1604 Tombaugh

1604 Tombaugh, provisional designation, is a rare-type Eos asteroid from the outer region of the asteroid belt, approximately 32 kilometers in diameter. It was discovered on 24 March 1931, by American astronomer Carl Otto Lampland at Lowell Observatory in Flagstaff, Arizona, in the United States. It was named after the discoverer of Pluto, Clyde Tombaugh.

Classification and orbit
Tombaugh is a member of the Eos family that orbits the Sun in the outer main-belt at a distance of 2.7–3.3 AU once every 5 years and 3 months (1,920 days). Its orbit has an eccentricity of 0.10 and an inclination of 9° with respect to the ecliptic. Its observation arc begins at Flagstaff, one year prior to its official discovery observation at Lowell Observatory. It had been previously identified at Heidelberg as in 1920, and as  in 1930.

Physical characteristics
Tombaugh is classified as an X-type asteroid. It is also classified as a rare XSCU type in the Tholen, and as a transitional Xc type in the SMASS taxonomy.

Rotation period
In April 2010 and November 2012, rotational lightcurves of Tombaugh were obtained from photometric observations at Oakley Southern Sky Observatory, Australia, and at Bassano Bresciano Observatory, Italy. Lightcurve analysis gave a rotation period of 7.047 and 7.056 hours with a brightness variation of 0.16 and 0.35 magnitude, respectively (U=2+/2+).

These periods supersede previous results obtained by astronomers Claes-Ingvar Lagerkvist (1975), Richard P. Binzel (1984) and Krisztián Sárneczky (U=1/2/2).

Diameter and albedo
According to the surveys carried out by the Infrared Astronomical Satellite IRAS and the Japanese Akari satellite, Tombaugh measures 28.78 and 32.33 kilometers in diameter, and its surface has an albedo of 0.138 and 0.104, respectively. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.0933 and a diameter of 32.25 kilometers with an absolute magnitude of 10.65.

Naming
This minor planet was named for American astronomer Clyde Tombaugh (1906–1997), famous for his discovery of Pluto in 1930. The discovering Lowell Observatory named this asteroid on the occasion of a symposium on Pluto, held in 1980. When Tombaugh examined the photographic plates during the trans-Saturnian search program at the Lowell Observatory, he also marked over 4,000 minor planets on these plates. The approved naming citation was published by the Minor Planet Center on 1 April 1980 (M.P.C. 5280).