A solar-powered drone has been lost at sea after a record-breaking flight lasting eight days between late April and early May. The crash also marks the untimely demise of the pioneering aircraft Solar Impulse 2, which previously performed the world’s first solar-powered crossings of the Atlantic and Pacific oceans before becoming an uncrewed test platform for US military missions.

The carbon-fiber aircraft could perform such feats of aeronautical endurance while running solely on renewable energy and batteries because of a 236-foot (72-meter) wingspan—comparable to a Boeing 747 jumbo jet’s wings—covered with more than 17,000 solar cells. The company Skydweller Aero purchased and modified the original Solar Impulse 2 aircraft to become a test platform for “perpetual uncrewed flight” with the capability of carrying up to 800 pounds (363 kilograms) of payload.

Skydweller Aero was conducting test flights for maritime patrol mission scenarios with the US military, and the company also holds contracts with the Navy and Air Force. So the Skydweller drone was operating in that capacity when it took off on its final flight in the early morning hours of April 26.