Gravity assist

A gravity assist is a specialized spaceflight maneuver used to change a spacecraft's path or speed.

In orbital mechanics, this is often called a swing-by or a gravitational slingshot. Scientists use this technique to alter the velocity of a probe relative to the Sun. This is vital because space missions must manage a strict propellant budget. This budget is known as the delta-v budget. It represents the total change in velocity available from the fuel carried on board.

Because lifting extra fuel from Earth is so difficult, saving propellant is a primary goal of mission design.

The mechanism of a gravity assist relies on the relative movement of a planet. As a spacecraft enters the gravitational sphere of influence of a planet, it is pulled by that planet's gravity. The assist is provided by the motion of the gravitating body itself. If the spacecraft's trajectory is designed to pass behind the planet, it will gain speed. If the trajectory passes in front of the planet, the maneuver acts as a braking maneuver to decrease speed.

This process allows a spacecraft to tap into the kinetic energy of a planet as it orbits the Sun.

To understand the physics, consider the frame of reference. In the planet's own frame of reference, the probe leaves with the same speed it had when it arrived. However, when viewed from the Solar System's frame of reference, the speed change is very apparent. A helpful analogy is a tennis ball bouncing off a moving train. If you throw a ball at a train approaching at 50 km/h, the ball's departure speed relative to the platform is much higher. The ball adds the train's velocity to its own. In space, the planet acts like that moving train, providing a massive boost to the smaller probe.

This exchange of energy follows Newton's Third Law. Any gain in kinetic energy by the spacecraft results in a corresponding loss for the gravitational body. The linear momentum gained by the spacecraft is equal in magnitude to the momentum lost by the planet. However, the mass of a planet is many orders of magnitude larger than a spacecraft. For example, a typical probe might weigh one metric ton. Jupiter has a mass of almost 2 x 10^24 metric tons.

A one-ton probe passing Jupiter would only cause the planet to lose about 5 x 10^−25 km/s of velocity. This effect is utterly imperceptible and can be ignored in most calculations.

The history of this concept involves many different researchers. Yuri Kondratyuk suggested using moons to accelerate spacecraft in a paper dated 1918–1919. Friedrich Zander also demonstrated a deep understanding of this physics in 1925. In 1956, Gaetano Crocco calculated interplanetary journeys using multiple gravity assists. The first actual use of the maneuver occurred in 1959. The Soviet probe Luna 3 used a gravity assist to photograph the far side of the Moon. Later, Michael Minovitch developed techniques at NASA's Jet Propulsion Laboratory that led to the famous Planetary Grand Tour.

One of the most significant applications was the discovery of a rare planetary alignment. In 1964, Gary Flandro studied the outer planets and found a specific alignment of Jupiter, Saturn, Uranus, and Neptune.

This alignment allowed for a multi-planet mission that could visit all four giants. Without gravity assists, such a mission might have taken forty years. Because of the assists, the Voyager probes could complete the tour in less than ten years. This shows how gravity assists can drastically reduce mission duration.

There are physical limits to how these maneuvers are used. The main limit is that planets are rarely in the correct positions for a specific destination. A similar alignment of the outer planets will not occur again until the middle of the 22nd century. Another limit is the distance of closest approach. The magnitude of the velocity change depends on how close the probe gets to the planet. The atmosphere of a planet can also limit how close a spacecraft can safely fly. If more speed is needed than gravity provides, engineers use a rocket burn at the periapsis. This is known as the Oberth effect, where burning fuel at the highest velocity provides the most kinetic energy.