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2.5 The Satellite Launch, Orbit and Operations

An obvious advantage for any accurate measurement is the location of the instrument in a benign environment. Accordingly, the scientific requirements of the GAIA mission call for a very quiet operational orbit, in terms of thermo-mechanical stability as well as instrument's exposure to radiation.

The selected operational orbit is a small-extension Lissajous or Halo orbit around the L2 Lagrangian point of the Sun-Earth system, 1.5 million km from Earth, the same selected for FIRST and PLANCK and proposed for the NGST. The main characteristics of such an orbit that make it preferable with respect, e.g., to the geostationary orbit planned for, but non achieved by, the Hipparcos satellite, are a low radiation flux due to the long distance from Earth and a stable thermal environment due to the continuous and uniform illumination from the Sun ([Pace 1997]). The long distance from Earth, however, severely limits the affordable data tranmission rate to the ground (see Section 2.6).

The spacecraft is designed for launch by Ariane V into a standard Geostationary Transfer Orbit (GTO). The transfer from the GTO to the operational orbit is done with a 400 N engine right after the separation of the launcher. A redundant set of 10 N thrusters is used for orbit correction during the transfer phase and for the final insertion into the operational orbit. During the operations, a pointing stability better than 2 mas/s over 1 s is required in order to follow the scanning law without blurring the star image on the focal plane. This is achieved thanks to a set of Field Emission Electric Propulsion (FEEP) microthrusters with a thrust of the order of 1 mN, which are used in combination with a wide field star sensor and the sky mappers of the astrometric instruments.

The selected operational orbit and the attitude control system provide a 6 year lifetime, including 8 months for the transfer between the GTO and L2 orbit and some margin, giving at least 5 years of observation without occultations of the field of view or eclipses of the Sun, and even more than that assuming a small correction manoeuvre after a few years of operations.


next up previous contents
Next: 2.6 Data Handling Up: 2. The GAIA Mission Previous: 2.4.2 The Spectrometric Instrument   Contents
Mattia Vaccari 2000-12-05