Astronomy and Astrophysics – Astronomy
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009iau...261.0910d&link_type=abstract
American Astronomical Society, IAU Symposium #261. Relativity in Fundamental Astronomy: Dynamics, Reference Frames, and Data Ana
Astronomy and Astrophysics
Astronomy
Scientific paper
Gaia is ESA's upcoming astrometry mission, building on the heritage of its predecessor, Hipparcos (1989-1993). The Gaia nominal scanning law (NSL) prescribes the ideal attitude of the spacecraft over the operational phase of the mission. As such, it precisely determines when certain areas of the sky are observed. From theoretical considerations on sky-sampling uniformity, it is easy to show that the optimum scanning law for a space astrometry experiment like Gaia is a revolving scanning with uniform rotation (60 arcsec/s for Gaia) around the instrument symmetry axis. Since thermal stability requirements for Gaia's payload require the solar aspect angle to be fixed (45 deg for Gaia), the optimum parallax resolving power is obtained by letting the spin axis precess around the solar direction. The precession speed (63 days period for Gaia) has been selected as compromise, limiting the across-scan smearing of images when they transit the focal plane, providing sufficient overlap between successive "great-circle" scans of the fields of view, and guaranteeing overlap of successive precession loops. With the above scanning law, with fixed solar-aspect angle, spin rate, and precession speed, only two free parameters remain: the initial spin phase and the initial precession angle, at the start of nominal science operations. Both angles, and in particular the initial precession angle, can be initialised following various programmatic criteria. Examples are optimisation / fine-tuning of the chemical-propellant budget, of the Earth-pointing angle, of the number and total duration of galactic-plane scans, or of the ground-station scheduling. This paper explores various criteria, with particular emphasis on the opportunity to optimise the scanning-law initial conditions to "observe" the most favourable passages of bright stars very close to Jupiter's limb. This would allow a unique determination of the light deflection due to the quadrupole component of the gravitational field of this planet.
Bruijne Jos de
Hoar John
Lammers Uwe
O'Mullane William
Prusti Timo
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