When many people hear the name ‘Sofia,’ they think of the bohemian capital of Bulgaria. But some of you will know that its also the name of the unique aircraft that uses the call sign “NASA747.” The Stratospheric Observatory for Infrared Astronomy (SOFIA) is an 80/20 joint project between NASA and the German Aerospace Center (DLR). SOFIA is the successor to NASA’s Kuiper Airborne Observatory.
SOFIA is unusual looking: a short and stubby aircraft with an outsized tail fin. It is one a small number of Boeing 747 Special Performance (SP) aircraft that were built as derivatives of the Boeing 747-100 series. These SP aircraft were shortened and specifically designed to fly higher and farther than any other 747 at the time. Due to the shortened fuselage, the tail fin needed to be made bigger to allow the rudder to have the same effect on the aircraft with a smaller turning moment. This led to the rather ungainly appearance of this series of aircraft.
The SOFIA airframe began life with Pan Am (N536PA) and was christened “Clipper Lindbergh” by Anne Morrow Lindbergh – the wife of legendary aviator Charles Lindbergh – on 20 May 1977, the 50thanniversary of Lindbergh’s historic flight from New York to Paris in 1927. In 1986, the aircraft was bought by United Airlines and was operated in a passenger role until December 1995, when the airframe was placed in storage near Las Vegas. In 1997, the aircraft was bought by NASA, which transformed it into the marvel it is today.
A number of corporations including L3 and Raytheon worked with NASA at a facility in Waco, Texas to convert the aircraft to an airborne observatory. To ensure success during the project, Raytheon bought another 747SP on which they first experimented and performed trials so the initial work wasn’t being done on the actual SOFIA aircraft.
To complete the circle linking the SOFIA airframe with Charles Lindbergh, at NASA’s invitation, Lindbergh’s grandson, Erik Lindbergh, re-christened the aircraft as “Clipper Lindbergh,” on May 21, 2007, the 80th anniversary of the completion of Charles Lindbergh’s trans-Atlantic flight.
The SOFIA needed to be a 747, and more specifically an SP variant, because the aircraft needed to house a 2.7 meter diameter infrared telescope and a huge gimbal stabilization system. Additionally – and crucially – the aircraft needed to cruise at 41,000ft in order to be above the majority of moisture in the atmosphere and provide optimum conditions for use of the infrared telescope. The 747 provided the size and the SP gives the ability to cruise at 41,000ft effortlessly.
The telescope compartment is extremely complex. The space must be cooled slowly to match the outside air temperature before the compartment is depressurized and the door opened. This procedure prevents thermal shock to the sensitive mirror in the telescope and other scientific instruments. When the door is open, the telescope is subjected to significant extremes of wind buffeting which are mitigated by a sophisticated 3-axis gimbal system. Once the mission is complete, the outer door is closed and the entire compartment is flooded with nitrogen to prevent condensation forming on the mirror lens and other instruments.
So what is SOFIA looking at? Primarily, the structures and atmospheres of planets. In 2016, SOFIA detected atomic oxygen in the atmosphere of Mars – a phenomenal observation. The height that the SOFIA telescope can achieve makes it possible for scientists to make observations that may not be possible with ground based telescopes.
NASA gave the aircraft a glass cockpit avionics upgrade in 2012 and has no current plans to retire this unique aircraft.