In-Orbit Experience of the Gaia and LISA Pathfinder Cold Gas Micro-propulsion Systems

Abstract

This paper presents in-flight experience of the cold gas micro-propulsion systems (MPS) used on-board the Gaia and LISA Pathfinder spacecraft. Gaia is an ESA Science cornerstone mission which is tasked with mapping one billion stars in the Milky Way to unprecedented precision. It is also expected to discover and chart 100,000’s of new objects including near-earth asteroids, exoplanets, brown dwarfs and quasars. The Gaia spacecraft was designed and built by Airbus Defence and Space. After a flawless launch on 19 Dec 2013, it was brought the circa 1.5 million km into L2 via a Soyuz Fregat burn. Additional delta-V was realized via a sequence of technically demanding orbit transfer manoeuvres using on-board chemical thrusters in thrust vectoring mode. Since early 2014, Gaia has been operating in a halo orbit around the second Sun-Earth Lagrange point that provides the stable thermal environment without Earth eclipses needed for the payload to function accurately. Starting in parallel to this and lasting six months, the spacecraft was fully commissioned and brought gradually up to the highest operational mode. The unique rate stability requirements for Gaia’s science mode (the standard deviation of its rate error is equivalent to one rotation every 420 years) lead to a high number of bespoke units, including a 106 CDD focal plane assembly, telescope-in-the-loop AOCS control and a cold gas micro-propulsion system which was developed by Thales Alenia Space Italia (TAS-I) and Leonardo Company. Continuously compensating for the solar radiation pressure torque in order to maintain an undisturbed scanning-law of the celestial sphere, a 3-year data set of MPS housekeeping telemetry was collected that offers the unique opportunity to characterize long-term performance of this novel fine pointing actuation system (thrust range 1 μN to 1000 μN at 0.1 μN resolution) in thermally stable conditions. Planned disturbances such as station keeping manoeuvres using coarse chemical propulsion as well as unplanned disturbances due to environmental effects such as micro-meteoroid impact also help characterize the cold gas system performance under stress due to sudden increase in thrust demand. LISA Pathfinder (LPF) is an ESA mission that demonstrates technologies needed for a planned ESA gravitational wave observatory. The LPF spacecraft, designed and built by Airbus Defence and Space, places two test masses in a nearly perfect gravitational free-fall, and controls and measures their relative motion with unprecedented accuracy. The laser interferometer measures the relative position and orientation of the masses to an accuracy of less than 0.01 nanometres, a technology shown to be sensitive enough to detect gravitational waves by the planned follow-on ESA mission, the Laser Interferometer Space Antenna (LISA). Launched on 03 Dec 2015, LPF reached its operational orbit around L1 in early 2016 where it underwent payload commissioning. The same MPS used by Gaia was selected for LPF fine attitude control to realize the extremely accurate free-fall trajectory of its test masses. After reaching the destination orbit, the propulsion module was separated. Since this point, the MPS was also the main actuator for station keeping manoeuvres where the cold gas thrusters are operated in “open-loop” realizing demanded forces. Approaching its nominal end of mission lifetime, LPF carried out dedicated test operations to characterize MPS performance under non-nominal conditions. The aim of this paper is to present the in-flight results that have been derived from post-processing of the Gaia and LPF housekeeping telemetry archives in terms of micro-thruster performances. MPS off-nominal events that have been encountered in-flight on individual or both spacecraft will be presented as well as mitigation actions that have been put in place to restore nominal conditions.