Eugene M. Shoemaker
Shoemaker, Eugene Merle
SHOEMAKER, EUGENE MERLE
(b. Los Angeles, California, 28 April, 1928; d. near Alice Springs, Australia, 18 July 1997), space science, astrogeology, studies of the populations of comets and asteroids, specifically those that could strike the Earth.
Shoemaker began a new science—the history of the solar system and the often violent interactions among the planets—called astrogeology. His early studies of lunar craters suggested that these features were the result of impacts, and that the crater record could provide an indication of the level of risk by impact to the Earth. These studies led to his training of astronauts for the Apollo program in the 1960s. Shoemaker’s interests then turned from the effects of impact to the objects themselves, the comets and asteroids that do the impacting. This phase of his career reached its climax in July 1994, when the comet he helped to discover, Shoemaker-Levy 9, collided with Jupiter in a cosmic and forceful demonstration of his ideas.
Early Life Gene Shoemaker’s interest in geology began with the gift of a set of marbles from his mother in 1935, when he was seven years old. These small toys contained some unusual stones like agate, and they set him off to his first geological field trips searching his family neighborhood collecting interesting rocks. The next summer Shoemaker traveled with his father to South Dakota’s Black Hills. The boy was so taken with the rose quartz and other minerals in the area that he gathered many samples. By the time the young Shoemaker entered fifth grade, he was being educated in Buffalo, NY, whose Museum of Science had a program that involved evening classes in sciences as diverse as mineralogy and biology. The course even used college-level textbooks. Shoemaker’s group went on field trips to a place south of Buffalo called Eighteen Mile Creek, where Shoemaker reveled in the rich trilobite collections in the Devonian rocks there.
After completing high school in Los Angeles, Shoemaker was accepted at Caltech, where he completed his undergraduate degree and where he also tried his hand as a cheerleader. He went on to Princeton for his PhD and continued with fieldwork with a search for uranium in a Northern Arizona field of old volcanoes called the Hopi Buttes. His earliest major discoveries as a geologist were deposits of uranium in the eroded volcanic vents of those long-extinct volcanoes. But it was the ancient volcanism itself that really captured the young geologist’s attention. The surrounding craters seemed similar to what he had seen in pictures of the Moon. Was it possible, he thought, that lunar craters were the result of a similar process?
In 1948, Shoemaker decided to go to the Moon. His dream began on the evening of 28 April, his twentieth birthday, as the Moon rose in the southeastern sky. A few days past full, it shone beautifully on the Colorado plateau near West Vancoram, where he was helping prepare for the diamond-drilling project that would search for a badly needed supply of uranium ore. As he drove the five miles from West Vacoram to Naturita, at the headquarters of the Vanadium Corporation of America, for his breakfast, he stared at the rising gibbous Moon and decided, then and there, that he wanted to go to the Moon. “I want to be one of the first people on the Moon. Why will we go to the Moon? To explore it, of course! And who is the best person to do that? A geologist, of course!” (quoted in Levy, p. 27). On that particular morning, the planet Jupiter, which would play a critical role in Shoemaker’s later life, shone just three degrees to the west of the Moon.
That same spring Shoemaker met Carolyn Spellman, the sister of his college roommate. Impressed with his passion, Carolyn listened to his explanations of the Earth that filled her with wonder. The couple was married in 1951 and in their early years together, Carolyn often joined Shoemaker in his field work. They were not married long before getting their first view of meteor crater in Arizona, the crater that would mark the first part of Shoemaker’s career. At the end of a long day of fieldwork the young couple drove quickly toward the site, but since they could not afford the admission fee, they drove up an access road on the west side of the crater. They left their jeep and crawled up the hill to the top of the butte. The sun was already setting over the structure, and the long shadows offered a magnificent view. Although they stayed just a few minutes, their first view of the crater was never forgotten.
Shoemaker’s early career was creative and filled with work in the field. The studies that he did on the Hopi Buttes would later prove useful in planning for the Apollo missions. Shoemaker thought that it was a good idea to look for a lunar site, like the Davy catena, that was thought at the time to be a structurally controlled string of volcanic craters. If that was true, then the astronauts might find xenoliths, or rock fragments blown out from beneath the lunar surface that might show the deeper structure of the Moon. The Davy Catena was not selected, but in 1993, the discovery of the chain of comets called Shoemaker-Levy 9 demonstrated that the catena was formed not from volcanoes but from an ancient comet that grazed the atmosphere of the Earth, broke apart, and then went on, in multiple pieces, to strike the Moon.
Two years after the young geologist’s visit to Meteor Crater, Shoemaker considered the formation again. His new research focused attention on small structures called evaporites dug up by an oil company a few dozen miles east of the crater. Evaporites are the signature of salt flats, and they led to a suggestion by geologist Dorsey Hager that the crater was the result of one that collapsed. Although Shoemaker doubted that theory, he decided to return to the crater and try to test the conclusion of a geologist, Grove Karl Gilbert, from several decades earlier. Gilbert’s idea was that the crater was the remains of a volcanic steam explosion. Shoemaker’s work led to an unexpected find: The glass was Coconino sandstone, the same rock that is present in such quantities in the Grand Canyon, but its quartz was fused somehow, the end result of a process that involves temperatures of about 1,500 degrees Celsius, some 300 degrees higher than the hottest lava flow. Meteor Crater could not be the result of volcanism, nor could the dynamics of a collapsing salt dome explain temperatures as hot as this. Shoemaker began to suspect that the only mechanism that could generate this much heat was the explosive impact of an asteroid from space as it struck the Earth.
On their way to the 1960 Geological Congress in Copenhagen, he and his wife stopped to explore Bavaria’s Rieskessel, a 24-kilometer diameter basin surrounding the town of Nördlingen. “I took one look at these rocks with a hand lens” Shoemaker recalled; “and there was no question these were impact rocks!” (Shoemaker, interview with author, 1993). The next morning the couple made their way to the St. George’s Cathedral, which dominated the village. As they looked up at the tall spires, Shoemaker took out his field lens and studied the stone of the cathedral. The building was made of rock that included suevite, a mineral that he believed had been shocked and partially melted by the Ries impact. This was a major discovery, the first find of a major impact on Earth: The object that fell there was not a stadium-sized rock but something the size of a village.
At the Dawn of the Space Age In the early 1960s, Shoemaker joined Gerard Kuiper and Harold Urey to build the scientific part of NASA’s Ranger program, designed as the first unmanned Moon landing. Then in 1963, Shoemaker was offered a highly coveted position as Principal Investigator of Surveyor’s television experiment. Where Ranger was a quick slam into the Moon, Surveyor would landing softly to begin a weeklong exploration. But first the Ranger craft had to show some success. After multiple failures, Ranger 7 was launched to the Moon in 1964. The Atlas launch vehicle performed flawlessly, and the Agena successfully put the Ranger craft into a trajectory to the Moon. For the next three days the group waited as the spacecraft approached 600 miles from the Moon. At that distance the cameras should turn on; cautiously, nervously, an engineer dispatched the signal to turn on the cameras.
“We have video!” came the excited announcement. It was an incredible moment. “Everyone was jumping up and down,” Shoemaker raved. “We didn't even know what the pictures were yet, but that didn't matter. There were pictures!” (Shoemaker, interview with author, 1992). On a very small scale, little Ranger was repeating a kind of episode that the Moon was very familiar with, an impact.
Ranger impacted near a ray from the giant crater Tycho, one of the Moon’s most recent large impact craters. Formed some 100 million years ago, the crater is the remains of an impact event of some long-gone comet or asteroid. As the incoming object struck, lunar rock shot out in all directions, landing again in a series of ray structures that stretch halfway around the Moon. Ranger 7 provided the scientists with a view of innumerable secondary craterlets that range in size from doors to rooms.
After two more successful Rangers, it was time to let the first Surveyor craft loose on the Moon’s prehistoric surface. On 30 May 1966, Surveyor soared aloft aboard an Atlas-Centaur rocket. As the craft approached a critical distance of sixty miles from the Moon’s Oceanus Procellarum, it was still racing along at 6,000 miles per hour. The retrofiring engine turned on exactly as scheduled, slowing the craft to 250 miles per hour. It shut off exactly on schedule. Surveyor was now using an onboard altimeter to check its progress. After the retrorockets fell away, three smaller rockets took over, firing until the craft slowed almost to a stop just thirteen feet above the surface of the Moon. To leave the surface below as pristine as possible, these rockets then shut down. Surveyor fell to a landing near the crater Flamsteed, the strong shock absorbers in its landing legs absorbing much of the shock. The spacecraft was on the Moon, its systems operating perfectly. In Mission control, the Shoemaker team was astounded. “It was the most surprised bunch of people you ever saw!” he recalled that incredible week as he watched the Sun climb over the craft’s landing site. (Shoemaker, interview with author, 1993).
To the Moon Almost seven years after this challenge, the United States was finally ready to send humans to the Moon, and Shoemaker played a major part of that effort. His prime hope, to go personally to the Moon, was dashed when he was diagnosed with Addison’s disease. His dream to go there ended during the summer of 1963, but not his role in the program. In 1965, he was appointed principal investigator for the field geology experiments for Project Apollo. It was a grand challenge, and put him in direct involvement with the astronauts and at the center of one of the most prestigious national scientific efforts in the history of the United States.
As training proceeded, Shoemaker and his team at the U.S. Geological Survey modeled a landing site on the Moon. Standing on the crater-strewn surface was a full-size mockup of the Lunar Excursion Module, or LEM, all arranged so that the geologists could practice good field techniques. “Some of those test pilots were very good observers,” Shoemaker remembers, adding how their flight training and alertness gave them the potential to be ideal field geologists—if they could get sufficient training. Under the direction of Gordon Swann, the astronauts also trained in places as diverse as Meteor Crater, Grand Canyon, and the volcanic islands of Hawaii and Iceland.
On 16 July 1969, three of Shoemaker’s field geology students—Neil Armstrong, Buzz Aldrin, and Michael Collins—waited to begin a field excursion of their own. In a roar of millions of gallons of burning kerosene, the Saturn 5 rocket beneath them surged to life and bore the three men away from Earth. Shoemaker and his wife were at Cape Kennedy watching. Armstrong and Aldrin remained on the surface at Tranquility Base for more than two hours. Armstrong noted big boulders more than two feet across. He thought they were basaltic, and added that they have probably 2 percent white minerals in them, white crystals. Armstrong’s 90-minute moon walk, in Shoemaker’s view, was one of the best of the entire Apollo program: “He saw more stuff, and he made more pertinent observations, in the precious little time he had on the surface, than some of the astronauts who followed him. Armstrong’s descriptions were lucid and accurate.” (Shoemaker, interview with author, 1992).
Even as the astronauts returned from man’s first visit to the Moon, Shoemaker’s interest in NASA’s program was declining. On 8 October 1969, thinking that he was talking off the record to a friendly Caltech audience, he announced his resignation from his work with Project Apollo. It was a sharp rebuke of NASA for considering the astronauts as passengers on a trip to the Moon, their tasks largely limited to setting up and switching on experiments. He believed that NASA wanted to get the men there and back without any mishaps, as cheaply and as safely as possible. His conclusion was that the space agency had no desire to improve Apollo so that the project would teach something about the Moon’s geology. Although he was somewhat correct in his assessment, he admitted later that Apollo’s geological yield was impressive, resulting in annual Apollo lunar science conferences that continued into the twenty-first century.
A Search for Cosmic Threats to Earth Shoemaker rejoined his alma mater, the California Institute of Technology, as a research associate in 1968, and shortly after became chairman of its Division of Geological and Planetary Sciences. During the last years of the 1960s, virtually all of his time was spent, vicariously at least, on the Moon, with the exception of some field trips to Meteor Crater with Caltech undergrads. The chairmanship of the division, however, was not a task that he could do by remote. It would require his full attention, and his moving from Flagstaff to California at least while the institute was in session.
It was during these busy days as chairman that Shoemaker’s research sights made a ninety-degree turn. His goal to study impacts in the solar system would remain the same, but the strategy would shift. Instead of studying the Moon’s record of impacts, he now turned to the impacting objects themselves, the comets and asteroids that threatened every world in the solar system. The beginning of this search for asteroids and comets dates from his seminal 1963 paper on “Interplanetary Correlation of Geologic Time.” The code in which these planetary histories are recorded, Shoemaker noted, consists of bodies of rock and rock debris. This code will be cracked by geologic mapping, for it is the spatial relationship of different bodies of rock that tells the sequence of events.
Shoemaker knew that in order for a statistical history of the Earth’s encounters with these bodies to make sense, many more Earth-approaching asteroids had to be found. He planned to apply for telescope time and conduct the science, but at this early stage he expected to do little
direct observing. It turned out that Caltech’s Palomar Observatory, home of the 200-inch reflector, then the world’s largest telescope, had a small 18-inch diameter Schmidt telescope that was only used occasionally. Shoemaker jumped at the prospect of putting this beautiful instrument to regular use. With funds still remaining from his role in the Apollo project, Shoemaker and his colleague Eleanor Helin drew up a proposal in 1972 for a preliminary search for asteroids in the vicinity of the Earth. At the time, he suspected that about two thousand asteroids two kilometers or larger, could be in orbits that could cross that of the Earth and hence be a threat to this planet, each one packing the wallop of a multimegaton bomb. Shoemaker calculated that by photographing 250 different fields of the sky each year, they would perhaps find some four collision candidates.
Despite slow progress at first, Shoemaker’s team doubled the count of Earth-crossing asteroids observed over the last century in just five years. He brought the tools of the geologist, including a stereomicroscope, to the program’s task of looking for rocks in the sky. The amazing aspect about Shoemaker during this period was that he could divide his time among many different projects and yet remain centered on his favorite theme of impact geology.
The program was immensely successful, especially after 1980, when Carolyn joined it. Her sharp eye led to the program’s many finds: During its run, which ended in June 1996, it yielded hundreds of asteroids, many of them on Earth-crossing orbits, and thirty-two comets. But all those successes paled before the events of March 1993.
Comet Shoemaker-Levy 9 and Jupiter On the partly clouded evening of March 23, Shoemaker, his wife, and David Levy took two photographs of an area of the sky on which, two days later, the comet Shoemaker-Levy 9 was found. At least as far back as 1929, the comet was in orbit about Jupiter. In July 1992 it passed so close to the giant planet that tidal forces tore it apart into twenty-one separate fragments. Two years later, in July 1994 the comet collided with Jupiter, for the first time ever giving people an idea of what happens when a comet hits a planet.
The discovery began a period that utterly dominated Levy’s and the Shoemakers’ lives for the next sixteen months. Carl Sagan paid his greatest tribute to the man whose vision made possible its discovery: “This is a most extraordinary find,” he said at a Colorado meeting in the fall of 1993. “There are those whose idea of a good time is to stay up on cold nights taking pictures. They get enormous pleasure out of it: [This team] has been at this before—Shoemaker-Levy 9 means that this is their ninth periodic comet find‖ .”
If the discovery of a comet that was split into twenty-one pieces because of a close encounter with Jupiter was unusual, the course of that comet after its discovery was extraordinary. On 22 May, the International astronomical Union’s Central Bureau for Astronomical Telegrams announced that a collision of the comet with Jupiter in July 1994 was likely. The events which followed proved the strength of Shoemaker’s ideas about the importance of impacts for a full week on media outlets around the world.
During the week of the comet impacts, Shoemaker was asked to lead a commission designed to set in motion a new era of automated comet and asteroid searches, a set of searches which would completely shut down the photographic technology that had led to his great success. During this period he and Carolyn continued their annual crater surveys they did each July and August in Australia. On 18 July 1997, three years to the day after the largest fragment of Shoemaker-Levy 9 collided with Jupiter, Shoemaker was killed in an auto accident in Australia.
Perhaps the best summary of Shoemaker’s own career comes from a lecture he gave a few weeks before his death, at Queen’s University in Canada. Shoemaker’s lecture was a beautiful summary of his life’s work, and it emphasized his research in cratering on the Moon, in Australia, young features like Meteor Crater and Wabar, and comets. He ended by suggesting that every 26- to 35-million-years, comets in the outer solar system are perturbed, or jostled, when the solar system crosses the densely populated plane of the Milky Way galaxy, and that people are very close to a plane crossing now. The isotope helium-3, once thought to be the product of volcanic eruptions, may actually be of interplanetary origin, having been brought to Earth by dust particles from comets. What caused this periodic flux in the numbers of comets blanketing the sky? Shoemaker suspected that as the solar system pushes its way, like a sine wave, up and down in the galaxy, it encounters the dense plane of this galaxy every thirty million years. The increased gravitational disturbances that are related to the plane crossing may increase the chances that Earth, and the other planets, get struck by comets that have been perturbed into the inner part of the solar system. Ending his presentation with his typical grand flourish, Shoemaker concluded that the study of the impact history of Earth is in reality a way to study the history of the motion of the Earth in the galaxy.
WORKS BY SHOEMAKER
With R. J. Hackman and R. E. Eggleton. “Interplanetary Correlation of Geologic Time.” In Proceedings of the Seventh Annual Meeting of the American Astronautical Society, Dallas, Texas, January 16-18, 1961. Advances in the Astronautical Sciences, vol. 8: New York, Plenum Press, 1963.
Ruth Northcott Lecture, Royal Astronomical Society of Canada, Queen’s University, June 1997.
“Impact Cratering through Geologic Time.” Journal of the Royal Astronomical Society of Canada 92 (1998) 297–309.
Gilbert, Grove Karl. “The Origin of Hypotheses: Illustrated by the Discussion of a Topographic Problem.” Presidential Address, The Geological Society of Washington, March 1895. See also “The Origin of Hypotheses, Illustrated by the Discussion of a Topographic Problem.” S cience, new series, 3 (1896): 1–13.
Levy, David H. Shoemaker by Levy: The Man Who Made an Impact. Princeton, NJ: Princeton University Press, 2000. Contains a complete Shoemaker bibliography.
Marsden, Brian. International Astronomical Union Circulars5800 and 5801, 22 May 1993. Collision of Shoemaker-Levy 9 with Jupiter was probable.
Sagan, Carl. Lecture to the Division of Planetary Sciences, American Astronomical Society, October 1993.
Shoemaker, Eugene Merle
American Astrogeologist 1928-1997
Eugene Merle Shoemaker was instrumental in establishing the discipline of planetary geology. He founded the U.S. Geological Survey's Branch of Astrogeology, which mapped the Moon and prepared astronauts for lunar exploration.
Born in 1928, Shoemaker's early fascination with the Grand Canyon led him to recognize that the powerful tool of stratigraphy could be applied to unraveling the history of the Moon. His research at Meteor Crater in Arizona led to an appreciation of the role of asteroid and comet impacts as a primal and fundamental process in the evolution of planets.
Shoemaker contributed greatly to space science exploration, particularly of the Moon. Although he hungered to become an Apollo astronaut himself, that aspiration was unfulfilled. Shoemaker was part of a leading comet-hunting team that discovered comet Shoemaker-Levy 9 and charted the object's breakup. Pieces of the comet slammed into Jupiter in July 1994—an unprecedented event in the history of astronomical observations. That same year, Shoemaker also led the U.S. Defense Department's Clementine mission, which first detected the possibility of pockets of water ice at the Moon's south pole.
While carrying out research on impact craters in the Australian out-back in 1997, Shoemaker was killed in a car accident. A small vial of the astrogeologist's ashes were scattered on the lunar surface, deposited there by the National Aeronautics and Space Administration's Lunar Prospector spacecraft, which was purposely crashed on the Moon on July 31, 1999, after completing its mission.
see also Apollo (volume 3); Asteroids (volume 2); Jupiter (volume 2).
Levy, David H. Shoemaker: The Man Who Made An Impact. Princeton, NJ: Princeton University Press, 2000.