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Spacecraft change their speed relative to the Sun by flying past moving planets or moons, whose gravity bends their paths. The encounter can increase or decrease heliocentric speed, depending on the flyby geometry. An asteroid flyby, by contrast, is not automatically a useful gravity assist: small asteroids may be scientifically interesting but too low in mass to bend a spacecraft’s trajectory measurably.
How a gravity assist changes a spacecraft’s speed
A gravity-assist flyby is a planned close pass by a moving planet or moon. The body’s gravity curves the spacecraft’s path. In a simplified frame moving with that body, the spacecraft leaves with approximately the same speed it had on arrival, but pointed in a different direction.
Viewed from the Sun, the result can be different. The planet or moon is moving in its orbit, so the spacecraft’s changed direction combines with the assisting body’s orbital motion. Depending on which side the spacecraft passes and the direction of the encounter, it can leave with greater or lower speed relative to the Sun. The spacecraft and body exchange orbital energy; the body’s change is imperceptible at mission scale.
That is why “slingshot” can be misleading if it suggests every flyby makes a spacecraft faster. A gravity assist may change speed, direction, or orbital plane—or some combination of these. The outcome depends on the encounter geometry, not simply on how close the spacecraft passes.
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Why an asteroid flyby may not provide an assist
“Flyby” describes a spacecraft passing an object; it does not guarantee that the object changes the spacecraft’s trajectory in a useful way. A small asteroid can be a compelling target for imaging and scientific study while exerting too little gravity to produce a measurable bend in the spacecraft’s path.
NASA/JPL’s Deep Space 1 asteroid flyby press kit says the asteroid encountered by the spacecraft had far too little gravity to bend its flight path measurably. A mission can therefore fly by an asteroid for science while using a planet for a major gravity assist.
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What two NASA missions show about assist outcomes
| Mission and encounter | Assisting body | Reported effect |
|---|---|---|
| Psyche, May 15, 2026 | Mars | Changed speed and orbital plane while the spacecraft was en route to asteroid Psyche; no onboard propellant was used for the assist. NASA/JPL reported a closest approach of 2,864 miles (4,609 kilometers) above Mars’s surface. |
| Lucy, December 2024 | Earth | NASA reported a Sun-relative speed increase of over 16,000 miles per hour (7.31 kilometers per second), sending Lucy toward the Jupiter Trojan asteroids. |
| Deep Space 1 asteroid encounter | An asteroid | The asteroid’s gravity was too small to bend the spacecraft’s path measurably, according to NASA/JPL. |
These are outcomes from particular missions and geometries, not standard values for gravity assists. For Psyche, mission principal investigator Lindy Elkins-Tanton of the University of California, Berkeley, described the purpose this way: “Ultimately, though, the only reason for this flyby is to get a little help from Mars to speed us up and tilt our trajectory in the direction of the asteroid Psyche.” NASA published the statement on May 8, 2026.
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How mission teams choose and verify a flyby
A gravity assist is not a free-form shortcut. Mission teams must arrange for the spacecraft and assisting body to meet at the right place and time, and navigate the encounter accurately. When comparing possible assists, the relevant questions include:
- How will the encounter change the spacecraft’s speed relative to the Sun?
- How much will it redirect the trajectory or alter the orbital plane?
- Where will the assisting body be when the spacecraft arrives, and how accurately can the spacecraft be navigated through the encounter?
- What propellant would otherwise be needed to achieve the desired trajectory?
Teams can check a spacecraft’s motion using radio tracking. A change in the radio signal’s Doppler shift indicates a change in relative motion. NASA reported that Psyche’s team analyzed Deep Space Network signals after the Mars encounter to assess the flyby’s effect. The NASA account of Psyche’s planned Mars assist also explains the mission’s intended speed and trajectory change.
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