I had already been working on the SOFIA project for some years, when back in 1998, a consortium of German research institutes (Max-Planck Institute of Radio Astronomy in Bonn, University of Cologne, Max-Planck Institute of Solar System Research and the DLR Institute of Planetary Research) decided to develop the German Receiver for Astronomy at Terahertz Frequencies (GREAT) as the Principal Investigator-class Science Instrument for the first generation at the SOFIA Observatory.

At this time, the aim was for the observatory to be operational by the end of 2001. It was not only the optimists who were expecting the GREAT spectrometer to soon enter operational service. Back then, who could have thought that it would take 13 years for GREAT to fly on SOFIA for the first time?

We finally achieved this in April 2011! SOFIA and GREAT are now ready for operation at the NASA Operations Center in Palmdale. The planning schedule envisages a total of four flights with GREAT on board SOFIA, starting with an Observatory Characterisation Flight (OCF) that also serves as a Commissioning Flight for GREAT, and three science flights as part of what is known as the Short Science programme.

Following successful completion of the first two flights with GREAT, which took place on 31 March / 1 April and 5 / 6 April 2011, I am now greatly looking forward to the third science flight.

Accompanying our SOFIA guests, science journalist Thomas Buhrke and Henning Krause, from DLR Corporate Communications, I will have the opportunity to tell you about 'SOFIA's Secrets', the GREAT instrument and the scientific objectives of the live astronomical observations on board the aircraft.

But there are still a few hurdles left to clear. At the start of the campaign, it became apparent that the US Congress might fail to reach agreement over the government budget, thereby threatening a possible 'government shutdown'. If this were to happen, there would be a long interruption in the SOFIA scientific flights. Furthermore, as the weather progressively worsened, on 8 April 2011, NASA decided to postpone the second scientific flight by a few days.

Fortunately, the government shutdown did not take place, and the flights with GREAT were resumed in the following week. Some rescheduling was needed: the participation of our guests on board SOFIA was moved from the third to the second flight.

The second flight was on 12 to 13 April 2011 and, luckily, the take off of the 53rd SOFIA flight took place exactly 50 years to the day of the first orbit around Earth by Yuri Gagarin in his spacecraft Vostok 1 back in 1961. Given that the weather had also taken a turn for the better, nothing stood in the way of a promising astronomy tour in the SOFIA 'convertible'.

The mission sequence has been accurately scheduled, but now we must perform most of the flight preparations. The 24-person flight crew is about to embark on a 15 hour mission, and will not arrive until this afternoon.

At 14:00 local time, SOFIA is towed out of its hangar and onto the apron, known as the 'ramp'. At around 15:30, the GREAT instrument is cooled with liquid nitrogen and helium, and the control software of the Mission Command and Control System (MCCS) is tested. The Crew Brief has been scheduled at 18:00. By this time, the SOFIA guests have arrived.

The flight plan is based- as always - around the scientific requirements. The Principal Investigator of the GREAT team, Rolf Gusten, describes the astronomical objectives in the GREAT Short Science Plan Flight #2 and presents the ambitious programme.

The different phases of this flight will take us from California (CA) across Nevada (NV), Idaho (ID), California (CA), Oregon (OR), Washington (WA), Idaho (ID), Montana (MT), North Dakota (ND), South Dakota (SD), Nebraska (NE), Kansas (KS), Oklahoma (OK), Texas (TX), New Mexico (NM), Arizona (AZ), Nevada (NV) and back to California (CA).

The selection of astronomical objects for the individual flight stages looks very promising indeed; calibration measurements of Saturn are performed at the start of the mission, followed by a programme including galactic and extra-galactic objects.

By 18:15, it is time for Flight Crew Entry, and at 19:00 hours, the door of the aircraft is closed in preparation for take-off. The time for the actual take off simply flies by. The take off was remarkably unspectacular - quieter than on a commercial airliner, but with powerful thrust from the jet engines. After about 15 minutes into the flight, the telescope hatch at the back of SOFIA opens, though almost nobody is aware of this at first.

The GREAT instrument is now ready to start work. It is operated simultaneously in two infrared channels, at low frequencies in the configuration L1a: 1.25 - 1.39 terahertz (about 230 microns) and L2: 1.82 - 1.92 terahertz (about 160 microns). This means that the emission lines of various molecules or transitions between both spectral bands can be observed simultaneously in the same spatial direction.

After a steep ascent to an altitude of approximately 37,000 feet, the pilots align the aircraft and hatch with Saturn and the first calibration measurements are performed. Based on this, the pilots then change the 'heading' of the aircraft to approx. 135 degrees.

It is impressive to witness the apparent leaning of the telescope during the turn. But if you were observing while seated on the dynamic part of the telescope, you would not move, and your angle of view would show the inside of the aircraft and its crew to be inclined at a surprisingly sharp angle.

After the calibration measurements have been carried out, the astronomical part of the observation flight begins: the search for interstellar molecules in galactic and extra-galactic objects. The first item on the programme is IC443 (IC stands for Index Catalogue), also known as the Jellyfish Nebula, a supernova remnant in the Gemini constellation, approximately 1.5 kiloparsec (roughly 5000 light years) away.

Supernovae constitute the final, explosive burning out of massive stars. But the shockwaves of these explosions compress the surrounding interstellar medium, initiating the first phase in the process of new star formation. The heart of the spectroscopists starts to beat faster: the first spectral evidence of interstellar carbon monoxide CO (12-11) appears on the data monitors, possibly also with OH lines. On this flight, GREAT and the SOFIA telescope really are living up to the expectations of the observers!

On Cep B, located at a distance of approximately 2400 light years, a galactic molecular cloud in the constellation of Cepheus exhibits carbon monoxide lines and, in the higher frequency channel, a clearly defined CII emission line, the single ionised carbon line constituting an important cooling line in interstellar gas.

This line is of utmost importance because stars form from dense, cool clouds that condense the interstellar material. The gas in a molecular cloud is also being ionised by the strong UV radiation from the surrounding hot, young first generation stars (spectral types O and B); the kinetic energy of the released electrons is continuously heating up the surrounding gas.

To enable any kind of cooling to take place, 'loss processes' are also required. Through collision-induced excitation of carbon atoms, some of the high levels of kinetic energy in the electrons can be converted into radiation energy. Bound electrons are excited and 'de-excited' by these collisions, resulting in the emission of low-energy photons, which do not cause further heating of the molecular cloud.

The CII lines are therefore key indicators of the places where stars are born. Another cooling line is the OI line emitted by neutral atomic oxygen, which is found at a frequency of 4.7 terahertz (63.18 microns). In the future it will be observable on the 3rd channel of GREAT, provided by the DLR Institute of Planetary Research.

The telescope is now pointed at M82 ausgerichtet, the famous irregular galaxy in the constellation of Ursa Major, located at a distance of approximately 14 million light years. At its heart is a powerful starburst event. M82 also exhibits carbon monoxide (CO) spectral lines.

To determine whether two molecular lines at different wavelengths originate in the same cloud or object, they must exhibit the same speed and direction of travel. With the help of the Doppler formula and the familiar resting wavelengths of these lines, the spectral lines can be presented in a radial velocity catalogue.

All related lines within a molecular cloud must then exhibit the same velocity. Following the standardisation of these details (known as LSR correction - Local Standard of Rest), it is possible to distinguish the composition of various clouds relatively accurately. In galaxies, the high individual system velocity and red shift usually give rise to an excessively large offset between the observed lines.

For M82, we see the lines at +210 kilometres per second. Since M82 is viewed edge on, its rotation will derive in the matter on one side moving away from us, and the matter on the other side toward us. But the insufficient spatial resolution of this galaxy with the beam of the GREAT instrument makes these lines appear to be widely dispersed.

During the flight over North Dakota, the pilots alter the heading once again by approximately 90 degrees. SOFIA is now flying south east and is ascending to an altitude of about 43,000 feet. Now, Sharpless 106 (S106), the hourglass-shaped cloud in the constellation of Cygnus (Swan) appears in our field of view. Located at a distance of 1500 light years, S106 is extraordinary.

It has two 'lobes' with a central dust disc and a central star. If it were not for the absorption of this dust, the star would be as hot as Sirius. The presence of several protoplanetary dust discs, or proplyds, in the vicinity has been verified with the Hubble Space Telescope. GREAT can also, for the first time, observe the C+ line from S106.

It is now 02:33. I suddenly feel exhausted, but the team is still highly motivated. We are now flying back West. During the last 3-hour leg of our flight we observe the Milky Way's galactic heart in the Sagittarius constellation (the Archer).

Here, we expect to observe high rotational transitions of the CO molecule in the circumnuclear disc around the main mass concentration in the centre of the Milky Way. The gas disc rotates at a speed of approximately 120 kilometres per second around Sgr A.

This means that GREAT can also observe the CO (10-9) lines first observed with HIFI on Herschel, verify the CO (11-10), CO (13-12) and CO (16-15) transitions, and even provide observations of OH and H2D+ lines. For these to be statistically significant, more observations need to be conducted in the flight scheduled for July.

H2D+ provides us with information about extreme physical conditions. Chemical models suggest that it is one of the last molecules to freeze out onto the dust grains and disappear from the gas phase, making it an excellent indicator of the dynamic processes at the earliest phases of the birth of a star (protostar), deeply embedded in the molecular clouds.

In the future, it will be possible to observe the heavy isotope deuterium, comprising one proton and neutron, in the HD combination on the 2.7 teraherz channel of GREAT. Due to the widespread assumption that deuterium was only able to arise primordially, that is, very shortly after the Big Bang, these observations will give some insight into its frequency, and therefore about some possible models of the cosmos.

Here, we can look forward to the final assessment of this data by the participating scientists at the Max-Planck Institute of Radio Astronomy in Bonn, and at the University of Cologne.

We land at about 06:05, and make our way to the debrief in the conference room. The faces of the crewmembers show that it's been a long night of hard work, but the successful observations make all the effort well worthwhile. Our guests on board SOFIA were captivated by the professional and productive atmosphere, and were not reluctant to repeat their adventure on board SOFIA in the future.