June 30 is Asteroid Day. The date is the anniversary of the 1908 Tunguska event, when a still-unexplained object exploded over Siberia, creating the largest impact in recorded history. For that reason, the day is mostly dedicated to the dangers of asteroids and meteors, and to promoting research to avoid catastrophic events. But the day is also about the opportunities that come with asteroids and meteors. For my first Photos of the Month article, I decided to look into our own collections and see what we’ve learned from these objects and their impacts.
In the photo above, Harvard astronomer Richard McCrosky poses next to a Baker super-Schmidt meteor camera. These super-Schmidts were the first cameras specifically made for photographing meteors. They were designed by Harvard Observatory research associate James G. Baker, and they began production in 1951. As the name suggests, they were based on the Schmidt telescope, a design which was invented in 1930 by German optician Bernhard Schmidt of the Bergedorf Observatory.
Richard McCrosky devoted his career to tracking meteors. He was a chief investigator in the Smithsonian Astrophysical Observatory’s Radio Meteor Project, was the scientist-in-charge of the Prairie Meteorite Network in the 1960s and 70s, and later was director of the Harvard-Smithsonian Center for Astrophysics' Oak Ridge Observatory. The Smithsonian Archives has records of both the Radio Meteor Project here and the Prairie Network here, and you can read an oral history interview with him here.
Fred Whipple was also an astronomer at Harvard College Observatory, responsible for discovering many meteors and comets. He is best known for the “dirty snowball” hypothesis of comets, a model which we still use to describe them today. He also invented the Whipple shield, a thin outer bumper which protects spacecraft from micrometeoroids while keeping the ship’s weight reasonable. Whipple’s papers are divided into several collections at Harvard University, the Smithsonian Archives, and the American Philosophical Society; you can read a 1977 oral history interview with him here.
In 1980, this team devised the Asteroid-Impact theory, also known as the Alvarez hypothesis, which asserts that the extinction of the dinosaurs was caused by a massive asteroid impact. Walter Alvarez, a geologist, had noted a sedimentary layer of clay with unusually high levels of iridium; an element uncommon on Earth but often found in asteroids. He explained to his father, the physicist and Nobel Laureate Luis Alvarez, that this layer also marked the point when the dinosaurs became extinct. Nuclear chemists Helen Vaughn Michel and Frank Asaro demonstrated the age of this iridium layer using a “neutron activation analysis” technique which operates on principles much like the more familiar carbon dating which is used to find the age of dinosaur fossils.
Ralph Belknap Baldwin had a wide-ranging career in physics, but he is best known for his work in astronomy. He is recognized as proving that the Moon’s craters are the result of asteroid impacts, and not volcanic. This work was published in his 1949 book The Face of the Moon, which was followed by a 1965 book, The Measure of the Moon. Looking up at the moon’s craters is such a fundamental part of human existence that I wonder at such a major discovery having been made so recently, and yet decades before humans visited the Moon. Baldwin’s papers are at both the University of Michigan and the Grand Valley State University. You can read his responses to our Center for History of Physics’s 1980 History of Modern Astrophysics Survey here and 1988 History of Geophysics Survey here, a 1989 oral history interview here, and his 1998 manuscript biography here.
Eleanor “Glo” Helin was a geologist and astronomer responsible for discovering nearly one thousand minor planets, asteroids, and other near-Earth objects. She originated and ran multiple asteroid surveys, including Palomar Observatory’s Planet-Crossing Asteroid Survey (PCAS) and the Jet Propulsion Laboratory’s International Near-Earth Asteroid Survey (INAS) and Near-Earth Asteroid Tracking program (NEAT).
Austrian-American astrophysicist Ernst Zinner helped launch a new field of science, studying the composition of objects in space by analyzing grains which had been carried to Earth on ancient meteorites. He worked for more than a decade to get reliable results from secondary-ion mass spectrometry (SIMS), as the ion spectrometry tools available at the start of his career could not be trusted. In 1987 he proved he had discovered material on Earth which predated the formation of our solar system and had come from outside of it. His work also confirmed the theories of fellow astrophysicist Donald Clayton, who in the 1970s had predicted that supernovae from different types of stars would create grains which could be identified by their isotropic properties. Ernst Zinner’s collection is at the San Diego State College Library. Donald Clayton’s papers are at Clemson University, and you can view his image collection as part of our Emilio Segrè Visual Archives here.
Carolyn Shoemaker found an interesting path to astronomy. She held degrees in history, political science, and English literature, and she taught high school social studies and English before becoming a stay-at-home parent. She only moved into astronomy after the kids had grown and moved out, when she began working alongside her husband, the geologist Gene Shoemaker. She found that she had remarkably keen stereoscopic vision and a great deal of patience. The Shoemakers focused on asteroids, comets, and other smaller objects in our Solar system. Carolyn Shoemaker personally discovered more than 500 asteroids, and 32 comets, which at the time was an all-time record. With her husband and their colleague, the amateur astronomer David Levy, she co-discovered the comet Shoemaker-Levy 9 which famously smashed into Jupiter in 1994.