Wednesday, around 5pm the 747 with SOFIA took off for the U.S. again. On board, Nasa test pilot Troy Asher. A couple of hours ago he had sent me an e-mail regardings some questions on flying the Stratospheric Observatory for infrared Astronomy (SOFIA).

Troy, what is different to fly the 747SP/SOFIA platform from flying a normal jumbo jet?

It is very much the same as flying a normal 747 or other very large jet with a few small exceptions. First of all, having this large 4m x 4.6m door open on the back of the airplane is hardly noticeable. Most people on the airplane cannot tell at all when the door is open or closed. The aerodynamics engineers who designed the door and the surrounding opening did a fantastic job.

Do you have to perform special flight maneuvers during measurement? When the 'door' is open?

The main difference between flying SOFIA and a normal jumbo jet is in the way we fly it during an astronomical observation. The flight maneuvers are not difficult, just very exacting and sometimes even monotonous.

We are very sensitive about the weather. Because the 2.5m mirror is a highly sensitive, one-of-a-kind object, we treat it with extreme care. If there is blowing dust, any kind of rain or other contaminants in the air, we do not open the door and in some instances, we won't even takeoff. We cannot fly in clouds with the door open and we try to avoid anything greater than light turbulence. Also, because the mirror is such a great reflector of focused light, we avoid flight during the daytime. We always plan to wait until sunset to open the door and always plan to be on the ground before sunrise, just in case the telescope door might not close when we ask it to. Direct sunlight on the mirror can create light so focused that it will damage other sensitive components of the telescope assembly or possibly even the aircraft structure. So it's a night-time job.

The telescope is pointed in large part by the attitude of the aircraft. In order to keep the telescope locked on to a far away object, we must maintain our heading to within 1-degree and keep our bank angles to no more than 2-degrees. Because it requires a curved flight path to stay locked on to a distant object, we are continuously making 1-degree heading changes by using very small bank angles...for 10 hours at a time. Also, airspeed changes must be made very smoothly as changing power settings on the engines can cause the nose of the aircraft to rise or fall which also interferes with telescope pointing. Don't make any sudden moves as a SOFIA pilot!

Because there tends to be less water vapor in the air at higher altitudes, we generally fly as high as the airplane will physically go. For a normal jumbo jet, you would fly at the most efficient altitude for the weight of the airplane, but not with SOFIA -- we fly as high as the engines will take us at any given time. This requires very precise airspeed control (which is 0.85 Mach number) because we have a 0.87 Mach upper speed limit, to reduce fatigue on the telescope door, and a lower airspeed of about 0.82 Mach, slower than which we have difficulty maintaining altitude. So the operating conditions for the aircraft lie in a very narrow band that has to be constantly monitored.

How does the crew schedule flights?

Planning for one of our missions is a very involved process. Our mission planners use a specialized computer system to calculate where the telescope would need to be pointed to observe a desired object. They work backwards taking into account orbital dynamics, winds, temperatures, and many other factors to calculate exactly where over the Earth's surface and at precisely what time the aircraft needs to fly so we can find the object we are looking for in the telescope. Their plan is continuously updated for changing weather conditions right up until takeoff time. This means that we never really know where the airplane is going to fly until just before takeoff. This makes coordinating flight plans with Air Traffic Control challenging and makes for some hectic times right before takeoff due to last minute changes.

Then, once we are flying, we must control our timing such that we arrive at the point where we will begin astronomy at exactly the calculated time. Once locked on, the aircraft must maintain precise headings and if the winds turn out to be different than we planned, the airplane does not travel to the points we originally planned. This creates other challenges with Air Traffic Control as well as other airplanes flying in our vicinity. Luckily, we always fly in the middle of the night, so Air Traffic Control is fairly kind to us due to lighter air traffic. That's one good thing about working the graveyard shift.