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Extension of radio interferometric baselines into space is inevitable if adiffraction-limited angular resolution defined by the Earth diameter at a givenobserving wavelength limits a pursuit of specific scientific goals. This wasunderstood in the early1960s, at the very dawn of the era of Earth-based VeryLong Baseline Interferometry (VLBI). Since then, three VLBI missions operatedin Space thus enabling baselines longer than the Earth diameter. These are theTracking and Data Relay Satellite Orbital VLBI experiment (1986 - 1988), theVLBI Space Observatory Program (VSOP, 1997 - 2003) and RadioAstron (2011 -present time). These Space VLBI (SVLBI) systems enabled studies of celestialradio sources with an unprecedentedly sharp angular resolution reaching 0.1nanoradian (tens of microarcseconds) and sharper. The first generation SVLBImissions provided cutting-edge results in several topics of modern radioastronomy. These include discoveries of ultra-compact galactic hydroxyl andwater vapor masers, radio emission in active galactic nuclei with thebrightness exceeding conventional theoretical limits, and detection of pulsaremission at meter wavelengths shedding a new light on the properties of theinterstellar medium. One of the Space VLBI missions, the RadioAstron, alsoventured into the domain of fundamental physics by sing its onboard Hydrogenmaser local oscillators for experimental verification of the EinsteinEquivalence Principle. The paper provides a brief review these discoveries andlessons learned over the first half a century of Space VLBI. This comparison,together with the review of technological achievements of the three SLBImissions, creates a fundament for projecting the development of space-basedradio interferometry into the next decades.

Radio Interferometers Larger than Earth: Lessons Learned and Forward Look of Space VLBI