This one was fun for me — it’s esoteric and nerdy. — Dan

Pictured above is the Space Shuttle Discovery, on its way to the International Space Station on October 23, 2007. As launches go, there’s nothing particularly notable about the above — many other launch photos look generally similar. As a result, nothing looks amiss, and that’s good, because nothing is. The launch went off fine and the mission was a success.

But there is something odd about the photo: everything — except the large missile-shaped object in the middle of the spacecraft — is white. That’s the external fuel tank — it is, basically, a big tank of rocket fuel, the contents of which are needed solely to get the shuttle into space. There’s nothing else in there. And, as you can see, it’s orange — a color you rarely see in consumer products, let alone massive, very visible scientific expeditions. And in fact, in the early space missions — such as Apollo 12, the launch of which can be seen below, that same part was white. What gives?

Well, science, of course.

Getting stuff into space is hard — it takes a lot of math, and a lot of fuel. And the bigger the thing you want to send into space, the more fuel you use. As Neil deGrasse Tyson points out, humans, their food, their equipment, etc. — that takes up only a tiny, tiny part of the entire contraption seen above; the rest is rocket fuel. The more the spacecraft carries upward, the more fuel you need. And as deGrasse further points out, the fuel itself needs to be propelled upward, so the more fuel you have, the more additional fuel you need to launch successfully. Which requires even more fuel, and so on. It scales exponentially, which is bad if you’re trying to save on fuel costs.

So NASA and other space agencies are very good about cutting mass wherever possible — the less the whole thing weighs, the less fuel you need. And the orange tank is orange for exactly that reason.

Rocket fuel isn’t the same as the gas we put in our cars. Per NASA (pdf), it’s composed of “liquid hydrogen fuel at minus 423 degrees Fahrenheit and the liquid oxygen tank at near minus 297 degrees Fahrenheit,” which is to say, very cold stuff. The world outside that tank is obviously much warmer, and the tank can be filled and idle, sitting in a big warehouse in Florida (typically the Vehicle Assembly Building) for months. So the external fuel tank needs to be insulated to keep the stuff inside cold. The spray-on insulation used by NASA is orange — and visible in the top photo.

That wasn’t always the case, though. Well, the color of the insulation has always been orange — that hasn’t changed. But the earlier days of the space program, NASA was concerned about the damage that ultraviolet light would do to the shuttle and its fuel tanks. To protect against this damage, the entire ship was painted white — including the external fuel tank.

After the first few Apollo missions, though, a NASA engineer named Farouk Huneidi revisited the decision. He demonstrated that the white paint wasn’t necessary to protect the external fuel cell. But it did come at a cost — the paint job was so extensive, the paint alone weighed roughly 600 pounds (270 kg). In an environment where every ounce mattered, saving that many was — literally — a huge relief.

Due to Huneidi’s work, the external fuel tank remained orange for decades, throughout the lifespan of the space shuttle program. And its legacy continues today; in the recent Artemis II launch, most of the fuel tank remained orange.

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Today’s Bonus fact: Okay, this is about painted oranges — there’s not a lot of trivia about orange-colored paint! — but close enough? In 1996, Michel Vaujour was in a Paris prison, serving his sentence for attempted murder and armed robbery, when he made a daring escape. Per CNN, he “forced his way onto the prison’s roof by wielding nectarines that were painted to look like grenades,” and then his wife — who had spent months taking flying lessons — picked him up in a helicopter she had learned to pilot. He was ultimately recaptured; there are no reports as to what was done with the painted fruit.

From the Archives: Orange Balls of Paint: Throw them at bad guys and the bad guys get marked.

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And thanks! — Dan