How does the slingshot effect work to change the orbit of a spacecraft?

摘要

Jeremy B. Jones, Cassini Navigation Team chief at the Jet Propulsion Laboratory in Pasadena, Calif., explains: Spacecraft taking advantage of a gravity assist use the same principles that underlie orbital changes occurring regularly among moons and smaller bodies in the solar system. Comets from outlying regions, for instance, are often thrown into the inner solar system by the major planets, frequently Jupiter. Absent any other influence, a moon or a spacecraft traces an elliptical path around a larger body, called the primary body, with constant orbital energy and angular momentum. But when a spacecraft comes close to a moon that is also circling the same primary body, the two smaller objects exchange orbital energy and angular momentum. Because the total orbital energy remains constant, if the spacecraft gains orbital energy, that of the moon decreases. Orbital period, the time required to complete one revolution, is proportional to orbital energy. Therefore, as the spacecraft's orbital period lengthens (the slingshot effect), that of the moon grows shorter.