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GOCE Mission

The Real Shape of Earth

The job of the GOCE mission,(1) just launched by the European Space Agency (ESA), is to measure the Earth's gravity field at every point and with unrivalled accuracy. This fundamental yet poorly known data will help determine the true shape of Earth.

If Man first believed the Earth was flat, then round–we might be in for yet another surprise: its shape is actually that of a lumpy potato.2 There's a hollow off South America, a bump to the north of Australia, and various lumpy bits here and there. This distorted shape is invisible both to the Earth-bound traveler and to the astronaut observing the blue planet in its atmosphere–yet it plays havoc with a host of measurements, for instance those of ocean currents or the movement of the Earth's crust. It is what makes the GOCE mission so important. This satellite will remain in orbit for 20 months. It will measure the gravity field, the cause of deformities, with the same precision (one part in a million) all over the surface of Earth, at a resolution of about a 100 km. Researchers from several CNRS labs3 are getting ready to process the data and include them in their models.

goce

© ESA - AOES Medialab

The GOCE satellite (artist's rendition), should be able to measure the gravity field with unprecedented precision and resolution on a global scale.



The quantity known as g (the acceleration due to gravity) is what relates mass to weight, and gives the Earth its shape. And if our planet isn't a smooth sphere, it is because g doesn't have exactly the same value all over the surface of Earth: Mass is not equally distributed inside the planet, and every point on its surface is not subjected to the same attractive force.
To get a better picture of the variations of g at the Earth's surface, scientists use an imaginary Earth called the “geoid.” It is important to note that this lumpy image of our planet corresponds to the mean level the ocean would have at rest. “If the whole of the Earth were covered with water, its surface would be the same as that of the geoid,” explains Michel Diament, of IPGP.4 “This means ocean level extends beneath the continents and is used as a reference for altitudes. For instance, the Mont Blanc summit is 4807 meters above the geoid.” So why do the world's oceans at rest show such hollows and bumps? The answer lies in the entrails of the planet. “If the Earth was immobile and made of just one material–i.e., if it was homogeneous–the geoid would be a sphere,” Diament says. “But our planet rotates, which gives it a flattened shape, and it is made of materials of different densities. The presence in one place of a magma reservoir at a depth of a few hundred meters, and in another of a sinking oceanic plate means that the density of the material beneath our feet varies, and with it the value of g.
Determining these hollows and bumps–with differences of up to a 100 meters–with the same precision all over the planet is no easy task. Indeed, local ground measurement can give the value of the gravity field with accuracy of one part in a billion, but for a large structure such as the Himalayas, it is necessary to have this type of high precision on a very large scale. “The aim is also to unify international reference systems so that, for example, the measurement of the altitude of a point means the same thing in Paris or Beijing,” adds Diament. To meet this challenge, GOCE is well equipped: on board, it carries a gradiometer, an instrument made up of six ultra-precise accelerometers built by the French Aerospace Lab Onera,5 completed by a GPS receiver. To preserve high resolution, GOCE was placed in a low orbit, at an altitude of 265 kilometers. At this distance, friction with the residual atmosphere makes it constantly lose altitude. The satellite therefore has to compensate for this by firing small ion thrusters. “This is a top notch Earth observation satellite,” says Diament admiringly. In fact, it was thought up nearly 30 years ago–in particular by Georges Balmino, today a researcher at CNES who has at last seen the fruits of his labor.

Azar Khalatbari

Notes :

1. Gravity Field and Steady-State Ocean Circulation Explorer.
www.esa.int/SPECIALS/GOCE/
2. View web site
3. IPGP (CNRS / Universités Paris-VI and VII / Université de la Réunion); Locean (CNRS / Université Paris-VI / MNHN / IRD); Observatoire de la côte d'Azur; Observatoire Midi-Pyrénées.
4.Institut de physique du globe de Paris (CNRS / Universités Paris-VI and VII / Université de la Réunion).
5. Office national d'études et de recherches aérospatiales.

Contacts :

Institut de physique du globe de Paris (IPGP).
Michel Diament,
diament@ipgp.jussieu.fr
Sébastien Deroussi,
deroussi@ipgp.jussieu.fr


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