SOFIA – A joint undertaking between NASA and DLR
Why a flying observatory?
Astronomical observations in the infrared (IR) and especially in the far-infrared wavelength range (FIR, 40 µm – 300 µm) can only be carried out from high altitudes. The reason for this is that water vapor in the Earth's atmosphere largely absorbs FIR radiation. NASA therefore operated aircraft-based observatories even before SOFIA, as FIR radiation can be measured at an altitude of 10 km and above. In the late 1960s, NASA began conducting observation flights at its Ames Research Center with a 0.3 m telescope on board a Learjet (LJO – Lear Jet Observatory). This can be seen as the precursor to the Kuiper Airborne Observatory (KAO, 1974-1995), which was put into service in 1974 and was built on the basis of a Lockheed C-141 Starlifter. It already had a main mirror with a diameter of 0.915 m, which was used to make some important astronomical discoveries (Uranus rings in 1977, Pluto's atmosphere in 1988).
Because there was also strong interest in the infrared wavelength range within the German astronomical community – Reinhard Genzel and Hans-Peter Röser had already made observations using the KAO – the Council of West German Observatories recommended German participation in SOFIA (Stratospheric Observatory For Infrared Astronomy) in 1985. Due to the many collaborations between the astrophysics groups at NASA and the DFVLR (a predecessor organization of the DLR) at that time, it made sense to launch SOFIA as a German-American joint project.
From idea to project
And so, in 1985, concrete discussions began on the realization of the SOFIA flying observatory. It was decided very early on that Germany's contribution would amount to 20% of the total project, but that this would not be in the form of financial resources, but rather in kind and in the form of labor. In return, German institutes would receive 20% of the observatory's observation time. Germany's contribution to the observatory would be the complete telescope — hardware, operation, maintenance, continuous improvement.
At the end of the 1980s, the key technical specifications for SOFIA were also defined as a result of various concept studies: The Boeing 747 SP was selected as the aircraft type, and the main mirror with a diameter of 2.7 m was to be made of Zerodur (Schott) due to its very low thermal expansion. Initially, it was planned to house the mirror in the front part of the aircraft, but this idea was rejected during the concept phase since it would have required two preasure gates (pressure bulkheads) in the fuselage.
The decisive factor in selecting the 747 SP, which was actually designed for extreme long-haul flights, was that it could climb to 37,000 feet immediately after takeoff and with a full fuel load, enabling astronomical observations to be made at that altitude. The maximum service ceiling was 45,000 feet, which would further improve the quality of the observations.
Conversion and commissioning
In 1997, NASA purchased an aircraft of this type from United Airlines, which had been built in 1977 and initially operated as a commercial aircraft by PanAm, and had it converted into an observatory by L-3 Communications in Waco, Texas. This conversion took almost ten years, until SOFIA's maiden flight took place on April 26, 2007. This was followed by flight testing at the NASA Dryden Flight Research Center at Edwards Air Force Base in California, starting in June 2007, first with the telescope door closed and later with it open. The activation of the telescope and the opening of the door were significant milestones in the commissioning of the observatory, as this demonstrated the basic functionality of the observatory with an inertially stabilized telescope. The first flight with astronomical observations finally took place on May 25/26, 2010, and SOFIA reached the “First Light” milestone. In the following years, flight testing was completed and the first astronomical observations were carried out (Early Science Program). The SOFIA cockpit was also upgraded with digital avionics during this period. The internal NASA milestone “Full Operational Capability” was finally reached in May 2014.
Operations
During its active service, SOFIA completed a total of nine observing cycles, each lasting approximately one year. Shorter maintenance phases were carried out during these periods, with longer ones (C/D checks) between two observing cycles. This enabled the observatory to complete a total of 783 scientific flights (not including test and transfer flights), recording data for a total of around 5,300 hours. Most flights were carried out from the operational base in Palmdale, California, but there were also regular deployments to other locations, mostly in the southern hemisphere. Christchurch in New Zealand became SOFIA's “second home.”
Observation operations were discontinued on September 29, 2022, following a joint decision by NASA and DLR after the successful completion of flight #921. SOFIA was transferred from Palmdale to the Pima Air and Space Museum in Arizona, where the aircraft is on permanent display. The telescope's three mirrors have all been removed and are expected to be on display as exhibits in the newly designed German Optical Museum in Jena from 2028.
Further detailed information on the SOFIA observatory and its technical performance can be found in the publication “SOFIA at Full Operational Capability: Technical Performance” by Pasquale Temi et al., published in 2018 in the Journal of Astronomical Instrumentation. The article “The SOFIA Telescope in Full Operation” by Andreas Reinacher et al. was also published in the same issue of the journal. This contains detailed information about the SOFIA telescope, its structure, and the function of its essential subcomponents. Comprehensive information about the entire SOFIA project can also be found in the richly illustrated book “SOFIA – Mission Infrared Universe” which is available for download via the University of Stuttgart's publication server (OPUS) in German language only.
Kontakt
Oliver Zeile
Dr.-Ing.Research Associate SOFIA Data Center
Manuel Wiedemann
Dr.-Ing.Research Associate SOFIA Data Center
Michael Hütwohl
Dipl.-Ing.Project Manager SOFIA Data Center