by Ralph W. Reid

May 14, 2008


Occasionally a question comes up in conversation like, "How can gravity be used to speed up or slow down spacecraft?" The material here is my attempt to explain at a very basic level how gravity can be used to speed up or slow down the movement of an object such as a spacecraft. Note that similar effects can be accomplished with objects made of magnets, but this document focuses on the effects of gravity as it can be used to navigate through a solar system. The details of the physics involved for gravitational and magnetic effects on objects can be expressed with calculus-level equations, but these details are beyond the scope of this document.

Basic Gravity

Gravity is a form of acceleration created by matter. The greater the mass of a given collection of matter within a given volume, the greater the gravitational effect that matter exerts on nearby objects. This gravitational force is an attractive force; that is, it pulls between things to try to bring them closer together. Gravity is what keeps loose objects and air from floating away from planet Earth into space. If an object is released close enough to Earth, that object will fall toward the Earth. In fact, the object and the Earth are both exerting a gravitational force on each other, but unless the object is substantially massive in relation to the Earth, the pull of the mass of the object will have no substantial effect on the Earth's position in space.

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.

Acceleration and Steering

If a spacecraft or other object being held above the surface of the Earth is released, it will move faster and faster (accelerate) toward the earth until it impacts the planet's surface. This is the result of the gravitational effect people experience every day while in the vicinity of Earth. However, while most spacecraft are not designed to impact relatively large objects such as planets or moons, they can pass close enough to such objects for gravity to have some effect on the course and speed of the spacecraft. If a spacecraft is thrown with some force parallel to the Earth's surface, the spacecraft will travel some distance before being drawn to the Earth's surface. If the spacecraft is thrown much harder in the same direction, it can travel much farther before it is drawn back to Earth. If a spacecraft is thrown hard enough parallel to the Earth's surface above the atmosphere, the spacecraft will not fall back to Earth for a long time and may circle (orbit) the earth several or many times before finally returning to the Earth's surface. Finally, if the spacecraft is thrown still harder, it can leave the vicinity of the earth entirely.

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.

For Further Study

The calculus techniques and physics equations needed for more precise predictions of how objects in gravity wells interact with each other are beyond the scope of this basic discussion. If you want to study these effects further, refer to your favorite astronomy, calculus, and physics resources, either online or off. Similar effects can be observed between magnetic objects, and these effects can be examined further in your own studies of physics.