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A spacecraft uses a slingshot maneuver—more precisely, a gravity assist—to change its path and its speed relative to the Sun or another central body. The planet’s gravity bends the spacecraft’s trajectory; because the planet is moving, that change in direction can transfer a small amount of the planet’s orbital energy to or from the spacecraft.
What happens during a gravity-assist flyby?
As a spacecraft approaches a planet, the planet’s gravity pulls it inward and accelerates it. The craft is moving fastest near closest approach. It then climbs away from the planet and slows again relative to that planet.
In an ideal, unpowered two-body flyby, the spacecraft leaves with the same speed relative to the planet that it had far before the encounter. Gravity has changed the direction of its velocity, not its far-away planet-relative speed. NASA explains this trajectory in its trajectory chapter and Basics of Space Flight primer.
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This is a flyby, not permanent capture: the spacecraft passes the planet and continues on a new trajectory.
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Why can the spacecraft gain or lose speed?
The planet is also moving—usually around the Sun. To describe the spacecraft’s motion relative to the Sun, combine its velocity relative to the planet with the planet’s own orbital velocity. Since the flyby turns the spacecraft’s planet-relative velocity, the resulting Sun-relative velocity can be different from the incoming one.
Depending on which side of the moving planet the spacecraft passes and the encounter geometry, it can gain or give up orbital energy. A gravity assist can accelerate, brake, redirect, or change the inclination of a trajectory; it does not always increase speed. The European Space Agency describes the same reference-frame effect in its gravity-assist explainer.
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The energy is not free. The spacecraft and planet exchange momentum and energy. The planet’s motion changes by an equal-and-opposite amount in the full system, but because the planet is so massive, its practical change is tiny compared with the spacecraft’s. NASA describes this exchange in its Cassini gravity-assists account.
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- Reference frame: A flyby may leave the craft’s far-away speed relative to the planet unchanged while altering its speed relative to the Sun or another central body.
- Flyby geometry: The side of the moving body that the spacecraft passes, the approach direction, and the distance at closest approach shape the direction change.
- Mission goal: Trajectory designers may seek a speed or energy gain, braking, a new direction, or a change in orbital plane or inclination.
- Mission constraints: The encounter must be timed and aimed to reach the next destination under the mission’s arrival, propellant, and later-maneuver requirements.
NASA’s gravity-assist simulator page cautions that “slingshot effect” is not the most precise description of the physical principle. The familiar image can introduce the idea, but the key is the combination of a trajectory deflection and a moving planet.
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How spacecraft have used gravity assists
OSIRIS-REx: changing speed and orbital plane
NASA reported that the spacecraft’s 2017 Earth flyby changed its velocity by 8,451 miles per hour (3.778 kilometers per second) and changed its direction to match the orbital plane of Bennu, which NASA described as tilted six degrees from Earth’s. That is a mission-specific velocity change, not a typical value for all gravity assists. NASA’s September 22, 2017 account quotes OSIRIS-REx project manager Rich Burns: “The total velocity change from Earth’s gravity far exceeds the total fuel load of the OSIRIS-REx propulsion system, so we are really leveraging our Earth flyby to make a massive change to the OSIRIS-REx trajectory, specifically changing the tilt of the orbit to match Bennu.”
Cassini: reaching Saturn and steering within its system
NASA says flybys of Venus, Earth, and Jupiter helped send Cassini to Saturn. Later, Titan flybys steered Cassini around Saturn and changed its orbital inclination. NASA reports that a typical close Titan flyby changed Cassini’s speed by around 800 meters per second relative to Saturn and zero relative to Titan. The different figures describe different reference frames, not a contradiction; they illustrate why a stated velocity change needs its reference frame.
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Does a gravity assist put a spacecraft into orbit?
No. A flyby changes a trajectory, but it does not automatically capture a spacecraft into orbit around the planet. Orbit insertion requires the spacecraft to arrive with suitable conditions and lose enough energy relative to the planet, often through a separate propulsion maneuver. NASA’s Cassini account describes the spacecraft using its main engine to enter Saturn orbit.
Gravity assists can also help a spacecraft shed energy. ESA’s Mercury example explains that BepiColombo can pass close to neighboring planets to reduce excess orbital energy rather than relying entirely on propellant to brake. Its outer-Solar-System example describes a Jupiter-bound mission gaining energy from Earth, Venus, and Mars. Those are different mission goals, not a single recipe for every trajectory.
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Can you demonstrate the idea?
NASA provides instructions for a Gravity Assist Mechanical Simulator. In it, a bearing ball rolls over an inclined glass tabletop and interacts with a magnet on a rotating disk. The setup uses magnetism as an analogy for a moving planet’s gravitational interaction; it is not a literal gravity simulation. NASA includes a parts list and handouts and notes that the device requires tuning and calibration.
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