Far Ultraviolet Imaging and Spectral Investigations on an Interstellar Probe

Far-UV imaging spectroscopy is a proven method for investigating key physical processes in both interstellar and interplanetary environments on spacecraft exiting the Solar System. The long history of such investigations includes Voyager (e.g., Lallement et al., Science, 2011) through New Horizons missions (Gladstone et al., GRL, 2018; Fig. 5 attached here), with the post-encounter extended mission science cases focused on heliophysics studies. SwRI's UVS/Alice family of far-UV imaging spectrographs provides an ideal low-resource strawman instrument for discussion of the potential science capabilities of an interstellar probe mission. New Horizons Pluto-Alice is actively studying the Lyman-α skyglow and plans routine sky-scans in the years ahead. Helium 58.4 nm emissions are similarly studied with the Lunar Reconnaissance Orbiter Lyman Alpha Mapping Project (LRO-LAMP) instrument (Grava et al., A&A, 2018), following up on IBEX discoveries. Science goals that a potential UV instrument addresses include searching for distant sources of interstellar neutral and ionized atom emissions, e.g., a potential wall of hydrogen at the boundary where the interstellar wind encounters the solar wind, oxygen ion 83.4 nm emissions (Gruntman et al., JGR, 2000) in the ribbon feature, etc. UV spectrographs are historically the instrument of choice for flyby missions exploring new Solar System targets of the sort an interstellar probe may encounter.

We advocate that an interstellar probe should include either an Ultraviolet Imaging Spectrograph (UVS) or Ultraviolet Imager (UVI) in a mission concept study. A UVS concept would include the improved spatial imaging capabilities developed for JUICE-UVS and Europa-UVS a decade later with long-slit field-of-view of 0.3°×7.5° (Retherford et al., SPIE, 2009; Davis et al., SPIE, 2019). A UVI concept based on Davis et al., SPIE, 2014 (10.1117/12.2057020) implements two off-axis mirrors to provide 0.07° resolution and field-of-view up to 40°×20°.