Tribute to Pluto's Discoverer

A memorial service was held today at New Mexico State University for Clyde Tombaugh, the astronomer who discovered Pluto. Tombaugh died on 17 January at his home in Las Cruces. He was 90.

Tombaugh spent much of his youth observing the sky. At the age of 22, he built a 9-inch telescope from discarded farm machinery and parts salvaged from his father's 1910 Buick. When he sent drawings of Jupiter and Mars to the Lowell Observatory in Flagstaff, Arizona, they offered him a job. Tombaugh helped search beyond Neptune for a "Planet X," which astronomer Percival Lowell had predicted based on perturbations in Neptune's orbit.

Using a machine called a blink-comparator, Tombaugh examined hundreds of photos of identical sections of the sky on different nights. In the blink-comparator, a light would rapidly switch between pairs of photos, making stars look fixed. Against this background, a planetary body moving in its orbit would appear to jump. On 18 February 1930, Tombaugh found the mystery planet, which the Lowell staff later named Pluto after the Greek god of the dimly lit underworld Hades.

"He did what some giants couldn't do," says Alan Stern, an astronomer at the Southwest Research Institute in Boulder, Colorado. "He was a very thorough observer." Tombaugh was an iron man, too: At age 90, he attended a lecture at New Mexico State University in a wheelchair, breathing from oxygen tanks. "I was floored by his energy level and his incisive questions," says Stern.

After finding Pluto, Tombaugh discovered six star clusters, two comets, hundreds of asteroids, several dozen clusters of galaxies, and one supercluster. He also earned bachelor's and master's degrees from the University of Kansas. After helping test captured German V-2 rockets in 1946 at White Sands Missile Range, he went to New Mexico State University, where he taught until 1973.

Tombaugh was an active astronomer to the end. When the Smithsonian Institution requested his first 9-inch telescope, Tombaugh demurred, saying he was still using it in his backyard. By his retirement in 1973, Tombaugh had confirmed the length of Mercury's day, measured the swirling vortex of Jupiter's Great Red Spot, and built a telescope in Ecuador to search for rocky debris circling the Earth.