NASA’s Boeing 747 Flying Telescope: Keeping the ‘SP’ Variant Airborne

NASA's 747SP aircraft (Photo: NASA)

The Boeing 747 aircraft is an iconic jet that has graced the skies for decades as the undisputed Queen of the Skies. The aircraft type spurred the global aviation industry as we know it. Unfortunately, fewer and fewer 747s are being seen in the skies carrying passengers.

A shift towards twin-engine aircraft that are more fuel efficient has diminished the popularity of this airplane. It will remain in passenger service for some time still and will continue to live on even longer as a cargo aircraft.

There is one use of the aircraft that isn’t very well known, however. In the mid-1990s, a joint project between NASA and the German Aerospace Center was commissioned to construct and maintain an airborne observatory. This project became known as the Stratospheric Observatory for Infrared Astronomy (SOFIA).

What better aircraft to suit a flying observatory than the 747? SOFIA is based on a Boeing 747SP, the variant of the aircraft specialized for ultra-long range flights by modifications such as a shorter fuselage allowing the plane to fly higher and faster.

Boeing’s 747SP

The specific SOFIA aircraft is a B747SP that was initially delivered to Pan Am on May 6, 1977 which named the aircraft after aviator Charles Lindbergh. The aircraft enjoyed nearly a decade under Pan Am before it was purchased by United Airlines in 1986.

United flew the aircraft for another nine years and retired it in 1995. It was then purchased by the Universities Space Research Association for the SOFIA project in 1997 and shortly purchased by NASA thereafter.

Raytheon was the primary contractor for the project and began work on the aircraft in 1998. An 18 ft tall by 13.5 ft wide door was installed in the rear of the aircraft. This door would be opened in flight to give the telescope access to the sky at the aircraft’s cruise altitude of 41,000 ft.

An altitude high enough that allows the telescope to be placed above most of the water vapor in the Earth’s atmosphere. Water vapor is mainly what prevents some infrared rays from being observed directly from the Earth’s surface. At the cruising altitude of the observatory, nearly 85 percent of the full infrared range is available.

The telescope is an 8.2 ft reflector telescope with an 8.9 ft primary mirror. It is placed behind a pressurized bulkhead where other scientific instruments are located. Further up the plane is where mission control can be found.

The opening in the fuselage to allow for the telescope to observe the sky doesn’t create too many issues in terms of aerodynamics. However, it is not the recommended location for an outdoor patio. Due to heavy winds at altitude, the telescope is lightweight and has a significant number of dampening devices to reduce vibrations.

SOFIA Takes Flight

The first flight of SOFIA took place in April 2007 to perform post-maintenance checks. Testing continued and in December 2009 the telescope door was fully opened for a portion of the flight. Routing observations began in 2010 and the observatory has reached full capability since then with about 100 flights per year across the globe.

SOFIA plays a critical role in our understanding of the universe and it’s nice to see the contribution of the Queen of the Skies go past just transporting people and cargo. Just recently scientists were able to use SOFIA to detect helium hydride, what is thought to be the first kind of molecule formed after the Big Bang. It was detected in a planetary nebula 3,000 light-years away and is the first-ever detection of the molecule in the modern universe.

This story was updated on Monday, April 22, 2019 at 12:27 p.m. ET to correct a grammatical error. 

Hemal Gosai

Hemal took his first flight at four years old and has been an avgeek since then. When he isn't working as an analyst he's frequently found outside watching planes fly overhead or flying in them. His favorite plane is the 747-8i which Lufthansa thankfully flies to EWR allowing for some great spotting. He firmly believes that the best way to fly between JFK and BOS is via DFW and is always willing to go for that extra elite qualifying mile.
Hemal Gosai