  Extension of radio interferometric baselines into space is inevitable if a
diffraction-limited angular resolution defined by the Earth diameter at a given
observing wavelength limits a pursuit of specific scientific goals. This was
understood in the early1960s, at the very dawn of the era of Earth-based Very
Long Baseline Interferometry (VLBI). Since then, three VLBI missions operated
in Space thus enabling baselines longer than the Earth diameter. These are the
Tracking and Data Relay Satellite Orbital VLBI experiment (1986 - 1988), the
VLBI Space Observatory Program (VSOP, 1997 - 2003) and RadioAstron (2011 -
present time). These Space VLBI (SVLBI) systems enabled studies of celestial
radio sources with an unprecedentedly sharp angular resolution reaching 0.1
nanoradian (tens of microarcseconds) and sharper. The first generation SVLBI
missions provided cutting-edge results in several topics of modern radio
astronomy. These include discoveries of ultra-compact galactic hydroxyl and
water vapor masers, radio emission in active galactic nuclei with the
brightness exceeding conventional theoretical limits, and detection of pulsar
emission at meter wavelengths shedding a new light on the properties of the
interstellar medium. One of the Space VLBI missions, the RadioAstron, also
ventured into the domain of fundamental physics by sing its onboard Hydrogen
maser local oscillators for experimental verification of the Einstein
Equivalence Principle. The paper provides a brief review these discoveries and
lessons learned over the first half a century of Space VLBI. This comparison,
together with the review of technological achievements of the three SLBI
missions, creates a fundament for projecting the development of space-based
radio interferometry into the next decades.
