Design of the Instrument and Telescope Control Units integrated subsystem of the ESA-ARIEL payload

The Atmospheric Remote-sensing Infrared Exoplanets Large-survey (ARIEL)(1) Mission has been recently selected by ESA as the fourth medium-class Mission (M4) in the framework of the Cosmic Vision Program. The goal of ARIEL is to investigate, thanks to VIS photometry and IR spectroscopy, the atmospheres of several hundreds of planets orbiting nearby stars in order to address the fundamental questions on how planetary systems form and evolve.(2) During its four-years mission, ARIEL will observe several hundreds of exoplanets ranging from Jupiter- and Neptune-size down to super-Earth and Earth-size with its 1 meter-class telescope.(3) The analysis of spectra and photometric data will allow to extract the chemical fingerprints of gases and condensates in the planets atmospheres, including the elemental composition for the most favorable targets. It will also enable the study of thermal and scattering properties of the atmosphere as the planet orbits around its parent star. The ARIEL telescope will feed, by means of a collimated beam, two separated modules hosted in the Payload(4) (P/L); the main instrument, i.e. the IR Spectrometer (AIRS), providing variable spectral resolutions in the range 30-180 for a waveband between 1.95-7.8 mu m and a combined Fine Guidance System (FGS) VIS-Photometer/NIR-Spectrometer module hosting 3 photometric channels in the range between 0.50-1.2 mu m to monitor the photometric stability of the targets. Two of these channels, commonly referred as the FGS, will also be exploited by the S/C as a prime/redundant system for fine guidance and closed-loop control. Integrated in this combined module is a further low-resolution (R similar or equal to 10) spectrometer in the 1.2-1.95 mu m waveband. The Payload is passively cooled to similar to 55 K by isolation from the Service Vehicle Module (SVM) via a series of V-Groove radiators and, to constrain the thermo-mechanically induced optical aberrations, the temperature of the primary mirror will be monitored and finely tuned thanks to an active thermal control system based on thermistors and heaters. They will be switched on and off to maintain the M1 temperature within +/- 1 K thanks to a proportional-integral-derivative (PID) controller implemented within the Telescope Control Unit (TCU), an electronics subsystem in charge of the active thermal stabilization of the detectors assemblies belonging to AIRS and FGS, besides M1. TCU, as baseline, shall also control the on-board IR calibrator by means of an accurate feedback-loop system, the M2 secondary mirror refocusing mechanism and will collect the housekeeping of the managed subsystems, forwarding them to the Instrument Control Unit (ICU), the main electrical I/F to the Spacecraft (S/C). The ICU processor will run the Application SW5 in charge of instrument management and data processing and it will feed and control the TCU, collecting all the needed housekeeping for the management of the monitored subsystems. ARIEL is highly complementary to other on-ground and space observatories (such as Kepler, TESS and PLATO) and will take advantage on the success of previous already flown missions, which will provide an optimized list of targets (similar to 500 planets at least). This paper will focus on the P/L warm electronics, on ICU and TCU architectures in particular.