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We present results from the first 22 GHz space very-long-baselineinterferometric (VLBI) imaging observations of M87 by RadioAstron. As a part ofthe Nearby AGN Key Science Program, the source was observed in Feb 2014 at 22GHz with 21 ground stations, reaching projected $(u,v)$-spacings up to$\sim11\,$G$\lambda$. The imaging experiment was complemented by snapshotRadioAstron data of M87 obtained during 2013--2016 from the AGN Survey KeyScience Program. Their longest baselines extend up to $\sim25\,$G$\lambda$. Forall these measurements, fringes are detected only up to $\sim$2.8 EarthDiameter or $\sim$3 G$\lambda$ baseline lengths, resulting in a new image withangular resolution of $\sim150\,\mu$as or $\sim20$ Schwarzschild radii spatialresolution. The new image not only shows edge-brightened jet and counterjetstructures down to submilliarcsecond scales but also clearly resolves the VLBIcore region. While the overall size of the core is comparable to those reportedin the literature, the ground-space fringe detection and slightlysuper-resolved RadioAstron image suggest the presence of substructures in thenucleus, whose minimum brightness temperature exceeds $T_{\rm B,min}\sim10^{12}\,$K. It is challenging to explain the origin of thisrecord-high $T_{\rm B, min}$ value for M87 by pure Doppler boosting effect witha simple conical jet geometry and known jet speed. Therefore, this can beevidence for more extreme Doppler boosting due to a blazar-like small jetviewing angle or highly efficient particle acceleration processes occurringalready at the base of the outflow.

RadioAstron Space VLBI Imaging of the jet in M87: I. Detection of high brightness temperature at 22 GHz