Posted December 11, 2001
Carolyn Shoemaker is a 'relative newcomer' to astronomy. Yet she is acknowledged as the most successful 'comet hunter' alive today. Jennifer Laing talks to this remarkable woman, who has stepped out from behind her late husband's shadow, and contributed significantly to our understanding of our solar system.
Astronomer Carolyn Shoemaker is aware of her unique status in astronomy's 'hall of fame'. "The subject of women in astronomy has long been one that [has] interested me. I know of quite a number in planetary science, which is certainly an astronomical variation, but not as many involved in what I think of as 'pure' astronomy."
Some may be surprised to learn that it is Carolyn, rather than her better-known late husband Gene, who has the distinction of being the world's leading and most prolific comet discoverer, work which she only took up in 1980, at the age of 51.
Her work in late adulthood as a 'comet hunter' was beyond Carolyn Shoemaker's imagination as a young girl. "As a child, I was not particularly involved with stargazing and [even] less with astronomy in general."
Neither of Carolyn's parents were scientists, with her father a poultry farmer and her mother a schoolteacher in her native New Mexico. A junior high school teacher who had majored in history and business at Chico State College, now the University of California, Chico, Carolyn Spellman's scientific destiny began when she met her future husband Gene Shoemaker at her brother's wedding in 1950. Shoemaker had been her brother's room-mate at the California Institute of Technology ('Cal Tech'), and was then working on a Ph.D. at Princeton University on metamorphic petrology.
Married at the age of 22, Carolyn then spent the next 25 years as "homemaker and mother" to her three children. Dr. Gene Shoemaker's work led him to play a leading role in organising geological activities for the lunar landings of the late 1960's and early 1970's, as chief scientist at the United States Geological Survey (USGS) Center of Astrogeology. He had wanted to go to the moon himself, but a diagnosis of Addison's disease prevented him from joining the astronaut program. Instead, he had to watch from the sidelines, as Harrison 'Jack' Schmitt become the first geologist to walk on the moon in 1972.
Carolyn Shoemaker's entry into astronomical history appears to have stemmed from something akin to the 'empty-nest syndrome.' She tells the story:
"I came to astronomy at age 51 after my 3 children had left home and when I was looking for something fulfilling to do. I asked my husband, Gene, who could and did spend most of his time working in geology and planetary science, for suggestions. It was he who thought I might be interested in the telescopic search for near-Earth asteroids. I slid gradually into planetary astronomy as a field, while working on his search program and loved it for the opportunity to keep on learning new things each day. Of course, the thrill of discovery of both asteroids and comets gave me a deep satisfaction." Gene's support was an important ingredient. "He always supported me in my efforts."
When Carolyn began her search for near-Earth asteroids and comets, she admits that it was considered an almost eccentric pursuit. The importance of this research, including understanding the potential risks of an impact for the future of life on Earth, was then not readily understood.
"Comets are the wild card when we consider the potential for impact on Earth. We cannot predict the coming of long-period comets well in advance, and it is necessary to learn more if we wish to defend our planet. We need to know a great deal more about their structure - are they solid bodies emitting gas and dust or are they unconsolidated flying snowballs easily broken apart, or are they all shades in between? Are some asteroids really extinct comets in which the action has been shut off? Could comets provide a source of water for space travellers? Did comets bring life to Earth or the nutrients for life?"
The work Carolyn Shoemaker carried out involved studying photographic plates and films taken 45 minutes to an hour apart of the night sky. The technique used a stereomicroscope, allowing the researcher to view two plates or films simultaneously. When one eye looks at one film and the other looks at the second film, the brain 'meshes' or melds the images together. Asteroids and comets appear to 'float' above the flat surface of the stars.
It is slow, methodical work, which requires training to discriminate between dust or grain of the film and near-Earth objects. David Levy, co-discoverer of Comer Shoemaker-Levy 9, has spoken of a typical night's work lasting 13 hours, as well as the hours scanning the pairs of films afterwards. Shoemaker's discovery rate is about 100 search hours per comet find, and, to date, she has found more than 300 asteroids and 32 comets.
She says that she has been successful at finding comets for several reasons. "The Palomar Asteroid and Comet Survey, of which I was a part, used film in photographing the sky and used stereopsis as a method of detection. I have good stereo vision. I worked with others who were also willing to work very hard to obtain the images we needed. Our search was conducted before the days of the large surveys and before the use of CCDs (charge-couple electronic devices) with telescopes. We searched a part of the sky where most comet hunters did not look, a part of the sky ideal for finding asteroids."
Her passion for her work must have also sustained her through the many long nights of tedium and painstaking work, combing through exposed films. "My real love for the night skies developed while observing at Palomar Observatory in California, and that love has never diminished." Carolyn Shoemaker has spoken about her ritual when she finds the latest comet. "I want to dance. I usually go to the staircase and call up to the observers at the telescope and yell, 'Yay-y-y-y-y!'" It's this exuberance of spirit which she is able to impart to the general public in lectures and public presentations around the world, and which may hopefully help to inspire a new generation of comet-hunters and/or astronomers.
In 1988, Carolyn Shoemaker was awarded, along with her husband, the Rittenhouse Medal by the Rittenhouse Astronomical Society for the discovery of comets. She also received an honorary doctorate of science from Northern Arizona University in Flagstaff in 1990, for her work with comets and asteroids.
Possibly her most famous discovery was that of Comet Shoemaker-Levy 9 in March 1993. The Shoemakers and their colleague David Levy found a comet which consisted of a chain of more than 20 pieces, described as looking like a 'string of pearls'. This comet had been pulled into fragments due to the gravitational pull of the planet Jupiter, and was fated to eventually collide with it. This occurred some 16 months later, in July 1994, when Comet Shoemaker-Levy 9 became the first comet to be recorded impacting Jupiter.
Carolyn Shoemaker acknowledges that "the discovery of comet Shoemaker-Levy 9 which impacted Jupiter was doubtless the most satisfying and significant achievement of my life in astronomy."
"Until then, no one had seen a comet so completely disrupted with its fragments all lined up and in orbit about a planet; no one had ever seen the impact upon a planet of a comet or an asteroid. This comet was nature's grand experiment for man to learn about the structure and makeup of comets, impact dynamics, the chemistry of Jupiter's clouds and, most importantly, to gain the awareness that, yes, objects can fall out of the sky and impact planets, including ours. The discovery of 31 other comets and of many near-Earth asteroids was also important as a foundation for our understanding of our solar neighbourhood and the origin of our solar system."
Part of the Shoemakers' research involved looking at the scars of the impacts of near-Earth asteroids, which had not been erased by such events as tectonic activity, and the couple travelled to Australia on a number of occasions to identify features caused by past impacts.
I recently discovered a link between the Shoemakers' work and that of a friend of mine, geologist Professor Vic Gostin, who travelled with Mars Society Australia on their expedition to the Red Centre, to scout for Mars-like environments here on Earth. Professor Gostin of Adelaide University had an asteroid named after him (Asteroid (3640) GOSTIN), which had been discovered by the Shoemakers in 1985. Gostin was accorded this honour due to his work on meteor impact craters in the Flinders Ranges.
The Shoemakers' rich and productive partnership, both from a personal and professional standpoint, ended tragically in 1997 on one of these field trips. The pair were involved in a car crash near Alice Springs, and Gene Shoemaker was killed instantly, while Carolyn sustained severe injuries. She eventually recovered, and continues her observation work today with David Levy and his wife Wendee near Tucson, Arizona.
When asked to rate her own individual contribution and impact on astronomy, Shoemaker is circumspect. "How does one stand back and evaluate themselves? I would like to think that I made an important contribution in planetary astronomy with my pursuit of comets, planet-crossing asteroids, and impact craters and with a role in the education of others as to the importance of these bodies.
Because my work preceded that of all but a small group involved in the search for near-Earth objects, it served to call more attention to them and to provide a base knowledge about the origin of our solar system and its solid bodies."
Jennifer Laing is a freelance science writer from Melbourne, Australia and PR Director of Mars Society Australia.