He continued his education at the University of Chicago where he obtained his doctorate in astronomy and astrophysics in 1956 under Professor Gerard P. Kuiper[citation needed].

[4] Gehrels pioneered the first photometric system of asteroids in the 1950s, and wavelength dependence of polarization of stars and planets in the 1960s, each resulting in an extended sequence of papers in the Astronomical Journal.

Gehrels initiated the Space Science Series of textbooks, was General Editor for the first 30 volumes of the University of Arizona Press, and set the style by participating in the editing of six of them.

Gehrels taught an undergraduate course for non-science majors in Tucson in the Fall, and lectured a brief version of that in the Spring at the Physical Research Laboratory in Ahmedabad, India.

Gehrels was requested by the Journal Nature to write a review on a book regarding Wernher von Braun, in which he quotes inmates of concentration camp Dora.