“…Since the early detection and imaging of the compact core and radio jet in M87 by VLBI technique (e.g., Reid et al 1982), the source has been observed by modern VLBI facilities, to study the structure, dynamics, and physical origin of the jet (see, e.g., Event Horizon Telescope Collaboration et al 2019 and references therein). Along with those studies, decades-long efforts have been made to improve angular resolution toward M87 by adopting orbiting antenna as a VLBI station, thus realizing a space VLBI array with a virtual aperture larger than the Earth diameter (see, e.g., Burke 2009;Hirabayashi 2012;Schilizzi 2012;Gurvits 2020;Gurvits et al 2021 for a review). Previous space VLBI programs including tests with the NASA Tracking and Data Relay Satellite System (Levy et al 1986(Levy et al , 1989Linfield et al 1989Linfield et al , 1990 and the VLBI Space Observatory Programme (Hirabayashi et al 1998(Hirabayashi et al , 2000 provided not only proof of the concept but also the possibility of imaging M87 at exceptionally high angular resolution (e.g., Dodson et al 2006).…”