| Minorplanet: | yes |
| 1999 Hirayama | |
| Background: |
|
| Discovered: | 27 February 1973 |
| Mpc Name: | (1999) Hirayama |
| Alt Names: | 1973 DR1935 GF 1940 EH 1951 FA1965 UF 1969 NB1975 NE |
| Named After: | Kiyotsugu Hirayama |
| Epoch: | 4 September 2017 (JD 2458000.5) |
| Uncertainty: | 0 |
| Observation Arc: | 77.24 yr (28,212 days) |
| Perihelion: | 2.7575 AU |
| Semimajor: | 3.1166 AU |
| Eccentricity: | 0.1152 |
| Period: | 5.50 yr (2,010 days) |
| Mean Motion: | / day |
| Inclination: | 12.529° |
| Asc Node: | 148.03° |
| Arg Peri: | 357.05° |
| Mean Diameter: | 33.77 km km km km |
| Rotation: | h h |
| Albedo: | 0.0617 |
| Abs Magnitude: | 10.610.9011.0 |
1999 Hirayama (prov. designation:) is a dark background asteroid from the outer region of the asteroid belt, approximately 34km (21miles) in diameter. It was discovered on 27 February 1973, by Czech astronomer Luboš Kohoutek at the Hamburger Bergedorf Observatory in Germany, and later named after Japanese astronomer Kiyotsugu Hirayama.
Hirayama orbits the Sun in the outer main-belt at a distance of 2.8–3.5 AU once every 5 years and 6 months (2,010 days). Its orbit has an eccentricity of 0.12 and an inclination of 13° with respect to the ecliptic. The spectrum of Hirayama matches a C-type classification on the Tholen taxonomic scheme, but with a "broad absorption band that can be associated to a process of aqueous alteration". That is, the surface appears to show some form of water modification.
This minor planet is named in honour of Japanese astronomer Kiyotsugu Hirayama (1874–1943), best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called Hirayama families. The lunar crater Hirayama is also named in his honour. The official was published by the Minor Planet Center on 15 October 1977 .
A rotational lightcurve of Hirayama was obtained at the Menke Observatory in February 2002. It showed a periodicity of hours, during which time the brightness of Hirayama varies by in magnitude . At the same time, photometric observations by astronomers Roberto Crippa and Federico Manzini gave a rotation period of 22.37 hours and a brightness variation of 0.47 magnitude . These results supersede an observation from January 2005, by Hiromi and Hiroko Hamanowa at their Hamanowa Astronomical Observatory, Japan, that gave a shorter period of 13.59 hours with an amplitude of 0.57 magnitude..
According to the surveys carried out by the Infrared Astronomical Satellite IRAS, the Japanese Akari satellite, and NASA's Wide-field Infrared Survey Explorer with its subsequent NEOWISE mission, the asteroid measures between 34.0 and 38.3 kilometers in diameter and its surface has an albedo between 0.053 and 0.088. The Collaborative Asteroid Lightcurve Link derives an albedo of 0.062 and a diameter of 33.8 kilometers with an absolute magnitude of 11.0.