The observatory, equipped with a 0.9-metre (36-inch) Cassegrain (Cassegrain reflector) reflecting telescope, was typically flown at an altitude of 12,500 metres (41,000 feet) to measure infrared radiation emitted by planets, stars, galaxies, and other celestial objects. Much of the infrared radiation in certain spectral regions is absorbed by water vapour and carbon dioxide in Earth's lower atmosphere and so cannot be detected by ground-based telescopes. The major discoveries of the Kuiper Airborne Observatory include the rings of the planet Uranus, stars in the process of formation, and molecules in interstellar space and in planetary atmospheres. A more sophisticated airborne observatory, a modified Boeing 747 named the Stratospheric Observatory for Infrared Astronomy (SOFIA) and developed by the United States and Germany, is scheduled to begin observations in 2010.