lambert

   求解Lambert问题。 输入：轨道上两个点的位矢R1、R2，以及从R1到R2所用的时间t(缺点是脉冲太大)。 输出：质点在R1端点的速度V1，质点在R2端点的速度V2. 此函数用于：在给定轨道上两个点的位矢R1、R2，以及从R1到R2所用的时间t。求解V1、V2。再调用前面的函数就可以确定轨道根数，及任意时刻质点的状态。(To solve the Lambert problem. Input: track two-point position vector R1, R2, and R1 to R2 used in the time t (The disadvantage is that the pulse is too large). Output: particle R1 endpoint speed V1, the particle in the R2 endpoint speed V2. This function is used: two points in a given orbit position vector R1, R2, and from R1 to R2 with the time t. Solve the V1 and V2. And then call the previous function can determine the orbital elements, and any moment of the particle' s state.)