Lowell Observatory
Flagstaff, Arizona
A Private Vision Producing Popular Science
Lowell Observatory, a vibrant research institution for nearly a century, has the longest and perhaps most colorful history among observatories of the American Southwest. This private, nonprofit research institution sits on a large mesa named Mars Hill, directly above downtown Flagstaff, Arizona, but even the short drive up is enough to firmly leave the town behind for the peaceful isolation of the observatory complex.
One of the earliest scientific outposts in the Arizona Territory, Lowell Observatory was founded in 1894 by Percival Lowell, a wealthy Bostonian eccentric securely in the top rank of uniquely American supporters of the sciences. Though influenced even now by Percival’s lifelong outsider status, the observatory has been the site of many important scientific discoveries, from the fundamental measurements of the recessional velocities of galaxies in 1912–1914, which ultimately inspired Hubble to make the observations that demonstrated that the Universe is expanding, to the culture-tweaking discovery of Pluto. The observatory then entered a relatively quiet period where its staff was dominated by three aging early heroes, until a one-two punch of new directors in the 1950s vaulted the observatory back to the prominence it once enjoyed.
| Web site | Address |
| www.lowell.edu www.lowell.edu/outreach |
1400 West Mars Hill Road Flagstaff, AZ 86001 |
| Phone (928) 233-3211 |
Today the observatory’s twenty-person scientific staff is a world leader in many areas of astronomy and space science, particularly in studies of galaxy evolution and the smaller bodies in our solar system: rocky asteroids and distant icy bodies revolving around the Sun in a region outside Neptune’s orbit known as the Kuiper Belt. Scientists at the Lowell Observatory typically use a wide variety of telescopes, ranging from the five primary research telescopes owned by Lowell to an assortment of spacecraft in Earth orbit and beyond.
The observatory is home to a melange of telescopes—four of the largest are located off Mars Hill at a second, darker site east of town called Anderson Mesa (see “For the Public,” below, for more information on Anderson Mesa). Mars Hill will always be known first as the home of the historic 24-inch (0.6-meter) Alvan Clark Telescope (named for the famous late-nineteenth- and early-twentieth-century craftsman of large telescope lenses), which remains the showpiece and heart of Lowell Observatory. Flush with his semiprofessional passion for Mars, Percival Lowell had the Clark Telescope installed on Mars Hill in July 1896 for a capital investment of $20,000, establishing what is generally acknowledged as the first permanent major scientific instrument in the Arizona Territory. The telescope dome once floated on salt water and was rotated by pulling on ropes, a distinctly nautical solution to smoothly rotate such a large mass; today, it uses tires from a 1954 Ford pickup truck.
Nearly two decades after it began work, V. M. Slipher used this refracting telescope to discover the first evidence that the Universe is expanding rapidly in all directions, as if the galaxies were the tiny fruit in a gigantic loaf of rising raisin bread.
During the early 1960s, the Clark Telescope was used by legendary planetary scientist Eugene Shoemaker and others to help create the basic drawings for a landmark effort to make comprehensive, detailed maps of the Moon sponsored by the U.S. Air Force. Soon after, Apollo astronauts were able to make some of their first good observations of their historic future destination through the Clark (as well as the McMath-Pierce Solar Telescope at Kitt Peak). The Clark refractor continues to excite the public today during the regular stargazing sessions hosted by the Lowell public outreach staff, and it is a frequent backdrop for many astronomy documentaries of all sorts.
The Clark Dome houses the Clark Telescope, purchased by Percival Lowell for studies of the solar system in 1896 from Alvan Clark and Sons in Cambridgeport, Massachusetts.
The Lowell Observatory 24-inch Clark Telescope.
Another historic, relatively small instrument at Lowell is a 21-inch (0.5-meter) telescope housed in a 1920s stone building on Mars Hill. It has been used with astounding patience and dedication to precisely monitor the brightness of Uranus and Neptune for more than fifty years, as well as Saturn’s moon Titan over Saturn’s full three-decade orbital cycle. Originally designed to monitor the total energy output of the Sun, this relatively tiny telescope was also used for historic work that defined the first widely accepted process for categorizing stars according to their colors (as measured through filters that isolated light at near-ultraviolet, blue, and visual wavelengths), and it produced a decades-long series of sky-brightness measurements that helped Flagstaff be named the world’s first International Dark-Sky City.
Similarly, the landmark discovery of Pluto was accomplished at Lowell using a 13-inch (0.3-meter) telescope optimized for photography and known as an astrograph. Kansan Clyde Tombaugh used the 13-inch to discover the much-prophesized “Planet X.” Later named Pluto at the suggestion of an English schoolgirl, it is now considered a so-called dwarf planet in a controversial redesignation passed by the members of the International Astronomical Union.
Long predicted to exist by Percival Lowell and others such as William Pickering, Pluto was detected using a machine to “blink” or compare the star fields on photographic plates taken on different nights. In early 1930, the patient Tombaugh used his eyes—not a computer!—to spot the relatively large change in position of the tiny speck of reflected sunlight—the expected signature of a candidate planet. From “blinking” images spanning several nights, he was able to see and measure the apparent motion of the relatively nearby Pluto, as compared to the vanishingly small movement of distant background stars, confirming it as a body lying beyond the orbit of Neptune. Both the astrographic telescope and the plates used to find Pluto are on display today in Lowell’s Rotunda Library Museum.
Flagstaff continues to have amazing contemporary links with Pluto and its companion moons. Pluto’s major moon, Charon, was discovered in 1978 by Jim Christy of the staff of the U.S. Naval Observatory in Washington, D.C., using observations made in Flagstaff. More recently, Marc Buie of the Lowell scientific staff was part of a team that used very long exposures taken by the Hubble Space Telescope to discover two tiny new moons of Pluto, since named Nix and Hydra. Will Grundy and other staff members at Lowell are involved in planning the observations of Pluto to be made by NASA’s New Horizons spacecraft. Already on its way, New Horizons will fly past the icy dwarf planet and its expanding coterie of moons in July 2015.
The 13-inch Pluto Discovery Telescope is open to visitors for specially arranged tours.
Why does Lowell Observatory exist at all? It is tied inextricably to the Lowell family (who continue serving as its mandated trustees), beginning with Percival. Known as a man of “pure action” from his college days at Harvard University until his death forty years later, Percival had an entirely separate career as an eminently respected scholar of Japan and the Orient (serving as the counselor of the first Korean mission to the United States) before astronomy took full hold of his imagination.
Percival’s astronomical obsession appears to have been inspired by several events, starting with a supposed viewing of Donati’s comet in 1858 when he was three years old. At age fifteen, he received a 2.25-inch (0.06-meter) brass telescope from his mother (on display today in the Lowell Rotunda Library Museum), which he first used on the roof of the family home in Brookline, Massachusetts.
Twenty-four years later, Lowell used this 30-inch-long instrument to view Gale’s comet—a comet discovered in the Southern Hemisphere but first sighted in the Northern Hemisphere by Andrew Douglass in Flagstaff. Douglass had been dispatched by Lowell to survey the rugged Arizona Territory for an appropriate site for a world-class observatory. (He sent word of the comet sighting back to Lowell via coded telegram, early evidence of the secretive passion that characterized Lowell’s approach to his astronomical pursuits.)
Lowell was also colored by his contact with the eccentric French astronomer Camille Flammarion, a prolific author and eager popularizer of scientific ideas who was a vocal advocate of the theory of the “plurality of worlds”—the idea that the conditions for extraterrestrial life are common. Whether this was accomplished by theological generosity or by fully agnostic chemistry was a matter of great discussion that continues to echo today. Lowell searched for evidence of life on other worlds, focusing his attention on Mars, where he believed (incorrectly as it turns out) that a web of fine features he observed on the planet’s surface was evidence of canals constructed by an advanced civilization. While the ubiquity of life beyond Earth still remains a subject of conjecture, the more than three hundred extrasolar planets detected by astronomers in recent years using several techniques would surely please and embolden the pluralists.
Lowell died on November 12, 1916, in Flagstaff, and he is buried on Mars Hill. His impressive cement tomb overlooks Flagstaff and resembles a large antique globe, evoking both the iconic power of an observatory and the grand dome of stars in the night sky. The funds spent on building the elaborate tomb were resented at the time of its construction by staff astronomers, who would have preferred that Lowell’s widow spend the money on new research equipment.
It is inscribed with a much-referenced quotation of Lowell’s emphasizing the “bodily isolation . . . and purity” demanded of those brave and lonely men who dare “to see into the beyond.” Percival lamented with undisguised pride that such creatures must suffer a hermit–like existence in order to do it, aloof from urban humanity and certain to be disbelieved when returning from the mountaintop with their newfound wisdom.
The future of Lowell Observatory is intimately entwined with a project that, although born of much more modern sensibilities, has again been supported significantly by the passion of one wealthy individual. The $40 million Discovery Channel Telescope (DCT), under final construction forty-five miles southeast of Flagstaff at a site in the Coconino National Forest known as Happy Jack, was born via an interesting marriage between the private observatory and the multimedia company Discovery Communications. In particular, it has benefited from a $6 million donation from John Hendricks and family. Hendricks, the founder and chairman of Discovery Communications, has a personal interest in astronomy and space science and has been a member of the Lowell Observatory Advisory Board for more than a decade.
The DCT, situated at an altitude of 7,760 feet, has a purposefully flexible design that makes it capable both of a very wide field of view (four times the apparent diameter of the full Moon) at its prime focus and aimed primarily for planetary science research, and of a different optical configuration (called Ritchey-Chrétien) that is better suited for research that demands high-resolution imaging or spectroscopy.
The primary mirror for the DCT weighs “only” 6,700 pounds, very light for a such a large mirror—the thin slab of ultra-low-expansion glass is 169 inches (4.3 meters) in diameter but less than 4 inches (100 millimeters) thick. It was cast and fused at Corning’s plant in Canton, New York, and polished to an accuracy of a few millionths of an inch by the University of Arizona’s College of Optical Sciences.
Construction at the site began in mid-2005, and it is planned that the telescope will be operational in 2010, at which point it would enjoy the honor of being the fifth-largest telescope in the continental United States. A study by the Center for Business Outreach at Northern Arizona University found that the DCT will contribute more than $576 million to the economy of the state of Arizona over its planned fifty-year useful lifetime—a surprising but noteworthy conclusion that bespeaks the importance of astronomy not only to “pure” research but to stimulating other scientific, technical, and economic activity.
Once operational, the DCT will certainly expand the legacy of Lowell Observatory’s groundbreaking discoveries of distant bodies in our solar system, and beyond. It should be easy to follow the project’s progress: The telescope and the research it enables will be the focus of ongoing educational television programs about astronomy, science, and technology to be aired on Discovery’s multimedia networks.
For the Public
Located just one mile west of historic downtown Flagstaff, Arizona, at an elevation of 7,200 feet, Lowell’s scenic Mars Hill campus draws more than 70,000 visitors a year from the local community and from Phoenix and California, whose residents look to escape the local summer heat.
The staff of the 6,500-square-foot Steele Visitor Center offers guided thirty-minute tours of the observatory daily on the hour from 10:15 A.M. to 4:15 P.M. (1:15 P.M. to 4:15 P.M. in winter).
The main exhibit hall and science center located in the Steele Visitor Center at Lowell Observatory features exhibits such as “The Galaxies” and “The Cosmos.”
The Rotunda Library Museum at Lowell Observatory houses many historic exhibits and a hands-on exhibit for children. The museum is part of a guided daytime tour for visitors and is often open during special events.
The John Vickers McAllister Space Theatre opened in May 2007. Tucked inside the existing visitor center exhibit gallery, the twenty-four-seat theater presents a twenty-minute narrated program during the daytime every hour starting at 10:00 A.M. through 4:00 P.M. (1:00 P.M. to 4:00 P.M. in winter). Lowell outreach staff make an effort to add lively music and timely news bits regarding the latest astronomical discoveries to the program, which is projected on a concave five-foot-diameter screen inside the planetarium-like venue. At night, the theater shows a shorter program to star-party guests designed to give them an introduction to what objects they’ll be trying to see in the sky later that evening.
Exhibits in the Rotunda Library Museum were restored in 2006. Subjects include the Lowell family, some of Percival Lowell’s sketchbooks and his calculations predicting the orbit of Planet X, the process of naming planet Pluto and a sampling of the letters the observatory received after the discovery of Pluto, current research news, and a children-focused exhibit on the workings of a telescope, with lens and eyepieces to play with.
Joining the Friends of Lowell Observatory support group is a great way to participate in special tours of Lowell Observatory’s semiremote dark site at Anderson Mesa, about 15 miles southeast of Flagstaff. It is the home of the 1.8-meter (71-inch) Perkins Telescope, the 1.1-meter (43-inch) Hall Telescope, the National Undergraduate Research Observatory, and the Naval Observatory’s Prototype Optical Interferometer. Known as NPOI, the interferometer is a collaboration between Lowell Observatory, the Naval Research Lab, and the U.S. Naval Observatory. The interferometer has been used to measure precise diameters and masses of stars and the often-complex properties of binary or multiple star systems.
The Perkins Telescope was originally located near Columbus, Ohio, where it was the third-largest telescope in the world when dedicated in 1935. It was moved to Lowell’s Anderson Mesa site in 1960 under an agreement between Ohio Wesleyan University, Ohio State University, and John S. Hall, who was then the director of Lowell, and has been steadily upgraded since then, most recently in a partnership with Boston University.
For Teachers and Students
Lowell staff, including the astronomers, share a dedication to involving teachers and students in the mission of the observatory. Staff scientist Deidre Hunter is particularly known for her active outreach programs with the Navajo and Hopi Nations.
Private, paid, one-hour daytime programs are available by reservation, including two multimedia presentations and a tour. Topics offered include the planets, comets/asteroids/dinosaurs, life on other worlds, and the expanding Universe.
Private evening programs, including exclusive access to the Clark Telescope, are also available.
A Talk with Robert Millis
Former director, Lowell Observatory
Born and educated in the Midwest, Robert Millis spent a summer at Lowell Observatory four decades ago as a graduate student and—seduced by the natural beauty of the area and the scientific freedom of the institution—he has been part of the staff essentially ever since. From 1989 to mid-2009, he served as director of the observatory. He is also a vocal advocate for the economic benefits of astronomy to the state of Arizona and the value of protecting its world-renowned dark skies.
When did you first encounter Lowell Observatory?
I first went to Lowell in 1965 while a graduate student at the University of Wisconsin, where the director at that time, John S. Hall, invited me to do some joint research with him. I joined the scientific staff in the fall of 1967 and have been there ever since.
Growing up in Illinois and not having traveled much, the western United States was a revelation to me, and Flagstaff in particular—its scenic beauty and cultural diversity. Lowell Observatory was very attractive because it was small, very collegial, and the science staff had tremendous freedom to work on whatever interested them. I just really liked the environment, and so did my wife. We arrived there on something like the fifth day of our honeymoon, so we encountered Lowell at a special time in our lives. I always imagined that I’d end up at a university, teaching, but time passed and it was a satisfying place to be, so thoughts of a university career just kind of faded away.
Robert Millis, former director, Lowell Observatory.
My wife and I lived on the observatory grounds for roughly thirty years. It was a very desirable place to raise our two kids. It’s a very park-like surrounding—the forest goes off for miles in one direction, but you could be in the heart of downtown Flagstaff in just five minutes. It’s kind of the best of both worlds. Some of Lowell’s major observing facilities are outside of the city at Anderson Mesa, but we spent most of our time on Mars Hill.
I gradually became part of management. Back in those days, the observatory entrusted positions of responsibility only to astronomers. For several years, I served as secretary/treasurer, supervising the bookkeeper who kept the financial records and managed grants and contracts. The total staff is small now, and it was smaller then. A lot of people tended to wear more than one hat. To a degree, that’s still true today.
Do your astronomers get special access to the observatory’s telescopes or do you also accept visiting astronomers?
We currently have twenty-one PhD scientific staff. First and foremost, our telescopes exist to serve them. We have always hosted visiting astronomers and will continue to do so, but staff has priority, though some staff members rarely if ever use our facilities. They go to the national observatory near Tucson [Kitt Peak] or other telescopes on Mauna Kea, and so forth.
Have you always been interested in solar system studies? Why do you find it so fascinating?
I came into planetary astronomy through the back door. It was not part of the curriculum at the University of Wisconsin. My thesis was on variable stars, but one thing I learned was how to do precision photometry [measuring tiny variations in starlight brightness]. So when I joined the Lowell staff, I began to look for applications in planetary science where my skills could be applied. I began a long history of studying the smaller bodies in the solar system, on through the satellites of the outer planets, asteroids and comets, ring systems, and Pluto, finally arriving out at the Kuiper Belt in the late 1990s. The unifying thread has been my interest in the smaller objects.
I tell people that I have a short attention span. I like to study things that change, that are dynamic. The moons of the outer planets show changes during their orbits, and there is tremendous variation among them. When I was first studying them, they were just specks of light in a telescope, not the intriguing worlds that the Voyager spacecraft showed us.
I was lucky enough to be part of the discovery of the rings of Uranus in 1977, which brought some NASA interest and funding. With Jim Elliot at MIT and others, we had great fun chasing occultations of stars (eclipses caused by an asteroid passing in front of a background star) to directly calculate the sizes of asteroids and help calibrate other observations. We were part of the first team that was able to observe the occultation of a star by Pluto, which enabled the first direct detection of the profile of its atmosphere (basically its density as a function of height).
Then along the way, Mike A’Hearn from the University of Maryland and I started on a long-term study of the group properties of comets, a program which persists today at Lowell under the leadership of Dave Schleicher. We’ve been able to observe about 120 comets, all on a very standard measuring system designed to determine differences in chemical composition between comets and get a better understanding of how comets respond to the changing level of solar heating as they go around their orbits. It turns out there’s a lot less diversity than one might have imagined, but there are a few real outliers [oddballs] and we still don’t really have a clear explanation of how they came to be so different in composition.
By the mid-1980s, I was beginning to get discouraged about the prospects for ground-based planetary astronomy. Then, the first Kuiper Belt object was discovered in 1992 and that opened up a whole new frontier for exploration of the bodies populating our solar system beyond the orbit of Neptune, an exploration which we are really just beginning. There are certainly substantial objects out there that have not yet been discovered, but they will be found over time.
We formed a multi-institution team that won access to the Mayall 4-meter telescope on Kitt Peak to do a survey to discover orbiting objects in the Kuiper Belt (known to astronomers now as “KBOs”). This program helped clarify the dynamical history of the Kuiper Belt (how the pull of gravity of the other planets in the solar system rearranged the orbits of KBOs over time) and provided a large sample of KBOs with well-known orbits that could be studied by telescopes on the ground and in space—a sample crucial for understanding the diversity of orbits and chemical composition among these objects, as well as their likely origin. It has been very satisfying, and Lowell astronomers continue to refine orbits so that our successors can study them years, decades, and centuries hence.
What are the roots of the Discovery Channel Telescope project?
The DCT was conceived during the early days of this period. The exploration of the solar system was very much on our minds. The desire to explore the Kuiper Belt and to contribute to the problem of finding near-Earth asteroids [asteroids whose orbits bring them close to Earth] were the primary reasons for the wide-field capabilities of the telescope.
Lowell Observatory celebrated its centennial in 1994, and it was a natural time to do some soul searching. We could have become more of a think tank, but we decided to continue to be an observatory with significance on a national and international scale. To do that, we decided that we needed a larger and more capable telescope.
Discovery Channel joined the project in 2003, and it really gathered steam. We had the resources to go full-speed ahead on building a 4-meter telescope—which, when you’re talking about something as challenging as a 4-meter telescope, is not very fast, I have learned.
What lies in the future for astronomical research at Lowell?
There is a transition occurring as my generation steps aside and another one steps up. Current staff and others will use the DCT and other facilities to continue our studies of the outer solar system. We’re also developing more and more expertise in searching for extrasolar planets using the transit technique [watching for the light of a star to dim ever so slightly when a planet passes in front of it in our view], led by people like Ted Dunham. Ground-based telescopes will be the key to sifting through the planet candidates found by the NASA Kepler mission. Other people like Deidre Hunter study star formation and evolution. Another project that began before I arrived and continues today is monitoring the activity level of about one hundred Sun-like stars to better understand the wide range in the energy output of such stars and, by implication, the Sun. I’m sure that will be part of what the DCT does.
What does the Discovery Channel bring to the project?
John Hendricks and his team originally brought the wherewithal to really get the project off the launch pad. What they bring now is the ability to shine a global spotlight on the facility, and the engineering, science, and technology that go into building and operating a modern telescope. My understanding is that Discovery will start with a two-hour documentary on the building of the DCT, and then follow up with programs about the research accomplished with the telescope. They are the experts.
What kind of access will the public have to the DCT?
I suspect there will be intermittent tours after we get operating, as soon as 2010. And there will be a live Web presence in the visitor center on Mars Hill, so a lot of public interaction with the DCT will be through television. It’s a pretty remote site, about forty-five miles and a good one-hour drive from Flagstaff, so the DCT is designed to be operable from anyplace in the world, and we believe most astronomers will choose to use it remotely.
What experience do you try to present to public visitors to Mars Hill?
We try to provide a genuine interaction with the visitor, some real person-to-person contact. We want people to go away feeling that Lowell Observatory is a unique and valuable institution with a rich history and a bright future—a bit of a “David” among the “Goliaths.” We pride ourselves on being a scientific institution with an open-door policy toward the public. We offer them an opportunity to be as much a part of the observatory as they want, from taking a tour all the way up to being a member of the advisory board.
We let as many people as possible look through the Clark refractor, and most people say it is a supremely satisfying experience. It looks like a telescope should, and the building is pretty funky. It is unusual that we are a private institution and not associated with a university or funded directly by the federal or state government. Most of the other private entities like Lowell have fallen by the wayside.
What is your perspective on the history of the observatory and what some might term its eccentricities?
The current staff feels a genuine connection right back to the founder, Percival Lowell. In part, that is because of the governance structure identified in his will. The observatory is guided by a sole trustee who must be a member of the Lowell family. For the last eighty years, the observatory has been guided by a family member. First it was Percival’s nephew Roger Lowell Putnam, then Roger’s youngest son, Michael, and now Michael’s brother, William Lowell Putnam. Their presence, and that of other family members who visit and talk frequently about “Uncle Percy,” gives everyone on the staff a pretty good grounding.
Clearly, Percival Lowell was dead wrong about the existence of intelligent life on Mars, but we can point at other things he initiated that bore tremendous scientific fruit. Nobody can dispute the discovery of the redshifts of galaxies took place using Lowell’s original 24-inch Clark Telescope, with the spectrograph he commissioned, by a young astronomer, Vesto Slipher, whom Lowell hired. Whether Pluto is a planet or not, it was discovered in a series of events that were set in motion by Percival Lowell. I like to invite people to stand on Mars Hill and look around at his legacy, and compare it to those of his detractors—Lowell, I think, comes out really well. I often say that Percival Lowell was the Carl Sagan of his day.
What has been the nature of the relationship between Lowell Observatory and the city of Flagstaff?
There has been a long and mutually supportive relationship. The community takes a lot of pride in the observatory and appreciates the visitation it attracts. The city and Coconino County came to the aid of the observatory and our neighbors, the U.S. Naval Observatory, in the mid-1980s when our dark skies were threatened by ongoing population growth by establishing some of the first outdoor lighting ordinances. It is a very astronomy-friendly city, with a large number of people with PhD’s. In my opinion, Flagstaff is fortunate to have places like Lowell, the U.S. Geological Survey, the U.S. Naval Observatory, and Northern Arizona University to serve as flywheels for the local economy.
Science Highlight
The Outer Bodies of the Solar System
One of the early landmark discoveries made at the Lowell Observatory occurred in 1930, when Clyde Tombaugh announced the discovery of a planet beyond Neptune: Pluto, a small, distant sphere of rock and ice about one-sixth the size of Earth. Over the subsequent decades, scientists working at Lowell have continued searching for other members of our solar system’s extended family. Their research has focused on discovering, analyzing, and categorizing the properties of asteroids, comets, and—more recently—a class of distant icy bodies called trans-Neptunian objects located in the so-called Kuiper Belt.
In retrospect, we consider Pluto to represent the first known example of this group. Collectively, this group of small bodies has a great deal to tell us about the origin and evolution of the solar system and the collisions that have shaped Earth’s history.
Over the years, Lowell astronomers have been at the forefront of efforts aimed at cataloging and quantifying the characteristics of asteroids: their sizes, shapes, orbital properties, and surface chemical compositions. These bodies are generally small in diameter, 100 meters to 10 kilometers (328 feet to 6 miles), and they are predominantly located in a loose belt orbiting between Mars and Jupiter. Understanding the properties of asteroids not only provides insights into how these small bodies were formed, but can also reveal complex gravitational interactions among planets and smaller bodies that shuffled the distribution of planetary orbits during the formative phases of our solar system.
Over the 4.6-billion-year history of the solar system, a few percent of the asteroid population has been “scattered” via gravitational interactions with larger bodies and thrust into orbits having a wide range of elliptical shapes and inclinations to the plane defined by the orbits of the major planets. Some of these asteroids are now in orbits that cross Earth’s orbit, creating the potential of powerful collisions of the sort that gave rise to the Tunguska explosion over Russia in June 1908 and, much earlier (65 million years ago), to the large impact crater below the Gulf of Mexico thought to have initiated the series of events that led to the extinction of the dinosaurs.
For the past decade, Lowell astronomer Ted Bowell has led a research program aimed at searching for the small subset of asteroids that have the potential of approaching close to Earth. To search for asteroids that pose a potential collision hazard, Bowell has developed an automated telescope at Lowell to search for relatively large “near-Earth asteroids” and determine their orbital trajectories with high precision. Surveys like Bowell’s point the way toward more sensitive imaging surveys capable of detecting objects down to a diameter of 150 meters (492 feet) or less; a survey called Pan-STARRS is under way in Hawaii, and a powerful all-sky telescope called the Large Synoptic Survey Telescope is being planned for a site in Chile.
Since the discovery of Pluto by Tombaugh, astronomers had speculated that it was not the only object located beyond the orbit of Neptune. In the early 1950s, pioneering planetary astronomer Gerard Kuiper suggested the existence of an extensive family of objects in these outermost reaches of the solar system. In 1992, his speculation was confirmed by astronomers David Jewitt and Jane Luu, who discovered the first evidence of these bodies in what was soon called the Kuiper Belt.
Subsequent observations have shown that the Kuiper Belt comprises perhaps 100,000 bodies of sizes above 100 kilometers (62 miles) in diameter, with the largest bodies approaching or slightly exceeding the size of Pluto (about 1,000 kilometers, or 621 miles, in diameter). This estimate is based on the statistical distribution of sizes among currently observable objects orbiting the Sun in the Kuiper Belt region. The Kuiper Belt undoubtedly contains a huge number of objects much smaller than 100 kilometers in size; such objects are faint and difficult to observe, so securing an accurate estimate is challenging.
Echoing an original conjecture of Gerard Kuiper’s, astronomers now believe that both the large and small members of the Kuiper Belt family were formed in the outer parts of the primordial disk of gas and dust that gave rise to the planets (Mercury through Neptune), which formed in the inner regions of the disk.
The smaller objects in the Kuiper Belt are currently thought to represent the source of comets (such as Halley’s comet), whose orbits periodically carry them into the inner solar system. Numerical simulations of the gravitational interactions between the outer planets and Kuiper Belt objects (KBOs) confirm that this scenario can occur by altering (via gravitational interactions with other, larger bodies in the solar system) some fraction of their orbits from their nearly initially circular form to highly elliptical paths that plunge into the inner solar system. Like their rocky cousins in the asteroid belt, some of these comets must have—over the history of the solar system—collided with Earth, possibly bringing the bulk of both the water and the organic material that led to the development of life on our planet.
The distribution of KBO orbits is also beginning to shed light on the evolution of planetary orbits over the history of the solar system. In part as a result of careful analysis of the statistical properties of KBO orbits, astronomers are beginning to entertain seriously the remarkable hypothesis that the planets Uranus and Neptune may well have formed between the orbits of Jupiter and Saturn and migrated outward over time. The agent driving their migration is thought to be the gravitational pulls of Jupiter and Saturn, the two most massive planets in our solar system. As Uranus and Neptune migrated outward, the gravitational force exerted by these two planets affected the orbits of Kuiper Belt objects, sending some of them outward beyond the orbit of Neptune and others inward toward Mars, Earth, Venus, and Mercury.
Some theorists speculate that the “epoch of maximum bombardment” some 300 million to 500 million years after the birth of the solar system, which gave the Moon and Earth most of their craters, was triggered by this massive redirection of the orbits of KBOs.
Already, a growing handful of Pluto-sized bodies with evocative names such as Sedna and Makemake have been found and mapped. Within a decade or two, we may find that our solar system is home to dozens more bodies of Pluto’s size or larger, rendering the comparatively lonely “Nine Planets” a quaint relic of the twentieth century.
The converted MMT, featuring a monolithic 6.5-meter primary mirror in place of multiple smaller mirrors.