Far beyond the familiar planets, a spacecraft called New Horizons continues its solitary passage toward the edge of the Sun's domain — carrying instruments that no other human-made object can replicate at such distances. A gap of just two million dollars a year now threatens to silence those instruments before they can witness the most consequential boundaries of our solar neighborhood. In the long history of exploration, few moments have been lost simply because the cost of listening was deemed too high; scientists are now asking whether this will become one of them.