SOFIA — the flying observatory
SOFIA carried a 2.5 m infrared telescope in a converted 747SP, observing through a door opened in flight at altitude. NASA ended the mission in September 2022.
SOFIA — the Stratospheric Observatory for Infrared Astronomy — was a joint project of NASA and the German Aerospace Center (DLR), with the Universities Space Research Association managing the science. It carried a 2.5 metre reflecting telescope inside a Boeing 747SP.
Why put a telescope in an aeroplane
Infrared astronomy has a problem: water vapour in the atmosphere absorbs most infrared light before it reaches the ground. Build the best infrared telescope in the world at sea level and it will not see much.
Space solves this, but a space telescope is enormously expensive, cannot be repaired easily, and cannot have its instruments swapped as the science moves on.
Flying is the compromise. At around 40,000 feet an aircraft is above roughly 99 percent of the atmosphere’s water vapour — enough to open up most of the infrared spectrum — and it lands every morning, so instruments can be maintained, upgraded and exchanged.
The open door
The engineering that makes SOFIA remarkable is the door. A large opening in the rear fuselage was opened in flight, exposing the telescope directly to the outside air at altitude.
That is not a trivial thing to do to a pressurised aircraft. The cavity had to be shaped so airflow across a large hole did not set up destructive buffeting or upset handling. The telescope had to be isolated from the airframe, because an aeroplane vibrates constantly and a telescope must hold its aim to a tiny fraction of a degree. And the observatory section was kept unpressurised, sealed off from the pressurised cabin where the crew and scientists worked.
Why a 747SP
The SP is the short, light, long-legged member of the 747 family, built originally for ultra-long thin routes. Those same qualities — high altitude capability and long endurance — are exactly what an airborne observatory needs, since useful observing time is time spent high and steady.
The airframe itself had a long previous life, flying for Pan Am and then United before conversion.
The end of the mission
NASA ended SOFIA on 30 September 2022, after a final science flight the night before. The decision was contested: the observatory was expensive to operate and its scientific return was judged not to justify the cost against other projects competing for the same budget.
The aircraft made its last flight on 13 December 2022, from Palmdale to Tucson, and is now preserved at the Pima Air & Space Museum.
| Manufacturer | Boeing |
|---|---|
| Variant | Boeing 747SP (SOFIA) |
| Status | Retired 2022; preserved at Pima Air & Space Museum |
Under tow. The airframe began life as a Pan Am and later United 747SP.NASA 
An earlier scheme, with the NASA and DLR partnership marked on the tail.NASA
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Frequently asked questions
- Why did SOFIA need to fly to observe?
- Water vapour in the lower atmosphere absorbs most infrared light, so ground telescopes cannot see those wavelengths. Flying at around 40,000 ft puts the telescope above roughly 99 percent of that water vapour.
- Did SOFIA really fly with an open door?
- Yes. A large door in the rear fuselage opened in flight once at altitude, exposing the telescope to the outside air. The cavity was aerodynamically shaped and the telescope isolated from the airframe so vibration did not blur images.
- Why was a 747SP used?
- The SP is short and light for a 747, which gives it the altitude performance and endurance the mission needed — long flights, high up, night after night.
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