5.1. Orbit and environment Gaia operates at the second Lagrange (L2) point of the Sun-Earth-Moon system. This saddle point is located ∼1.5 million km from Earth, in the anti-Sun direction, and co-rotates with the Earth in its one-year orbit around the Sun. Gaia moves around L2 in a Lissajous-type orbit with amplitudes of 120 000 km × 340 000 km and 180 000 km (in and perpendicular to the ecliptic plane, respectively) and an orbital period of ∼180 days; these numbers guarantee that the Earth is maximally 15◦away from the boresight of the phased-array antenna, which is used for science data transmission (Sect. 3.2.2). An L2 Lissajous orbit has several advantages over an Earth-bound orbit, for instance stable thermal conditions, a benign radiation environment, and a high observing efficiency in which the Sun, Earth, and Moon are outside the fields of view of the instrument. By design, the orbit of Gaia is not impacted by eclipses of the Sun by the Earth during its five-year nominal lifetime, although small (percent level) partial eclipses by the Moon typically occur once per year. In July 2019, a 14 m s−1 eclipse-avoidance manoeuvre is scheduled to ensure continued Earth-eclipse-free operations through to 2025. After more than two years in orbit, the L2 environment has not (yet) provided surprises:
1. The temperature of the spacecraft is generally as expected, with long-term trends visible owing to the seasonal variation in the spacecraft-Sun distance. 2. The rate of micro-meteoroid fluxes is generally as expected (with the attitude-control system reacting to momentum disturbances exceeding ∼10−5 N m s), essentially following the Grün interplanetary flux model (Grün et al. 1985). 3. The radiation damage to the CCD detectors is measured through first-pixel response in the along-scan profiles of lines of charge injection and through onboard counters registering the number of rejected detections in the sky mapper; this radiation damage is generally as expected, essentially showing a slow yet persistent degradation, caused by the omnipresent flux of high-energy Galactic cosmic rays (a handful of par-
ticles cm−2 s−1), combined with a handful of discontinuities
corresponding to solar activity and associated proton events (Kohley et al. 2014; Crowley et al. 2016a). The data also show a clear sign of the well-known anti-correlation between cosmic-ray fluxes and solar activity (e.g. Davis et al. 2001). Owing to the (so far) benign nature of the current solar cycle 24, however, the accumulated radiation dose has been significantly lower than expected, with only ∼4% of the predicted end-of-mission (five-year) dose having materialised so far during the first two years of operations (for a detailed description, see Crowley et al. 2016b). Radiation damage is hence not expected to strongly affect the long-term performance of Gaia.
5.2. Scanning law The scanning law, which describes the intended spacecraft pointing as a function of time, is one of the keys to the astrometric performance of Gaia. Following principles already worked out in the 1970s and employed for Hipparcos in the 1990s (e.g. Hoyer et al. 1981), Gaia scans the sky using uniform revolving scanning. Such a scanning law optimises the astrometric accuracy in the sense of maximising the uniformity of the sky coverage. Key ingredients of the nominal scanning law are as follows (Fig. 6; Lindegren & Bastian 2011):

[Figure: Fig. 7. Spin axis z makes loops around the Sun, which must overlap. A star at point a may be scanned whenever z is 90◦from a, i.e. on the great circle A at z1, z2, z3, etc. The scans intersect at a large angle, which allows the determination of two-dimensional object coordinates from one-dimensional measurements. Image courtesy Lennart Lindegren.]

ment z axis. Second, the spin phase Ω(t), which is the angle between the plane containing the Sun and the instrument z axis and the instrument zx plane; the fundamental xy plane is the plane through the two viewing directions (see Lindegren et al. (2016) for a definition of the scanning reference system). The governing equation for ν(t) equals