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Very long baseline radio interferometry (VLBI) with ground-basedobservatories is limited by the size of Earth, the geographic distribution ofantennas, and the transparency of the atmosphere. In this whitepaper, wepresent Capella, a tentative design of a space-only VLBI system. Using foursmall (<500 kg) satellites in two orthogonal polar low-Earth orbit planes, andsingle-band heterodyne receivers operating at frequencies around 690 GHz, theinterferometer is able to achieve angular resolutions of approximately 7microarcsec. Within a total observing time of three days, a near-complete uvplane coverage can be reached. All key components required for Capella - radiotelescope, receiver, sampler, recorder, atomic frequency standard, positioningsystem, data downlink, and pointing control system - are already available,some of them off-the-shelf; the science payload of each satellite has a mass ofabout 200 kg and consumes about 550 W of power. The data from the telescopescan be correlated on the ground using dedicated versions of existing Fouriertransform (FX) software correlators; in addition to the steps required by VLBIdata correlation and calibration in general, dedicated routines will be neededto handle the effects of orbital motion, including relativistic corrections.With the specifications assumed in this whitepaper, Capella will be able toaddress a range of science cases, including: photon rings around supermassiveblack holes; the acceleration and collimation zones of plasma jets emitted fromthe vicinity of supermassive black holes; the chemical composition of accretionflows into active galactic nuclei through observations of molecular absorptionlines; mapping supermassive binary black holes; the magnetic activity of stars;and nova eruptions of symbiotic binary stars - and, like any substantially newobserving technique, has the potential for unexpected discoveries.

Capella: A Space-only High-frequency Radio VLBI Network Formed by a Constellation of Small Satellites