Abstract：Observations of the masers in the course of RadioAstron mission yielded detections of fringes for a number of sources in both water and hydroxyl maser transitions. Several sources display numerous ultra-compact details. This proves that implementation of the space VLBI technique for maser studies is possible technically and is not always prevented by the interstellar scattering, maser beaming and other effects related to formation, transfer and detection of the cosmic maser emission.  

   For the first time cosmic water maser emission was detected with projected baselines exceeding Earth Diameter. It was detected in a number of star forming regions in the Galaxy and megamaser galaxies NGC 4258 and NGC 3079.

   RadioAstron observations provided absolute record of the angular resolution in astronomy. Fringes from the NGC 4258 megamaser were detected on baseline about 26.7 Earth Diameters (~340,000 km). This means that the angular resolution about 8 μas was directly achieved in the cosmic maser observations. Such resolution is sufficient to measure parallax of the water maser source in the nearby galaxy LMC.

   RadioAstron detected also the smallest structures ever observed in a Galactic maser. Analysis of the data on Cep A water maser indicates that the source contains features with the sizes about the diameter of the Sun. The space–Earth cross power spectrum shows two unresolved components smaller than 15 μas separated by 0.5 km/s in velocity and separated by about 160±35 μas. The limits on the diameters of the components (15 μas) correspond to a linear scale of 1.6e11 cm. The brightness temperatures are greater than about 2.e14 K, and the line widths are 0.5 km/s. Most of the flux density (～ 90 per cent) is contained in a halo of diameter 1 mas. We discuss possible interpretations for the compact structure. The sharpest “direct” linear resolution better than 4 million kilometers was achieved in observations of the maser in Orion.

   So, the major step from milli- to micro-arcsecond resolution in maser studies is done in the RadioAstron mission. Existence of the features with extremely small angular sizes is established. Further implementations of the space VLBI maser instrument for studies of the nature of cosmic objects, studies of interaction of extremely high radiation field with molecular material and studies of the matter on the line of sight are planned.

   Prof. Andrej Sobolev from the Ural Federal University in Russia. He is an expert on maser studies and space VLBI applications including with RadioAstron, a successfully running space based interferometry experiment studying pulsars, variable stars, masers and AGN.