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Archive Astronomy Questions of the Week - International Year of Astronomy 2009

What is the best place to 'park' a satellite?

Week 8

The position of Lagrangian point L2 in the Sun-Earth system

The mathematician Joseph-Louis Lagrange (1736-1813) could not yet have known anything about space exploration and satellite technology. Still, he made a mathematical discovery that is of great importance to modern space research: If two differently sized bodies, such as the Earth and the Sun, orbit around each other, there are five special points in their gravitational field. These so-called Lagrangian points (designated as L1-L5) move with the bodies. Moreover, the gravitational and centrifugal forces are exactly balanced in these positions. A third body with a much smaller mass, such as a satellite, that is located at one of these points will have no external forces acting on it. This means that it will always stay in the same spot (relative to the two other bodies), even without using its own propulsion system.

Lagrangian points help to save fuel

The two European space telescopes Herschel and Planck are scheduled to be launched in April 2009. Both satellites will have their own orbit around Lagrangian point L2. From this position, they will measure heat radiation in space. The L2 point is located outside the Earthâ€™s orbit. Here, the gravitational pull of the Earth and the Sun balance the centrifugal force of its own motion around the Sun. Thus, Herschel and Planck will only need minimal amounts of fuel for carrying out minor orbital corrections while they are in this position.

Apart from saving fuel, 'parking spot' L2 offers other advantages: The antenna used to transmit data to Earth is always pointing in the right direction, and its transmission power does not need to be continually adjusted since the satellite's distance from the Earth remains constant. Thanks to the fact that space telescopes at L2 stay in the same position relative to the Sun and the Earth, it is also easier to shield them from the sunlight that would otherwise perturb their orbit.

Contact
 Dr.-Ing. Christian Gritzner German Aerospace CenterSpace Agency, Space Science Tel.: +49 228 447-530 Fax: +49 228 447-706
Last update: 02/02/2009 16:07:31