The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 23 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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One of the perturbing forces is due to the fact that the Earth is not a perfect sphere. The slightly irregular mass distribution inside the Earth changes the gravity force, for instance near 75 degrees East, as shown here in yellow. The sum of the Earth's gravity and the centrifugal force does not vanish. Instead a small force remains, pulling the spacecraft towards the 75 degree meridian. If the satellite is not continuously pushed back, it will librate about this - or its antipodal - equilibrium point. Note that there are stable points at 75 degrees East and 105 degrees West.