The Event Horizon Explorer: A New Mission to Detect a Black Hole’s Photon Ring

Explosive technological growth has created a radio interferometry renaissance, enabling a series of increasingly capable facilities and leading to scientific breakthroughs including the first images of black holes. However, the angular resolution of these facilities is now approaching a fundamental limit, determined by interferometric baselines limited to the diameter of the Earth and severe atmospheric absorption at frequencies above a few hundred GHz. I will describe how these same technological breakthroughs, paired with advances in free space optical communication, are now enabling a new mission — the Event Horizon Explorer (EHE) — that will extend the existing ground array to space. This mission will improve upon the resolution of the Event Horizon Telescope by an order of magnitude, achieving the sharpest angular resolution in the history of astronomy and sufficient to reveal the bright “photon ring” that is predicted to be a universal feature in black hole images. The EHE will make precise measurements of black hole parameters, will provide sharp new tests of general relativity, and will measure the masses and horizon-scale structure of hundreds of new supermassive black holes. We will present the EHE science goals, their associated requirements, and the implications for the next decade of experimental relativity using astronomical observations.