didymos with its moonArtist impression

(65803) Didymos (1996 GT) is an Apollo asteroid discovered on April 11, 1996 by Spacewatch at Kitt Peak. It has a satellite orbiting it with a period of 11.9 hours, hence the appellation "Didymos", meaning "twin". Following are some describing parameters:

Semi-major axis: 1.644 AU
Orbital Period: 770.14 days
Allows to cheaply achieve a 2:1 resonance (2 s/c revolutions : 1 asteroid revolution)
Eccentricity: 0.384
Inclination: 3.4 deg
Geometric albedo: 0.147
Diameter primary: 800 m
Diameter secondary: 150 m
Separation: 1050 m
Orbital period secondary: 11.9 h (almost circular orbit)
Orientation of the mutual orbital plane is known (2 solutions)
Propulsion selection and interplanetary transfer


From Radar observations: preliminary shape model of the primary (principal axis view). Lambda and beta are pole coordinates (two solutions exist for the moment).The secondary (not imaged) estimated to be more elongated from other data (courtesy of Lance Benner).