Spacecraft orbits around asteroids for global mapping

A study is presented on spacecraft orbits around asteroids, the purpose of which is to observe asteroids globally with high resolution. Objects of the study are asteroids smaller than 100 km in size with orbital radii between 1.5 and 3 AU. If the radiation pressure is the only perturbation, a frozen orbit whose Keplerian mean elements do not change exist. There are two kinds of frozen orbits: one is the orbit in the orbital plane of the asteroid, which is not adequate for high-resolution observation, and the other is the solar plane-of-sky orbit, which is adequate for the asteroids whose radii are below 2 or 3 km. The polar orbit, which is suitable for the global observation and has the merit that effects of the oblateness disappear, was also studied. Basically, the maintenance of the polar orbit requires only making the eccentricity zero. The maintenance was investigated by numerical integrations for the averaged orbital elements considering the solar radiation pressure and the oblateness, When the ratio of the characteristic angular velocity due to the radiation pressure and the one due to the oblateness multiplied by the eccentricity limit for the maintenance is smaller than about 1, the maintenance is not necessary.