From a Learjet to SOFIA: A Brief History of the World’s Largest Flying Telescope

If you ask someone what a Jumbo Jet can do, you will likely get one of the following two answers: transport passengers or haul cargo. Both of those are, of course, correct.

But, did you know that there is a Boeing 747 that helps us better understand the universe?

That’s exactly what Stratospheric Observatory for Airborne Astronomy, the world’s largest airborne observatory also known as SOFIA, and its predecessors do. In this article, I explore the origins of airborne astronomy, as well as the history of the Boeing 747SP modified into the observatory and of the SOFIA project itself.

SOFIA Christchurch
SOFIA, the aircraft that helps us better understand the universe.


This is the second article in a four-part series detailing the history and operations of the world’s largest airborne observatory, Stratospheric Observatory for Infrared Astronomy also known as SOFIA. The following articles are included in the series:

  1. Introduction: Experiencing SOFIA’s Southern Deployment 2018 in Christchurch – An introduction to the series giving an overview about my experiences with SOFIA.
  2. From a Learjet to SOFIA: A Brief History of the World’s Largest Flying Telescope – A brief look at the history of airborne astronomy and a look at how SOFIA came to be.
  3. “Per Aspera Ad Astra:” The Complexities of Operating SOFIA – A detailed look at the variety of teams involved in the operation of SOFIA and the challenges faced by them.
  4. NASA 747, Cleared for Take-Off, Destination Mars: Observing a SOFIA Mission – A briefing-to-landing account of my flight onboard SOFIA.


A Brief History of Airborne Astronomy

While airborne astronomy traces its origins back to the 1920s, it wasn’t until the 1960s that it went from mainly observing solar eclipses to more advanced infrared airborne astronomy focused on studying planets and solar systems.

It was around the same time that space telescopes were being put into service for the first time as well. Putting an infrared telescope into space eliminated the problem of distortion caused by the lower levels of atmosphere, however, it came at a significant cost.

Not only the actual cost of developing and launching the telescope into space, but also the cost of lost flexibility. In other words, once it was built and launched, it became very difficult, costly, and nearly impossible to maintain it or make any modifications to it.

Similarly, if new discoveries requiring further verification through infrared observations were made, space telescopes weren’t the most practical solution given the decades it takes to develop one and deliver it to space.

Hence comes the flying telescope.

It was Dr. Gerard Kuiper, a Dutch-American astronomer, that came up with the idea of and tested putting an infrared telescope onboard an aircraft.

First, he used a telescope pointed out of a window onboard NASA’s Convair CV-990 to collect a variety of data including data showing that the clouds of Venus were not made of water which was a widely held belief at that time.

Thanks to those results, a proposal for an exposed telescope to be installed onboard one of NASA’s aircraft was accepted. The result was a Learjet equipped with a 12-inch reflecting telescope installed in place of one of the aircraft’s windows. The aircraft did its first observations in 1968.

NASA Learjet Observatory
The Learjet Observatory. The 12-inch telescope was removable, permitting use of the plane for other research programs. (Photo credit: NASA)

Having proven the concept and collected valuable data, the following year, NASA decided to step up its airborne astronomy game by approving the Kuiper Airborne Observatory (KAO) project to develop a flying telescope with a 36-inch diameter mirror.

While initially the telescope was to be installed on a CV-990, as the project progressed, Lockheed C-141 was chosen instead.

The Kuiper Airborne Observatory did its first research flight in 1974 and, as mentioned during the talk at Ara Institute of Canterbury I had a chance to attend, throughout its lifetime, it took part in several important projects. Some of those included the discovery of the rings of Uranus, of Pluto’s atmosphere, and providing early evidence of a black hole in the galactic center of the Milky Way.

As NASA’s success with flying telescopes grew, so did its ambitions.

And so, in 1995, it was time for the Kuiper Airborne Observatory to retire so that budget and resources would be freed for the next big thing, the Stratospheric Observatory for Infrared Astronomy – SOFIA.

Kuiper Airborne Observatory Retires 1995
Kuiper Airborne Observatory staff in front of the observatory as it headed for retirement in 1995. (Photo credit: NASA)

If you are interested in learning more about the history of airborne astronomy, make sure to check Milestones in Airborne Astronomy: From the 1920s to the Present written by Wendy Whiting Dolci, one of the former KAO Mission Directors.


From Hauling Passengers to Watching Galaxies Form

The idea to equip a Boeing 747 aircraft with a telescope came out as early as in the late-1970s, decades before SOFIA took off for the first time.

The next big milestone came in 1984 when Stratospheric Observatory For Infrared Astronomy (SOFIA) Preliminary Feasibility Study was issued, and in 1986 when an agreement with DFVLR, the predecessor of DLR, NASA’s German equivalent, was made.

After that, the project was slowed down by budget cuts and the reunification of Germany, however, in the mid-1990s, it got back on track.

To keep the aircraft acquisition cost within budget, NASA bought a second-hand 747SP, a subtype of the 747 offering increased range at the expense of smaller capacity, from United Airlines in October 1997.

United Airlines 747SP
One of United Airlines’ Boeing 747SPs in the early 1990s. (Photo credit: Raj Changela)

While the airframe’s present use is about as exciting as it gets for a 747, its previous life was notable as well.

The aircraft that currently operates as SOFIA with tail number N747NA, was the 306th 747 built. What makes it even more interesting, though, is that it was the 18th of just 45 747SPs ever built.

The aircraft that would become SOFIA flew for the first time on April 25, 1977 and within a couple of weeks of its first flight, it was delivered to Pan Am – SP’s launch customer – as its 6th airframe of the type.

As its delivery took place on May 20, the 50th anniversary of Charles Lindbergh’s first transatlantic solo flight, Pan Am decided to name the aircraft Clipper Lindbergh and got the aircraft christened by Anne Morrow Lindbergh, Lindbergh’s widow.

SOFIA Pan Am Clipper Lindbergh
Clipper Lindbergh during its time with Pan Am. (Photo credit: Boeing)

The aircraft, registered N536PA, officially entered into service on May 6, 1977 – half a century after Lindbergh’s historic flight. During its decade long life with Pan Am, the aircraft was used on routes such as New York – Tokyo, the route the aircraft was originally designed for.

The aircraft started a new chapter in its life in 1986, when Pan Am sold it to United Airlines. Not long after that, it got re-registered to N145UA. Former Clipper Lindbergh lasted in airline service for about another decade, until United Airlines placed it out of service and stored it at the end of 1995.

Unlike other similar aircraft whose fate was to remain stored or get scrapped, 747SP no. 18 got lucky as it found a new life just a couple of years later when NASA acquired it to turn it into SOFIA.


A Decade-Long Modification Process

When the idea of SOFIA was floated around for the first time, NASA planned to get the necessary modifications done by Boeing. However, by the time the project came to life, Boeing had lost its interest in “one-off” modifications like that, and so NASA had to look elsewhere.

The company of choice was L-3 Communications Integrated Systems and so, soon after its acquisition, the 747SP was ferried to L-3’s facilities in Waco, Texas.

There, the whole aircraft underwent significant modifications with the most prominent one being the addition of the telescope door that could be opened in-flight to the rear part of the fuselage.

NASA SOFIA Modifications
The 747SP undergoing modifications in Waco, Texas. (Photo credit: L-3 Communications, Platform Integration)

Because of the complexity of the modifications, L-3 even purchased a part of another United 747SP’s fuselage in order to create a full-size mock-up of the modifications it was about to make on the actual aircraft.

As for the 100-inch telescope itself, it was manufactured in Germany by MAN Technologie and Kayser-Threde, and ferried to Waco for installation in 2002.

After about a decade of work, the modifications were finished and the telescope was installed successfully, and SOFIA was born.

SOFIA Telescope
A close-up of the SOFIA telescope. (Photo credit: NASA/Tom Tschida)


The Rebirth of Clipper Lindbergh

Tying onto its history with Pan Am, the aircraft was re-christened as Clipper Lindbergh before entering into service with NASA. This time, Charles and Anne Lindbergh’s grandson, Erik Lindbergh, christened the 747SP in a ceremony that took place on May 21, 2007, 80 years after Charles Lindbergh’s famous flight.

Erik Lindbergh christening SOFIA as Clipper Lindbergh in 2007. (Photo credit: NASA/Tom Tschida)

From there on, a period of testing including the aircraft’s first flight with the telescope door completely open on December 18, 2009, followed before the aircraft embarked on its new life of gazing into space.

By this time, not only the aircraft and the telescope, but also some of the instruments designed to collect the data using the telescope were built. And so, SOFIA performed its “first light” flight on the night of May 25 and 26, 2010, with the first targets being Jupiter and the Messier 82 galaxy.

The first science flight followed on December 1, 2010, and it was the first in a series of flights that aimed to “validate SOFIA’s capabilities and demonstrate the observatory’s ability to make observations not possible from Earth-based telescopes.”

Since then, the aircraft flew hundreds of missions out of its base in Palmdale, California, as well as from other locations – mainly Christchurch in New Zealand which the observatory and its staff call their home nearly every northern hemisphere summer.

In fact, the aircraft is based in Christchurch right now on its fifth Southern deployment where I had a chance to learn about it in great detail and where it will fly more than 25 missions observing the southern sky.

NASA SOFIA at Christchurch Airport
SOFIA at Christchurch airport back in June 2018.

While this is the end of the second instalment of this four-part series, stay tuned. In the next part, I will talk about SOFIA’s operations which requires a concerted effort of countless teams on the ground and in the air.

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