Very Long Baseline Interferometry (VLBI) provides the finest angular resolution of all astronomical observation techniques. However, observations with Earth-based instruments are approaching fundamental limits on angular resolution. These can only be overcome by placing at least one interferometric element in space. In this paper, several concepts of spaceborne VLBI systems are discussed, including TeraHertz Exploration and Zooming-in for Astrophysics (THEZA) and the Black Hole Explorer (BHEX). Spaceborne VLBI telescopes have some of the most demanding requirements of any space science mission. The VLBI system as a whole includes globally distributed elements, each with their own functional constraints, limiting when observations can be performed. This necessitates optimisation of the system parameters in order to maximise the scientific return of the mission. Presented is an investigation into how the impact of the functional constraints of a spaceborne VLBI telescope affect the overall system performance. A preliminary analysis of how these constraints can be minimised through optimisation of the spacecraft configuration and operation is also provided. A space-based VLBI simulation tool (spacevlbi) has been developed to model such missions and its capabilities are demonstrated throughout the paper. It is imperative that the functional constraints are considered early in the design of the future space-based VLBI systems in order to generate feasible mission concepts and to identify the key technology developments required to mitigate these limitations.

The black hole Sagittarius A∗ (Sgr A∗) is a prime target for next-generation Earth-space very long baseline interferometry missions such as the Black Hole Explorer (BHEX), which aims to probe baselines on the order of 20 Gλ. At these baselines, Sgr A∗ observations will be affected by the diffractive scattering effects from the interstellar medium (ISM). Therefore, we study how different parameter choices for turbulence in the ISM affect BHEX's observational capabilities to probe strong lensing features of Sgr A∗. By using a simple geometric model of concentric Gaussian rings for Sgr A∗'s photon ring signal and observing at 320 GHz, we find that the BHEX-ALMA baseline has the required sensitivity to observe Sgr A∗ for a broad range of values of the power-law index of density fluctuations in the ISM and the inner scale of turbulence. For other baselines with moderate sensitivities, a strong need for observations at shorter scales of 13.5 Gλ is identified. For this purpose, an orbit migration scheme is proposed. It is modeled using both chemical propulsion (CP)-based Hohmann transfers and electric propulsion (EP)-based orbit raising with the result that a CP-based transfer can be performed in a matter of hours, but with a significantly higher fuel requirement as compared to EP which, however, requires a transfer time of around 6 weeks. The consequences of these orbits for probing Sgr A∗'s space-time are studied by quantifying the spatial resolution, temporal resolution, and angular sampling of the photon ring signal in the Fourier coverage of each of these orbits. We show that higher orbits isolate space-time features while sacrificing both signal lost to scattering and temporal resolution, but gaining greater access to the morphology of the photon ring. Thus, we find that orbits between the low Earth regime and the reference BHEX orbit can provide rich access to Sgr A∗'s parameter space.

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow “photon ring” that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole’s spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole’s spin. In addition to studying the properties of the nearby supermassive black holes M87^∗ and Sgr A^∗, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.

Very long Baseline Interferometry (VLBI) provides the finest angular resolution of all astronomical observation techniques. The Earth-based Event Horizon Telescope (EHT) has demonstrated this in recent years with the landmark achievement of resolving the shadows of the supermassive black holes M87∗ and SgrA∗. However, these observations also showed that the science case for further sharpening the resolution of astrophysical studies is far from being exhausted. The only way to overcome fundamental limits on angular resolution of Earth-based arrays is to place part of or the entire interferometer in space. In this paper, several concepts of spaceborne VLBI systems are discussed including, TeraHertz Exploration and Zooming-in for Astrophysics (THEZA) and the Black Hole Explorer (BHEX). Spaceborne VLBI telescopes have some of the most demanding requirements of any space science mission. The VLBI system as a whole includes globally distributed elements, each with their own functional constraints, limiting when observations can be performed. This necessitates optimisation of the system parameters in order to maximise the scientific return of the mission. End-to-end mission simulations are an indispensable tool in conducting such an optimisation. Presented is an investigation into how the impact of the functional constraints of a spaceborne VLBI telescope affect the overall system performance. A preliminary analysis of how these constraints can be minimised through optimisation of the spacecraft configuration and operation is also provided. A space-based VLBI simulation tool has been developed to model such missions and its capabilities are demonstrated throughout the paper. It is imperative that the functional constraints are considered early in the design of the future space-based VLBI systems in order to generate feasible mission concepts and to identify the key technology developments required to mitigate these limitations.

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.