A shot of the SOFIA on the ramp at Palmdale in the early evening. © Charles Cunliffe – Global Aviation Resource.

The SOFIA airborne telescope provides a unique capability to observe the heavens.  Charles Cunliffe was able to see the aircraft up close and personal on two occasions with the media event held at Stuttgart and to participate in a mission from Palmdale.  In this feature for Global Aviation Resource, Charles details the history of the programme and describes the mission experience.

Introduction to SOFIA, its Mission and History

The Stratospheric Observatory for Airborne Astronomy or SOFIA for short, is a joint project between the US space agency NASA, and its German counterpart DLR. It is the world’s largest airborne observatory and the only telescope to observe wavelengths from 28 to 320 micrometres. The SOFIA project consists of a Boeing 747SP that has been highly modified to carry a 2.7m diameter, 17 tonne telescope. The aircraft operates at high altitudes, typically stratosphere level (from 38,000ft to 45,000ft), which allows SOFIA to observe above 99% of Earth’s water vapour which is a major advantage over ground-based telescopes. The aircraft spends the majority of the year at Palmdale Regional Airport (PMD) in California as part of NASA’s Armstrong Flight Research Center, inside Building 703, which was once home to the production line of the Rockwell B-1 bomber. During summer in the northern hemisphere, the aircraft usually redeploys and spends June to July based at Christchurch International Airport, New Zealand to explore the southern hemisphere.

Airborne astronomy offers many advantages over space satellites. Satellites are extremely costly and time consuming to develop and launch, whist virtually impossible to modify or maintain in orbit without the assistance of astronauts. Airborne observation allows for more flexibility at a cheaper cost. The observation instruments onboard can be quickly changed to suit the mission and modified however needed. This negates the issues of using multiple satellites for different missions and the extensive development costs that come with it. The mission data is downloaded as soon as the aircraft lands and made available to the public shortly afterwards for scientific analysis (and interpretation).  SOFIA’s mobility allows researchers to observe from almost anywhere in the world. This flexibility permits studies of transient events that often take place over oceans where there are no ground-based telescopes. This has permitted astronomers on SOFIA to study eclipse-like events such as in 2015, when Pluto passed between a distant star and the Earth creating a shadow near New Zealand.

The origins of the SOFIA programme can be traced as far back as 1965, when Dutch astronomer Gerard Kuiper used a NASA Convair 990 for performing infrared observations of Venus, whose research pioneered airborne astronomy. Due to the success of Kuiper’s findings, only three years later, a proposal to modify a NASA operated Learjet with a 12-inch telescope onboard was conjured up by American physicist Frank Lowe was accepted and tested. The Learjet would go onto make significant discoveries on Jupiter and Saturn on how they sourced energy. As the research and discoveries grew, so did the requirement for a larger telescope.

Enter the Kuiper Airborne Observatory (KAO) project, a highly modified C-141A Starlifter (N714NA) operated by NASA, based at Moffett Field, California. The telescope size had tripled from the Learjet’s 12-inches to 36-inches and due to the C-141’s cruising altitude, it could climb above almost all of the water vapour in the Earth’s atmosphere, allowing observations in infrared – a significant advantage over ground-based platforms where infrared radiation is blocked before reaching the ground. The KAO C-141A would go onto operate for over 20 years and made several major discoveries about our galaxy and other galaxies such as, the first sightings of the rings of Uranus and how star formations are developed. The aircraft was retired in 1995 and succeeded by the SOFIA programme.

Telescope with the GREAT instrument fitted for the media day. © Charles Cunliffe – Global Aviation Resource.

Telescope operator stations. © Charles Cunliffe – Global Aviation Resource.

The telescope with the EXES instrument attached. © Charles Cunliffe – Global Aviation Resource.

The proposal to fit a larger aircraft mounted telescope inside of a Boeing 747 was presented as early as 1984, decades before SOFIA would fly. The system concept was published in 1987 and a partnership between NASA and DFVLR, (since renamed DLR – ‘German Aerospace Center’) was created. The project was met with delays from the outset, the reunification of Germany and NASA budget cuts let to a 5-year delay. In 1996, a memorandum of understanding was finally signed between NASA and DLR to construct SOFIA and the project was back in the green.

The aircraft selected was a Boeing 747SP (Special Performance) a highly modified version of the 747 optimised for ultra-long-range flights, first flown in 1975. The 747SP is technically a derivative of the 747-100 but features major modifications such as a shortened fuselage (only 747 version to be ‘shrunk’), redesigned flaps and taller vertical stabiliser. These modifications allowed the 747SP to fly further, faster and higher than any 747 model at the time. 45 747SP aircraft were ultimately produced, including MSN 21441, line number 306, N536PA. This 747SP (number 18 of 45 produced) was delivered to Pan Am on 6th May 1977 and was christened ‘Clipper Lindbergh’ by Anne Morrow Lindbergh, Charles Lindbergh’s widow. Clipper Lindbergh would serve the airline for only 11 years before being purchased by United Airlines in 1986 and re-registered N145UA. The aircraft operated with UA for almost another decade before being stored in 1995. In 1997, the aircraft was saved from scrapping and acquired by NASA registered N747NA to be converted into the SOFIA project.

Aircraft Conversion Process and the Project Today

In 1998, N747NA flew to Waco, Texas to begin the extensive conversion process by L-3 Communications Integrated Systems. The most major part of the conversion process was cutting a hole in the left-hand rear side of the aircraft to house the telescope. To ensure the conversion was viable, L-3 purchased a fuselage section from another 747SP (N141UA) to use as a full-size mock-up. L-3 designed and installed the 18ft long by 13.5ft wide door to cover the telescope. As the door is required to be open in-flight for all SOFIA observation missions, to give the telescope access to the sky, this required the rear pressure bulkhead to be moved forward – meaning the telescope is integrated into the new bulkhead. The main telescope assembly is therefore unreachable during flight; however, the instrument-section is reachable throughout the flight. The distance from the very front of the aircraft back to the telescope is 37m. This is just as far as the first powered flight by the Wright Brothers in Kitty Hawk in 1903 and there is a plaque onboard the aircraft showing this.

When in-flight, the open cavity has no significant influence on the handling characteristics of the aircraft except for a small fuel burn penalty. This is mainly due to the fact airflow travels up and over the telescope cavity, guided by forward fuselage ramp area. There is also a telescope cavity aperture that directs away most of the air trying to enter the cavity.

The German designed and built telescope was constructed by a consortium of MAN Technologie and Kayser-Threde GmbH and consists of three mirrors – primary, secondary and tertiary mirrors. The telescopes 2.7m diameter primary mirror is made of a unique glass material called Zerodur, that has almost zero thermal expansion. This is an advantage as it means the mirror is unaffected by the warm temperatures on the ground in California, nor the extreme cold by the altitudes SOFIA operates at. To reduce weight, honeycomb-shaped pockets were milled out the back of the primary mirror to make it approximately 80% lighter than similar sized mirrors on ground-based observation platforms.

The entire telescope arrangement including primary mirror assembly, the main optical support and the suspension assembly were put together in Augsburg, Germany in 2002. After successful tests, the telescope assembly was transported from Germany to the United States onboard an Airbus Beluga, where it was installed in the Boeing 747SP in 2004 and initial ground-based observations were made.

After numerous setbacks including cost increases and a project review where NASA suspended funding, SOFIA finally made its first flight on 26th April 2007. 30 years after the aircraft was originally christened Clipper Lindbergh by Pan Am. The aircraft was re-christened on 21st May 2007, by the same name by Erik Lindbergh, Charles Lindbergh’s grandson at NASA’s invitation. After several post maintenance shakedown flights in Waco, the aircraft moved to the Armstrong Flight Research Center at Edwards Air Force Base to begin flight testing. On 18th December 2009, SOFIA performed its first flight with the telescope cavity door open, lasting for two minutes of the 79-minute flight. The telescope saw ‘first light’ (a term used to describe the first use of a telescope to take an astronomical image), on 26th May 2010. Initial observation flights began in the winter of that year and the observatory achieved full operational capability in 2014.

SOFIA being prepared for flight. © Charles Cunliffe – Global Aviation Resource.

The cockpit of SOFIA on the media day. © Charles Cunliffe – Global Aviation Resource.

A telescope operator at work. © Charles Cunliffe – Global Aviation Resource.

In 2012, the aircraft received a major flight deck upgrade with most out-dated analogue instruments replaced to form a glass cockpit. The upgrade also included new avionic systems to improve SOFIA’s efficiency, operability and compliance with current airspace regulations.

The overall SOFIA project is funded 80% by NASA and 20% by DLR. As DLR owns the telescope they are responsible for its daily operation and maintenance, whereas NASA is responsible for all aircraft related items. Additionally, DLR fund the longer aircraft maintenance visits to Hamburg, Germany with the last heavy maintenance visit taking place in 2018. A little-known fact is that SOFIA’s livery was funded by DLR, as well as 4 of the 16 engines the project currently owns.

The final-say on the aircraft is always with NASA as they are the owner of the aircraft. While DLR as the owner of the telescope has the final say on the telescope – requiring both teams to work extremely closely together on all major project decisions. Day-to-day flying decisions are usually triggered by aircraft technical state, along with weather, crew staffing and ATC restrictions.

To get accepted to fly a scientific mission on SOFIA is not simple. Observation proposals are chosen amongst many with an oversubscription rate of 5:1. The process of gaining observing time starts with a ‘Call for Proposals’, issued by the Universities Space Research Association (USRA) on behalf of NASA. The Call is open to all qualified astronomers, in the U.S. and outside the U.S. However, German institutions are exempt and have their own “Call for Proposal” administered by the German SOFIA Institute (Deutsches SOFIA Institut; DSI) on behalf of DLR. All observing proposals that are scientifically well-justified through scientific peer review will be considered for selection and are ranked by the SOFIA Science Mission Operations (SMO) Directorate in priority order ‘1’,’2’ & ‘3’. Priority 1 = “will do” – these proposals are the highest ranked and offer the best scientific return. Priority 2 = “should do” – these projects are likely to be completed, but they are not a top priority and will not be re-scheduled if there is a flight cancellation. Priority 3 = “do if time” the lowest ranked priority, these proposals will be added to the flight plan when no higher ranked targets are available. There are several factors used in the decision on which missions will be flown on the SOFIA program, including the overall scientific merit of the proposed investigation, the broader scientific impact of the investigations to astronomy and the degree to which the investigation uses SOFIA’s unique capabilities.

Operating SOFIA

One of the main strengths of SOFIA is its mobility: observations of any special, spontaneous astronomic event around the globe can be covered within a very short notice. This has been demonstrated in the past when the observatory rapidly deployed to Daytona Beach, Florida in 2017 to study the occultation of Neptune’s Moon – Triton. SOFIA’s flight planners designed the flight plan to put the aircraft in the centre of the shadow for approximately two minutes as Triton aligned in front of a faraway star.

Since reaching Full Operational Capability in 2014, the SOFIA Project has around 100 to 120 flights scheduled per year, with 75% being flown from Palmdale and the other 25% from Christchurch. Flights are mainly dependent on maintenance inputs, but also on the available budget for fuel and manpower. With an annual dispatch-rate of about 75% to 85%, this usually results in 80-100 executed science flights per year – flying on average 4 days a week.

A typical observation flight out of Palmdale would consist of a minimum crew of around 12 :

  • 3 x Flight crew = 2 pilots and a flight engineer.
  • 2 x Mission Directors (I & II) – to ensure the mission is going as planned by co-ordinating with the flight deck and science teams.
  • 2 x Safety Technicians– responsible for the safety of everyone onboard the aircraft and its systems.
  • 2 x Telescope Operators– ensuring the telescope is focused and locked to the correct targets at the correct time.
  • 3 x Science Instrument Operators – this team is individual for each science instrument and consists of experts who ensure the function and data collection of the science instrument.
  • Guest Observers – They can join the science flight if they wish but do not need to. SOFIA works autonomously like a satellite in this regard, where the data comes to the scientist automatically without any need for the scientist to fly with SOFIA. Their presence during a flight is just an additional “offer” from the Project to the scientist and many of them appreciate this experience. A typical flight hosts 1-2 guest observers.

Mission lengths are dictated by strict crew duty limits as per the same as any commercial airline. The crew duty time for the flight crew is 14 hours – this includes ‘check-in’ time usually 2.5 hours before take-off to conduct the mission brief and planning, and ‘check-out’ time, at least half an hour after the aircraft lands. The duty time can be extended up to 16 hours with one extra pilot. Based on the above, the maximum flight duration for any SOFIA mission on a 3-flight deck crew is around 11 hours. During a typical week with 4 flights, Crew one will fly Monday and Wednesday and Crew two will fly Tuesday and Thursday, giving way over 24 hours in-between flights.

The Telescope and its Instruments

The 17-tonne telescope onboard SOFIA is mounted inside the aircraft through a system of spherical bearings and shock absorbing pressure barriers. The main bearings are about 1m in diameter and float on a pressurised oil layer to allow electrical motors to move the telescope within fractions of a second. This is necessary to ensure a certain pointing stability and to not lose the astronomical target out of sight. With no telescope motors running and all breaks open, the telescope could actually be moved by hand.  Vibrations from the aircraft’s engines are mainly absorbed by “rubber tires” –designed by German car tire manufacturer, Continental. The secondary mirror is also manoeuvrable and is a key component in further increasing SOFIA’s pointing stability.

The SOFIA Project operates and maintains 6 science instruments, these instruments allow SOFIA the flexibility to observe many different studies. Examples such as, supernovae, the supermassive black hole in the galactic centre of the Milk Way, astrochemistry used to identify spectral fingerprints of the processes leading to the universes water creation, planetary nebulae, comets, star formations and planets. SOFIA’s telescope can have one instrument attached at a time and takes about 40 hours to change between instruments – each are required to be cryogenically cooled before fitting to the telescope.

  • FIFI-LS (Field Imaging Far-Infrared Line Spectrometer) – Studies the interstellar medium where matter from old stars gets recycled to new stars at wavelengths of 51 – 120 microns.
  • GREAT (German REceiver for Astronomy at Terahertz Frequencies) – works like a very high frequency radio receiver detecting light waves not light particles. Dual channel instrument with a range of 63 – 612 microns.
  • EXES (Echelon-Cross-Echelle Spectrograph) – enables the study of molecular hydrogen, water vapour, and methane with a range of 4.5 – 28.3 microns.
  • FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope) – uses a dual-channel mid-infrared camera and spectrograph sensitive from 5 – 40 microns to study celestial objects such as planets and star forming regions.
  • HAWC+ (High-resolution Airborne Wideband Camera) – is far-infrared camera and imaging polarimeter operating between 50 – 240 microns and is used to study interstellar magnetic fields.
  • Under development – HIRMES (HIgh Resolution Mid-infrarEd Spectrometer) – covers the 25 – 122 micron wavelength region. Used to study the formation of planetary systems over a spectral range rich in ionic, atomic, and molecular lines.

Stuttgart Media Day – SOFIA’s First Science Flight in Europe

In September 2019, SOFIA made a highly publicised visit to Stuttgart, Germany for its first European observation flight. The premise of the science flight was for researchers to explore the areas around black holes and to investigate the question of whether dark energy is causing the universe to expand at an ever-increasing rate. The mission after heading west out of Stuttgart Airport (STR), routed via France and the UK to the Swedish coast and further south over the Baltic Sea, to cross Poland, the Czech Republic, Austria, Slovenia, Croatia, the Adriatic Sea, and Italy – almost as far as south Sicily. The observation flight required extensive co-ordination and teamwork with many stakeholders and was executed with great success.

I spoke with Clemens Plank, SOFIA Project Engineer at DLR about the complexities of getting SOFIA over to Europe and the planning behind the science flight from Stuttgart. Clemens goes on to say: “For the science flight to take place, SOFIA required diplomatic overflight clearance from 15 European countries. This certainly kept our embassies and international affairs office busy. Re-assuring them that we are looking upwards and not downwards was a common question that had to be overcome! The SOFIA aircraft is not FAA nor EASA certified but is NASA-self-certified which confused things even more!”

 “Air Traffic Control was naturally required to be informed and coordinated for this special flight. European ATC certainly had no idea about the needs of SOFIA (a big requirement is the need to stay exactly on-time and on-track and as high as possible). It was a combination of educating ATC, making the scientific requirement of the flight obvious and coordinating everything with Euro Control.

 Also, we had to ‘book’ military airspace. Fortunately, continental European danger areas are usually without military activity at night and at this altitude. It was easier in some countries than in others… It’s impossible in the USA to ‘book’ military airspace as a civil aircraft so it was a big learning curve for the Project. We avoid military areas in the USA as much as we can, but this was not possible in the tight European Airspace.”

Mission directors keeping an overview of the science flight underway. © Charles Cunliffe – Global Aviation Resource.

The Orion star formation showing what the telescope is looking at. © Charles Cunliffe – Global Aviation Resource.

Lined up and ready for departure from Palmdale. © Charles Cunliffe – Global Aviation Resource.

SOFIA was suffering technical issues before departing to Stuttgart and the flight required an extensive re-time in-order to be able to operate the science flight. Performance issues meant that SOFIA had to take-off from Palmdale before 09:00L in order to get to Stuttgart, Clemens explains more: “As the distance between PMD-STR is quite long, this required SOFIA to carry a lot of fuel in order to make it non-stop. (On the way back to PMD it was clear that we would need a fuel-stop in Minnesota due to the headwinds – the 17t telescope and thus a shifted Center of Gravity (CoG) doesn’t come without some disadvantages…). With reaching our Maximum Take-off Weight (MTOW) and a daytime take-off we ran into new issues which usually do not affect our missions: the take-off weight isn’t a fixed number but a function of air temperature, winds, humidity, etc. Which is a very strict world for SOFIA with the many modifications performed to the airframe. Higher air temperature after 9am meant lower air density – equal to a lower MTOW– equal to less fuel – equal to less distance – equal to a stop somewhere in Minnesota again – equal to a second crew needed in Minnesota to take over from there which wasn’t available on such short notice – equal to another day of mandatory crew rest in Minnesota – equal to a late arrival in STR on Tuesday evening at the earliest. This would have been most likely too late to get everything ready for a science flight on Wednesday.”

 “Our mounted science instrument, HAWC+, would have also needed a refill of liquid helium in Minnesota, which isn’t easily available in just any supermarket. We were preparing for refilling and servicing HAWC+ in STR – and probably would have managed to take off for the science flight on Wednesday if super urgent. This therefore would have led to all the aircraft visits for the public and science community being cancelled which was the main reason for coming to STR. This would have led to the decision to stay in PMD and execute science flights with the available crew there instead.”

 “So why didn’t we just take-off before 9am in PMD then? Because this would have resulted in an arrival before 6am LT in STR which is prohibited by law (noise restriction). We already had a waiver for the science flight on Thursday night, but not for the now needed early morning arrival on Monday. So, I had a very busy Saturday evening to get a hold of the Stuttgart government in order to get a last-minute waver for Sunday night. Everyone I was in touch with before only had office-phones and no mobiles… The waiver was finally emailed to me on Sunday morning. Fortunately, this was just in time to email our flight crew in PMD that they must take their alarm clock seriously and the aircraft can take off in 6hrs!”

SOFIA finally arrived into Stuttgart on Monday, September 16th, 2019, at 04:14L – a delay of just under 24 hours. The aircraft was open for pre-booked public tours on 16th, 17th and 19th September, with the debut European science flight taking place on the evening of 18th September.

For the first time to the public, the telescope cavity door was opened on the ground which was surprising due to the high risk of particles and debris. This again required extensive planning from the SOFIA team, as the position of the sun at the time of door opening was required to be researched and aircraft parked close to SOFIA had to be towed further away before starting their engines.

Flying on SOFIA

Mission Brief:

Having been warmly welcomed to Building 703 at Palmdale Regional Airport, I was able to take some images on the ramp before the mission briefing. The shots had to be taken in-front of the sun due to the sensitive background across the airfield – owing to the many defence companies who operate at PMD – including a rumour that the B-21 Raider is being developed at the airport.

After about 10 minutes photography, it was time for the mission briefing. Tonight’s flight would be the first in the series for the EXES instrument (EXES Flt 658 04FEB20) focusing on the Orion Constellation. The briefing started promptly at 15:45L. Led by the Mission Director, who proceeds to introduce each member of the crew and their role onboard to the guest observers, the aircraft status which included aircraft defects (such as one toilet sink is in-op for the flight). The telescope was cleared as good to go by the telescope team, but with the telescope gate valve slightly leaking. This was cleared with the EXES team as a non-issue and that observations can continue as usual. All systems are good to go.

The plan is to take off on runway 07 and initially head north east to 36,000ft. Airspace issues are not a problem this evening (the airspace in this part of California can get very busy with many airfields near each other). There is also a 12 to 53 minutes take off delay window built in for this flight plan, which allows the aircraft to maintain timings for the mission targets. If the planned take off time is running behind schedule, the flight crew can exercise a delay to the mission start which essentially cuts various waypoints off the planned route to allow SOFIA to catch up and crucially be on time for the next possible mission targets. Most of the research will be conducted over the Pacific Ocean, routing the aircraft close to Hawaii – therefore Honolulu International Airport is one of our diversion airfields once we reach the point of no return to mainland USA.

In the climb out of Palmdale, the Primary Flight Displays. © Charles Cunliffe – Global Aviation Resource.

Telescope operators and scientists at work during the mission. © Charles Cunliffe – Global Aviation Resource.

EXES scientists at work interpreting the data from the instruments. © Charles Cunliffe – Global Aviation Resource.

The aircraft is in great shape tonight; however, the weather is not. There is a SIGMET for severe turbulence to south and east of Palmdale, with various aircraft reports throughout the day echoing this. Low level waves of turbulence in the atmosphere stretch from surface level to 12,000ft until 1600L and is expected to continue. Furthermore, there is light to moderate turbulence across most of California and Nevada. At our planned take off time of 17:30L, there should only be a few clouds to 30,000ft and 12-18 knot wind and a temperature of 4c at the airport. 

As previously mentioned, a major advantage of SOFIA is the aircrafts ability to fly above 99% of the water vapour in the atmosphere. The focus on atmospheric water vapour is an important part of the mission brief. The water vapour is given a value rating, the lower the rating the less water vapour in the atmosphere. As we continue the climb throughout the mission, the water vapour rating is forecast to drop. However, at 36,000ft to 37,000ft, the vapour is pretty ‘soupy’ and but as the mission continues and we climb to 39,000ft to 43,000ft, the water vapour rating is in the low teens; yet this is still considered high for February in Palmdale.

This evening at troposphere level, there is a big ridge of high pressure with a strong Jetstream. In turn, the ridge brings a high amount of moisture and big cirrus clouds around 39,000ft to 40,000ft. As we are planned to cruise at 43,000ft for most of the mission targets, the clouds should not be an issue. The Jetstream is blowing well to the north, bringing strong winds of around 80 to 100 knots across parts of Washington state on our return into Palmdale.

During the brief by the flight crew, we learn that there are no NOTAMs or ATC issues to be concerned about tonight. The plan is to use Victorville Airport (VCV) as a landing alternate if Palmdale should be unavailable and we are planned to return with approximately 11 tonnes of fuel.  The doors are planned to be closed at 16:35L with a mini tow from the ramp to the engine start point, with engine start commencing at 16:55L to coincide with a take-off time of 1730L. We are planned to land at 03:24L in the morning, in total a 9-hour 54-minute flight.

When leaving the NASA ramp at Palmdale, the 747SP can only start the inboard engines and taxis as such until approaching the runway threshold. This is due the fact taxiway ‘Sierra’ (leading from the NASA ramp to the main airport complex), is very narrow and the outbound engines overhang the taxiway, meaning if they were running there is a higher chance of FOD ingestion. We will taxi up Sierra, cross runway 25 and make a left turn onto taxiway Bravo to hold short for runway 07 to start the outboard engines and run through the take-off checklist.

The most important part of any SOFIA mission is the science. The mission brief plan is handed to every member of crew before departure, which not only contains the flight plan but also where the mission targets are (shown as waypoints), the forecasted weather, exact co-ordinates and times for the target subject and what elevation the telescope will be pointing at during time on target.

Flight plan for the mission. © Charles Cunliffe – Global Aviation Resource.

Today’s flight is called “Karsten”, the first in the series for the K-Campaign.  Every year in the SOFIA programme is treated as a cycle starting in April 2020 marks the seventh cycle, being the 7th year since full operational capability was achieved. Each cycle is divided into 10-15 campaigns, with every instrument being mounted to the telescope on each campaign in a rotating schedule – depending on the scientific request. Like how the USA names hurricanes, the campaign names go through each letter of the alphabet in order – starting A,B,C and so on.

The science will start by focusing on a very bright star formation called Omicron Ceti to get the boresight, focus and slip rotation for the telescope set-up. These stars are a mass of evolved stars late in their life and are oxygen rich – which are dumping a lot of material into interstellar union. We will be studying the photosphere and what they are emitting. This is a continuation of a programme performed in October 2018, by using the large spectral range that SOFIA grants EXES access too. 

Then we will look at Orion IRc2 which is a very interesting and complex region, where young stars warming up cosmic dust are emitting molecules that are establishing a very molecular rich region. This is a continuation from a spectral survey that the team have been working at the shorter wavelengths with SO2 being the primary molecule of interest.

Next, we will observe IC63 which is a region where a hot star is illuminating a gas cloud, and we will be looking for hydrogen molecular emissions. Then we go to W3RS5 which is a young star and the focus here will be on hydrogen cyanide complementing ground-based work.

The final observation will be RXBoo which is an evolved star and we will be looking for carbon dioxide, which is something that EXES has only recently been finding successfully from SOFIA. We will see if it can be detected on this science flight, and the data from this will be available to the public immediately.

A member of the flight crew suddenly states the SIGMET for severe turbulence has been extended to 1945L which would lead to a 2-hour delay to the mission – way past our 12 to 53 minutes take off delay window. It is decided today’s mission is to be cancelled and re-scheduled.

The mission is fortunately rolled over 24 hours and the targets are the same as the previous night’s mission. The weather has completely changed within the 24-hour slip, as the turbulence causing the cancellation is now non-existent and we are ready to fly. The exact mission timeline is detailed below – it takes over 24 hours to prepare the aircraft and complete a SOFA mission as the aircraft flies on average four times a week, this is a fantastic testament to the teams that keep SOFIA mission ready.

Mission timeline (all times local):

Timeline for EXES Flt 658 (5 FEB 20) 

1730 Departure Time
9+54 Flight Duration
0600 Start Pre-Flights
1000 EXES Cryogen Service
1000 TA Cavity Closed
1100 TA Bearing Oil Cooling Air
1130 DOF Tag up on hangar floor
1200 Upload Overlays/Mission Systems and TA Start up
1200 Fuel Aircraft
1330 Mission Systems DOFs Complete/MOPS checkouts begin
1515 Crew Briefing
1510 TO Final Balance
1530 Start APU (Power Transfer)
1545 Mission Briefing
1545 Crew Ready
1635 Doors Closed + Mini-Tow
1655 Engine Start
1710 Taxi
1730 Take-off
0324 Landing
0700 Data Transfer

Onboard the Aircraft

After the mission brief ends, we promptly head out and board the aircraft ready for near 10-hour flight. Onboard the aircraft, we are given a safety brief by the two safety technicians who are responsible for arming the slides, passenger welfare and overall safety onboard the aircraft. A unique feature flying on SOFIA, is that whenever you are away from the seats, you must always have your EPOS (Emergency Passenger Oxygen System) with you. This is a self-contained hood with a built-in oxygen tank used in the event of fire or decompression

After the end of the safety brief, the Mission Director kindly asks me would I like a flight deck jumpseat for take-off – which I gratefully accept! Climbing the spiral staircase from the main deck to the flight deck really brings home the nostalgic feeling of flying on a classic 747. This evening’s pilots are both ex-USAF KC-135 Stratotanker crew, whilst the flight engineer is still current with the 747-400 Super Tanker Project.

The aircraft is towed slightly forward on the ramp (mini-tow manoeuvre), where we proceed to start engines 2+3 around 10 minutes behind schedule due to some last-minute paperwork. “NASA747 Heavy, ready for taxi”. We proceed to taxi up ‘Sierra’ until the Flight Commander notices the gate separating the NASA ramp to the taxiway is only slightly ajar. This required the rapid response of the ground crew to come out and physically open the gate in-order for the aircraft to taxi past. The Flight Commander radios the Mission Director to say we need to execute a 25-minute take-off delay to the mission. As this is still within our 12 to 53-minute window it is fine for the flight to continue.

We reach the holding point for runway 25 and proceed to start the outboard engines. Exactly 25 minutes delayed we are thundering down the runway into the sunset. We proceed to make a sharp right-hand turn after departure and are shortly given clearance to our initial cruising altitude of 38,000ft and a heading towards the Pacific Ocean. What surprises me the most is that the entire climb to cruise was flown entirely without autopilot. The aircraft does not have an autothrottle, the flight engineer plays a delicate balancing game with the throttle levers to make sure each engine is producing the same level of thrust throughout the flight.

Final approach back in to Palmdale. © Charles Cunliffe – Global Aviation Resource.

SOFIA on the ramp after the mission is completed. © Charles Cunliffe – Global Aviation Resource.

The ex-Frys 747SP is seen at Mojave. It is stored to provide spare parts for SOFIA. © Charles Cunliffe – Global Aviation Resource.

As we are reaching cruising level, it is time for the telescope door to be opened as per instruction from the Mission Director. There is a CDDS (Cavity Door Drive System) control panel at the flight engineers’ station which uses two switches, enable/disable and the turn knob to open the cavity door. In a few seconds the door is wide-open, and observations can commence. As briefed, there are no aerodynamic changes or buffeting from the open cavity – you really would not know it’s open unless told!

To conduct our observations, ATC grants the aircraft permission to deviate up to twenty miles off our filed flightplan. This is something the Mission Director took advantage of throughout the flight, with frequent calls of “1-degree right, left etc.” heard throughout the flight on the intercom between the flight deck and Mission Director to keep us on precise target. The Mission Director(s) (I) & (II) are the communications link between the mission team and flight deck, but also keep the mission on track with regular announcements on how much time we have left on each leg of the flightplan.

This flight has three telescope operators onboard, being the first flight in the EXES series, the team has taken this opportunity to conduct some in-flight training in case any troubleshooting is required. The telescope operators are working hard to make sure we stay ‘locked on’ to the correct target and to make sure the telescope is functioning correctly using their multitude of monitors.

During the flight I learn that the heat from engine #1 sometimes reflects infrared light on certain instruments. This makes ‘ghosts’ appear on the infrared picture from the telescope, therefore modifications had to be performed to the cavity and the secondary mirror structure to try to counteract this. The telescope is also extremely sensitive to light and can not be exposed to direct sunlight, therefore the telescope cavity door is always closed in-flight before sunrise. Also built into the flight plan is the illumination levels from the Moon on each target. Light pollution from the Moon is bad for observations, so flights conducted under a full Moon are rare.

Next to the telescope operators sit both the instrument team and data specialists who are armed with laptops analysing the data as fast as it comes. Real-time computer processing is another nice advantage of SOFIA to make sure the data being received is useful and accurate.

Towards the nose of the aircraft, next to the spiral staircase, sits the galley, with coffee making facilities, two fridges and microwave. Apart from a fridge generously stocked with water, catering on-board is strictly BYOF (bring your own food). This area also has a counter where team members share and distribute snacks owing to the great camaraderie within the Project.

Back in the flight deck and almost ready to start our descent from 45,000ft, I notice the Flight Commander wearing an oxygen mask, who then takes it off and the second pilot puts on his. This is a NASA rule that above 40,000ft the pilots must take it in turn to wear their oxygen masks in case of a rapid decompression.

With so little time remaining, the seat belt signs are switched on and we begin a gradual descent back into PMD. As with the climb, the entire descent is flown by hand. On long final, the runway lights are switched on from SOFIA itself, as PMD ATC tower is unmanned in the early hours of the morning and we touchdown nine and a half hours after getting airborne the previous evening.

The Future of the SOFIA Programme

In 2020, around 150 SOFIA science flights were scheduled to operate with a C-check planned in the autumn at Hamburg. With the COVID-19 crisis, this has effectively paused the programme and all science flights are currently suspended to ensure the safety of all staff. It is uncertain if SOFIA will travel to Christchurch again this summer, but the team remain hopeful and are investigating alternatives including Argentina, Chile, Samoa or Tahiti.

The SOFIA observatory was designed for a 20-year mission lifetime and the project team are very confident that the aircraft can operate until 2034 or even beyond with good aircraft and telescope maintenance. The project owns 12 spare Pratt and Whitney JT9D engines, a 747SP stored at Mojave Air and Spaceport – N747A ex Fry’s Electronics Inc – where the most sensitive spare parts are stored in the hangar at PMD, another stored 747SP in Arizona and if needed, another 747SP for spare parts could be purchased. Spare landing gear has become a challenge for the aircraft, especially in-regards to their overhaul facilities, but the project is well equipped with conditioned landing gear for now. SOFIA remains in contact with other SP operators worldwide, as well as Boeing, collaborating to ensure the aircraft can operate for as long as possible.

At the time of publishing, the aircraft has 80,511 hours and 10,872 cycles with a MTOW of 315,700kg and an empty weight of 172,365kg.

I would like to sincerely thank Clemens Plank (SOFIA Project Engineer, DLR) for his help in putting this article together and hospitality, Dr Dörte Mehlert (University of Stuttgart) for arranging my flight onboard SOFIA and the Stuttgart Media Day visit and the entire SOFIA Project team I met during my stay in Palmdale.