Event Horizon Explorer: Hybrid Space-Ground VLBI of Black Holes

The Event Horizon Explorer will observe black holes and their surroundings with space-ground Very Long Baseline Interferometry at submillimeter wavelengths. It builds upon the Event Horizon Telescope (EHT) and its successor, the next-generation EHT (ngEHT). Going to space offers improved VLBI resolution to study the black hole photon ring. Some engineering challenges include designing a suitable antenna and receiver, precision timing and downlink communications. We present the science and technical challenges in the context of current NASA programs and the overall landscape for astrophysics funding in the coming decade.