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In orbital mechanics and aerospace engineering, a gravitational slingshot, gravity assist maneuver, or swing-by is the use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically in order to save propellant, time, and expense. Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed and/or redirect its path. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft.〔(Basics of Space Flight, Sec. 1 Ch. 4, NASA Jet Propulsion Laboratory )〕 It was used by interplanetary probes from ''Mariner 10'' onwards, including the two ''Voyager'' probes' notable flybys of Jupiter and Saturn. == Explanation == A gravity assist around a planet changes a spacecraft's velocity (relative to the Sun) by entering and leaving the gravitational field of a planet. The spacecraft's speed increases as it approaches the planet and decreases while escaping its gravitational pull (which is approximately the same). Because the planet orbits the sun, the spacecraft is affected by this motion during the maneuver. To increase speed, the spacecraft flies with the movement of the planet (taking a small amount of the planet's orbital energy); to decrease speed, the spacecraft flies against the movement of the planet. The sum of the kinetic energies of both bodies remains constant (see elastic collision). A slingshot maneuver can therefore be used to change the spaceship's trajectory and speed relative to the Sun. A close terrestrial analogy is provided by a tennis ball bouncing off the front of a moving train. Imagine standing on a train platform, and throwing a ball at 30 km/h toward a train approaching at 50 km/h. The driver of the train sees the ball approaching at 80 km/h and then departing at 80 km/h after the ball bounces elastically off the front of the train. Because of the train's motion, however, that departure is at 130 km/h relative to the train platform; the ball has added twice the train's velocity to its own. Translating this analogy into space, then, a "stationary" observer sees a planet moving left at speed ''U'' and a spaceship moving right at speed ''v''. If the spaceship has the proper trajectory, it will pass close to the planet, moving at speed ''U + v'' relative to the planet's surface because the planet is moving in the opposite direction at speed ''U''. When the spaceship leaves orbit, it is moving at speed ''U + v'' relative to the planet's surface but in the opposite direction (to the left). Since the planet is moving left at speed ''U'', the total velocity of the spaceship relative to the observer will be the velocity of the moving planet plus the velocity of the spaceship with respect to the planet. So the velocity will be ''U + (U + v) = 2U + v ''. This oversimplified example is impossible to refine without additional details regarding the orbit, but if the spaceship travels in a path which forms a parabola, it can leave the planet in the opposite direction without firing its engine, and the speed gain at large distance is indeed ''2U'' once it has left the gravity of the planet far behind. This explanation might seem to violate the conservation of energy and momentum, but the spacecraft's effects on the planet have not been considered. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, though the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small. These effects on the planet are so slight (because planets are so much more massive than spacecraft) that they can be ignored in the calculation.〔(The Slingshot Effect ), Durham University〕 Realistic portrayals of encounters in space require the consideration of three dimensions. The same principles apply, only adding the planet's velocity to that of the spacecraft requires vector addition, as shown below. Due to the reversibility of orbits, gravitational slingshots can also be used to reduce the speed of a spacecraft. Both Mariner 10 and MESSENGER performed this maneuver to reach Mercury. If even more speed is needed than available from gravity assist alone, the most economical way to utilize a rocket burn is to do it near the periapsis (closest approach). A given rocket burn always provides the same change in velocity (Δv), but the change in kinetic energy is proportional to the vehicle's velocity at the time of the burn (see Oberth effect). So to get the most kinetic energy from the burn, the burn must occur at the vehicle's maximum velocity, at periapsis. Powered slingshots describes this technique in more detail. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gravity assist」の詳細全文を読む スポンサード リンク
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