In outer space, the gravitational force created by the mass of an asteroid, moon, planet, star or black hole can be by far the strongest force exerted on a nearby object. The gravity wells created by various moons and planets in our solar system have been used to steer and accelerate some spacecraft after they have been launched from earth. If an object such as a spacecraft is close to a larger object such as a moon or planet, the object will start moving toward that moon or planet. By having a spacecraft pass close to a much larger object such as a moon or planet, the spacecraft's velocity and direction can be changed.
During solar system navigation, the path and velocity of spacecraft are frequently affected as the spacecraft pass close to larger objects such as planets or moons. As a spacecraft passes by a planet, the gravity of the planet at first has very little influence on the direction and velocity of the spacecraft, then much more influence as the spacecraft passes closest to the planet, and then decreasing influence as the spacecraft moves farther away. The gravity of the planet pulls on the spacecraft, causing the spacecraft to take a path which curves around the center of the planet's mass. The result is that the spacecraft leaves the vicinity of the planet on a different course than the one it had as it approached the vicinity of the planet.
Of course, planets and moons are not stationary objects. Because they move, a spacecraft can be aimed at the path of one of them to cause the spacecraft to accelerate toward the planet or moon as the planet or moon passes by. If a planet crosses through space shortly before a spacecraft reaches that same space, the spacecraft will be pulled forward in its path, resulting in an increased velocity. If a spacecraft passes through space shortly before a planet reaches that same space, the spacecraft will be pulled backward in its path, resulting in a decreased velocity. The course of the spacecraft will also be changed because the planet is not always directly in front of or behind the spacecraft. Therefore, the spacecraft will be slowed down or sped up as its direction is also changed by the gravitational pull of the planet. Precise calculations can be performed to predict the velocity and direction changes the spacecraft will undergo as it passes a planet, and these predictions can help determine the best approach path of the spacecraft to achieve the desired velocity and direction changes.
Multiple gravity wells created by planets and moons have been used in some exploratory missions in this solar system. Among these missions have been the Voyager spacecraft which used Jupiter, Saturn and some of their moons to reach exploration targets further out in the solar system, and the Galileo spacecraft which used a couple of planets in the inner solar system to accelerate to Jupiter to explore the planet and its moons. Ongoing explatory missions which are using the gravity of planets and/or moons include explorations of Saturn and its moons and Pluto and its moons. The gravity of planets and moons is likely to continue to play a significant role in exploration of the solar system by various spacecraft for a very long time.