  We present the Black Hole Explorer (BHEX), a mission that will produce the
sharpest images in the history of astronomy by extending submillimeter
Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and
measure the bright and narrow "photon ring" that is predicted to exist in
images of black holes, produced from light that has orbited the black hole
before escaping. This discovery will expose universal features of a black
hole's spacetime that are distinct from the complex astrophysics of the
emitting plasma, allowing the first direct measurements of a supermassive black
hole's spin. In addition to studying the properties of the nearby supermassive
black holes M87* and Sgr A*, BHEX will measure the properties of dozens of
additional supermassive black holes, providing crucial insights into the
processes that drive their creation and growth. BHEX will also connect these
supermassive black holes to their relativistic jets, elucidating the power
source for the brightest and most efficient engines in the universe. BHEX will
address fundamental open questions in the physics and astrophysics of black
holes that cannot be answered without submillimeter space VLBI. The mission is
enabled by recent technological breakthroughs, including the development of
ultra-high-speed downlink using laser communications, and it leverages billions
of dollars of existing ground infrastructure. We present the motivation for
BHEX, its science goals and associated requirements, and the pathway to launch
within the next decade.
