Kirkwood's most significant contribution came from his study of asteroid orbits. When arranging the then-growing number of discovered asteroids by their distance from the Sun, he noted several gaps,[3] now named Kirkwood gaps in his honor, and associated these gaps with orbital resonances with the orbit of Jupiter. Further, Kirkwood also suggested a similar dynamic was responsible for Cassini Division in Saturn's rings, as the result of a resonance with one of Saturn's moons. In the same paper, he was the first to correctly posit that the material in meteor showers is cometary debris.
Kirkwood also identified a pattern relating the distances of the planets to their rotation periods, which was called Kirkwood's Law. This discovery earned Kirkwood an international reputation among astronomers; he was dubbed "the American Kepler" by Sears Cook Walker, who claimed that Kirkwood's Law proved the widely held Solar Nebula Theory. The "Law" has since become discredited as new measurements of planetary rotation periods have shown that the pattern doesn't hold.
Altogether he wrote 129 publications, including three books. The asteroid 1951 AT was named 1578 Kirkwood in his honor and so was the lunar impact crater Kirkwood, as well as Indiana University's Kirkwood Observatory. He is buried in the Rose Hill Cemetery in Bloomington, Indiana, where Kirkwood Avenue is named for him.
