ARIEL's design builds on the thermal design heritage from ESA's Planck mission, and is a modified version of that proposed for EChO (the Exoplanet Characterization Observatory), a previous Cosmic Vision candidate that was considered for the M3 launch opportunity in 2024 (for which the planet-hunting PLATO mission [PLAnetary Transits and Oscillations of stars] was ultimately selected).
The design has been simplified, and its mass reduced, to meet programmatic constraints — unlike EChO, which also targeted temperate exoplanets, ARIEL will preferentially focus on warm and hot ones, thus requiring a simpler payload. The launch mass of the entire spacecraft is approximately 1300 kg.
The spacecraft is designed with two distinct modules: the Service Module (SVM) and Payload Module (PLM). These two modules are thermally isolated from one another — the SVM sits at the 'bottom' of the spacecraft. Three V-Grooves, composed of an Aluminium-skin/Aluminium-honeycomb sandwich, and three pairs of low conductivity Glass Fiber Reinforced Plastic (GFRP) bipod struts sit on top of the SVM to support the PLM (including the optical bench, the telescope and the instruments). This thermal shield assembly allows the complete PLM to be passively cooled to ~55K (optical bench, telescope and instruments). In addition, a Neon Joule-Thomson cooler allows the AIRS long wavelength channel to be cooled to ~42K.
The PLM comprises an off-axis Cassegrain telescope (with a primary mirror of approximately 1.1 m × 0.7 m) with a third beam collimating mirror, the ARIEL infrared spectrometer (AIRS) covering the 1.95 – 7.80 μm wavelength range, and a Fine Guidance System (FGS) module with three narrow-band visible to near-infrared photometer channels (two used as guidance sensors as well as for science) and a low-resolution near-infrared spectrometer.
The structure of the PLM is based on a horizontal telescope configuration. The complete optical bench and all mirrors will be made out of Aluminium.
The propellant tank is accommodated inside the SVM, while the solar panels, thrusters and high gain antenna are accommodated underneath. ARIEL will use a hydrazine monopropulsion system. ARIEL's Attitude and Orbital Control Systems (AOCS) will be three-axis stabilised and wheel-based, operating the wheels in a narrow angular speed range, away from any peak vibration mode and away from any possible amplification frequency of the spacecraft structure to mitigate the impact of micro-vibrations on the pointing budgets.