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

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.

Because a spacecraft is much, much smaller than a moon, the effect on its orbit is far greater than on that of a moon. For example, the Cassini spacecraft to Saturn is about 3,000 kilograms, whereas Titan, the largest of the ringed planet's satellites, weighs some 1023 kilograms. The effect of a slingshot maneuver on Cassini is thus about 20 orders of magnitude greater than that on Titan.

A spacecraft that passes “behind” the moon gets an increase in its velocity (and orbital energy) relative to the primary body, which gives the appearance of a slingshot throwing it into a larger orbit. We can also fly a spacecraft “in front” of a moon, to decrease its velocity (and orbital energy). Moreover, traveling “above” or “below” a moon can alter the direction of the spacecraft's velocity, modifying only its orbital orientation (and angular momentum magnitude). Intermediate flyby orientations change both energy and angular momentum. Of course, all such adjustments precipitate an inverse change in the energy and angular momentum of the moon, but its larger mass results in changes so small that they are undetectable among all the other forces that affect a moon's orbit.

Where does wind come from?

Chris Weiss, assistant professor of atmospheric science at Texas Tech University, provides this answer:

Simply put, wind is the motion of air molecules. Two concepts are central to understanding what causes wind: air and air pressure.

Air contains molecules of nitrogen (about 78 percent by volume), oxygen (about 21 percent), water vapor and other trace elements. All these air molecules move about very quickly, colliding readily with one another and with any objects at ground level.

Air pressure is defined as the amount of force that these molecules impart on a given area. In general, the more air molecules present, the greater the air pressure. Wind, in turn, is driven by what is called the pressure gradient force.

Changes in air pressure, such as those caused by the dynamics of storm systems and uneven solar heating, in a given horizontal area force air molecules from the region of relatively high air pressure to rush toward the area of low pressure.

The areas of high and low pressure displayed on a weather map in large part drive the gentle breezes we usually experience. The pressure differences behind this wind are only about 1 percent of the total atmospheric pressure, and these changes occur over the range of multiple states. The winds in severe storms, in contrast, result from much larger and more concentrated areas of pressure change.

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