(21821) Billryan Main-belt Asteroid
Discovered on 12 October 1999 by LONEOS at Anderson Mesa.
William H. Ryan (b. 1962) is an astrophysicist at the New Mexico Institute of Mining and Technology's Magdalena Ridge Observatory. Ryan discovered that (3782) Celle has a companion, making it the first identified binary vestoid; and later, that vestoid (3703) Volkonskaya is also binary.Naming citation (20080718/MPCPages.arc), IAU / Minor Planet Center, via NASA/JPL
Where Billryan is today
Seen from above the solar system on 2026-10-14: Billryan is 2.94 au from the Sun and 3.82 au from Earth. Computed from JPL's orbital elements.
Facts
| Number | 21821 |
| Provisional designation | 1999 TN36 |
| Orbit class | Main-belt Asteroid |
| Absolute magnitude (H) | 14.84 |
| Orbital period | 5.04 years |
| Semi-major axis | 2.94 au |
| Perihelion – aphelion | 2.67 – 3.21 au |
| Eccentricity | 0.0904 |
| Inclination | 2.89° |
| Closest distance between its orbit and Earth's (MOID) | 1.69 au |
Data: NASA/JPL Small-Body Database and the IAU Minor Planet Center.
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