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RadioAstron is a 10 m orbiting radio telescope mounted on the Spektr-R satellite, launched in 2011, performing Space Very Long Baseline Interferometry (SVLBI) observations supported by a global ground array of radio telescopes. With an apogee of ∼350 000 km, it is offering for the first time the possibility to perform μas-resolution imaging in the cm-band. Aims. The RadioAstron active galactic nuclei (AGN) polarization Key Science Project (KSP) aims at exploiting the unprecedented angular resolution provided by RadioAstron to study jet launching/collimation and magnetic-field configuration in AGN jets. The targets of our KSP are some of the most powerful blazars in the sky. Methods. We present observations at 22 GHz of 3C 273, performed in 2014, designed to reach a maximum baseline of approximately nine Earth diameters. Reaching an angular resolution of 0.3 mas, we study a particularly low-activity state of the source, and estimate the nuclear region brightness temperature, comparing with the extreme one detected one year before during the RadioAstron early science period. We also make use of the VLBA-BU-BLAZAR survey data, at 43 GHz, to study the kinematics of the jet in a ∼1.5-yr time window. Results. We find that the nuclear brightness temperature is two orders of magnitude lower than the exceptionally high value detected in 2013 with RadioAstron at the same frequency (1.4 × 10 13 K, source-frame), and even one order of magnitude lower than the equipartition value. The kinematics analysis at 43 GHz shows that a new component was ejected ∼2 months after the 2013 epoch, visible also in our 22 GHz map presented here. Consequently this was located upstream of the core during the brightness temperature peak. Fermi-LAT observations for the period 2010â '2014 do not show any Î 3-ray flare in conjunction with the passage of the new component by the core at 43 GHz. Conclusions. These observations confirm that the previously detected extreme brightness temperature in 3C 273, exceeding the inverse Compton limit, is a short-lived phenomenon caused by a temporary departure from equipartition. Thus, the availability of interferometric baselines capable of providing μas angular resolution does not systematically imply measured brightness temperatures over the known physical limits for astrophysical sources.© 2017 ESO. ; Scientific results from data presented in this publication are derived from the EVN project code GA030C (ground array), and from the RadioAstron mission project code RAKS04C (space segment). J.L.G. and A.F. acknowledge support by the Spanish Ministry of Economy and Competitiveness grants AYA2013-40825-P and AYA2016-80889-P. A. A. and M.A.P.T. acknowledge support by the Spanish MINECO through grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P, cofunded with FEDER funds. E.R. acknowledges support from the Spanish MINECO through grants AYA2012-38491-C02-01 and AYA2015-63939-C2-2-P and from the Generalitat Valenciana grant PROMETEOII/2014/057. T.S. was funded by the Academy of Finland projects 274477 and 284495. The research at Boston University was supported in part by NASA Fermi Guest Investigator grant NNX14AQ58G. S.S.L. was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. NRF-2016R1C1B2006697). Y. Y. K. was supported in part by the Alexander von Humboldt Foundation. This research has made use of data from the OVRO 40-m monitoring program (Richards et al. 2011) which is supported in part by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G and NSF grants AST-0808050 and AST-1109911. ; Peer Reviewed