Clyde William Tombaugh (1906–1997)

Clyde Tombaugh, known for his discovery of Pluto in 1930, was born on 4 February 1906 in Streator, illinois, and died of congestive heart failure on 17 January 1997 in his home in Las Cruces, New Mexico.

Clyde spent his childhood on farms in Illinois and Kansas. He was introduced to astronomy by his uncle, who owned a 3-inch telescope. After graduating from high school, Clyde made his own 9-inch reflector and began an active program observing the planets.

In 1928, Clyde sent a set of drawings of Mars to Lowell Observatory, seeking suggestions to improve his technique. At that time, the administrators were planning to launch a systematic program to seek the ninth planet. Recognizing a skilled observer and knowing that Clyde had worked under winter conditions, they offered him a job at Lowell, where he began work in January 1929.

Clyde had the privilege of bringing a new telescope on line and developing an observing procedure for seeking a distant planet. In order to avoid asteroids, he developed a plan to obtain plate pairs, separated by 3-6 days, when the selected star field was near the meridian. He reasoned that at this time the motion of the other bodies would be nearly perpendicular to his line of sight and, while the displacement of distant planets among the stars would be due mainly to the earth's orbital motion, the additional motion of asteroids would allow them to be excluded from the search.

Although Percival Lowell had predicted a planet, based on orbital positions of Neptune and Uranus, Clyde's search had been underway for 10 months before he found Pluto on plates exposed on 23 and 29 January and blinked on 18 February. The discovery was announced on March 13, 1930. Pluto was disappointingly faint. Various suggestions, such as assuming the surface was a smooth glassy sea of ice that generated specular reflection, were made in an effort to reconcile this faintness with Lowell's prediction. The lack of agreement spurred Clyde to continue his systematic search for other bodies.

After receiving the Edwin Emery Slosson Scholarship and joining the AAS in 1931, Tombaugh began college at the University of Kansas in 1932 and continued to work at Lowell during the summer. He received BA and MS degrees from Kansas in 1936 and 1939, and an honorary DSc from Arizona State College in 1960. At KU, he met Patricia (Patsy) Edson, a philosophy major. They married in 1934 and had two children, Annette and Alden. Clyde is also survived by five grandchildren and eight great-grandchildren.

After graduation, Clyde returned to Lowell and carried out a systematic search for other bodies in the outer solar system. This photographic program, centered on the ecliptic, covered 75% of the sky. As Clyde would point out, this study would have found planets brighter than 16.5 mag., and, had there been an earth-sized planet within 100 AU, it would have been detected. Although the general public reveres a discoverer, the null result from this systematic study is far more important. It has shaped the thinking of the astronomical community for more than 50 years. It has influenced both our models of solar system formation and our search for planetary systems around other stars. Byproducts of the study included the discovery of a cataclysmic variable (TV Corvis), six star clusters, two comets, observations of a number of asteroids and clusters of galaxies, and the discovery of one supercluster.

During World War II, Clyde taught navigation. Following the war, he came to White Sands Proving Grounds and developed tracking systems to determine flight paths and characteristics of rockets as we entered the space age. In 1955, he moved his operations to New Mexico State University, and from 1953 to 1958 directed a search for natural earth-orbiting debris. This program was carried out in Ecuador, where the axis of the telescope was mounted parallel to the ground and the drive was capable of various rates. Rates were selected for objects at assumed distances, and resulting exposures contained reference star trails and assured maximum exposure of faint sources. Again a systematic search yielded a null result. This time the result was welcome; near space was not hostile to manned activity.

At New Mexico State University, Clyde assembled a team that provided a systematic set of planetary images that have been used to support the Mariner, Viking, and Voyager missions. Tombaugh was instrumental in designing and obtaining funding for the university's Tortugas Mountain Observatory, a 24-inch telescope (see picture) that captured its first images in 1967 and is still in service taking data for NASA. Bradford Smith, team member on the Mars Mariner and Viking missions and Principal Investigator for the Voyager imaging team, began his career with Clyde's group. Brad continued Clyde's involvement in the space program, and we at NMSU continue his work with involvements in exploration of the giant planets and the search for other planetary systems.

From 1955 to retirement in 1973, Tombaugh taught both geology and astronomy classes. His commitment was contagious and his interest and dedication to public education did not flag as he entered retirement. He continued to be a strong influence on our students and made an amazing effort to satisfy the demands of the public. In 1980, in collaboration with Patrick Moore, he published his version of the discovery of Pluto, Out of the Darkness: The Planet Pluto.

Clyde reacted to lagging professional opportunities for young scientists by committing Patsy and himself to raising funds to establish the Clyde W. Tombaugh Fellowship at New Mexico State. Assisted by Bernard McNamara, he toured the US and Canada from 1985 to 1990, presenting public lectures and raising funds.

Clyde remained involved in NASA activities. He received the NASA Distinguished Public Service Medal in 1980 in recognition of his efforts. He followed plans for Mars exploration with avid interest and collaborated with the Jet Propulsion Lab team to define a feasible mission to Pluto. In recent years, he worked with camera crews to define the significance of such a mission and retained strong hopes for continuing space exploration.

Clyde was a warm and stimulating friend who encouraged inventive thinking and perseverance. To the last of his life, when he was not inflicting puns on his friends and colleagues, he was enthusiastically following new developments. We will miss him, the only true plutocrat of our time. It is expected that Tombaugh's scientific papers will be archived at NMSU.

Photo (available in PDF version): Clyde Tombaugh in 1979, with the 24-inch Planetary telescope and planetary camera at New Mexico State University, Las Cruces (courtesy Reta Beebe).