4.1. Launch and early-orbit phase Gaia was launched from the European space port in French Guiana by a Soyuz-STB launch vehicle with Fregat upper stage on 19 December 2013 at 09:12:19.6 UTC. Initially, the coupled Fregat-Gaia upper composite was placed on a 180 km altitude parking orbit, after which a single Fregat boost injected Gaia into its transfer orbit towards the second Lagrange (L2) point of the Sun-Earth-Moon system. Soon after Gaia separated from the Fregat, it autonomously pointed itself towards the Sun and initiated the deployment of the sunshield assembly and the release of the launch bipods between the service and payload modules. One day later, a turn-and-burn orbit manoeuvre with size ∆V = 23.5 m s−1 was conducted to remove the stochastic launcher dispersion, after which the switch-on of service-module units and the ten-day payload decontamination (heating) phase was started. The launch and early orbit phase with extra groundstation coverage and 24-hour manned shifts of operators at the Mission Control Centre (Sect. 5.3) lasted four days. After completion of the decontamination phase, on 3 January 2014 the scientific payload was switched on and overall system tuning began (Sect. 4.2). The transfer to L2 took 26 days from launch and the insertion burn into the Lissajous orbit (Sect. 5.1) was split in two parts, separated by one week (7 and 14 January 2014), with a total ∆V of 166.3 m s−1.
4.2. Commissioning and performance verification The commissioning and performance-verification phase was coordinated by ESA and the industrial prime contractor, Airbus DS, and was supported by scientists in the data processing and analysis consortium, in particular, the payload experts (Sect. 6). This phase started with a period during which Gaia was initialised and its performance was iteratively improved (Milligan et al. 2016) and ended with a period during which Gaia was operated for a few weeks in ecliptic-pole scanning mode (Sect. 5.2) to allow the science ground-segment verification of the scientific performance of Gaia (Els et al. 2014). Early activities in this process included (initial) focussing, spin-rate adjustments, and tuning the various settings of the onboard attitude-control loop, which matches the rotation of Gaia with the fixed TDI rate. The overall conclusion of the commissioning phase was that nearly all subsystems behaved nominally and some even better than expected. Examples of properly functioning subsystems are the focal plane assembly (noise, linearity, bias, cross-talk, etc.), the onboard data handling (including compression and prioritisation of science data), the onboard detection and windowing of sources, the phased-array antenna and link budget, the pointing and spinrate performance achieved by the combined attitude control and micro-propulsion subsystems, and the Rubidium atomic master clock.

This is the case despite the latter showing occasional, presumably stress-relief induced steps in the Rubidium lamp light level, at the level of 10−3 V, and occasional frequency jumps, at the level of ∆f/ f ∼5 · 10−12, sometimes correlated with light level changes.

However, three particular points came to light.

First, the latch valve of chemical thruster 3B was found stuck in closed position, later found, most likely as a result of a tiny leak of propellant inside the valve cap, i.e. the mechanical housing containing the valve circuitry.

The leak itself is thought to be caused by a minuscule crack in a flexure sleeve, allowing NTO or MMH chemical propellant to leak into the actuator electronic assembly, causing circuit failure to the valve actuator coils and the microswitch.

As an immediate mitigation, thruster 3B was removed from the onboard failure detection, isolation, and recovery logic such that thruster 3A would always have been used in case of safe mode.

To recover robustness against the loss of redundancy of chemical thruster 3A, which had become a single-point failure in this configuration, a new AOCS survival mode was developed and implemented in the central software; this mode uses the torque authority provided by a slight misalignment of thrusters (originally designed only for attitude-control manoeuvres) to maintain three-axis spacecraft control in case of thruster 3A failure.

Second, the biases of the mass-flow sensors of the micropropulsion thrusters, which means the offsets achieved with zero cold-gas mass flow, were found to be drifting.

Such a drift in itself can be calibrated, although initially at the expense of science time because such a calibration initially required switching to chemical-propulsion control.