The first step results of the study on the orbits of 1.5-3 AU orbit radii around asteroids with maximum radii below 100 km performed aiming at the global observation of the asteroids are reviewed in this issue. Firstly, the frozen orbit, which can be realized in case that the solar radiation pressure alone is the perturbation, was studied and found that, among the two kinds of the frozen orbits, the orbit in the orbital plane of the asteroid is feasible for the asteroid with a radius between 20-50 km and the solar-plane-sky orbit is feasible for the asteroid with a radius below 2-3 km. It was also found that the increase in the velocity of about 100 m/s during one year is necessary to maintain the orbit by canceling the orbital change due to the oblateness by maneuvers. Secondly, the polar orbit which is adequate for the global observation of asteroids and it is not affected by the oblateness was studied and found that the increase in the velocity needed during one year is less than 1 m/s and the polar orbit is advantageous observation, fuel cost and operation. Taking into account of the result of study on the polar orbit maintenance by the numerical integration of the variation rate of the averaged orbital element, it was concluded as follows: (1) for asteroids whose radii are less than 2 or 3 km, the solar plane-of-sky frozen orbit is the first candidate. The expected resolution is in the order of cm-10 cm; and (2) for asteroids whose radii are larger than 2 or 3 km, the polar orbit is the first candidate. The expected resolution is in the order of 10 cm-1 m.
小惑星周回軌道の研究
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