ESA GNC Conference Papers Repository

This paper details in-flight mission operations experience from ESA's Gaia spacecraft, with a particular emphasis on the evolution of solar radiation pressure torque due to environmental changes and a shift in thermo-optical properties of the sun-shield's MLI material. Tasked with mapping 1 billion stars to unprecedented precision (to the micro-arc-second level, comparable to the width of a coin on the Moon as viewed from Earth), this ESA's Science cornerstone mission is also expected to discover and chart 100,000's of new objects including Near Earth Asteroids, exoplanets, brown dwarfs and quasars. After a flawless launch on 19 Dec 2013, Gaia was brought the circa 1.5 million km into L2 via a sequence of technically demanding orbit transfer manoeuvres using on-board thrusters in thrust vectoring mode. Starting in parallel to this and lasting 6 months, the spacecraft was fully commissioned and brought gradually up to the highest operational mode. The commissioning phase involved detailed checks also of the relatively high number of bespoke units, including the 106 CDD focal plane assembly, telescope-in-the-loop AOCS control and micro-propulsion which is continuously compensating for the solar radiation pressure torque in order to maintain an unprecedented rate stability (the rate error on Gaia is equivalent to one rotation every 420 years). Operating this ultra-precise mission brings up subtle effects in housekeeping telemetry of the AOCS subsystem, that are normally not observed on conventional spacecraft. This paper focuses on the evolution of solar radiation pressure (SRP) torque measured by Gaia over its 3 years in orbit around L2. Apart from seasonal effects, solar activity and partial lunar eclipses, the main driver for SRP torque and cold gas usage is the long-term change of thermo-optical properties of the sun-shield's MLI material whose characterisation and prediction is crucial for mission lifetime estimation.