EnVisS camera for the Comet Interceptor mission (ESA-JAXA): polarimetric observations of cometary dust.

Comet Interceptor (CI) is an ESA mission in cooperation with the Japanese Aerospace Exploration Agency (JAXA). It aims to provide the first-ever in-situ (as opposed to ground-based observation) characterisation of a long period comet, which could be a dynamically-new comet or an interstellar object, and to perform the first simultaneous multi-point exploration of a cometary coma and nucleus.The science of the mission encompasses two main themes: Comet Nucleus Science and Comet Environment Science. More specifically, the key questions to address are:1) Comet Nucleus Science - What is the surface composition, shape, morphology, and structure ofthe target object?2) Comet Environment Science - What is the composition of the coma, its connection to the nucleus(activity) and the nature of its interaction with the solar wind?The CI mission consists of the main spacecraft (S/C A) and the two probes (named Probe B1 and Probe B2, respectively). The Comet Interceptor payload complement includes remote sensing and in situ measurement instruments accommodated on-board the main S/C and the two probes, so as to enable performing multi-point observations of the selected object during the fly-by.Comet Interceptor will be launched together with the Ariel mission, to the Sun-Earth second Lagrange point, L2. Following a waiting phase at L2, used to select the actual target object and to optimise the related transfer orbit, CI will cruise to the encounter and release the two probes shortly before performing the fly-by. The duration of the waiting phase depends on the actual target and its maximum length is estimated to be four years. After their release, the two probes will perform autonomous operations, relaying the scientific data back to the main spacecraft. The maximum duration of the CI mission, from launch to the end of the post-encounter phase, is six years.The payload for spacecraft A and B2 is provided by ESA Member States. Among these, on the B2 probe, the full sky camera EnVisS (Entire Visible Sky) will study and map the comet dust coma environment from an advantageous point inside the coma itself. EnVisS has been designed with a very wide 180 Field of View (FoV), which, by exploiting the B2 spacecraft rotation, enables the reconstruction of full-sky images. EnVisS adopts a flexible push-broom/push-frame imaging technique, wherein slices of the sky will be acquired while the probe rotates. Subsequently, on-ground, the acquired slices will be stitched together to form a full-sky image. The camera will be equipped with a 3-strip filter assembly transmitting the visible wavelength range 550-800 nm. One of the strips is a high-transmission broadband filter, while the others are linear polarization filters. Thanks to these filters, EnVisS will measure the intensity, the degree of linear polarization and polarization angle orientation of the light scattered by the dust particles in the comet coma, providing an unprecedented extended coverage of the phase angles.