贴一张nasa的图说明问题。该图演示旅行者飞跃木星的速度提升。
The planet's own motion is a key. A gravity assist with Jupiter involves not a stationary planet as considered above, but a planet with enormous angular momentum as it revolves around the Sun. In the diagram at right, Jupiter's motion along its solar orbit has been illustrated with a vector colored red (simplified, of course: Jupiter revolves along an arc, not a straight line. Imagine the Sun situated below the bottom of the diagram). The spacecraft acquires this Sun-relative vector, or a significant portion of it, during its interaction with Jupiter.

You can see how the red vector is added to VIN and VOUT. The resulting vector shows how the spacecraft's velocity, relative to the Sun, takes on a nice boost from Jupiter. Notice how rotation of the vector from VIN to VOUT (the bending of the spacecraft's path by the planet's gravity) helps increase the result. This trajectory bending is the other key.
【 在 feng321 的大作中提到: 】
: 飞行器，和行星，差不多是一个相向的 相对运动，而不是行星静止、只有飞行器靠近的运动。比如飞行器和行星差不多相差100米的时候，两者相向运动，关闭发动机，让引力提供加速的动力。当加速到50米（最近的效果最好的距离）的时候，开启发动机，改变降落的飞行方向，这时候引
: χ鸾ゼ跣　Ｕ庋纸谑×巳剂希俣扔值玫搅思涌欤钦庋穆穑壳敫呤种附
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修改:SlO FROM 124.64.17.*
FROM 124.64.17.*