New challenges for a refurbished observatory
Maintenance is connected with waiting. No wonder then that, near the end of SOFIA's five-month heavy maintenance visit at Lufthansa Technik AG in Hamburg, the researchers could hardly wait for the whole procedure to be completed. I was curious to find out exactly how scientific investigations would improve in the entirely refurbished airborne observatory. SOFIA has been back in service since 13 January, and it is already clear that the conditions for observation on board the recently maintained aircraft have been significantly enhanced.
The interior of SOFIA is remarkably smooth and quiet as it rolls to the assigned runway at its home base in Palmdale, California. There is running water on board and a small kitchenette to brew coffee – something that both flight and science crews had previously longed for during their long nocturnal flights. However, the German infrared spectrometer GREAT, installed on the telescope for six flights, means that the ban on microwaves and mobile phones remains in place. The receiver would otherwise immediately detect their radiation.
The flight schedule for this 10-day January campaign involves more than 20 observation projects. The plan is to complete a rapid series of flights, three per week – a major challenge for the GREAT team and the staff at NASA, DSI and USRA. "We have set ourselves an ambitious observation programme for the first three flights, referred to as the 'consortium phase'," explains GREAT Principal Investigator Rolf Güsten from the Max Planck Institute for Radio Astronomy in Bonn (MPIfR). Here, the scientists intend to investigate high-resolution spectra of neutral oxygen (at 4.7 terahertz) and singly ionised carbon (at 1.9 terahertz), as well as carbon monoxide molecules at high frequencies in galactic and extragalactic sources, to determine the properties of these molecular clouds.
Heinz-Wilhelm Hübers and his assistant, Heiko Richter, from the DLR Institute of Planetary Research are also part of the team on the consortium flights. The local oscillator, a quantum cascade laser used in conjunction with GREAT, was developed in the Berlin-based institute. Hübers explains that the laser proved reliable during its first in-flight deployment in May 2014: "This technology allows high-resolution spectroscopic investigations at frequencies in the terahertz spectral range. We will continue to make contributions to optimising the GREAT instrument."
The research flights conducted thus far have yielded promising results. For instance, they identified the presence of atomic oxygen, singly ionised carbon and carbon monoxide in the direction of the Flame Nebula (NGC 2024) in the constellation of Orion. "The gas there has the density of butter," explains Urs Graf, a project principal investigator from the University of Cologne.
The gas observation programme will dominate the second week of flights. Here, the experienced team from MPIfR and the University of Cologne will operate GREAT, a highly sophisticated instrument of extraordinary technical complexity, while the guest observers on board will monitor the progress of the measurements.
I am particularly looking forward to observations conducted on the supernova remnant IC443, also known as the 'Jellyfish Nebula', in the constellation of Gemini. The purpose here is to investigate the possible presence of iron hydride, a compound with the constituent parts hydrogen and iron. The GREAT instrument has already succeeded in performing an identical investigation on the mercapto radical. Evidence for the presence of iron hydride would be another step forward, and important in determining the conditions under which metal hydrides form. Non-detections, referred to as upper limits, are also valuable when addressing questions of cosmic chemistry. Although somewhat unpopular among astronomers – there are no spectral lines to identify – upper limits nevertheless permit conclusions to be drawn on the state of interstellar matter. We will certainly look forward to the results of the data analysis.