Space. Peering up at it on a clear winter's evening presents a magnificent, abundant, thought-provoking vista of impossible grandeur. Civilisations have watched and charted the heavens above us for millennia, but nobody has had such an advanced understanding of space as our own. Immense technological strides in the last half-century have allowed us to explore our universe in greater detail than ever before. There's a constant drive for new technology as astronomers seek to understand our cosmos even further. The latest tool available is a Boeing 747.
This 747 is unlike any other. It carries the world's largest airborne telescope. At 40,000 feet the aircraft's rear upper fuselage section opens up and the telescope emerges, pointing to the heavens. This is the Boeing 747 Stratospheric Observatory For Infrared Astronomy, or SOFIA.
The first thing that may spring to mind is: why is an airborne telescope needed? Certainly there are many powerful telescopes on Earth, but the water vapour in much of our planet's atmosphere hinders visibility - even in the driest areas where there are observatories such as the Atacama Desert in Chile. However, the stratosphere - 40,000-45,000 feet up - is above 99% of Earth's water vapour, meaning the views afforded by an airborne telescope flying at that height are that much clearer.
An aircraft has other advantages over Earth-based telescopes. It has the flexibility to be positioned in a particular place at short notice if astronomers want to focus on a particular area of sky to capture a specific event or point of interest. It's also easier and cheaper to upgrade than space-based telescopes with new equipment as it becomes available.
So what is the 747 SOFIA actually for? The aircraft, a 747SP, is fitted with a 100-inch (2.5m) diameter far-infrared telescope - bigger than the one in the Hubble Space Telescope. This enables astronomers to see light sources at the furthest reaches of the spectrum - invisible to human eyes - in mid-infrared, far-infrared and sub-millimetre ranges. The 747 SOFIA's bread-and-butter will be detecting wavelengths as small as 0.3 to 1,600 microns, which are impossible to view from Earth.
Why's this important? As Professor Brian Cox said in his recent BBC series 'Wonders of the Universe', "light tells the story of the universe". By measuring light sources and where they come from, astronomers are able to measure the size of celestial objects, what they're made of and their distance from Earth. That goes for everything, from objects within our solar system, to interstellar space and beyond. Light is everything to understanding our place in the cosmos. Different colours represent different wavelengths of light, which enables astronomers to detect how distant objects are - and therefore their age. Many of these wavelengths are invisible, which is why sophisticated telescopes like that fitted into the 747 SOFIA are required.
Already a network of space-based telescopes exists to do this. The Great Observatories are four large, powerful telescopes designed to look into deep space. These are the Hubble (which observes visible light and near-ultraviolet light), the Compton (gamma rays), Chandra (x-rays) and Spitzer (infrared spectrum). With its far-infrared and sub-millimetre capabilities the 747 SOFIA naturally complements these telescopes, which is why NASA has placed the 747 SOFIA in the class of the Great Observatories. By being able to peer into the most distant reaches of the universe, it will be in the vanguard of space exploration. Who's to know what it might discover or help astronomers understand?
NASA is certainly excited. As well they might be, given that it's years late and, at $1.1 billion, thousands of dollars over-budget. However, the 747 SOFIA is certainly going to be a valuable tool. NASA says SOFIA's "unparalled astronomical science capabilities will help answer many fundamental questions about the creation and evolution of our universe". It's expected to help explore the nature of the black hole in our own Milky Way galaxy. Besides the crew of pilot, co-pilot and flight engineer, SOFIA missions see a 20-strong team working in the front fuselage area comprising a flight director, telescope operator, technicians and an 'investigator crew' of astronomers who monitor the telescope.
This isn't the first airborne telescope. Planetary scientist Dr Gerard Kuiper's work in the mid-1960s in mounting a 12-inch telescope in the window of a Convair 990 led to the development of NASA's Kuiper Airborne Observatory (KAO), a modified C-141 Starlifter fitted with a 36-inch telescope, which during a 21-year lifespan contributed much to our understanding of the universe. It discovered the rings of Uranus, water in comets, the dust ring around the centre of the Milky Way and numerous galaxies.
The KAO was retired in 1995, leaving a gap in astronomers' capability which SOFIA now fills. But with a telescope three times larger than the KAO, not to mention today's computing and imaging software capabilities, the SOFIA offers a significant optical quality improvement on its predecessor and a far more powerful way in which to look into infinity.
The SOFIA project is a joint venture between NASA and the DLR (Deutsches Zentrum fur Luft- und Raumfahrt, or German Aerospace Centre). The aircraft is based at NASA's Dryden Aircraft Operations Facility at Palmdale, California. SOFIA's operations are planned jointly by the Universities Space Research Association (USRA) and the DLR SOFIA Institute (DSI), under the leadership of the SOFIA Science Project at NASA's Ames Research Center at Moffett Field near San Jose, California. The aircraft is anticipated to have a lifespan of 20 years, and will also be used by educational institutions to test newly-developed instrumentation.
The aircraft has been a long time in coming. Such has been the complexity of this project that it's now fifteen years since NASA and the DLR signed the Memorandum of Understanding to develop SOFIA. Indeed, NASA first floated the concept of a 747 as an airborne observatory during the mid-1980s. The 747SP used as the SOFIA platform is a former Pan Am and United Airlines aircraft, and was acquired by NASA in October 1997.
At 20 tonnes, the telescope carried by the aircraft is the largest ever taken aloft. Naturally, given this size and the type of missions being flown, the 747SP required significant modification. This was undertaken at Waco, Texas, by L-3 Integrated Systems. So exactly how did they do it?
Firstly it is necessary to understand the 747 SOFIA's flight profiles. A typical mission for the aircraft lasts 10 hours, meaning there is an open cavity in the aircraft's rear fuselage for a considerable length of time. More pointedly, that cavity measures 14.5 feet wide and has a circumference of 16 feet. With the wind speeds and pressure in the aircraft's operating environment leaving it quite literally open to potentially damaging acoustic vibrations that would impact on the aircraft's stability and fatigue life, it was vital to design the cavity so that airflow around the aircraft and control surfaces wouldn't be disrupted.
To find out L-3 created a computer model of the 747SP fitted with the cavity and telescope, and hired a standard 747SP fitted with strain gauges. This enabled engineers to understand how the modifications would affect the 747SP's structural integrity and calculate stability. Consequently, when it came to actually modifying the aircraft, engineers knew exactly what to do. L-3 reverse-engineered the 747SP, adding several skin layers to strengthen the fuselage and re-routing power and control cables to the rear fuselage and tail. Additionally L-3 purchased a stored 747SP and undertook a 'dummy run', re-engineering its rear fuselage and creating the cavity and door mechanism so any pitfalls in the installation process could be identified.
With the mathematical models and practices complete, work on modifying the aircraft began in 2004. The telescope itself had been assembled by the DLR at their centre in Bonn. Comprising two mirrors, one made of a glass-ceramic composite called Zerodur and the other from silicon carbide, the component parts were completed in 2000 and carefully assembled over the following two years. It was transported to L-3's Waco facility in an A300-600ST Beluga in 2002, and undertook its first ground-based tests two years later.
Following the delicate process of installing the telescope in the 747SP and initial tests, the aircraft flew for the first time from Waco on 26 April 2007, moving a month later to the Dryden Flight Research Center at Edwards. In January 2008, during an intensive initial test period, the aircraft was moved to Palmdale. The telescope cavity was fully opened for the first time on 18 December 2009 and first used to collect data on 26 May 2010, taking images of Jupiter and the galaxy Messier 82.
Although full operational capability is expected only during 2014, routine science observation and data collection flights began in earnest in December 2010 and are now continuing apace. In April, equipped with a far-infrared spectrometer called GREAT, the telescope captured some impressive views of the Omega nebula (a cloud of dust, or stellar nursery, in which new stars are formed) and the IC 342 galaxy, located 11 million light years from Earth. Understandably keen to vaunt the 747 SOFIA's capabilities Juergen Stutzki, one of the investigators on the mission, commented: "These observations give us unique information about the physical processes and chemical conditions in the stellar nurseries. SOFIA will give us new and deep insight into how stars form".
After a very long wait, and a very large amount of resources and engineering skill and effort, a unique resource is now available to astronomers. What will it help us to understand or even discover? The insights provided over history by airborne telescopes and the 747 SOFIA's capabilities suggests this most unique variant of the famous 'Jumbo' will provide much that will inform and astonish.
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