Tom Gehrels (February 21, 1925 – July 11, 2011), was an American astronomer

Astronomer

An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...

, Professor Planetary Sciences, and Astronomer at the University of Arizona

University of Arizona

The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...

, Tucson.

Gehrels, who was born at Haarlemmermeer

Haarlemmermeer

Haarlemmermeer is a municipality in the Netherlands, in the province of North Holland. It is a polder, consisting of land reclaimed from water, and the name Haarlemmermeer means Haarlem's Lake, still referring to the body of water from which the region was reclaimed in the 19th century.Its main...

, the Netherlands

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

, pioneered the first photometric system of asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s in the 1950s, and wavelength dependence of polarization

Polarization

Polarization is a property of certain types of waves that describes the orientation of their oscillations. Electromagnetic waves, such as light, and gravitational waves exhibit polarization; acoustic waves in a gas or liquid do not have polarization because the direction of vibration and...

 of star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s and planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s in the 1960s, each resulting in an extended sequence of papers in the Astronomical Journal

Astronomical Journal

The Astronomical Journal is a peer-reviewed monthly scientific journal owned by the American Astronomical Society and currently published by Institute of Physics Publishing. It is one of the premier journals for astronomy in the world...

.

He discovered, jointly with the husband and wife team of Cornelis Johannes van Houten

Cornelis Johannes van Houten

Cornelis Johannes van Houten was a Dutch astronomer, sometimes referred to as Kees van Houten.Born in The Hague, he spent his entire career at Leiden University except for a brief period as research assistant at Yerkes Observatory...

 and Ingrid van Houten-Groeneveld

Ingrid van Houten-Groeneveld

Ingrid van Houten-Groeneveld is a Dutch astronomer.In a jointly-credited trio with Tom Gehrels and her husband Cornelis Johannes van Houten, she was an extremely prolific discoverer of many thousands of asteroids...

, over 4000 asteroid

Asteroid

Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, including Apollo asteroid

Apollo asteroid

The Apollo asteroids are a group of near-Earth asteroids named after 1862 Apollo, the first asteroid of this group to be discovered by Karl Wilhelm Reinmuth...

s, Amor asteroid

Amor asteroid

The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of the Earth from beyond, but do not cross it. Most Amors do cross the orbit of Mars...

s, as well as dozens of Trojan asteroid

Trojan asteroid

The Jupiter Trojans, commonly called Trojans or Trojan asteroids, are a large group of objects that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of the planet's two Lagrangian points of stability, and , that respectively lie 60° ahead...

s.