The gravitational slingshot, also known as a gravity assist or gravitational assist maneuver, is a technique used in spaceflight to increase or decrease the speed of a spacecraft by utilizing the gravity of a planet or other celestial body. It may seem counterintuitive that a spacecraft can gain speed from a planet while it eventually slows down due to gravity, but it is due to the conservation of energy and momentum.
When a spacecraft approaches a planet, it enters the planet's gravitational field. As it gets closer to the planet, the planet's gravity pulls on the spacecraft and causes it to accelerate. This acceleration increases the spacecraft's speed.
As the spacecraft moves around the planet, it follows a curved trajectory due to the gravitational pull. After passing the planet, the spacecraft starts moving away from it. At this point, the planet's gravity starts to slow down the spacecraft, gradually reducing its speed.
However, the important aspect to note is that the spacecraft's gain in speed while approaching the planet is greater than the loss in speed while moving away from it. This difference occurs because the planet's gravity is acting as a "slingshot," transferring some of its momentum to the spacecraft.
The gravitational slingshot maneuver takes advantage of the fact that the planet's gravity field is in motion as the spacecraft passes by. By carefully planning the trajectory and timing, the spacecraft can effectively "borrow" some of the planet's orbital energy and momentum to increase its speed relative to the Sun or change its trajectory.
In this process, the planet itself is not significantly affected by the spacecraft's interaction. The mass of the spacecraft is generally negligible compared to the mass of the planet, so the planet's orbit remains mostly unchanged.
By utilizing gravitational slingshot maneuvers with multiple planets, spacecraft can achieve significant increases in speed while conserving onboard fuel. This technique has been successfully employed in various space missions, including those to outer planets, such as the Voyager missions and the New Horizons mission.
In summary, although the spacecraft does slow down due to the planet's gravity, the gravitational slingshot effect allows it to gain more speed than it loses, resulting in a net increase in velocity.