A NASA project to develop giant helium-filled balloons able to support scientific observations on the edge of space. The balloons are designed to operate above 99% of the Earth's atmosphere for up to 100 days. The prototype carries a trans-iron galactic element recorder (TIGER), a payload weighing 990 kg, to study outer space. The ULDB is the largest single-chamber, high-pressure balloon ever flown, with a diameter of 59 m and a height of 35 m.

The first two ULDB launches, from Australia in February and March 2001, were unsuccessful. During the first, the balloon developed a leak in its thin plastic skin after a few hours and crashed when brought down by remote control. The second was forced down by shifting high-altitude winds less than 24 hours into its flight. NASA is planning future ULDB missions to study the Sun and find new planets. In March 2003, an ULDB was launched in Australia and terminated after 12 hours due to gas loss.