Experimental results from the ST7 mission on LISA Pathfinder

Abstract

The Space Technology 7 Disturbance Reduction System (ST7-DRS) is a NASA technology demonstration payload that operated from January 2016 through July of 2017 on the European Space Agency's LISA Pathfinder spacecraft. The joint goal of the NASA and ESA missions was to validate key technologies for a future space-based gravitational wave observatory targeting the source-rich milliHertz band. The two primary components of ST7-DRS are a micropropulsion system based on colloidal micro-Newton thrusters (CMNTs) and a control system that simultaneously controls the attitude and position of the spacecraft and the two free-flying test masses (TMs). This paper presents our main experimental results and summarizes the overall the performance of the CMNTs and control laws. We find that the CMNT performance to be consistent with pre-flight predictions, with a measured system thrust noise on the order of 100 nN/ √ Hz in the 1 mHz ≤ f ≤ 30 mHz band. The control system maintained the TM-spacecraft separation with an RMS error of less than 2 nm and a noise spectral density of less than 3 nm/ √ Hz in the same band. Thruster calibration measurements yield thrust values consistent with the performance model and ground-based thrust-stand measurements, to within a few percent. We also report a differential acceleration noise between the two test masses with a spectral density of roughly 3 fm/s 2 / √ Hz in the 1 mHz ≤ f ≤ 30 mHz band, slightly less than twice as large as the best performance reported with the baseline LISA Pathfinder configuration and below the current requirements for the Laser Interferometer Space Antenna (LISA) mission.

LISA Pathfinder platform stability and drag-free performance

“…In table II, comparison between April 2016 and January 2017 data sets allows to appreciate the consistency between the "noise runs" and the time-invariance of the sensor and actuator performances. It is worth mentioning that independent studies by [18] also show consistent results and similar performances for the cold-gas thrusters at different times of the mission (September 2016 and April 2017).…”

“…The S/C force noise curve (turquoise) is due to the micro-thruster noise, for movements along X and rotation around Y. This has been demonstrated by comparing the colloidal and cold gas micronewton thruster systems independently and jointly [18]. Note that the presence of a strong GRS sensing noise component, around 0.1Hz, is due to the control strategy.…”

“…to a Galilean frame and the micro-propulsion system, a set of six cold gas thrusters (a technology already flown in space with ESA's Gaia mission [5] and CNES 's MICROSCOPE mission [4]), allows S/C displacement and attitude control along its six degrees of freedom. Note that LPF also has a NASA participation, the Space Technology 7 (ST7 ) mission, contributing a set of eight colloidal thrusters and the electronics/computer that control them [18].…”

Lunar Gravitational-wave Antenna

“…Figure 2 shows an optomechanical and cryomagnetic seismometer concept for LGWA, together with the noise performance of the Mars Insight Very Broadband (VBB) sensors and of the LISA Pathfinder (LPF) mission (Armano et al 2016;Anderson et al 2018), which is the best acceleration sensitivity ever achieved at mHz frequencies. The Mars Insight model combines the position and velocity readouts of the suspended masses.…”

Electrospray ionization enters the final frontier: Mass spectrometry's role in understanding electrospray thrusters and their plumes

“…While this mission is testing a combustion‐based thruster, ionic liquids have also been used for electrospray thruster missions. For instance, ESA's LISA‐Pathfinder mission tested an electrospray system, using an ionic liquid propellant (1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide), which performed very successfully …”