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Spacecraft change their speed relative to the Sun by flying past a moving planet or moon at a carefully chosen angle. The encounter can trade a tiny amount of the body’s orbital energy for a substantial change in the spacecraft’s speed or direction. An asteroid flyby, by contrast, may be scientifically valuable without providing enough gravity to alter a spacecraft’s path measurably.
How a gravity assist changes a spacecraft’s speed
A gravity assist is a planned close encounter with a moving body. Its gravity curves the spacecraft’s path as it passes. In a simplified frame moving with the assisting body, the spacecraft’s inbound and outbound speeds are approximately equal: gravity changes its direction, not its speed in that frame.
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But the body itself is moving around the Sun. Viewed from the Sun, the spacecraft’s changed direction combines with the assisting body’s orbital motion, so the spacecraft can leave with a different heliocentric speed. The exchange is with the planet’s or moon’s orbital energy; the body’s change is imperceptible at mission scale. The encounter geometry determines the result: a spacecraft may gain or lose speed relative to the Sun, and its direction or orbital plane can change too. A gravity assist does not automatically speed a spacecraft up. NASA describes these effects in its Psyche mission press kit.
Why a flyby of an asteroid may not be a gravity assist
“Flyby” describes passing an object, not necessarily getting a useful trajectory change from it. A spacecraft can pass an asteroid to image and study it, while the asteroid’s gravity is too weak to bend the spacecraft’s path measurably.
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That was the case for Deep Space 1’s asteroid encounter: NASA/JPL says the asteroid was too small to produce a measurable bend. The mission’s science flyby should not be confused with a major planetary gravity assist. NASA/JPL’s Deep Space 1 asteroid flyby press kit describes the encounter; NASA’s Lucy mission resources also distinguish asteroid encounters from trajectory-changing assists.
What recent missions show
| Mission and encounter | Assisting body | Reported effect |
|---|---|---|
| Psyche, May 15, 2026 | Mars | Changed speed and orbital plane on the way to asteroid Psyche; no onboard propellant was used for the assist. Closest approach was 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 | Asteroid | The asteroid’s gravity was too small to bend the spacecraft’s path measurably. |
The figures describe specific mission encounters, not a standard speed change or a typical assist. Psyche’s Mars encounter illustrates that a mission can seek both a speed adjustment and a change in trajectory. As Lindy Elkins-Tanton, principal investigator for NASA’s Psyche mission at the University of California, Berkeley, put it: “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 comment on May 8, 2026, ahead of the encounter, in its Psyche mission report.
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How mission teams confirm the speed change
Spacecraft teams can track motion using radio signals. As a spacecraft moves toward or away from Earth, the frequency of its transmitted signal shifts through the Doppler effect. NASA says Psyche’s team analyzed signals received through the Deep Space Network after the Mars encounter to assess the flyby. This tracking helps establish the spacecraft’s motion; the planned trajectory and measured outcome depend on the encounter, rather than on a universal speed-change figure. See NASA/JPL’s report on Psyche’s Mars flyby.
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Mission planners choose a flyby based on what the spacecraft needs and what the encounter can deliver. The outcome depends on the assisting body’s position and motion at the time, the spacecraft’s approach path, and how precisely the encounter can be navigated. A useful assist can change Sun-relative speed, redirect the spacecraft, adjust its orbital plane, or combine those effects. Its value also depends on mission constraints, including the propellant that would otherwise be needed for a trajectory change.
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For a spacecraft headed to an asteroid, the asteroid may be the destination for science while a planet supplies the major gravity assist along the way. Lucy’s Earth encounter and Psyche’s Mars encounter show why the distinction matters: the moving planet can reshape a heliocentric trajectory, while a small asteroid’s gravity may be negligible for that purpose. NASA’s December 2024 Lucy gravity-assist update reports the mission-specific speed increase; it should not be treated as a general benchmark for other spacecraft.
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