2391 Tomita, provisional designation 1957 AA, is a Nysian asteroid from the inner regions of the asteroid belt, approximately 15 kilometers in diameter. The asteroid was discovered on 9 January 1957, by German astronomer Karl Reinmuth at Heidelberg Observatory in southern Germany. ^[10] It was named after Japanese astronomer Kōichirō Tomita. ^[2]

Tomita is a member of the Nysa family, which is named after its most massive member 44 Nysa. It orbits the Sun in the inner main-belt at a distance of 2.1–2.8  AU once every 3 years and 10 months (1,393 days). Its orbit has an eccentricity of 0.13 and an inclination of 3 ° with respect to the ecliptic. ^[1]

A first precovery was taken at Lowell Observatory in 1929, extending the body's observation arc by 28 years prior to its official discovery observation at Heidelberg. ^[10]

Tomita has been characterized as a stony S-type asteroid by the Collaborative Asteroid Lightcurve Link (CALL), and as a carbonaceous C-type asteroid by Pan-STARRS' photometric survey. ^{[3] [9]}

According to the surveys carried out by NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, the asteroid measures between 15.07 and 19.4 kilometers in diameter and its surface has a low albedo between 0.03 and 0.07, respectively. ^{[4] [5] [6] [7] [8]}

As CALL considers the body to be of a stony composition, it assumes a much higher albedo of 0.21 and calculates a diameter of 9.2 kilometers, as the higher the asteroid's reflectivity (albedo), the shorter its diameter at a constant absolute magnitude (brightness). ^[3]

In December 2013, two rotational lightcurves were obtained for this asteroid from photometric observations. They gave a rotation period of 7.9533±0.0005 and 8.435±0.079 hours with a brightness variation of 0.14 and 0.15 in magnitude, respectively. ( U=3/n.a.). ^{[a] [b]}

This minor planet was named in honor of Japanese astronomer Kōichirō Tomita (1925–2006), long-time observer at the Tokyo Astronomical Observatory, and a discoverer of minor planets and comets himself. Tomita was also known as one of Japan's principal popularizer of astronomy. ^[2] The official naming citation was published by the Minor Planet Center on 14 April 1987 ( M.P.C. 11748). ^[11]

• Asteroid Lightcurve Database (LCDB), query form ( info Archived 16 December 2017 at the Wayback Machine)
• Dictionary of Minor Planet Names, Google books
• Asteroids and comets rotation curves, CdR – Observatoire de Genève, Raoul Behrend
• Discovery Circumstances: Numbered Minor Planets (1)-(5000) – Minor Planet Center
• 2391 Tomita at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 2391 Tomita at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters