Spacecraft save fuel by getting a boost from gravity

By Linda Hermans-Killiam

It takes a lot of fuel to escape Earth's gravity. The more a spacecraft weighs, the more fuel rockets need to launch them. Spacecraft also have to carry fuel to help them travel through space. Every bit of fuel adds weight and cost to space missions. Fortunately, there is a way to help spacecraft travel around the solar system, without using much fuel.

How is this done? Through a clever maneuver called a gravity assist!

With gravity assist, a spacecraft aimed at a faraway destination first flies close to a nearby planet. When the spacecraft gets close, the planet's gravity causes it to fall faster and faster until it reaches its closest point to the planet. After that, it keeps going and slows down because of the planet's gravity. However, the planet's own motion and its gravity have changed the spacecraft's speed and direction. In this way, gravity assist can help a spacecraft speed up or slow down by thousands of miles per hour!

In 1974, Mariner 10 was the first spacecraft to use gravity assist to reach another planet. It flew by Venus to reduce its speed so it could go on to Mercury. The Voyager 2 spacecraft used a gravity assist from Jupiter to propel it towards Saturn. It then used gravity from Saturn to get to Uranus, and then used Uranus to get to Neptune. More recently, New Horizons used a gravity assist from Jupiter to reach Pluto in much less time than a direct flight from Earth.

By using gravity assist, spacecraft can also visit asteroids and comets. NASA's OSIRIS-REx spacecraft will fly by Earth in September 2017. It will get a gravity assist from Earth to propel it towards asteroid Bennu. It will gather a sample from the asteroid in 2020 and return it to Earth.

Large moons can also help with gravity assists. NASA's Galileo spacecraft used gravity assists from Jupiter's large moons to visit other moons. The Cassini probe used gravity assists from Saturn's largest moon Titan. This helped it travel among the other moons and rings of Saturn.

These are just a few examples of missions that have used gravity assist. Future missions will continue to use this clever method for traveling through space. We may have to fight gravity to get into space, but we can work with it to explore the solar system.

Learn more about gravity! Visit the NASA Space Place: https://spaceplace.nasa.gov/what-is-gravity.