The Comet Interceptor EnVisS Instrument: Wide-Field Imaging of the Solar Wind's Interaction with a Comet

The Comet Interceptor mission was selected in June 2019 by the European Space Agency as its next planetary project. The mission will launch in 2028, and will perform a flyby of an as-yet-undiscovered comet approaching the Sun for the first time since its formation. As well as the primary spacecraft which will encounter the comet at a safe distance, two sub-spacecraft will venture closer to the target's nucleus. It is planned that one spin-stabilized sub-spacecraft will carry the Entire Visible Sky instrument, or EnVisS. This innovate camera will instantaneously image a near-180 degree swath in a plane containing the spacecraft spin axis. With each rotation of the spacecraft, EnVisS, which will contain no moving parts, can produce a map of the entire sky. Several narrow-band linear filters will be juxtaposed and bonded with the instrument's imaging sensor, allowing the camera to concurrently produce all-sky images at each filter's wavelength range. Together with studies of the dust and neutral gas, one of the key scientific aims of the instrument is to image the large-scale distribution of cometary pickup ions in the solar wind surrounding the comet. To achieve this, at least one of the EnVisS filters will target the visible light emission from a common cometary ion, probably H2O+. We expect that EnVisS will obtain several images of the sky on approach to the comet, during the traversal of the coma, and, if the sub-spacecraft survives its passage through the potentially hazardous inner coma, also post-encounter. These images should provide maps of an ion tail at unprecedented spatial resolutions, as well as plasma structures within the coma. Multiple image sequences should reveal the three-dimensional distribution of ions, capturing the dynamics of the plasma tail, as well as possible transient features such as ion tail rays. These structures' morphologies and dynamics can be compared to in situ particles and field measurements on all three spacecraft platforms to help decipher the nature of the comet-solar wind interaction. Data from EnVisS will be complemented by ground-based observations of the target comet. We explain in greater detail the observation technique employed by EnVisS and describe the instrument's design. We also discuss the challenges of the planned observations using this device's novel measurement technique.