Recent advances in technology coupled with the progress of observational radio astronomy methods resulted in achieving a major milestone of astrophysics - a direct image of the shadow of a supermassive black hole, taken by the Earth-based Event Horizon Telescope (EHT). The EHT was able to achieve a resolution of ∼20μas, enabling it to resolve the shadows of the black holes in the centres of two celestial objects: the supergiant elliptical galaxy M87 and the Milky Way Galaxy. The EHT results mark the start of a new round of development of next generation Very Long Baseline Interferometers (VLBI) which will be able to operate at millimetre and sub-millimetre wavelengths. The inclusion of baselines exceeding the diameter of the Earth and observation at as short a wavelength as possible is imperative for further development of high resolution astronomical observations. This can be achieved by a spaceborne VLBI system. We consider the preliminary mission design of such a system, specifically focused on the detection and analysis of photon rings, an intrinsic feature of supermassive black holes. Optimised Earth, Sun–Earth L2 and Earth–Moon L2 orbit configurations for the space interferometer system are presented, all of which provide an order of magnitude improvement in resolution compared to the EHT. Such a space-borne interferometer would be able to conduct a comprehensive survey of supermassive black holes in active galactic nuclei and enable uniquely robust and accurate tests of strong gravity, through detection of the photon ring features.

Recent advances in technology coupled with the progress of observational radio astronomy methods resulted in achieving a major milestone of astrophysics - a direct image of the shadow of a supermassive black hole, taken by the Earth-based Event Horizon Telescope (EHT). The EHT was able to achieve a resolution of approximately 20 microarcseconds, enabling it to resolve the shadow of the black hole in two celestial objects, M87* and SgrA*. This pioneering result paves the way for a multitude of astrophysical research of galactic and extragalactic objects with unprecedented sharpness. The EHT results also mark the start of a new round of development of next generation Very Long Baseline Interferometers (VLBI) which will be able to operate at millimetre and sub-millimetre wavelengths. The inclusion of baselines exceeding the diameter of the Earth and observation at as short a wavelength as possible is imperative for further development of ultra-sharp astronomical observations. This can be achieved by a spaceborne VLBI system. TeraHertz Exploration and Zooming-in for Astrophysics (THEZA) is a concept of such a system, prepared in response to ESA's call for its next science program Voyage 2050. THEZA's goal is to improve upon the angular resolution of the next generation of the Earth-based EHT by an order of magnitude. We consider the preliminary mission design of the THEZA spaceborne interferometer, specifically focused on the detection and analysis of the pattern of photon rings, forming in a black hole observable image as a consequence of extreme gravitational deflection of light. This phenomenon is highly informative for deciphering the properties of space-time in strong gravitational fields and determining key characteristics of black holes. Earth, Sun-Earth L2 and Earth-Moon L2 orbit configurations for the space interferometer system are presented, optimised for the study of photon rings around supermassive black holes. It is shown that a THEZA mission operating in each of these configurations can detect the first order photon ring interferometric signature, enabling the mass and spin of black holes to be more accurately measured than with ground-based systems. Performing multi-epoch monitoring of the ring and associated emission would also enable tests of general relativity. Such a space-borne interferometer system will open up a new area of astrophysical observation, until now unreachable with Earth-based systems observing at the shortest possible wavelengths and past space interferometers operating at longer wavelengths.