The spring of 1993 was a wet one in southern California - good for wild flowers, but lousy for watching comets. 'It was a terrible time up there on the mountain,' recalls Carolyn Shoemaker, who regularly scans the sky from the observatory on Mount Palomar with husband Gene and amateur astronomer David Levy. 'In January we had one good night. In February, one good hour.'

In March, the team got off to a better start with a gloriously cloud-free night. But then Gene developed the evening's first photographs of the sky, and shocked them all. They were totally black, ruined. Somebody had opened the box of film, and unwittingly exposed it to the light. 'Our hearts just sank,' says Carolyn. 'It was the only film we had. We could see the whole night going down the drain - and it might be our only good night that month.'

Today, however, they would drink the culprit's good health. If that box of film hadn't been to hand the following night - when the weather was bad again and they didn't want to risk wasting good film on the sky - they might not have pulled out a few sheets that were only marginally damaged, and photographed the spectacular Periodic Comet Shoemaker-Levy 9 1993e on course for Jupiter.

Carolyn smiles when she recalls how it all came about. Since she took up astronomy more than a decade ago, she's been helping her husband to track down and plot the Solar System's asteroids (the belt of rocky satellites between Mars and Jupiter), searching for ones with unusual orbits that could hit a planet, Earth included. 'Old Eagle-Eye', as Gene calls her, scans the photographs they take at Palomar, trying to distinguish, amid myriad celestial distractions, the unexpected sparkle of the Sun's reflection from an unknown asteroid, which would appear on the developed negative as a black smudge.

At the same time, Carolyn is hunting for comets, the fast-moving clumps of dust and ice that stream across the Solar System on elongated orbits, their surfaces vaporizing as they near the Sun and reflecting its light. In 1983, she chalked up her first comet - Shoemaker 1983p. Carolyn now has 32 comets to her name, more than any other astronomer, living or dead. Gene, who works with her most of the time, has 29 to his.

What's her secret? In part, it's the telescope. The old 18-inch Schmidt at Palomar may be useless for scrutinizing deep space, the preserve of the observatory's main 200-inch Hale telescope, but it is ideal for surveying a wide swath of sky. In part, it's sheer perseverance - the Shoemakers watch the sky seven nights a month, year in, year out - with Levy for half the time.

For Gene, however, it's his wife's innate skills. 'Carolyn is a natural observer, she has a natural gift,' he says. 'With those films, you're literally looking for needles in haystacks - you're picking out images in fields that have tens of thousands of stars, galaxies - and you have to do it fast, because you cover a lot of sky. It's an athletic feat.'
 

Building blocks

It's a feat with a purpose, too. 'The Solar System is more than the planets,' says Brian Marsden, director of the International Astronomical Union's Minor Planet Center in Cambridge, Massachusetts, which verifies and catalogues newly discovered comets and asteroids. 'These smaller bodies are the building blocks. By studying them, we can learn about the evolution of the Solar System. And as we see, they sometimes do interesting things, like crash into Jupiter.'

Marsden's views are shared by Louis Friedman, executive director of the Planetary Society, in Pasadena, California.

'We've begun to understand that comets have had - if I may use a pun- - a huge impact on the evolution of the terrestrial planets,' says Friedman. 'I think that coordinated surveys are very important, and the Shoemakers have been leaders in this whole area.'

Gene, a geologist, has devoted his life to understanding asteroids and comets and how they created so many of the pocks and pimples on the planets. Though he recently retired from the US Geological Survey, he still has an office at the organization's field station at Flagstaff, Arizona, which he shares with his wife. And, aptly enough, the office looks as if a meteorite had hit it, with every available surface strewn with paper.

Carolyn has never been on the staff of the USGS, though she now holds a lecturer's post at Northern Arizona University and is a staff scientist at the Lowell Observatory, both in Flagstaff. Before developing her interest in astronomy, she was at home raising the couple's three children. Before that, she taught social sciences and English at high school after graduating in history and political science.

'The idea of keeping house and doing volunteer work after they had grown did not appeal to me,' she says. 'I needed something that absorbed me and interested me as much as geology did Gene.'

Discovering comets certainly seems to fit the bill. 'Each one is exciting and has its own thrills,' she says. 'You just get this real feeling of joy whenever you discover one.'

Her favorite, not surprisingly given all the excitement, is the comet bound for Jupiter, the one that would have got away but for that exposed film. Two sheets from the box, which were damaged only round the edges, captured the comet in action.

'It was terribly exciting,' Carolyn recalls. 'I had just about abandoned all hope of finding anything. Then I came across this very strange-looking object. I thought it had to be a comet, but it was the strangest comet I had ever seen because it was bar-shaped. I turned to the others and said: 'I don't know what this is. It looks like a squashed comet.' We were all sort of stunned.'

The unusual shape, it transpired, was due to the fact that many little comets, not one, were traveling through space together, strung out in a line like a glistening string of pearls. Other, more powerful telescopes turned their sights on Carolyn's find, and discovered 21 fragments in all. And the path of the comet became clear as more positions were plotted. It was orbiting Jupiter, hijacked (no one knows when) by the huge planet's gravitational forces from a larger orbit around the Sun. Working back from the orbits of the fragments, Marsden calculated that it was probably one large comet until these forces ripped it asunder in July 1992. Most exciting of all, the comet was now on a collision course with the giant planet.

As they wait for the fireworks, Carolyn and Gene press on with their regular work. Each month, they load up their car and head off through the Arizona and Nevada deserts, only stopping for a spot of dinner and dancing on their way to Palomar. They take tranquillizers each morning at the telescope so that they can sleep during the day and work each night; back in Flagstaff, their body clocks slide back to normal again.

When they work at Palomar, everyone with them pitches in. Sometimes it is Levy (who has 21 comets to his name, 13 found with the Shoemakers), and sometimes it is Henry Holt, a retired USGS geologist. 'I may be the first one to see something new, but Gene, David and Henry are the discoverers, too,' insists Carolyn. She says it is mostly they who keep the telescope centered on a solitary star as a reference during each 8-minute exposure, change the films, set the clocks and develop the films. It is they, she adds, who shoot swath after swath of the sky, making sure to double back and retake each one 45 minutes later.

Then comes Carolyn's main job. She must spend hours hunched over a microscope peering at film in an office at Palomar or in a dark, curtained corner of a room in the USGS buildings at Flagstaff. Methodically, she marches the films back and forth under the lens, searching for the telltale signature of an asteroid or a comet, which she marks with red ink. A clear winter night can yield 60 films, and even Carolyn takes 20 minutes to scan each pair, which gives her 10 hours of work at least.
 

Spots before your eyes

The key to finding a comet or asteroid, naturally enough, is its movement. Those near enough to Earth to be spotted will visibly glide across the sky in the 45 minutes that elapse between the two exposures of the same patch. So it will be in a different position when the second photograph is taken. When the two films are placed next to each other and viewed under a stereomicroscope - one by the left eye, and the other by the right eye - anything that has moved during the time lapse will appear to 'float' above the backdrop of 'stationary' stars. The farther an object moves, the higher above the stars it floats.

While this may sound straightforward, doing it is quite another matter. Carolyn encouraged me to scan a film to see if I could find anything, and I didn't need to be asked twice. The prospect of discovering Mestel 1994p glued me to the microscope until eventually, there it was; a small, fuzzy blob floating before my eyes. Carolyn sat down to take a look, and gently revealed that I had discovered a speck of dust.

Dust, blemishes in the emulsion, satellites, false images caused by freakish 'sparks' of light in the telescope all complicate Carolyn's job. Then there are strange 'ghost' images made by the sky's brightest objects, such as Sirius and even Jupiter itself, and the much-hated (and much-avoided) Milky Way fields, with all their stars. Inevitably, some asteroids and comets are missed, much to Carolyn's chagrin. 'I think everyone would like to be the best at what they do,' she says, and undoubtedly she wants to be the top comet-finder ever.
 

Record breaker

In the years since she discovered her first comet, she has broken record after record. Though Carolyn can now claim to have discovered more comets than anyone alive, she needs six more to beat the all-time record of Jean-Louis Pons, a 19th-century French amateur astronomer. Pons had only 26 comets named after him, but discovered 11 more that do not bear his name.

Strictly speaking, Shoemaker has already beaten Pons's record if you count the comet heading for Jupiter as 21 mini-comets. Carolyn tried this tack on Marsden at the Minor Planet Center.

'There, I'd be at 52, which would be pretty exciting,' she says, grinning.

'And, after all, each one is on its own separate orbit and is doing its own thing, so it seemed reasonable to me.' But not to Marsden, evidently. 'He just laughed and said 'No'.'

Carolyn has no idea when she might pass Pons's record. Her best haul of comets was seven in a single year, but there's been more than one bad spell when she thought she would never find another comet in her life. In those lean times, the atmosphere is so tense that all she has to do is cough for one of the team to swing round and cry, 'What have you found?'

Then there are the exhilarating moments. In 1988, she was looking for a known asteroid close to the Milky Way and discovered another comet, just in time for her birthday on 24 June. That same year, she and Gene were trying to confirm one of Levy's independent discoveries but, looking at the wrong part of the film, stumbled upon another, brand new comet. The two comets had broken apart 13 000 years ago.

But the biggest thrill of all was last year's first glimpse of Shoemaker-Levy 9. The ensuing excitement is particularly gratifying for Carolyn, given astronomy's remote focus these days.

'Astronomers per se have long since left the Solar System behind - it's become 'old hat',' she says. 'They're interested in galaxies, the edge of the Universe, in black holes, quasars, strange radio signals - lots of fascinating things, but all very distant.'

When she and Gene first went to Palomar, she recalls, astronomers had a hard time understanding why anyone would be interested in asteroids or comets. Now telescopes all over the world will be watching their comet collide with Jupiter.

'We don't know if it is going to go in and just go pfff, that's it, or whether we're going to see something major,' she adds. 'But we hope it will be a grand show. If I'm going to lose a comet, then I want it to go out with fireworks.'