[ INTRO ] In 1997, NASA bought a Boeing 747SP for what might be both a super cool and super absurd purpose.

Working with the German Aerospace Center, they converted it into a mobile, flying observatory, with a giant hole in the back for an infrared telescope to look out at the universe. This was the Stratospheric Observatory For Infrared Astronomy, or SOFIA for short.

After over a decade of modifications, its science mission ran from 2010 to 2022. And the transformation it went through to get there was a bit of an engineering marvel, with engineers solving one seemingly impossible problem after another. In many ways, planes are the perfect middle ground between telescope s on Earth and telescopes in space.

They fly above the weather and above a good chunk of Earth’s atmosphere, which not only distorts the light coming from space… it can completely block certain wavelengths. While some short infrared wavelengths can make it through, ground-based telescopes are almost useless for studying medium or long infrared wavelengths. And one of the main culprits is water vapor.

But if you can get up to an attitude of 14 kilometers, you’re above about 99% of those pesky molecules. So an infrared telescope on a plane can see almost as well as it could from space, but without all the trouble and expense of getting it into space. Another benefit is it can come home at the end of every expedition.

Some space telescopes are so far away they’re basically impossible to repair or upgrade. Plus, planes can fly just about anywhere. They aren’t fixed in place or stuck in a specific orbit.

So if you want to study something that can only be seen from a certain spot on Earth, and only lasts a short amount of time, you can take your telescope wherever it needs to go. But despite these advantages, an airplane isn’t exactly an ideal environment for a telescope. The 747SP is a shorter version of the famous 747, which means it can stay in the air for longer, and a longer flight means more time to collect data.

But this kind of plane was built to carry people, not a 2.7-meter, 17,000-kilogram telescope sitting in the back. So engineers had to reinforce the plane’s fuselage to handle having a giant hole carved into one side… and then completely rewire it so that no wires passed through that hole. But they couldn’t just reinforce everywhere equally, because SOFIA would need the largest door ever built into the side of a working plane.

As that door opened mid-flight, it would change how the rest of the plane was balanced, and the reinforcement had to be stable no matter the door’s position. Meanwhile, that mid-flight hole came with its own suite of challenges… …Like the threat of exposing the crew, the scientists, and their delicate instruments to low-pressure, negative 40-degree air whipping past the plane at 80% the speed of sound. The solution was to build a 74-centimeter thick bulkhead, to separate the telescope from the rest of the plane.

But the bulkhead couldn’t be a solid piece of metal. If it were, light couldn’t get from the telescope on one side to the instruments on the other. So on top of helping SOFIA handle huge swings in temperature and pressure, the bulkhead had a tube running down the center that let light through, but not let air in or out.

And what about turbulence? Planes vibrate a lot even during smooth flights. A giant hole in the side of the plane would make things even worse.

Since telescopes have to stare at their targets for seconds on end, if not longer, a bumpy plane ride would turn everything SOFIA studied into a smudge. Which would completely defeat the purpose. One way engineers tackled that problem was by making the plane a little wider right before the door.

It looked a little funny with the door closed, but air followed the bump almost like a ramp: It glided past the opening instead of diving into it, keeping the air around the plane as calm as possible. On top of that, the tube that let light pass through the bulkhead housed equipment that let the telescope rest on a cushion of air, separating it from the vibrating fuselage around it. So SOFIA’s telescope was nice and cushioned.

And it was also kept balanced by adding or removing steel plates in particular spots depending on the instruments being used. But the challenges kept piling up. During each flight, the telescope had to constantly adjust where it was looking both as the plane flew and Earth turned, and as it switched from one target to the next.

All of those changes and corrections were carefully controlled by a system of gyroscopes, motors, and computers. And of course the plane’s navigator had to make sure SOFIA was following the trajectory that had been meticulously planned out. Those flight plans also included the headache of making sure each target was on the left side of the plane, where the telescope could see it, without accidentally taking SOFIA anywhere it wasn’t allowed to fly.

Unfortunately, all of that innovation made SOFIA unexpectedly expensive to build. And it was also pretty expensive to run. Between fuel, maintenance, crew salaries, and various other expenses, it cost about 83 million dollars per year.

In the past, NASA’s project leaders decided that that money was well spent on a telescope that can do what no others could at the time. But in April 2022, they changed their minds. Six months later, SOFIA would be decommissioned .

Originally proposed as a twenty year mission, the telescope operated with its full complement of instruments for only eight years. SOFIA may have taken its final journey, but it leaves behind an amazing scientific legacy… including the incredible engineering that went into making it. Since SOFIA was too expensive, here’s a much cheaper alternative: This month, you can get a Messenger pin through the SciShow Space Pin of the Month Club.

Messenger will only be available through October, so anyone who wants this pin should get it before November. When November comes, we’ll have a whole new pin to share. You can reserve your Messenger pin at DFTBA.com/SciShow.

Your support through the Pin of the Month Club keeps SciShow Space up and running, rather than going the way of SOFIA. Thank you! [ OUTRO ]