Ballast in gas ballooning is used to control buoyancy (affecting altitude) during flight. Ballast is frequently in the form of sand or water and is carried aloft at launch. When the pilot needs to adjust the balloon’s altitude, a small amount of ballast is jettisoned from the gondola which reduces the gross weight of the balloon and allows it to rise to a new pressure altitude (when it is desired for the balloon to descend lifting gas is released from a valve at the top of the envelope). The balloon will remain at that altitude until there is another dynamic change in the lift equation.
It is often stated that the ballast is to feed the gas balloon and well before all ballast has been expended, the aircraft must be safely back on the ground. The use of large amounts of ballast to execute a major ascent will shorten the potential duration of a flight