Swing-by maneuver
A swing-by maneuver is a deliberate close flyby of a spacecraft past a planet, during which the planet's gravitational pull accelerates or redirects the probe. This allows space missions to save huge amounts of fuel and reach destinations that would otherwise be unattainable.
Space probes cannot carry an unlimited amount of fuel. Every kilogram of fuel must first be hauled up from Earth itself. That’s why mission planners use a trick: they let the probe fly closely past a planet. The planet’s gravitational pull draws the probe into a new direction and changes its speed in the process. This deliberate flyby is called a swing-by maneuver. The probe is handed a boost of momentum without having to fire its engines.
Why probes wouldn’t get far without a planetary boost
The journey to the outer planets is extremely long. A rocket would have to accelerate a probe very strongly to reach Jupiter or Saturn directly. For heavy probes, the power of today’s rockets is often simply not enough. A swing-by shifts part of this work onto the planets themselves.
The most famous example is the two Voyager probes starting in 1977. They took advantage of a rare alignment of the outer planets that occurs only about once every 175 years. Voyager 2 flew past Jupiter, Saturn, Uranus, and Neptune in succession. Without this chain of flybys, the journey to Neptune would have taken around thirty years instead of twelve.
Modern missions depend on this too. The European probe BepiColombo is flying to Mercury and requires nine flybys of Earth, Venus, and Mercury to get there. That sounds cumbersome, but it is the only affordable solution. Carrying enough fuel for the same change in trajectory would make the probe far too heavy.
Where the free boost comes from
A planet pulls the probe in as it approaches and accelerates it. As the probe departs, the planet slows it back down by exactly the same amount. So from the planet’s perspective alone, the flyby achieves nothing at all. What matters is that the planet itself is racing around the Sun. Jupiter moves at around 13 kilometers per second.
If the probe flies past behind the planet, it gets redirected into the planet’s direction of motion. Measured from the Sun’s frame of reference, it then leaves the planet faster than it arrived. This gain comes from the planet’s orbital momentum. The planet itself slows down minutely in the process, but it is billions of times heavier than the probe. This deceleration is immeasurably small.
A vivid comparison is a tennis ball thrown against a moving train. The ball bounces back faster than it arrived, because the train is moving. The train is barely slowed down at all. If the probe instead flies past in front of the planet, it loses speed. This effect is also used deliberately, for example to slow down on the way toward the Sun.
Swing-bys in mission news and popular culture
When a space agency reports an Earth flyby, there is almost always a swing-by behind it. The probe Juno raced past Earth at an altitude of about 560 kilometers in 2013 to pick up speed for Jupiter. Such events are planned down to the minute. An error of just a few seconds can throw the trajectory off by millions of kilometers.
The solar probe Parker Solar Probe also works this way, though in reverse. It uses several Venus flybys to shed speed and get closer to the Sun. That’s because flying toward the Sun is energetically harder than flying away from it. This surprises many people and is a common misconception.
The maneuver often appears in films, frequently depicted inaccurately. It should not be confused with aerobraking, in which a probe skims a planet’s atmosphere to slow down. In a swing-by, the celestial body is only circled without any contact. For the space industry, the technique is a hard-nosed cost factor: fuel saved means more room for scientific instruments.