How Far Can an Interstellar Probe "See" Using ENAs?

Are there any Energetic Neutral Atoms (ENAs) emitted by sources outside the heliopause, extra-heliospheric ENAs? If yes, we cannot "see" them because are effectively "blind" being inside the heliospheric bubble. Extra-heliospheric ENAs get stripped or photoionized while traveling through the Local Interstellar Medium (LISM) and then inside the heliosphere. Moreover, for an observer inside the heliosphere the extra-heliopsheric ENAs, if any, may not be distinguishable from the ENAs produced at the heliospheric interface. But if an observer is outside the heliosphere, how far can one "see" using ENAs? The higher energy, the lower stripping cross sections, the further away can one "see". Without assuming any sources we estimate the propagation distances for extra-heliospheric ENAs. ENAs in the keV range propagate over 1000 au, 100 MeV over 100 000 (3 ly), a distance to nearest stars, relativistic ENAs can propagate up to 20 ly, that is the size of the local interstellar cloud. An interstellar probe equipped with a proper ENA instrumentation and traveling distances to 500 - 1000 au would be the only mean to investigate these extra-heliospheric ENA emissions. Moreover, large travel distances make possible ENA parallax observations and thus measurements of the distances to ENA emitting structures. We thus argue that deep sky ENA surveys outside the heliopshere could be a highly exploratory science objective for an interstellar probe. We also suggest possible sources of extra-heliospheric ENAs and discuss diagnostic techniques.