Gravitational assistance

The gravitational assistance , in the field of the Astronautics, is the use of the effect of the gravitational field of a celestial body on the Flight Path Vector of a Space engine, when the Trajectoire was designed to benefit from it. The term corresponding in English is gravity assistance .

Explanation

Initially, the space engine approaches planet and ends up returning in its zone of influence (it enters its Sphère of Hill). The gravitational field attracts more and more the space engine which sees its speed increasing: it “falls” towards planet and thus accelerates. Of course, the trajectory of the machine was established so as to avoid a collision with selected planet. It thus exceeds planet healthy and except and leaves gradually the gravitational field of this one, by losing gradually speed. At the end of the overflight (when it leaves the sphere of Hill), the space engine lost as much speed during the phase of exit than it gained some during the phase of entry. You undoubtedly ask for yourselves the interest of such an operation which apparently was used for nothing. However its speed changed considerably, in size and direction.

From the point of view of planet, the machine follows a hyperbolic trajectory, traced of its entrance point in the sphere of Hill of planet to her exit. At these points, the speed of the machine is the same one, but its orientation is different. From the point of view of the Sun, the orientation and the magnitude the speed of the machine changed. This allows is to send the machine more far from the Sun (it then borrows energy and angular momentum from planet), or contrary to decreasing its orbit (the machine gives energy and angular momentum to planet). It is precisely by this mechanism that the planets can capture Comet S or eject Astéroïde S of the solar system.

Example

Let us take as example Voyager 2, which made the turn of giant planets. The spaceship was launched on a standard orbit of transfer of Hohmann towards Jupiter. If Jupiter had not been there at the time of the arrival of the spaceship, this one would have continued on its orbit and would have returned towards the Ground.

However, the arrival of the space probe was carefully calculated so that it passes behind Jupiter in its orbit around the Sun. When the spaceship arrived under the influence of the field of gravity of Jupiter, it fell in direction from Jupiter, increasing its speed to the maximum at the time of the narrowest approach towards Jupiter. Since all the masses in the universe attract each other, Jupiter accelerated the spaceship appreciably during all the phase of approach (and the spaceship slowed down Jupiter in its orbit, in a negligible way of course), since the spaceship approached by behind. Exceeding Jupiter, the spaceship during its phase of distance slowed down relative in Jupiter, until leaving the zone of gravity of Jupiter. Its speed relative to Jupiter at the time of its departure of this zone was identical at its speed of arrival, by conservation of energy. But relative with the Sun, its direction was changed. It left the surroundings joviens carrying an increase in the Angular momentum stolen with Jupiter. Jupiter gravity was used to connect the spaceship to the enormous reserve of angular momentum of planet. This technique was repeated with the approach of Saturn and Uranus.

Reference

Decree of the February 20th 1995 relating to the terminology of sciences and space technology.

See too

gravitational Support
List of the articles relating to astronautics

External bonds

Transfers of orbit and gravitational assistance
skeletal Simulation of the gravitational assistance (animation Flash)

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