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Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA's F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum Δ V capability of 600 ms − 1 . Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission's science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule. [ABSTRACT FROM AUTHOR]

ARIEL, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was selected as the fourth medium-class mission in ESA's Cosmic Vision program. ARIEL is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 micron and operating in cryogenic conditions. Fabrication of the 1.1 m aluminum primary mirror for the ARIEL telescope requires technological advances in the three areas of substrate thermal stabilization, optical surface polishing and coating. This article describes the qualification of the three procedures that have been set up and tested to demonstrate the readiness level of the technological processes employed. Substrate thermal stabilization is required to avoid deformations of the optical surface during cool down of the telescope to the operating temperature below 50 K. Purpose of the process is to release internal stress in the substrate that can cause such shape deformations. Polishing of large aluminum surfaces to optical quality is notoriously difficult due to softness of the material, and required setup and test of a specific polishing recipe capable of reducing residual surface shape errors while maintaining surface roughness below 10 nm RMS. Finally, optical coating with protected silver must be qualified for environmental stability, particularly at cryogenic temperatures, and uniformity. All processes described in this article have been applied to aluminum samples of up to 150 mm of diameter, leading the way to the planned final test on a full size demonstrator of the ARIEL primary mirror. [ABSTRACT FROM AUTHOR]

The STereo imaging Channel (STC) is the first push-frame stereo camera on board an European Space Agency (ESA) satellite, i.e., the ESA-Japan Aerospace eXploration Agency mission BepiColombo. It was launched in October 2018, and it will reach its target, Mercury, in 2025. The STC main aim is to provide the global three-dimensional reconstruction of the Mercury surface. STC, the stereo channel of spectrometer and imagers for Mercury Planetary Orbiter BepiColombo-Integrated Observatory System, is based on an original optical design that incorporates the advantages of a compact unique detector instrument and the convenience of a double direction acquisition system. In fact, STC operates in a push-frame imaging mode and its two optical sub-channels will converge the incoming light on a single focal plane assembly allowing to minimize mass and volume. The focal plane of the instrument is housing six different filters: two panchromatic filters in the range 600-800 nm and four broadband filters with central wavelengths in the range 420-920 nm. In this paper, the geometrical calibration of the instrument, including the optical setups used, will be described. The methods used to derive the focal lengths, the boresights, and the reference systems of the different filter models are presented, and the related distortion results are discussed. The STC off-axis configuration forced to develop a distortion map model based on the RFM (rational function model). In contrast to other existing models, which allow linear estimates, the RFM is not referred to specific lens geometry, but it is sufficiently general to model a variety of distortion types, as it will be demonstrated in this particular case. [ABSTRACT FROM AUTHOR]

• The paper describes the STC channel and its role in the BepiColombo mission. • The on-ground performance include the on-ground resolution and detector response. • The in-flight performance include the SNR and global coverage of Mercury surface. • Conclusions and future steps. The Stereo Imaging Channel (STC) is one of the channels of the Spectrometer and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIO-SYS) onboard the ESA BepiColombo mission to Mercury. STC is a double wide-angle camera designed to image each portion of the Mercury surface from two different lines of sights, whose main aim is to provide panchromatic stereo-image pairs required to generate the Digital Terrain Model (DTM) reconstruction. In addition, selected surface areas will be acquired in color. This work presents the expected STC on-ground and in-flight performance describing the preliminary evaluation of some key parameters: the optical performance, the on-ground resolution and detector response, the achievable Signal to Noise Ratio (SNR) for different integration times and observation strategies and the global coverage of panchromatic filters during the entire scientific phase. The estimation of the SNR has been made using the STC radiometric model with Hapke reflectance model for Mercury surface and the SPICE toolkit software. The SPICE toolkit software with kernel for BepiColombo mission has been used also for the estimation of the on-ground pixel dimension and the global coverage all over the mission. [ABSTRACT FROM AUTHOR]