The fiery spectacle made the Shoemakers, the most successful living comet-hunters, even more famous. But Dr Helin, who was first to note the comet five days before them, through the same telescope at the Palomar Observatory, California, has not become a household name.
Comet hunting is an intensely competitive business. Unlike minor planets (asteroids) and planet features such as craters, comets are named after their discoverers, conferring on them something approaching immortality.
And since discoveries are recognised by a first-past-the-post system (the first confirmed report received by the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts), astronomers must report quickly if they want to become immortal.
The Shoemakers (Gene, now retired, a leading authority on meteor craters; Carolyn, a geologist) are quick to report. Dr Helin, despite being a planetary astronomer at Nasa's jet propulsion laboratory at Caltech, is not so quick. Mrs Shoemaker has put her name on 32 comets, Dr Helin on 12.
After Dr Helin had introduced Gene Shoemaker, a former employer of hers, to the Palomar Planet-Crossing Asteroid Survey 21 years ago, he decided that he preferred his wife's object-spotting from photographs to hers. That was 14 years ago. Since then, Dr Helin and the Shoemakers have not been on speaking terms. Their teams are still members of the same Nasa-funded project, but they work on different programmes, hiring the telescope for separate six-day stints.
The day Dr Helin's team took a photograph containing the first image of the comet, 19 March 1993, was not her happiest. For a start, it happened to be deadline day for her proposal to Nasa for further funding, and her envelope containing the application had ended up in the wrong department, where the personnel were on holiday. Distracted, she merely noted the fuzzy patch on the film as a 'curiosity'.
Five days later, the Shoemakers, taking their turn at the 58-year- old, 18in Schmidt telescope, spotted the same fuzzy trail. Within 36 hours, on the strength of the single observation, they had claimed the comet by electronic mail.
Even 36 hours is unusually dilatory for the Shoemakers. But, they explain, the sky had clouded, delaying a second, confirmatory observation needed to detect not only position but direction and velocity. They begged Jim Scotti, of the Spacewatch project in Kitt Peak, Arizona, to use his more powerful telescope. He confirmed the comet and e-mailed the Central Bureau in the early hours of 26 March. Later that day, the bureau's circular named the comet, at the Shoemakers' request, 'Shoemaker-Levy'. David Levy is a long-serving member of the team.
News of Dr Helin's find might never have been published if the bureau's English-born director, Dr Brian Marsden, had not sought confirmation of the comet from her. His 1 April circular lists the position co-ordinates of her three observations of it on 19 and 21 March. A footnote says: 'The Mar. 19 images were actually noted at the time.' Such is the fame that comet Shoemaker-Levy has brought Dr Helin.
She is bitter, particularly as the log-book at the Palomar Observatory, in which astronomers list their activities, is available to everyone using the observatory, including the Shoemakers. Dr Helin's log entries reveal that she photographed the same part of the sky in which, five days later, the Shoemakers also photographed the comet. Why, when desperately in need of confirmation, did the Shoemakers choose not to telephone her? Did they fear it might prompt her to claim the comet as her own discovery?
Mrs Shoemaker told me: 'We didn't even think of telephoning her. We did not expect her to have it (the comet) because she had not reported it. In any case, she would not have been able to see more than us: a more powerful telescope was needed. We do not generally look in the log to see what she has done. We did when we heard she was unhappy, but found nothing significant.'
Dr Helin said: 'I don't think I'll ever get over this. They should have contacted me and asked, 'Will you check your film?' But they were afraid of losing it to me. Instead, they did everything else to get it confirmed. Then it was wow-ee] and all the press coverage. Is that professional?
'All I want is recognition that the comet was discovered on that telescope a few days before them, but they will not give it.'
Mrs Shoemaker retorted: 'In order to discover something, you have to report it. And to report it, you have to recognise it. When Dr Helin first saw the comet she did not recognise it for what it was.'
Caught in the crossfire between competing claims, Dr Marsden of the Central Bureau is looking forward to the triennial meeting of the International Astronomical Union in the Hague next month when the rules for naming comets will be debated. One faction wants to abolish proper names and revert to a code of letters and numbers.
Meanwhile, Dr Marsden is fielding a daily average of 50 items of e-mail, mostly claims of new sightings. In the interests of instant communications, he is expected to make on-the-spot recognitions of discoveries and attributions of names. One of the 'nebulous areas' that irks him is the convention, flouted by the Shoemakers, that comet claimants should make observations on at least two nights. (In the case of asteroids, this 'back-up' rule is firmly upheld by the IAU.) Three of the Shoemakers' last four finds, Dr Marsden said, had been submitted as single observations: 'It annoys me. We expect better things from professionals.'
Mrs Shoemaker said of Mr Scotti, whose observations confirmed Shoemaker-Levy 9: 'We did not want to cut him out. But he could not be the discoverer because we told him where to look.' Dr Helin retorts: 'They gave him a wrong position and a wrong direction. It takes my breath away.'
The liveliest banter between Dr Marsden and the Shoemakers is over the number of names allowed for each comet. The 16-letter limit for asteroid names does not apply. The Shoemakers would be happy to see quadruple names honouring their team. In 1988, they successfully pressed Dr Marsden to allow the 22-letter triple 'Shoemaker- Holt-Rodriguez'. Since then he has dug his heels in: 'The system is unsatisfactory and being abused. Things have got out of hand.'
He still prefers names to numbers because they encourage lone amateurs. Multiples should be abandoned, he believes, unless they include the names of independent confirmers such as Dr Helin and Mr Scotti, who he regards as an 'extended team'. In 1992, he shortened the sought-for 'Shoemaker-Levy-Holt' to 'Shoemaker' on the grounds that independent discoverers might come to light.
In May, he cut out Levy-Spahr (Tim Spahr is a student in the team), again leaving only 'Shoemaker', on the grounds that Mrs Shoemaker alone had discovered the comet on film at the family home in Flagstaff, Arizona, 10 hours' drive from the telescope. 'It's got a bit touchy,' Mrs Shoemaker said. 'We seem to be caught in a bind.'
Dr Marsden finds naming asteroids less contentious: about 1,361 of the total of 6,057, orbiting between Mars and Jupiter, still lack names. As director of the Minor Planet Center, he knows by heart the rules of the IAU's Minor Planets Names Committee: no glorification of politics or the military until a century has elapsed, no tastelessness, no unpronounceability.
Without his connivance, this month's dedication of Minor Planet 3834 as Zappafrank might not have occurred. The original ambition of Dr John Scialli, a psychiatrist of Phoenix, Arizona, was to attach the musician's name to one of the only three planets known to orbit another star, a pulsar in the constellation Virgo. They were discovered in 1992-94 by Alexander Wolszczan, a Pole.
But there is no precedent for assigning names outside the solar system. Moreover, an American national radio contest favoured naming the planets after the Three Stooges. Finally, an electronic message sent to Dr Scialli by one of Mr Wolszczan's lab assistants revealed that the eminent astronomer was not smitten by Zappa, having been observed listening to Deep Purple.
Dr Marsden promised Dr Scialli: 'I will try to find you an asteroid.' The Czech observatory in Klet had the very thing: an unnamed asteroid discovered by a former assistant in 1980. They donated it.
A more straightforward job in planetary nomenclature is that of Joel Russell, cartographer for the US Geological Survey. His copy of the two-volume Index of Women of the World (1970) supplies most of the traditionally female names approved for planet features by the IAU's Venus committee. Among his choices were Simone de Beauvoir and Barbara Hepworth. Vainest nomenclature is that of the International Star Registry: pounds 47.50 a go to name stars, which are not named by any official body.
But it is hard keeping up with the Shoemakers: 16 of their 173 asteroids form a celestial family tree named after themselves; fathers, mother, mother-in-law, sister, brother, aunt, uncle, son, daughters, sons- and daughters-in-law and grand-daughter.
Dictionary of Minor Planet Names by Lutz D Schmadel (Springer-Verlag, 1993).
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