A solar-powered aircraft uses solar energy to electrolyze on-board water to produce hydrogen. The hydrogen fills various on-board tanks, causing the aircraft to become lighter than air. The hydrogen is also used to operate a fuel cell which provides power for electrical equipment, including a motor for turning a propeller. Water produced as waste by the fuel cell is recycled for use in the production of hydrogen. When hydrogen is removed from the tanks, either because it is consumed by the fuel cell or because it is compressed and pumped out of the tanks, air returns to the tanks, and the aircraft becomes heavier than air. The aircraft can thus be made to climb and descend by making it lighter than air, or heavier than air. The aircraft emits no harmful substances into the environment. The aircraft can remain aloft indefinitely, limited only by an insignificant amount of leakage of hydrogen and water.