Hot-air balloon

From Memory Alpha, the free Star Trek reference.

A hot-air balloon is an aircraft consisting of a fabric enclosure filled with air that is a higher temperature than the air outside the enclosure. The lower density of the hot air inside provides lift for the craft.

A hot-air balloon was used by Kelemane in an attempt to send a letter to the object in the sky known as the "Ground Shaker". (VOY: "Blink of an Eye")