环月探测器利用轨道近月点高度不断降低的规律实现软着陆

Soft Landing of Lunar Orbiter Using the Descending of Orbit Perigee＇ s Altitude

由于月球自转较慢，其非球形引力位有一显著特征：扁率小，其动力学扁率J2=O（10^-4），而其南北半球极不对称（包括形状和质量分布），反映这一不对称的奇次带谐项J2l-1（l≥2）中若干项与J2的差别只有1个量级，即｜J2l-1/J2｜=O（10^-1）。由于这一特征，低轨月球卫星的轨道偏心率e有变幅较大的长周期变化△el，从而使得轨道的近月点高度在一个时间段内不断降低，而且这一现象还与轨道倾角i有关。例如初始平均高度分别为100km、75km、50km的近圆轨道，i=45°时，分别于49天、41天和21天后，近月点高度hp即降为0，轨道器在近月点处与月球以相切的状态相碰。而当i=90°时，则分别于172天、104天和61天出现上述相碰的状态。这种状态正可以被利用（加以制动）使探测器在月球上实现软着陆。

As a result of slow rotation of the moon, its gravity field has the following characteristics., small dynamical oblateness J2=O（10^-4）; and several of the odd zonal items J2l-1 （l≥2） which reflect asymmetry of mass in north and south direction have only one order difference from J2, i. e. ,｜J2l-1/J2｜=O（10^-1）. Consequently, the orbit eccentricity e has a large amplitude variation △el in its long-periodic term, which can lower the altitude of perilune constantly during a certain period of time, and this characteristics is also related to orbit inclination. For example, hp of near circular orbits with average altitude of 100km, 75km, 50kin will lower to zero after 50, 41 and 21 days respectively when i=45°. This means that the orbiter will arrive at the moon along tangential direction to its surface. When i=90°, this situation will come true after 172, 104 and 61 days. This state could be used for soft landing of a spacecraft on the moon.