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Item Orbit design for mitigating interstellar scattering effects in Earth-space very long baseline interferometry observations of Sagittarius A(EDP Sciences, 2025) Tamar, A.; Hudson, B.; Palumbo, D.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 G?. 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 G? 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. © The Authors 2025.Item Fundamental physics opportunities with future ground-based mm/sub-mm VLBI arrays(Springer Science and Business Media Deutschland GmbH, 2025) Ayzenberg, D.; Blackburn, L.; Brito, R.; Britzen, S.; Broderick, A.E.; Carballo-Rubio, R.; Cardoso, V.; Chael, A.; Chatterjee, K.; Chen, Y.; Cunha, P.V.P.; Davoudiasl, H.; Denton, P.B.; Doeleman, S.S.; Eichhorn, A.; Eubanks, M.; Fang, Y.; Foschi, A.; Fromm, C.M.; Galison, P.; Ghosh, S.G.; Gold, R.; Gurvits, L.I.; Hadar, S.; Held, A.; Houston, J.; Hu, Y.; Johnson, M.D.; Kocherlakota, P.; Natarajan, P.; Olivares Sánchez, H.; Palumbo, D.; Pesce, D.W.; Rajendran, S.; Roy, R.; Saurabh; Shao, L.; Tahura, S.; Tamar, A.; Tiede, P.; Vincent, F.H.; Visinelli, L.; Wang, Z.; Wielgus, M.; Xue, X.; Yakut, K.; Yang, H.; Younsi, Z.The Event Horizon Telescope (EHT) Collaboration recently published the first images of the supermassive black holes in the cores of the Messier 87 and Milky Way galaxies. These observations have provided a new means to study supermassive black holes and probe physical processes occurring in the strong-field regime. We review the prospects of future observations and theoretical studies of supermassive black hole systems. Current ground-based very-long-baseline interferometry (VLBI) arrays like the EHT and proposed future extensions like the next-generation Event Horizon Telescope will greatly enhance the capabilities of black-hole imaging interferometry. These enhancements will open up several previously inaccessible avenues of investigation, thereby providing important new insights into the properties of supermassive black holes and their environments. This review describes the current state of knowledge for five key science cases, summarising the unique challenges and opportunities for fundamental physics investigations that future mm/sub-mm VLBI developments will enable. © The Author(s) 2025.