He was involved in establishing the Spacewatch program for finding near-Earth asteroid ( NEO = Near Earth Object).
Gehrels discovered several comets, including 64P/Swift-Gehrels, 78P/Gehrels, 82P/Gehrels, and 90P/Gehrels.
He was also involved in the discovery of over 4329 asteroids, including the Apollo asteroid (1864 ) and Daedalus ( 5011 ) Ptah, the Amorasteroid ( 4587 ) Rees, and several asteroids from the group of Trojans.
He made most of his later discoveries, together with the astronomer Cornelis Johannes van Houten couple and Ingrid van Houten - Groeneveld as part of the Palomar - Leiden Survey. This Gehrels made extensive sky shots with the 48'' -Schmidt camera of the Mount Palomar Observatory. The photographic plates were shipped to the Netherlands and evaluated at the University of Leiden for the determination of asteroids.
During World War II Gehrels was a member of the Dutch resistance.
Tom Gehrels is the father of Neil Gehrels.
- On the Glassy Sea: An Astronomer 's Journey, Tom Gehrels, ISBN 0-88318-598-9
- A review of comet and asteroid statistics, Tom Gehrels, Earth Planets Space, 51, 1999 ( PDF)