Planetary Vision

Black holes prefer to remain invisible. They reflect no light, emit no light�the biggest and brightest ones are so far away that they carve out an angle about the same as that of an atom held at arm�s length. For decades, it was clear that it would take a planet-sized telescope to see a black hole�not an aggregation of many images of Earth put together in a mosaic: This was a fully integrative problem that would demand telescopes spread across the planet. In April 2019, the Event Horizon Telescope Collaboration (EHTC) achieved just this: Some 200 collaborators, and telescopes on five continents synthesized an Earth-sized instrument. Exploiting the spinning of Earth as part of the apparatus, the EHTC recorded the billion-degree plasma around the event horizon: The first image of a black hole. Some of us started asking: What if we had a virtual telescope larger than the planet? Could we peer deep into the doughnut-like image to glimpse not hot gas but a thin, bright ring of pure light orbiting the black hole�light gathered from all parts of the universe that the black hole could see? In this sense, the black hole watches us while we watch it. The tantalizing possibility of detecting and measuring the photon ring has set in motion planning for a space mission (the Black Hole Explorer, or BHEX) that would launch a radio telescope in a 25,000-kilometer orbit around Earth, integrated with terrestrial radio telescopes. It would produce the highest-resolution images in the history of astronomy.