When an airplane flies at an altitude of about 41,000 feet (12,500 meters), it is above 99 percent of Earth’s atmospheric water vapor. Since water vapor absorbs incoming infrared radiation, being above the vapor makes it possible to make many infrared observations. At the same time, airborne telescopes are easily accessible for repairs, upgrades, and real-time adjustments, unlike space telescopes. Also, the airplane can be flown to different parts of the world and operated at different places, offering greater flexibility than a terrestrial telescope.