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

Abstract: 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.