Gravity assists are performed to save fuel at the expense of time. They have been used by many spacerockets throughout history.
Without gravity assists, the well known Voyager Grand Tour would never have been possible. Jupiter gravity assists helped Cassini reach Saturn. However, they have been largely used for smaller space missions. For example, ESA's Rosetta probe used two Earth and one Mars gravity assist to reach a comet and NASA's MESSENGER probe used many Earth, Venus and Mercury flybys to enter Mercury orbit, and the Parker Solar Probe performed several Venus flybys to help approach the Sun.
During a Mercury mission, the rocket might reach Mercury's sphere of influence, but too fast. In order to decrease speed, three or more Mercury flybys should be needed.
In addition, when a rocket leaves Mercury, a flyby can increase its aphelion to the orbit of Venus.
Mercury has a very small sphere of influence, very close to the sun and little gravity. Because of this, gravity assists are difficult to perform.
Venus is largely used for gravity assists. The planet has enough gravity, and with one or two gravity assists, a rocket coming from Earth can be sent to Mercury and a rocket coming from Mercury can be sent to Earth.
In addition, Venus is used for rockets that have only little fuel left, that cannot reach the Earth in one flight. A Mercury Return Mission or a Venus Return Mission might use all or nearly all fuel of a spacerocket. However, if the astronaut waits for a correct planetary alignment and uses a Venus flyby, it can rise the aphelion of its rocket to reach Earth orbit.
The Earth can be used for a rocket passing from Venus to Mars or from Mars to Venus. However, this is something done rarely.
More often, an Earth gravity assist can be used for a rocket in heliocentric orbit, to get to Venus, Mercury or Mars, saving fuel.
NASA sent the probe OSIRIS-REx to have an Earth gravity assist, in order to reach an asteroid. The rocket Deep Impact performed several Earth flybys to gently adjust its orbit, to encounter a comet.
Warning: The Moon might sometimes get in the way of an Earth gravity assist.
The Moon has little gravity, but still can be used for a gravity assist. One or two Moon flybys can send a rocket into heliocentric orbit. NASA's STEREO-A and STEREO-B probes used a Moon flyby for this reason.
Mars was used for Dawn and Rosetta missions for a gravity assist. A Mars flyby can send a rocket into the asteroid belt or Jupiter. In addition, a Mars flyby can lower the perihelion closer to Earth.
The two small moons of Mars have no gravity. A flyby cannot change the orbit of a rocket, due to the lack of gravity on the moons.
Jupiter has a very high gravity, which makes it useful for gravitational assists, such as when NASA used it's gravity to give Voyager 2 a boost in speed to allow it to reach Saturn, however in the base solar system this gravitational boost is useless as no objects beyond Jupiter's orbit has been added.
Jupiter's gravity can be very useful as it can give you a very large gravitational assist which can send you to reach other planets and stars but only if you are using a custom solar system which adds them in.
Warning: When using Jupiter's gravity you will going to want to be careful when getting close due to Jupiter's four moons (like Io) as they have quite large hill spheres and that could easily disrupt your gravitational assist.
In some simulators, like the Kerbal Space Program, it is possible to use a Sun gravity assist, a thing that in reality is not possible (except for an interstellar mission, for a rocket approaching from another star). In Spaceflight Simulator, this is not possible, unless there are neighboring stars in a custom solar system pack.
To perform a gravity assist, there are a few things that must be considered:
- A flyby with a planet close to the perihelion will usually lower the perihelion.
- To get closer to the Sun, enter the sphere of influence of the target planet from between the planet and the Sun.
- A flyby with a planet close to the aphelion will usually rise your aphelion.
- To get away from the Sun, enter the sphere of influence of the target planet from a point away from the Sun.
- The closer you get to the planet, the stronger will be the effect. Beware not to hit the surface or enter the atmosphere.
- At closest approach, you can fire the engines forwards to increase the effect and backwards to decrease the effect. You will take advantage of the Oberth effect and save fuel.
When the path of your rocket will intersect the sphere of influence of a planet, you can see on map view that the trajectory ends. If you zoom in to the planet, you can see the path of your rocket inside the sphere of influence or hill sphere. The path you see is what your rocket will follow when the planet will be at its target. You have to use some imagination and take the planet from where it is, to put it where it will be during flyby.
Only when you enter a planet's sphere of influence, you can see the path your rocket will have after flyby.
Watch the path your rocket will have around the planet.
- If the rocket, after the flyby, will exit the sphere of influence away from the Sun, the rocket will turn to an orbit closer to the Sun.
- If the rocket, after the flyby, will exit the sphere of influence towards from the Sun, the rocket will turn to an orbit further from the Sun.
The same happens if the rocket performs a Moon flyby, in relation with the Earth, or the Galilean moons of Jupiter.
In order to perform gravity assists, a rocket has to do trajectory correction maneuvers and deep space maneuvers.