  High resolution imaging of supermassive black holes shadows is a direct way
to verify the theory of general relativity at extreme gravity conditions. Very
Long Baseline Interferometry (VLBI) observations at millimeter/sub-millimeter
wavelengths can provide such angular resolution for supermassive black holes,
located in Sgr A* and M87. Recent VLBI observations of M87 with the Event
Horizon Telescope (EHT) has shown such capabilities. The maximum obtainable
spatial resolution of EHT is limited by Earth diameter and atmospheric phase
variations. In order to improve the image resolution longer baselines are
required. Radioastron space mission has successfully demonstrated the
capabilities of Space-Earth VLBI with baselines much larger than Earth
diameter. Millimetron is a next space mission of the Russian Space Agency that
will operate at millimeter wavelengths. Nominal orbit of the observatory will
be located around Lagrangian L2 point of the Sun-Earth system. In order to
optimize the VLBI mode, we consider a possible second stage of the mission that
could use near-Earth high elliptical orbit (HEO). In this contribution a set of
near-Earth orbits is used for the synthetic space-ground VLBI observations of
Sgr A* and M87 in joint Millimetron and EHT configuration. General-relativistic
magnetohydrodynamic models (GRMHD) for black hole environment of Sgr A* and M87
are used for static and dynamic imaging simulations at 230 GHz. A comparison
preformed between ground and space-ground baselines demonstrates that joint
observations with Millimetron and EHT significantly improve the image
resolution and allow the EHT+Millimetron to obtain snapshot images of Sgr A*
probing dynamics at fast timescales.
