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Discovery
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| Discovered by | Luboš Kohoutek |
| Discovery date | February 27, 1973 |
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Designations
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| Named after | Kiyotsugu Hirayama |
| Alternate name(s) | 1935 GF; 1940 EH; 1951 EY1; 1951 FA; 1965 UF; 1969 NB; 1973 DR; 1975 NE |
| Minor planet category |
Main belt |
| Epoch June 14, 2006 (JD 2453900.5) | |
| Aphelion | 516.937 Gm (3.456 AU) |
| Perihelion | 416.446 Gm (2.784 AU) |
| Semi-major axis | 466.692 Gm (3.120 AU) |
| Eccentricity | 0.108 |
| Orbital period | 2012.590 d (5.51 a) |
| Average orbital speed | 16.81 km/s |
| Mean anomaly | 26.858° |
| Inclination | 12.489° |
| Longitude of ascending node | 148.349° |
| Argument of perihelion | 355.105° |
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Physical characteristics
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| Dimensions | 34.0 km |
| Mass | 4.1×1016 kg |
| Mean density | 2.0 g/cm³ |
| Equatorial surface gravity | 0.0095 m/s² |
| Escape velocity | 0.0180 km/s |
| Rotation period | ? d |
| Albedo | 0.10? |
| Temperature | ~158 K |
| Spectral type | ? |
| Absolute magnitude (H) | 10.6 |
1999 Hirayama is an asteroid. It is named after Kiyotsugu Hirayama, the Japanese astronomer 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.
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