A stratospheric airship is a powered airship designed to fly at very high altitudes (30,000–70,000 ft or 10 – 20 km). Most designs are remote operated aircraft/unmanned aerial vehicles (ROA/UAV). To date none of these designs have received approval from the FAA to fly in U.S. airspace.
Read more about Stratospheric Airship: Stratospheric Airship Projects