Orbit design for mitigating interstellar scattering effects in Earth-space very long baseline interferometry observations of Sagittarius A*

¹ Department of Physics, National Institute of Technology Surathkal, Karnataka 575025, India
² Faculty of Aerospace Engineering, Delft University of Technology, 2629HS Delft, The Netherlands
³ KISPE Limited, Farnborough, United Kingdom
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Black Hole Initiative at Harvard University, 20 Garden Street, Cambridge, MA 02138, USA

^⋆ Corresponding author: adityatamar@gmail.com

Received: 18 October 2024
Accepted: 9 April 2025

Abstract

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.