• Home
  • CLYDE TOMBAUGH: Return to Pluto - New Horizons 2015

CLYDE TOMBAUGH: Return to Pluto - New Horizons 2015

Friday, July 10, 2015

Clyde Tombaugh, discoverer of Pluto, may rank as one of the best-known Kansans, as well as an alumnus of the University of Kansas. He was born in Feb. 1906 in Streator, IL, but moved with his family in 1922 to Burdett, KS. An avid amateur astronomy buff, Tombaugh was known for building his own telescopes from available farm equipment and investing long hours of observing to improve his equipment and his familiarity with the skies. He hoped to attend KU in 1929, but the loss of his family's crops in a hailstorm coupled with the Depression made that impossible. While pursuing his avocation for astronomy, he contacted Lowell Observatory for feedback about his sketches of observations of the planets made through his homemade telescope and, surprisingly, was offered a job at the Observatory, assisting in the rekindled search for Planet X, the predicted ninth planet of the solar system, expected to orbit the Sun beyond the planet Neptune.

The task of detecting a planet centered on the comparison of photographic images of an area of the sky taken over multiple evenings. The images were recorded on glass plates; an example of the size of the plate can be seen in the photo at left where Tombaugh is carrying the plate holder for the telescope as he enters the observatory. If one compared the location of a planet (or comet or asteroid) from one night to the next it could be expected to move, unlike the fixed stars. To detect this motion, the primary instrument in use was a device called a blink comparator, which held two glass plates of the same field of view taken on different nights, illuminated by a light source. A stereoscopic viewer with an internal mirror allowed the observer to switch from viewing one plate to the other and then back again repeatedly every few seconds. If, for example, a star was varying in brightness, it would blink on and off as it went bright then faint. For planets, comets and asteroids, the point of light would appear to jump from one position to the next and back again. An image of the blink comparator used by Tombaugh is shown at left, while the picture on the right shows Tombaugh at the device. For a simulation of how the comparator worked, check out this link.

The job of taking the plates and surveying the plates was mind-numbing and exhausting. However, the search wasn't random. The area of the sky which was the focus of the project was a location predicted to contain the planet by Percival Lowell, the wealthy Bostonian who founded the observatory with the explicit goals of studying Mars and searching for planet X. The predicted location was based upon deviations in the orbit of Neptune, supposedly caused by gravitational pertubations due to the next planet out in the solar system. These deviations were expected because a similar approach had been used in the 19th century to discover Neptune, a large Jovian planets whose mass is significant enough to cause Uranus to deviate over time from its predicted orbital path. The bizarre aspect of the story is that when Pluto was discovered in Feb. 1930 in the general area predicted by Lowell, it was recognized that it was too low in mass and too far removed from Neptune to be the source of the orbital discrepancies; later studies found that the orbital deviations did not exist but were due, instead, to observational errors. In that sense, Pluto's discovery was serendipitous.

With the discovery of Pluto, Clyde Tombaugh became world-famous for his astronomical Clyde Tombaugh at the 1980 dedication of the KU Observatoryprowess despite having only a high school education. Thanks to a scholarship from KU, Tombaugh eventually returned to Lawrence where he met his wife Patsy and received a BA degree in 1936 and an MA in Astronomy in 1939. For a more complete history of Tombaugh and his KU connection, check out the article at this link on KU History. Clyde Tombaugh was a dedicated promoter of both amateur and professional astronomy, playing a critical role in development of the astronomy program at New Mexico State University, where he served as a faculty member for decades, and at KU, where the campus observatory carried his name (phot at right: Clyde Tombaugh at the 1980 dedication of the KU Observatory) until the 27-inch telescope on the roof of Lindley Hall was shut down in 2002. This telescope, used by Clyde for his master's thesis, still lives at Farpoint Observatory outside Topeka, under the operation of the Northeast Kansas Amateur Astronomical League. The telescope was rebuilt and upgraded with modern technology (photo at left) and used by NEKAAL for public observing and to search for Near-Earth-Objects, asteroids detected by their motion against the background of fixed stars.

The Pluto Controversy: A Short Version

As noted earlier, while Pluto was discovered in the area of the sky predicted by Lowell, it quickly became apparent that the planet was too far and too small to meet the characteristics expected for the ninth planet of the solar system. Moreover, Pluto was found to have an unusually inclined. non-circular orbit which carried it inside the orbit of Neptune and well away from the plane defined by the other planets. Because of its low surface brightness and small angular size on the sky, additional information about the fundamental properties of the planet remain difficult to obtain for decades. The decline in the status of Pluto bears strong similarities to the 19th century discovery and decline of Ceres, predicted to exist between the orbit of Mars and Jupiter to maintain the numerological pattern defined by the Titius-Bode Law. Once it was realized that Ceres was much smaller than the other terrestrial planets and that thousands of objects populated the asteroid belt, it was reclassified as simply a large asteroid.

For Pluto, the decline reached something of an extreme with the tongue-in-cheek publication of an editorial by Dessler and Russell in 1980 promoting the importance of studying Pluto soon because, based upon its declining mass estimates over the 50 years it had been studied, one could predict that it would disappear in the near future. The turnaround in recognizing Pluto's relevance for planetary astronomy occurred about the same time with the discovery of Pluto's biggest moon, Charon, in 1978 by James Christy. It is now known that Pluto has at least 5 moons. Equally important, between 1979 and 1999, Pluto was located closer to the Sun than Neptune, 20 years out of the 248 years it takes Pluto to orbit the Sun once, making observations optimal.

However, the most critical breakthrough came in 1992 with the discovery by David Jewitt and Jane Luu of the first object in the Kuiper Belt, an extensive torus of subplanetary size objects that appear to be a frozen debris left from the formation of the solar system. The number of objects categorized to date is over a thousand. one of which, Eris, is larger than Pluto. The discovery of this vast reservoir of primordial solar system material eventually helped provide justification for New Horizons and study of the region of the solar system beyond Neptune, a zone which extends a thousand times farther than the orbit of Neptune to the true edge of the solar system defined by the Oort Cloud. So, while the General Assembly of the IAU in 2006 declined to accept the recommendations of its own committee and redefined the term planet in a manner which excluded Pluto, from a scientific standpoint, the significance of Pluto now and in the future as the definitive representative of a class of objects which should supply more insight into the origin and evolution of the solar system than the traditional eight planets is firmly established. The flyby of the Plutonian system on July 14, 2015 will be the first step in the revolution of what we know about trans-Neptunian objects. If New Horizons survives the encounter, it will be transmitting its data from the flyby for at least a year, with long-term plans to redirect the craft to travel to at least one additional TNO before ending its mission. The person who would be least surprised by the resurrection and rehabilitation of Pluto is Clyde Tombaugh, who understood that important things sometimes come is small packages.

(For an extended look at New Horizons and its mission, check out the NASA mission web site at

  Clyde Tombaugh Memorial Window Unitarian Universalist Church of Las Cruces, NM










The Tombaugh Fund was established and funded between 1986 and 1992 thanks to the generous donation of an anonymous supporter and matching donations from faculty and alumni of the Department of Physics and Astronomy. The Fund is named in honor of KU's most famous astronomy alumnus, Clyde Tombaugh, discoverer of the planet Pluto, and the namesake of the campus observatory. The Fund was set up to support the needs of the undergraduate majors within the Astronomy program and occasional equipment needs of the Observatory. A prime focus of the Fund was the support of undergraduates involved in research over the summer months; Tombaugh Summer Interns received $1000 for living expenses to remain in Lawrence and work with a faculty member on an astrophysically-related project. In 2006, the award was changed to a four-year scholarship of up to $1000/yr for an undergraduate Astronomy major. The Fund was returned to an undergraduate research fund in 2012.

The success of the Tombaugh Fund has only been possible due to the generosity of a wide array of individuals linked by a common  interest in maintaining and improving the quality of the Astronomy program at KU, especially in expanding the opportunities available to the heart and soul of the program, the students. Donations to the Tombaugh Fund are always needed and always welcome, no matter what the size. Clyde Tombaugh himself generously contributed to the support of the Fund  in 1995 by donating autographed copies of his biography by David Levy. Of the original 100 signed copies, only a handful remain and are available to any donor who contributes $500 or more to the Tombaugh Fund.

Donations to the Tombaugh Fund can be made directly and safely on-line at the link below:

Clyde W. Tombaugh Student Research Fund

If you have any interest in supporting Astronomy at KU, feel free to contact  the Department of Physics and Astronomy by email or phone at
physics@ku.edu or 785 - 864 - 4626
or the
KU Endowment Association: Jenna Goodman
j207g595@ku.edu or 785 832 7417
Prof. Bruce Twarog
or 785-864-5163