NASA’s New Horizons spacecraft has recently begun sending back the first color images of Pluto and Charon, and they are spectacular. But prior to the probe making its close approach to Pluto on July 14th, scientists have been scouting Pluto’s atmosphere using a 98-inch telescope mounted inside a highly modified Boeing 747SP.

A combined effort between NASA and the German Aerospace Center (DLR), the aircraft is called SOFIA (registration N747NA, callsign “NASA747”), which stands for Stratospheric Observatory for Infrared Astronomy. SOFIA probably isn’t the best-known 747 belonging to NASA, but it is enabling valuable scientific observations because of its unique capabilities.

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And while SOFIA has been flying astronomy missions since 2010, the idea of putting a telescope in an airplane to study the stars is almost 100 years old.

Telescope Planes

In the early 1920’s, Sherman Mills Fairchild developed the then-revolutionary electrically-driven K-3 aerial camera for mapping and reconnaissance missions. While the cameras were originally intended to be pointed towards Earth and not skyward, this ultimately spurred the earliest airborne astronomy flights from biplanes during the 1920’s and 1930’s, which were undertaken primarily for observing solar eclipses.

On September 10th, 1923, the U.S. Navy attempted to measure the centerline of a solar eclipse from the air using K-3 type mapping cameras and hypersensitized film aboard 16 different aircraft flying simultaneously. While precise details about all of the aircraft that were used during the attempt are elusive, one of the aircraft involved was reportedly a Felixstowe F5L “flying boat” biplane.

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The convergence of smooth running jet aircraft, improved telescope technology and infrared sensors in the 1950s and 1960s led to the field of airborne astronomy taking on research beyond eclipse observations. In 1968, Gerald Kuiper (yes, that Kuiper) aimed a 12-inch telescope through the window of a NASA Learjet. Today, Kuiper is revered as a pioneer in airborne astronomy and planetary science, his work spurring a series of unique aircraft built to carry telescopes aloft.

One of NASA’s early airborne telescopes was the Galileo Observatory, a converted Convair 990 airliner that supported scientists at the Ames Research Center. That aircraft met an untimely demise in a 1973 mid-air collision with a US Navy P-3C Orion while on final approach at Moffett Field. A replacement aircraft called Galileo II was built, but unfortunately it was destroyed in a fire following an aborted takeoff in 1985.

Despite such unfortunate luck with the two Galileo Observatory aircraft, NASA operated the Kuiper Airborne Observatory (KAO) from 1974-1995, a Lockheed C-141A Starlifter modified with a 36-inch reflecting telescope. SOFIA is KAO’s follow-on program, offering scientists increased capabilities via a higher performance aircraft with a larger aperture telescope.

The Kuiper Airborne Observatory and SOFIA on the tarmac at NASA’s Ames Research Center.

Meet SOFIA

SOFIA’s donor aircraft is a Boeing 747SP widebody airliner, one of only 13 current airworthy examples. The 747SP (SP stands for “Special Performance”) was originally designed to compete with the Douglas DC-10 and Lockheed L-1011 tri-jets in the early 1970’s. Lacking a competitive product in their lineup at the time, Boeing chose to offer airline customers a scaled-down version of the 747 instead of scaling up a smaller jet.

Boeing introduced the 747SP in 1973 with changes including a fuselage shortened by over 48 feet, lighter wings, solid flaps and the removal of under-wing canoes. All of this added up to an empty 747SP weighing around 45,000 pounds less than an empty 747-200, making the jet ideal for long-range intercontinental flights. Because the shorter, lighter 747SP retained the same four engines and as the original 747, it could also fly faster and higher.

These special characteristics of the Boeing 747SP lend themselves perfectly to SOFIA’s mission, as they enable long loiter times and extended range, all while flying at higher altitudes. At an operating height of 41,000-43,000 feet, SOFIA flies above over 99 percent of the atmosphere’s water vapor, giving SOFIA opportunities to gaze into the heavens with clarity rivaled only by telescopes in orbit.

The specific 747SP aircraft that was selected for modification for the SOFIA program originally entered commercial service with Pan American World Airways in 1977, where it was christened Clipper Lindbergh, a name it still officially retains today. Pan Am sold the aircraft to United Airlines in 1986, for whom it operated in commercial service until 1995, when it was sent to storage.

SOFIA wearing an early livery during a 1998 test flight.

In 1997, it was retrieved from storage and NASA acquired it for conversion into an airborne observatory. Raytheon began the first step of SOFIA’s conversion in 1998 by installing a 13.5 foot wide retractable door behind the wing on the aft side. SOFIA’s door arcs 18 feet upward along the fuselage and can retract in flight, protecting the highly sensitive onboard instruments from the sun until conditions are ideal for data collection.

Beyond the huge door that reveals the enormous telescope onboard, SOFIA was modified with heavy shock absorbers, pressure bulkheads and counterweights to accommodate the telescope instruments. The aircraft’s interior was also retrofitted to provide space for educators to work during missions. Throughout the course of the program, SOFIA will invite thousands of science teachers, planetarium scientists and others to fly onboard. This ensures that the benefits of SOFIA’s science missions will reach as many people on Earth as possible.

SOFIA remained in budgetary and developmental purgatory until late 2009, when the optical systems were finally integrated into the airframe and it was first flown with the door open. Routine scientific flights began in 2010, and full capabilities were set to come online in 2014. Then, NASA abruptly announced that they would drastically cut SOFIA’s funding request for FY 2015, indicating that they planned to place the aircraft into storage and that, “savings from SOFIA can have a larger impact supporting other science missions.”

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A 2014 report from NASA’s Inspector General found that SOFIA is one of the most expensive programs in NASA’s science portfolio. With total program life cycle costs estimated at $3 billion, SOFIA costs more than $100,000 per planned research flight hour to operate. After publicly stating that SOFIA’s “contributions to astronomical science will be significantly less than originally envisioned,” the program hung in limbo for about a year, when suddenly NASA changed their minds in early 2015.

For now, the program appears to be on stable budgetary footing, with the aircraft having flown regularly throughout the first half of the year. Even so, NASA’s spastic decision-making and SOFIA’s estimated $1 million per mission costs should illustrate that SOFIA could easily be a sacrificial lamb for future NASA budgets. Pulling the plug on SOFIA as soon as the program finally starts to perform science missions is hasty and ignores the unique capabilities that no other observatory can provide.

Seeing What We Can’t See

SOFIA is currently deployed to Christchurch, New Zealand until July 2015 and is observing parts of the sky that aren’t visible from the Northern Hemisphere with four main instruments, more than ever before. Hopefully the program will be allowed to continue unfettered now that it is finally mature enough to generate substantial scientific observations.

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While there is no argument that orbital telescopes are ideally situated to capture images that improve our understanding of the universe, there are a few areas where airborne telescopes have advantages over orbital telescopes. Instruments aboard SOFIA are far easier to maintain and service, whereas missions to repair orbital telescopes (such as STS-125 in 2009) are hugely expensive and risky.

Additionally, SOFIA is not vulnerable to the ever-increasing risk of space junk, whereas the Hubble Repair mission in 2009 had a one-in-221 chance of colliding with orbital debris (although NASA deemed this risk acceptable). During the mission, a four inch piece of debris from a recently-exploded Chinese weather satellite came less than two miles away from the Hubble telescope and Space Shuttle Atlantis.

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The ability to place optical instruments in the ideal place and time to observe rare astronomical events is central to appreciating the value of an asset like SOFIA. In 2011, SOFIA was at the right place at the right time to observe Pluto’s occultation in 2011. Notably, NASA says that it was the only observatory capable of doing so in the world at the time.

Ground-based telescopes can only observe a certain tract of the sky, and orbital telescopes aren’t easily repositioned. However, a telescope mounted on an intra-atmospheric aircraft that can produce a snapshot of the sky at precisely where and when desired is a unique capability, and one that is worth preserving in the event that orbital telescopes become incapacitated.

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Night after night, SOFIA prowls the skies while making observations about comets, the life cycles of distant stars, the formation of planets and the chemical makeup of interstellar space. Requests from the scientific community for time aboard SOFIA far outpace the number of flight hours available, showing how the aircraft is hugely versatile for studying our celestial neighbors both near and far away.

Throughout the last century of flight, airborne telescopes have clearly proven their worth to the science community, and SOFIA should remain the pinnacle of airborne telescope technology for many years to come, especially seeing as NASA now has two retired 747 Shuttle Carriers to use for spares free of charge. The big flying telescope also sits as yet one more reminder of just how versatile the 747 design remains almost 50 years after its first flight.

Image credit: Top shot - NASA/Wikicommons, SOFIA side profile close-up - Reed Saxon/AP, Felixstowe F5L - Public domain/Wikicommons, #NASAbeyond graphic - NASA/Wikicommons, KAO/SOFIA on tarmac - NASA/Wikicommons, Boeing 747SP original livery - Public domain/Wikicommons, SOFIA interior - NASA/Wikicommons, SOFIA early livery test flight - NASA/Wikicommons, Hubble STS-125 - NASA/Wikicommons, SOFIA side profile in flight - NASA/Wikicommons, Bottom tarmac rear shot - NASA/Wikicommons