SOFIA, the Stratospheric Observatory for Infrared Astronomy

To overcome the drawbacks of ground-based telescopes when studying the solar system – interferences, weather – a partnership between NASA and the German Aerospace Center (DLR) has put together a stratospheric observatory based on a modified aircraft carrying a 2.5-meter reflecting telescope which flies to heights between 38’000 and 45’000 feet or above 99% of the infrared blocking atmosphere of the Earth.

The successor to the Kuiper Airborne Observatory, SOFIA – based upon a Boeing 747SP – can thus reach the skies from almost any location around the globe and can take advantage of its mobility to survey events that take place over locations on Earth that are not accommodated with any telescopes (such as oceans). This American part of the airborne observatory is operated and managed by the Universities Space Research Association (USRA), whereas NASA was awarded the aircraft’s development.

SOFIA Telescope
SOFIA takes off from Hamburg, Germany, following a heavy maintenance visit at Lufthansa Technik.

On the German side, the Deutsches SOFIA Institute (DSI) primarily takes care of telescope and science-related considerations. After research and development, sent to the stratosphere to observe stellar events such as star-forming regions, comets, or nebular, it was on May 26, 2010, that SOFIA saw first light.

History and development

SOFIA is not the first airborne observatory. Such technology goes back to 1965 when NASA modified its first aircraft, a Convair 990, to perform infrared observations of Venus. This was followed by the development of the Learjet Observatory, a telescope based on a Learjet 24B aircraft, by the Ames Research Center to observe Jupiter and nebulae through a 12-inch infrared telescope. However, NASA now wanted to increase the scale of its observational capacity and began developing the Kuiper Airborne Observatory equipped with a 36-inch telescope whose operations, in turn, led to the discovery of the ring system around Uranus. 

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In 1984 was issued the proposal for an observatory mounted upon a larger aircraft to which Germany would contribute 20%, and the agreement was then signed, bringing SOFIA to life. The manufacturers contracted to build the telescope were Zerodur, who built the 2.5-meter diameter primary mirror, complemented by work from Schott AG and REOSC, the accuracy of the mirror was increased to 8.5 nanometers. The main components were assembled in Germany and shipped to the USA, where the first tests were effectuated on board an Airbus Beluga.

In 2004, SOFIA completed its ultimate ground-based test by taking an image of the star Polaris, more commonly known as the North Star. On the 18th December 2009, the aircraft performed its first test flight with the door in the fuselage fully opened during 2 minutes of the 79-minute long flight. SOFIA started returning images in 2010 when routine flights began, reach full capacity in 2014 with around 100 flights per year, flown three or four nights a week.

The SOFIA telescope

SOFIA is equipped with a 2.5 m diameter Cassegrain reflector telescope which is design for infrared astronomical observations. When at cruising altitude, the telescope can intercept 85% of the total infrared range available. The telescope is equipped with a 2.7-meter diameter parabolic primary mirror as well as a hyperbolic secondary mirror. Initially, the SOFIA telescope carried nine instruments for various purposes of infrared and optical astronomy covering wavelengths from 1 to 655 μm and 0.3 to 1.1 μm.

These include a near-infrared camera, the FLITECAM, an instrument covering the mid-infrared spectrum: the FORCAST, and the HAWN, which covers the far-infrared spectrum. To overcome turbulent winds and the vibrations of the aircraft, the SOFIA telescope was designed to be extremely lightweight and mounted with bearings in pressurized oil to isolate it. To circumvent vibrations, a system of gyroscopes, high-speed cameras, and magnetic torque motors serve to counter the movements.

Visible (left) and infrared (right) images of the Horsehead Nebula.The infrared image has been taken by SOFIA’s upGREAT instrument.
Credits: Left: Dylan O’Donnell; Right: NASA/SOFIA/J. Bally et. al

The SOFIA aircraft

The aircraft on which the telescope and observatory are based is a Boeing 747SP wide-body, now registered as N747NA, whose aft end of the fuselage (close to the tail) was modified to be opened and give the telescope a wide enough opening. The SP – “Special Performance” – declination of the 747 allows it to fly higher, faster, and over longer distances at once. Before becoming SOFIA, the aircraft was operated by Pan American World Airways, delivered to them in 1977 as a commercial passenger aircraft under the registration N536PA.

In honor of aviator Charles Lindbergh, the aircraft was christened Clipper Lindbergh. United Airlines then purchased the aircraft in 1986, receiving the new registration, N145UA, remaining in service until 1995. In 1997, the Universities Space Research Association purchased the aircraft. A year later, the aircraft’s installation would begin, and a 4.1 by 5.5-meter door was created on the left side of the aircraft’s fuselage that can be opened mid-flight and was constructed to have no significant impact on aerodynamics. Later that year, NASA purchased it from them to conduct a series of tests. Finally, in 2012, the aircraft was upgraded to a glass-cockpit and new systems and updated software.

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Research and scientific goals

As it was designed, SOFIA aims to observe the infrared universe studying, for example, planetary atmosphere and surface composition, the evolution, structure of comets, and the composition and processes of areas of star formation. In 2016, in studying the atmosphere of Mars, SOFIA detected atomic oxygen, and in 2017 a star occultation (hidden by another object passing between it and the observer) of a trans-Neptunian object located in the Kuiper Belt – 486958 Arrokoth – an observation which ground-based telescopes failed to conduct.


Most recently, in October 2020, SOFIA – amongst other observatories – reported the detection of molecular water on the surface of the moon. SOFIA’s unusual situation gives the opportunities – both its mobility and reachable heights – are many and give us one more key in unraveling physics and astronomy questions that remain unanswered.

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