Gravity assist

The English term swing-by - including fly-by, Slingshot, Gravity - Assist ( GA), gravity deflection, gravitational maneuvers or Gone swing maneuver called - is a method of interplanetary space, in which a relatively lighter spacecraft ( such as a spacecraft ) close to a massive body ( as a planet ) flies by.

The direction and the speed of the probe can be changed as the planet undergoes no measurable change in its orbit because of its much larger mass. Seen from a great distance, apply to the encounter the laws of elastic collision.

Principle

Fly the probe by the gravitational field of a planet, it will be deflected, reducing their flight direction and speed is changed. Depending on the conditions of the flyby of the planet transmits a small portion of kinetic energy from its orbital momentum to the probe or vice versa. That is, depending on whether the probe flies past, decreased or increased their speed in the reference frame in which the planet moves in front of or behind the planet. Since it is a three-body problem, the path changes are not analytically but only numerically calculate. Generally, the web of the planetary change can be neglected, because its mass is much greater than that of the probe.

The same effect could also occur if a lighter planet or an asteroid passes a heavier planet in the gravitational field. However, since the mass of the lightest planet compared to the heavier is not negligibly small changes in this case, the heavier planet 's sun orbit noticeably (albeit less strongly than the lighter).

A swing-by represents a special kind of flyby

Examples

A planet orbits the Sun in an orbit. A probe is to be accelerated by using a swing - bys, approaches from behind the planet and its orbit is happening behind him before she passed him. As long as the probe herfliegt behind the planet, the attractive force acts on it accelerating. Characterized it is also faster compared to the sun. Then they overhauled the planet and is then slowed down by him. With respect to the planetary speed change is symmetrical with respect to the sun, the speeds transform but different because the same occurs with deflection. Thus, an increase in speed is left at the end.

A probe is to be braked using a swing - bys, approaches from behind the planet and flies past him by his orbit happened to him. So you come to the planet opposite to the closest approach. The probe is decelerated relative to the sun. Thus remains a reduction in speed, even if - the probe has become faster and removal again slower in the planet approach - as described above.

Similarly, the change in the plane of the web is possible.

Objectives

The speed of change can be used to brake the sun for more probe targets or accelerate solar distant targets.

The change of direction can also be used to leave the plane of the ecliptic and to direct probes into a polar orbit around the sun. In this way, space probes can reach trajectories that are not or only with a significantly greater energy expenditure of the probe drive otherwise be possible.

History

The mathematical foundations were laid in 1961 by Michael Minovitch. The first swing-by maneuver was performed in 1970 during the Apollo 13 mission. The crew was able to save back to earth again by a swing-by maneuver around the moon following the explosion of an oxygen tank. In February 1974 Mariner 10 was the first spacecraft that performed a swing-by maneuver, it was slowed down enough by the flyby of Venus to reach the planet Mercury. This could on the one hand, the spacecraft started with a (opposite the Titan IIIC) cheaper Atlas Centaur and on the other hand, Venus will mitbesucht. Today, almost all interplanetary space probes that do not have the Mars or Venus to the final destination, use this technique.

Uses

Swing - bys can be used to shorten the flight times of probes. Voyager 1 and 2 were, for example, by a swing-by of Saturn around 18 km / s speeds and thereby reached the third cosmic speed. Without swing-by Voyager 2 had more than used twice as long to reach Neptune.

Swing - bys are often not used to shorten the travel time, but space probes to start with carrier rockets that are too weak for the direct flight to your destination. To get still the goal, then the spacecraft must perform one or more Swing - bys in order to achieve the necessary speed. Thus, the flight time is significantly longer than a direct flight. Most often, the reason for this procedure that a larger launch vehicle would be more expensive than the longer mission time. Only sometimes, such as Cassini -Huygens, the probe is so heavy that even is not enough, the largest launch vehicle for a direct flight.

Especially the second reason results in large detours. For example, the Cassini- Huygens was first then put on the way to Saturn from Venus twice and once from the earth to the required speed.

Rarely Swing - bys are used to the inclination to change as much as in the Ulysses solar probe to leave the plane of the ecliptic.

The unrealized Solar Probe should be brought by a swing-by at Jupiter on a polar sun orbit whose perihelion only three solar radii should be located above the solar surface and its aphelion would have found itself at the level of the orbit of Jupiter. It would serve not only the inclination as Ulysses changed greatly, but also could have been extremely slowed down.

Swing - bys in Animation

The red curve in each case the lower part shows the speed of the spacecraft over time.

Swing-by to reduce speed