Tombaugh draws the diameters of Earth and Mars to illustrate the planets’ comparative sizes. Earth's diameter is 7900 miles against Mars diameter of 4215 miles. On his Mars diagram, Tombaugh draws what would seem to be his understanding of the pitted Martian surface. His observations of Mars led him to conclude that Mars had been hit by asteroids causing observable craters due to the planet's proximity to the asteroid belt. The Mariner 4 space probe affirmed his hypotheses. The astronomer best known for identifying Pluto has signed, "Clyde W . Tombaugh" in a bold hand. A fine, visually instructive illustration.
The asteroid 1604 Tombaugh discovered in 1931 is named after him. He discovered 14 asteroids, beginning with 2839 Annette in 1929, mostly as a byproduct of his search for Pluto and his further searches for other celestial objects.