The universe – infinite distances. The milky way – home to several 100 billions of stars. And more are coming into existence all the time, especially in the dark areas of the milky way that is shown here stretching majestically across the skies of New Zealand.
Yet, spectacles such as these remain invisible to the human eye that can only perceive optical information. They become visible to beings whose eyes are sensitive to light in the infrared range. Air humidity in the stratosphere above New Zealand is extremely low in winter, which provides a better view into the infrared universe than almost anywhere else on Earth. That’s why astronomers from the University of Stuttgart spend several weeks “down under” each year to research the exact process of star formation. Their platform of choice for this is the SOFIA flying observatory, whose creation and technical and scientific milestones are documented on the following pages – as is the enthusiasm of researchers working on and with the SOFIA.
The Stratospheric Observatory for Infrared Astronomy (SOFIA), is a converted Boeing 747SP with a 2.7 metre telescope on board. This flying observatory, the only one in the world, is a collaborative project by the German Aerospace Center (DLR) and the National Aeronautics and Space Administration (NASA). The University of Stuttgart’s DSI (Deutsche SOFIA Institut = German SOFIA Institute) coordinates the German side of the scientific operations, and, working with the American colleagues, is predominantly responsible for the maintenance of the telescope and three of the other eight scientific instruments currently installed in the SOFIA. The ingress of infrared light from space is primarily blocked by water vapour in the atmosphere, and ground-based instruments can only pick up this cosmic radiation to a limited degree. That’s why, several times a week, the SOFIA-based astronomers take to the skies – or, more precisely, to the stratosphere. There, at an altitude of around 13 kilometres, the influence of the Earth’s atmosphere is negligible, which means that the cosmic infrared radiation can be observed without obstruction. The SOFIA’s home base is the NASA Armstrong Flight Research Center in Palmdale, California.
The frame of the SOFIA telescope is made of carbon fibre- reinforced plastic. During observations, when the door is open and the telescope mirrors are deployed, it is subject to temperatures of around minus 50 degrees Celsius and buffeted by headwinds generated by a flying speed of about 800 kilometres an hour.
The light from the telescope's mirror system is directed through the pressure bulkhead via the so-called Nasmyth Tube – shown here under construction – to the scientific instruments mounted on the side of either the telescope or the bulkhead which are inside the pressurised cabin. The Nasmyth Tube is made of carbon fibre reinforced plastic (CFRP) and has a wall thickness of just 30 millimetres. The fibres are wound around a form in different directions to ensure strength and a high load-bearing capacity.
Two cranes were required to stabilise the telescope mounting assembly, which weighs some 9300 kilogrammes, so that it could be installed precisely within the fuselage.
The centrepiece of the telescope is its mirror, which weighs around 750 kg. It is made of Zerodur®, a kind of ceramic glass that maintains its shape and dimension even under the greatest temperature fluctuations. In June 2008, the mirror was coated with aluminium for the first time to optimise its reflective characteristics. The aluminium layer is just 0.00015 millimetres thick, which is about 1/300 the thickness of a human hair, and weighs slightly more than 2 grammes, which is about one seventh the amount of aluminium used in a drink can. This image shows two members of a coating team using their own reflections to check the results of their work.
In the spring of 2013, the SOFIA team put the Field-Imaging Far-Infrared Line Spectrometer (FIFI-LS) into operation. Under the auspices of Professor Alfred Krabbe, his colleagues at the Institute of Space Systems prepared the instrument in Stuttgart for operations in the stratosphere. This image shows Alfred Krabbe, Felix Rebell, and Leslie Looney (back row from left to right) as well as Sebastian Colditz and Bill Wohler (front row from left to right) on board the SOFIA waiting excitedly for the first observation results. Researchers can not only use the FIFI-LS to observe the genesis of stars, but also the properties of the interstellar medium i.e., the material between the stars, both in our own and in distant galaxies.
As a 3D spectrometer, FIFI-LS makes highly efficient use of the expensive observation time on board the SOFIA. Not only does it take pictures, but also uses a highly sophisticated system of mirrors to detect spectral information for each pixel, which allows it to assign a wavelength to the recorded radiation. It is only the wealth of information captured by the FIFI-LS that makes it possible to identify and better understand the special physical processes happening in outer space. The instrument operates in the far-infrared range of between circa 45 to 210 micrometres and can observe the interstellar medium and star formation areas both in our own milky way and in neighbouring galaxies.
Stars never come into existence in isolation. Instead, a few hundred or even thousands of them always emerge simultaneously from some enormous cloud of gas and interstellar dust. This dust accumulates around star formation zones and screens off the view into the active centres of our galaxy. Thus, whenever we look at the Orion constellation with the naked eye, what we completely fail to see is one of the most active star formation areas (M42) in the entire milky way. However, it is visible to “eyes” that are able to see in the mid-infrared range. The FIFI-LS instrumentation, which was developed at the University of Stuttgart, allows the astronomers to measure the strength of the 146 micrometre line of atomic oxygen. Not only can the scientists use this data to estimate the volume and spatial distribution of oxygen within the star formation zone, but also the velocities, the pressure and the temperature of the corresponding gas.
Whenever starts such as our own sun form, they are always surrounded by a disc of material from which a planetary system like our own can form. Yet, for a long time it was not clear whether things worked the same way for stars that are 20 or 30 times heavier than the sun. However, in combination with data gathered from other observatories, infrared data from the SOFIA – recorded using the University of Stuttgart’s FIFI-LS instrument – have shown conclusively that this could well be possible. Just like their low mass siblings, massive stars can form by first gathering the surrounding gas and dust into a disc around them. Bit by bit the material on the inner edge of this disc then falls into the young star at the centre, thereby increasing it mass whilst the energy released during the processes radiates out. However, this increase in mass is not continuous; it occurs in a series of growth spurts because, rather than being distributed evenly, the material that forms the stellar disc occurs in clumps. When these clumps then fall into the central star – as observed using the FIFI-LS – they cause a sudden increase in brightness.
Occasionally, an object from our solar system crosses the path of a distant star covering it for a brief interval and casting a faint shadow on the Earth. Unfortunately, these shadows rarely coincide with the location of an observatory during the night. But the SOFIA can be positioned flexibly and can fly to the precise spot where such shadows will be cast, and it is then that the astronomers on board can “misuse” the occluded star as a bright, distant lamp to illuminate the atmosphere of the gnomon from behind. This was done during the Pluto occultation in June 2015, which was observed from the SOFIA using the Focal Plane Imager Plus (SPI Plus), which was developed by the DSI. Scientists can use the decrease and increase in brightness at the beginning and end of the occultation to draw useful inferences about the atmospheric structure and layering of the planet in question. If the star, planet and observer are exactly aligned during maximum occultation, then the illuminated atmosphere bundles the starlight like a focusing lens and produces the so-called “central flash”, which is clearly visible in the FPI+ data from 2015 – in itself proof positive that the SOFIA pilots succeeded in positioning the aeroplane at the correct coordinates at precisely the right time. The fine atmospheric structures are also visible in the image captured by the New Horizons space probe two weeks later – e.g., simultaneously at astronomic time scales.
The SOFIA is a flying laboratory and all components require regular maintenance. All scientific instrumentation on board, such as the FIFI-LS, is continuously checked for serviceability and carefully prepared by the scientists prior to each new mission on board the flying observatory. A cooling system is required to ensure that the infrared instruments do not simply register the ambient thermal radiation of their immediate environment. In this image we see Christian Fischer of the University of Stuttgart’s DSI topping up the liquid helium as a coolant to prepare the far infrared spectrometer for the pending system check.
Just as cars need a regular MOT, all aircraft need to pass the so-called D-check on a regular basis and the SOFIA’s turn came in 2014. It is a shorter version of the jumbo jet, which means that it can fly higher than its bigger cousin and can even reach the stratosphere. Lufthansa Technik in Hamburg is one of the few wharfs that is licensed to service aeroplanes of this type. The SOFIA underwent a thorough check up, which not only involved removing all engines and testing them in the noise-protection hangar, but also inspecting all structural components for potential cracking, testing the cockpit electronics and renewing stanchions and windows as required. Five months later Lufthansa Technik returned a fully serviced and certified aeroplane to NASA under whose flag the SOFIA officially flies, ready to embark on many more flights to the stratosphere in the service of astronomy.
Not only are students at the University of Stuttgart given the opportunity to fly on board the SOFIA, for example, in preparation for their bachelor, masters or doctoral dissertations, teachers from all over Germany can also apply to join a flight as part of a unique educational programme. The basic idea is to provide teachers with a chance to network with researchers and technicians and to enable them to experience scientific research first hand. The hope is that they will then go on to draw upon this authentic experience in conjunction with a broad palette of topics from the natural and engineering sciences to inspire their students with a long-term enthusiasm for these subjects. In the final analysis, this special advanced teacher training, which is unique in Germany’s research topography, represents an effective investment in the future.