Gene Shoemaker (1928-1997) founded a new branch of planetary science that her termed "astrogeology," which revolutionized our understanding of the solar system and the process of planetary evolution. He was the first to produce conclusive evidence that planet Earth had been bombarded by asteroids throughout its formative history, and that there was a very real likelihood of such an event occurring in the future.
Any doubt that this might be another brand of doomsday speculation vanished in 1994, when people worldwide watched such a cosmic spectacle from the safety of their homes and offices. Television stations broadcast live footage as Comet Shoemaker-Levy 9 torpedoed into the giant planet Jupiter in an incendiary hail of 21 fragments. The largest of these, dubbed "fragment G" struck Jupiter with a force equivalent to six trillion tons of TNT, or about 600 times the entire estimated nuclear arsenal of the world. The impact produced a fireball, rising 3000 km above the marbled clouds of the Jovian atmosphere.
Born April 28, 1928 in Los Angeles, California, Eugene Merle Shoemaker spent part of his childhood in Buffalo, New York. Due to the scarcity of jobs at the time of the Depression, his father had to settle for a job as a physical education instructor at Civilian Conservation Corps camps in Wyoming. His mother taught at Buffalo State Teacher's College. Always a student in a hurry, Shoemaker attended evening classes for high school students at the Buffalo Museum of Science while he was still in elementary school.
His lifelong interest in stones was sparked at the age of seven, when his mother presented him with a set of natural-stone marbles. The family's summer get-togethers brought the young boy in contact with the landscape of the West. There, his growing fascination with rocks led him to gather river stones and spend hours smashing them to look at their inner structure. Later, as he told Astronomy magazine's Rex Graham, "I really got into minerals and gemstones."
After his parents moved back to Los Angeles, Shoemaker completed high school in two years. In 1944, at the age of 16, he enrolled at the California Institute of Technology. He excelled in that university's competitive environment and acquired both his bachelor's degree and master's degree in geology within three years. Close friend and colleague, David Levy, remembers that Shoemaker loved to say "I squirted in and out of Caltech in 2 and 2/3 years."
A self-styled "rock-knocking geologist," Shoemaker began working for the United States Geological Survey (USGS) in the summer of 1948, mapping uranium-bearing formations. These were typically old volcanic vents in western Colorado. While prospecting for uranium, Shoemaker closely studied the structure of volcanic vents, teaching himself in the process to distinguish between craters caused by underground forces and those punched out by an impact from above.
Between projects for the USGS, Shoemaker completed graduate work at Princeton University, earning a Ph.D. for his geological mapping of the mile-wide Meteor Crater outside Flagstaff, Arizona. Daniel Moreau Barringer had proposed in 1906 that the crater had been caused by a meteorite, but had failed to find sufficient evidence to substantiate his theory. Shoemaker closely examined the structure and layers of ejected rock and debris at the crater and compared his observations with those of craters formed by nuclear detonations in the Nevada desert. The similarities were compelling proof that Meteor Crater had indeed been formed by an explosive impact from outer space and not by a volcano, as had been previously believed.
In 1960, along with USGS colleagues Edward Chao and Beth Madsen, Shoemaker isolated and named the uniquely compacted mineral that they found at Meteor Crater "coesite." Coesite is a variant of silica created by the enormous heat and pressure of a nuclear-scale shock wave. Shoemaker had found the equivalent of a DNA fingerprint for extraterrestrial impact. The science of astrogeology was born.
Shoemaker turned his attention to the Ries basin on the western border of Bavaria, north of the German city of Augsburg. The basin is a circular depression 17 miles across. He traveled there in 1961, with his wife and mother. While visiting a church in the town of Nordlingen, which sits in the Ries basin, Shoemaker reportedly scratched its walls to see what it was made of and was delighted to find traces of coesite.
Shoemaker subsequently explored the strange rock formations of the Ries basin for a week, and sent pieces back for analysis to USGS colleague, Edward Chao. All the samples contained coesite. The world's first giant impact crater had been conclusively identified. Since then, geologists have identified more than a hundred impact craters worldwide. Shoemaker also demonstrated that the moon's craters were formed by comet and asteroid collisions, not volcanic eruptions.
In 1973, Shoemaker established the Palomar Planet-Crossing Asteroid Survey together with Caltech geologist, Eleanor Helin. Their observations from 1973 to 1982 revealed three distinct types of earth-crossing objects distinguished by the placement of their orbits relative to that of the Earth. Although about 70 earth-crossing asteroids have been positively identified, Shoemaker estimated that there could be as many as 2,000 of these interstellar vagrants, any one of which might collide with the earth in the indeterminate future.
Shoemaker was one of the earliest proponents of the theory that a catastrophic asteroid or comet impact may possibly have annihilated most of life on Earth 65 million years ago, including the dinosaurs. His findings paved the way for other scientists to find geological evidence of that event, now commonly known as the K/T boundary. In 1980, Luis and Walter Alvarez from the University of California at Berkeley discovered this evidence in the town of Gubbio, Italy. They found an unusual sedimentary layer of gray-red claystone, less than an inch thick The layer sharply separated rocks and beds with fossils from the Cretaceous period, the age of the dinosaurs, and fossils from the later Tertiary period, the age of mammals. This layer has since been identified at more than 70 sites worldwide, strongly suggesting the occurrence of a worldwide cataclysm that produced raging firestorms and a prolonged winter that blocked photosynthesis in plants.
Mesmerized by the hope of being the first geologist to walk on the surface of the moon, Shoemaker was deeply involved in the space program from the earliest days. His dream ended when he was diagnosed in 1962 with Addison's disease, a rare affliction of the adrenal glands. Although it was successfully treated with steroids, the condition disqualified him from becoming an astronaut. Years later, he would watch as Apollo-17 lifted off Cape Canaveral, taking his friend and fellow geologist, Harrison Schmidt, to the moon.
His deep disappointment in no way interfered with the energy and vision that Shoemaker contributed to the Space Agency's efforts to put a human on the moon. During the 1960's, he led teams investigating the structure and history of the moon and in developing geologic mapping methods from telescope images. Shoemaker asserted that astronauts walking on the moon would not sink into its surface. He was proved right by the initial unmanned lunar missions, the Ranger and Surveyor probes. Shoemaker was the investigator in charge of the geological fieldwork carried out during the first Apollo missions, and was also involved in suggesting the landing sites for those missions.
From 1962 to 1975, Shoemaker combined astrogeology research for the USGS with teaching at the California Institute of Technology (Caltech). One of his students at Caltech, Dr. Susan Werner Kieffer, remembered him as being one of the most generous and intellectually honest mentors she had ever known. Shoemaker chaired Caltech's Division of Geological and Planetary Sciences from 1969 to 1972. In an obituary for Science magazine, Kieffer reminisced about Shoemaker's love of field mapping, and about the river-rafting expeditions that he took his students on through the Grand Canyon to teach geological processes. "No planet in the solar system escaped his observant eyes," she wrote, recalling his interpretation of plumes observed on Io and Triton, satellites of the planets Jupiter and Saturn.
Mary Chapman, a colleague at the USGS recalled her conversation with a newcomer who, upon hearing Shoe-maker's laughter and animated conversation at a meeting, asked: "Who is that loud guy?" She replied "That is the god of planetary geology, and we all know that gods don't whisper."
Shoemaker was married in 1950 to Carolyn Spellman. They had heard about each other through Carolyn's brother Richard, who was Shoemaker's roommate at Caltech. In his personal account of the Shoemakers, Richard Preston of the New Yorker magazine wrote that they continued corresponding until Shoemaker began graduate school at Princeton, when Carolyn suddenly stopped writing. She finally responded to his insistent letters stating that she had assumed that he would stop being interested in her now that he was at Princeton. In response, Shoemaker invited her to go camping with him on the Colorado Plateau that summer. Carolyn's mother, Hazel, decided she would accompany them. It was on that trip that Shoemaker proposed to Carolyn, while driving with her into town to pick up supplies; "in a strategic move," they had left Carolyn's mother in a motel room. They married a year later.
After her marriage, Carolyn quit her job as a teacher and devoted herself between 1952 and 1983 to raising their three children. When Eleanor Helin left in 1982 to launch her own asteroid search, Carolyn learned how to use the 18-inch Schmidt telescope at Mount Palomar, and joined her husband in meticulously sifting through sections of the night sky captured on film. "I love his enthusiasm for life," Carolyn said to Rex Graham of Astronomy magazine, "His enthusiasm makes me enthusiastic."
In the 1980's, Shoemaker had come up with the ingenious idea of using a geological tool, the stereomicroscope, as an aid to spotting moving objects. To quote Richard Preston "They took strings of photographs, lapped at the edges like rows of fish scales. Every forty minutes they backtracked the telescope and rephotographed the places where they had just been, to make stereo pairs." Carolyn would then view the stereo pairs through the microscope. If one of the black dots were a moving object, its image would appear to "float", or practically jump into view. Together, the Shoemakers made a prolific team, discovering 32 comets and several hundred asteroids.
David Levy, an amateur astronomer with a degree in English literature from Acadia University in Nova Scotia, had always wanted to collaborate with Shoemaker, whom he idolized. They met in 1988 at an asteroid conference in Tucson, and worked together for the next five years.
On the night of March 23, 1993, they were sitting in the Palomar observatory waiting for a break in the clouds. Levy suggested using film that had been slightly damaged around the edges because he saw no point in wasting good film under such bad observing conditions. They photographed a region in the constellation Virgo that included Jupiter, which was then at its apogee, or farthest point from the sun. Two days later, when Carolyn scanned the images captured on the damaged film, she found a curious object that looked to her "like a squashed comet." They immediately sent a computer message to David Marsden, director of the Minor Planet Center in Cambridge, Massachusetts, to claim discovery of a new object. Formally named Periodic Comet Shoemaker-Levy 9, the comet instantly became the focus of astronomers around the world. On May 23, 1993, Marsden tripped another surge of excitement by announcing that the comet was set on a collision course with Jupiter.
Further study suggested that the comet had been just another cosmic traveler until it was captured in an orbit around Jupiter by that planet's gravity. At a critical point in the asteroid's path, gravitational forces had pulled it into 21 fragments that appeared "like a string of pearls." These were now falling into Jupiter like steel filings flying toward a powerful magnet. The Shoemakers and David Levy were elated at this unexpected payoff for all the cold nights spent in painstaking observation. Shoemaker reportedly exclaimed "What are the odds that this would happen in our lifetimes, with Hubble (telescope) fixed, Galileo (space probe) nearing Jupiter, infrared detectors having come of age, and before the (research) money has run out? Folks, I think we've been privileged to witness a miracle!"
Over the years, Shoemaker argued the wisdom of setting aside more government funding to search for potential "earth-hitters." In an April 1997 paper published in the Journal of Geophysical Research, he predicted a 10-kilometer crater every 100,000 years and a 20-kilometer crater every 400,000 years. Though not all planetary scientists accepted Shoemaker's estimates, the fireworks from Comet Shoemaker-Levy did much to make the astronomical community more receptive to his ideas. The National Geographic Society subsequently produced a documentary on his life and work titled "Asteroids: Deadly Impact."
Given current observational tools, the Shoemakers pointed out that even if they found an object heading straight for the Earth, it would appear motionless until it was very close, by which time it would be too late to do anything. Such an impact has occurred within recent memory. On June 30, 1908 a white-hot fireball, that was possibly a 200-foot wide asteroid, passed over Siberia and exploded in a brilliant mushroom cloud over the Tunguska Valley. Fifty miles from the point of impact, a tent full of nomads was picked up and tossed through the air. Forests over hundreds of square miles were reduced to ashes and the roar from the shock wave broke windows and crockery up to 600 miles away. Shoemaker's great worry was that an asteroid impact might be mistaken for a nuclear strike, and possibly produce knee-jerk retaliation with a real nuclear device.
In an ironic twist of fate, it was a deadly head-on collision that claimed the world's pioneering asteroid-hunter. Shoemaker and his wife Carolyn were on one of their annual visits to Australia. They had undertaken these visits to systematically examine potential impact craters discovered in recent years through satellite-based remote sensing imagery. On July 18, 1997, they were driving their pickup on a gravel road 310 miles north of Alice Springs, Australia. As they rounded a curve with acacia bushes on either side, a Land Cruiser vehicle appeared directly in front of them. Carolyn would later recall "It was instantaneous— no time to be worried or afraid or avoid it." Carolyn survived with multiple fractures, but Shoemaker, age 69, died of his injuries.
His story might have ended there, were it not for the devotion of Caroline Porco, one of Shoemaker's former students and a collaborator on the Voyager mission to the outer planets. Knowing that being denied the opportunity to go to the moon had been the biggest disappointment in Shoemaker's life, she proposed that a portion of his remains be sent to their final resting place on the moon as a final tribute.
Upon obtaining his wife's consent, it was decided that the polycarbonate capsule containing an ounce of Shoemaker's ashes would be launched on board the Lunar Prospector, bound for a yearlong moon-mapping mission. Around the capsule was wrapped a piece of foil marked with an image of Meteor Crater in Arizona, where it all began; an image of comet Hale-Bopp which was the last comet that he and his wife observed together; and a passage from William Shakespeare's Romeo and Juliet. After a moving dedication, family and friends watched the Athena rocket slowly rise from its launch pad with its special cargo on the evening of January 6, 1998. On July 31, 1999, Lunar Prospector's mission ended and the craft crash-landed on the Moon. Shoemaker had touched the Moon at last.
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