June 19, 2018
In Discovering Pluto historian William Sheehan along with his co-writer Dale P. Cruikshank uncover the behind-the-scenes history of the enigmatic and much discussed icy orb at the edge of our solar system. Today, William answers our questions about the outer Solar System.
Why do you think Pluto has so captured the public imagination since it was first identified by Clyde Tombaugh in the 1930s?
From times immemorial, there were five planets—wanderers—tracing movements across the starry background of the night sky. After the Copernican revolution, the Earth of course became a planet, like the others traveling around the Sun, but still Saturn marked the outer boundary of the Solar System, and the stars were at unfathomable distances beyond. This ancient picture changed in 1781, when William Herschel discovered Uranus. Suddenly the scale of the Solar System had doubled, and within a few short years other astronomers began to discover new planets, as they were then called; these were the asteroids between Mars and Jupiter. Inspired either by the discovery of Uranus itself or by the first asteroids, Keats wrote the stirring lines,
“Then felt I like some watcher of the skies/When a new planet swims into his ken.”
Uranus began after a few years to wander inexplicably off course, and this led two mathematical investigators—John Couch Adams in England and Urbain Jean Joseph Le Verrier in France—to use the discrepancies in its movements to calculate the position of an unseen planet beyond—Neptune—whose optical discovery was made on the basis of these calculations in Berlin in September 1846. This was seen at the time as the greatest achievement of Newtonian celestial mechanics—the discovery of a planet, “with the stroke of a pen.” Adams and Le Verrier were duly enshrined in the pantheon of astronomical greats. Few developments in astronomy were awarded greater accolades than this, literal discovery of a new world.
Le Verrier himself thought that there might be another planet inside the orbit of Mercury, and even gave it a name, Vulcan; it does not exist, and never did—the movements for which its existence was invoked were explained by Einstein on the basis of the General Theory of Relativity in 1915. However, Uranus continued, apparently, to be wandering off course, even after Neptune was entered into the equations. Several astronomers, of whom Percival Lowell was the most celebrated, developed an elaborate program to track down another putative planet—Planet X—which might be indicating its presence as Neptune had done for Adams and Le Verrier. Lowell was an extraordinarily colorful and interesting figure, who is best remembered for founding the Southwest’s (and Arizona’s) first major observatory in Flagstaff, and for his exciting and provocative theories about the canals of Mars, which won over the general public (and inspired science fiction writers like Wells and Burroughs) but were harshly criticized by many professional astronomers. Lowell’s motivations in searching (secretively for the most part) for “X” were complex, and included the hope that recapitulating the great feat of Adams and Le Verrier would restore his prestige in the eyes of other astronomers. Unfortunately, Planet X was undiscovered when he died in November 1916.
The story of how later the search was resurrected at his observatory, how a self-taught farm boy from Kansas (Clyde Tombaugh) was hired to carry out the mind-numbing and backbreaking work of searching for it on photographic plates exposed in all weather under the stars, and how Clyde found a planet that at first was hailed as the incarnation of the icy planet of Percival’s dreams in 1930 provided the perfect coda to the story of frustrated ambition redeemed by faith and hard work. The planet was also the first discovered by an American, and came just as the Great Depression—and the rise of Fascism in Europe—were getting underway, so that the world, and Americans in particular, were in need of “good news.” In the end, Pluto proved to be a most peculiar planet, and was shown—rather definitively by Dale Cruikshank and David Morrison who in 1976 discovered the presence of methane ice on the surface—to be smaller than the Earth’s moon, and has now been seen as a kind of dual object—planetary in some ways, including rotundity and having an active (if extraordinarily odd) geology, but also the largest of the Kuiper Belt Objects which roam the outer Solar System.
What was the most surprising thing you uncovered during your research for Discovering Pluto?
The most surprising thing was what the New Horizons probe found when it passed by Pluto in July 2015. Most people, including me, had probably expected a cold and inert world, not perhaps unlike that the celebrated British astronomy writer Patrick Moore had invoked in 1955, “Beyond all doubt, Pluto is the loneliest and most isolated world in the Solar System—cut off from its fellows, plunged in everlasting dusk, silent, barren, and touched with the chill of death.” Far from it; instead, areas of Pluto show evidence of quite recent geological activity, with changing “land” forms that consist of exotic ices—including recently, methane ice dunes in Sputnik Planitia. It is also exciting to see—on an actual body in the Solar System—examples of the behavior of these ices that has already been elucidated in the laboratory.
You have written many books about planetary science, including Planet Mars. What keeps you coming back to writing these histories?
I have been fortunate in having been born just before Sputnik went into orbit around the Earth, and being consciously aware as the first spacecraft set out for the Moon and planets. I acquired my first small telescope in the mid-1960s, at a time when visual observations by amateur astronomers were still often better than the most detailed photographs by professional astronomers at the great observatories, and when it still seemed that amateurs might contribute usefully to their study. When I started out, Mariner 4 had not yet passed by Mars (July 1965, fifty years to the day before New Horizons made its Pluto flyby!), and it was still possible—just—to believe in Percival Lowell’s canals of Mars! Mariner 4—which showed there were craters on Mars—brought what seemed at the time to be a Great Disillusionment; almost like finding out (and I was at that age, just ten or so) that Santa Claus didn’t exist.
Robert Burnham, Jr, who wrote the Celestial Handbook series, and used the Pluto telescope for the proper motion study at Lowell Observatory in the 1960s, was a mentor, and encouraged me to look at Comet Ikeya-Seki in October 1965—it remains the most spectacular comet I have ever seen. This shows how important an interest of a professional can be in encouraging a young person. After a number of years, I was invited (in the summer of 1982) to Lowell as a guest investigator with a somewhat tentative project of trying to understand how observers like Lowell could have seen canals on Mars when obviously there are no canals. Art Hoag was the director then, and Bill Hoyt, who had written the landmark book Lowell and Mars published by University of Arizona Press, was in-residence historian. While there—and coming into contact with the observing books of Lowell and his associates, and seeing how their visions of canals gradually unfolded and became elaborated over time—and also observing directly through the Clark telescope they had used, I had a flash of insight—I was in the NAU library at the time; I remember it as if it were yesterday–that the key aspect no one had recognized was that because of fluctuating seeing the canals were seen only in brief intervals of a fraction of a second or so. All the observers of the canals—including Clyde Tombaugh, who graciously corresponded with me on his experiences—described this. Thus the phenomena of the canals could be related to experimental psychology. I had always been drawn to interdisciplinary work—this has become the fashion now but at the time represented an aspiration that was more honored in the breach than the observance, simply because the various disciplines had become so developed and complex that it was difficult for anyone to master them at the same time. In any case, I came away from Lowell with the thesis of a book—Planets and Perception—which I drafted during the summer of 1983, while living in a small town in southern Minnesota, just across from the Iowa border, with no resources more than those I brought with me and the Carnegie library with a six-foot shelf of books on astronomy, physics, and math. In retrospect, I think I was crazy to tackle such an ambitious project more or less alone and unaided; had I been in a graduate program, I might have worked for ten years on it, but I finished the draft in several months and then—put it away in a desk drawer as I began my medical studies.
Eventually, I got around to submitting it—and did so only to one publisher, the University of Arizona Press. Though they quailed a bit at the cross-disciplinary nature of the thing—and had to send it out to three academic reviewers, two astronomers and one psychologist!—they graciously accepted it. It was published thirty years ago in November, and I didn’t know what to expect. I was in my internship then. In January, I got a good review from Richard Baum in the Journal of the British Astronomical Association, which I remember reading in the on-call room, and thought that I was lucky to get that. By early May, I was at morning rounds on the psych service the VA in Minneapolis; the attending physician, Charlie Dean, who subscribed to Nature, rather casually congratulated me on the review the book had received in Nature by the renowned historian of science, Albert Van Helden. I was over the Moon. Of course, the book had many faults which I see only too clearly now—how could it not?—but proved to be quite seminal in its small way, and I will always be grateful to the University of Arizona Press for taking a chance with an unknown scholar and a very experimental piece of work and believing in it.
But to get back to your question, this book has defined my lifelong career interest—and until recently, when I retired, I have been both a practicing psychiatrist, interested in the brain and the way we “know,” and a historian of astronomy—and have found the history of Solar System studies during this period of time to be perhaps the most important thing we as a species have done. It has been our Parthenon, our Cathedrals. Obviously there are a lot of writers who understand this, and have devoted themselves to the documentation of this wonderful era, including many of the scientists who have been in the forefront of research (like Dale Cruikshank, my co-author of Discovering Pluto). But I think my background in psychiatry has given me a somewhat unique perspective on the human angle of this story, and that story—the exploration of the Solar System—is, after all, passionately and irreducibly a human story, whose grandeur and magnificence far exceeds the explorations (and too often bloody) conquests of previous eras. It collectively represents some of the best aspects of human nature, Something, I would add, that we desperately need to affirm and reaffirm at the present time, when it is too easy to be disillusioned about our species in light of some of its more unsavory aspects. These are things that keep drawing me back to this subject.
With the New Horizons space probe back in action after a 6-month break, what do you think will be the next chapter in Pluto’s history?
We are looking forward to New Horizons’ close approach to another KBO, MU69, which most of us expect to show only an ancient battered landscape. But as with Pluto, we are foolish not to expect to be pleasantly surprised, and perhaps we will discover fresh patches of surface exposed by a recent collision with another KBO, in which case we may have an opportunity to see deeper into the interior where so much of the early history of the Solar System lies hidden.
What are you working on now?
I have been working on a series of books on each of the planets for Reaktion Press in Great Britain—so far I have finished Jupiter and Mercury, and am on to Saturn. I am also working on a book on Mars with Jim Bell for the University of Arizona Press, who is the PI on the camera system for the 2020 Mars rover (and sample return mission). As with the Pluto book written with Dale, I will be covering mostly the historical backgrounds, in this case how we came to know Mars (including our long tendency to see it as the image of the Earth, or even of Arizona!), while Jim will take over the torch and bring to it his unrivaled knowledge of the spacecraft era. I always prefer, by the way, if possible, to work in collaboration, as it not only provides an opportunity for me to greatly extend the range of my own knowledge but also is as much more enjoyable for the shared companionship.
William Sheehan is a historian of astronomy and psychiatrist. His many books include Planets and Perception, Worlds in the Sky, and The Planet Mars, also published by the University of Arizona Press. Asteroid No. 16037 was named in his honor.