Which mission maneuver is a gravitational assist?

An unpowered planetary flyby changes the craft's solar-relative trajectory. Exactly. A gravity assist uses a close encounter with a moving body to alter a trajectory.

Why can a flyby change speed relative to the Sun?

The planet moves around the Sun, so changing the craft's direction relative to it changes the solar-frame motion. Exactly. The apparent puzzle disappears when the planet's motion and the reference frame are included.

What role did Venus play for Mariner 10?

It was a scientific target and a flyby that redirected the mission toward Mercury. Exactly. Mariner 10 studied Venus and used its gravity before the first Mercury encounter.

From the archive · September 9, 2026

Borrowing motion from a planet

How can a spacecraft change course without carrying all the fuel for the turn?

01 · Word

Gravitational assist

Pronounced grav-ih-TAY-shuh-nul uh-SIST

noun

A planetary flyby that changes a spacecraft's path and speed relative to the Sun

Examples

Mariner 10 used a gravitational assist at Venus to reach Mercury.
A mission planner may accept a longer route to gain a useful gravitational assist.

Origin

The term belongs to orbital mechanics and is also shortened to gravity assist or called a planetary swingby. Assist describes the encounter's usefulness, not an exception to conservation laws. The planet is moving too, so the encounter can change the spacecraft's motion relative to the Sun.

02 · Idea

The moving planet changes the exchange

Approach a planet and its gravity bends your path. In an ideal unpowered flyby, you leave with the same speed far from the planet that you had on arrival, measured relative to that planet. But the planet itself is moving around the Sun. A change of direction in one frame can become a change of speed in another.

The European Space Agency explains the maneuver as an exchange of energy and momentum. The spacecraft's tiny mass makes the change in the planet's orbit negligible for practical purposes. Depending on the geometry, a flyby can increase or decrease the craft's solar-relative speed. Getting inward toward the Sun can require shedding orbital energy.

A planet can change your journey without becoming your destination.

The useful resource is the planet's motion as well as its gravity.

Limits

A flyby needs the right alignment and a precisely targeted trajectory. It cannot provide an arbitrary push in any direction. Timing, navigation, radiation, and the atmosphere constrain how closely a spacecraft can pass.

03 · Moment

Venus opens the road to Mercury

Venus, February 5, 1974; Mariner 10 turns toward the inner planet

NASA's Mariner 10 passed Venus on February 5, 1974, gathering images and measurements while using the encounter to redirect its trajectory toward Mercury. Venus was both a scientific target and a means of reaching another world. The spacecraft then made its first Mercury encounter on March 29.

Spacecraft: Mariner 10
Encounter: February 5, 1974
Next target: Mercury

The caveat

Popular descriptions call the maneuver a slingshot, which can imply a simple speed boost. The actual change depends on the reference frame and geometry. Mariner 10's achievement was a carefully designed transfer, not a universal recipe for accelerating whenever a planet is nearby.

Mariner 10 became the first mission to use an interplanetary gravity assist to reach another planet. It later returned for additional Mercury flybys. The route demonstrated that a mission could be designed around encounters that altered its orbit rather than treating every planet as the endpoint of a direct journey.

The connection

A flyby exchanges momentum with a moving planet, changing a spacecraft's path and solar-frame energy.