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.

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.