The SHARPEx space-to-space VLBI experiment
Space is the next frontier for ultra-high resolution imaging of astrophysical sources with very long baseline interferometry (VLBI). The limited size of the Earth and atmospheric absorption and turbulence at high frequencies prevent order-of-magnitude resolution improvements from current black hole imaging with the ground-based Event Horizon Telescope. A two or three-element submm space-based array like SHARP would image the thin black hole photon ring and measure the black hole spin and spacetime. Here, we present the SHARP Experiment (SHARPEx) mission concept. SHARPEx will demonstrate key enabling technologies for space-to-space VLBI, including clock syntonization between two satellites, on-board correlation, and sub-wavelength relative orbit determination. SHARPEx will consist of two CubeSat or SmallSat platforms carrying small (~1 m diameter) antennas, observing at cm wavelengths. While SHARPEx will be a technical demonstrator, a SHARPEx-type follow-up mission with enhanced intersatellite link properties (range and bandwidth) may produce images of bright AGN with unprecedented resolution and fidelity.