4951 Iwamoto

4951 Iwamoto, provisional designation, is a stony, synchronous binary asteroid and slow rotator from the inner regions of the asteroid belt, approximately 5.5 kilometers in diameter. It was discovered on 21 January 1990, by Japanese astronomers Yoshikane Mizuno and Toshimasa Furuta at Kani Observatory (403) in Japan.

Orbit
Iwamoto orbits the Sun in the inner main-belt at a distance of 1.9–2.6 AU once every 3 years and 5 months (1,239 days). Its orbit has an eccentricity of 0.17 and an inclination of 8° with respect to the ecliptic. It was first identified as at Lowell Observatory in 1931, extending the body's observation arc by 59 years prior to its official discovery observation at Kani.

Physical characteristics
In the SMASS classification, Iwamoto is a common S-type asteroid.

Diameter and albedo
According to the survey carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, Iwamoto measures 5.192 and 5.515 kilometers in diameter, and its surface has an albedo of 0.218 and 0.186, respectively. The Collaborative Asteroid Lightcurve Link adopts Petr Pravec's revised WISE-data, that is, an albedo of 0.1844 and a diameter of 5.528 kilometers with on an absolute magnitude of 13.74.

Slow rotator
From 25 December 2006 to 23 March 2007, photometric observations of Iwamoto were obtained by the international community of photometrists at Badlands Observatory (SD, USA), Ondřejov Observatory (Czech Republic), Modra Observatory (Slovakia), Carbuncle Hill Observatory (RI, USA), Sonoita Research Observatory (AZ, USA), Kharkiv Observatory (Ukraine), McDonald Observatory (TX, USA), Ironwood Observatory (HI, USA), Leura Observatory (Australia), Skalnaté pleso Observatory (Slovakia), Shed of Science Observatory (MN, USA), Pic du Midi Observatory (France).

Lightcurve analysis gave a rotation period of 118 hours with a brightness variation of 0.34 magnitude (U=3). In May 2011, astronomers Etienne Morelle, Raoul Behrend obtained another lightcurve with a concurring period of 118 hours and an amplitude of 0.38 magnitude.(U=3). With such a long period, Iwamoto is also a slow rotator, as the vast majority of asteroids have a much shorter rotation period of 2.2 to 20 hours.

Binary system
During the photometric observations in 2006/7, it was revealed that Iwamoto ("primary") is a synchronous binary system with a minor-planet moon ("secondary") orbiting it every 4.917 days (or 118 hours, which identical to the primary's rotation). Based on the secondary-to-primary mean-diameter ratio (Ds/Dp) of at least 0.76, it was estimated that Iwamoto and its moon measure 4.0 and 3.5 kilometers, respectively. The diameter of Iwamoto has since increased to 5.5 kilometers (see above). The "Jonstonarchive" estimates that the moon has a semi-major axis of 31 kilometers.

Naming
This minor planet was named in honor of Japanese astronomer Masayuki Iwamoto (born 1954), a discoverer of minor planets at the Tokushima Observatory (872). The approved naming citation was published by the Minor Planet Center on 5 March 1996 (M.P.C. 26763).