  Very long baseline radio interferometry (VLBI) with ground-based
observatories is limited by the size of Earth, the geographic distribution of
antennas, and the transparency of the atmosphere. In this whitepaper, we
present Capella, a tentative design of a space-only VLBI system. Using four
small (<500 kg) satellites in two orthogonal polar low-Earth orbit planes, and
single-band heterodyne receivers operating at frequencies around 690 GHz, the
interferometer is able to achieve angular resolutions of approximately 7
microarcsec. Within a total observing time of three days, a near-complete uv
plane coverage can be reached. The technology for all key components required
for Capella - radio telescope, receiver, sampler, recorder, frequency standard,
positioning system, data downlink, and pointing control system - is already
available, partially off-the-shelf. Making realistic assumptions, the science
payload of each satellite has a mass of about 190 kg and consumes about 460 W
of power. The data from the telescopes can be correlated on the ground using
dedicated versions of existing Fourier transform (FX) software correlators.
With the specifications assumed in this whitepaper, Capella will be able to
address a range of science cases, including: photon rings around supermassive
black holes; the acceleration and collimation zones of plasma jets emitted from
the vicinity of supermassive black holes; the chemical composition of accretion
flows into active galactic nuclei through observations of molecular absorption
lines; mapping supermassive binary black holes; the magnetic activity of stars;
and nova eruptions of symbiotic binary stars - and, like any substantially new
observing technique, has the potential for unexpected discoveries.
