - Periodic Comet Shoemaker-Levy 9 1993e (it was the ninth comet discovered jointly by the Shoemakers and Levy which travelled around the Sun in a short-period orbit; and the fifth comet discovered in 1993, hence 'e', the fifth letter).

When other astronomers trained their telescopes on this curious find, they noticed that Shoemaker-Levy 9, or S-L 9 for short, was in fact a string of 21 comet fragments stretched out in a trail nearly 200,000 kilometres long. S-L 9 had two unusual features. First, it had 21 separate nuclei like pearls on a necklace; no comet observed had broken into as many pieces. Many comets break up when they come close to the Sun, but they usually break into two or three pieces. Second, comets usually orbit the Sun; S-L 9 was orbiting Jupiter.

Astronomers also calculated that S-L 9 would inevitably crash into Jupiter in July 1994. Carolyn Shoemaker is not used to losing her comets. 'If I am going to lose a comet, then I want it to go out with fireworks', she hoped. That's just what happened: S-L 9 died in an extraordinary cosmic firework, which Carolyn watched from a safe distance of a billion kilometres.

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