In mid-January 2026, NASA’s massive Artemis II Space Launch System rocket — stacked with its Orion crew capsule and sitting atop a towering mobile launcher — began a slow trip from the huge Vehicle Assembly Building to Launch Pad 39B at Kennedy Space Center. The crawl took nearly 12 hours, covered about 4 miles, and marked a major milestone as humanity prepares to send astronauts around the Moon for the first time in decades.

You probably saw videos or photos of the Artemis rocket being rolled out to the launch site. If you looked closely, you probably noticed that it wasn’t just the rocket sitting on that enormous crawler. It was carrying a massive launch platform and service structure as well. You might have wondered why they do it that way?

Rocket Science Starts with a Crawl

What if I told you that one of the most important steps in getting a rocket off the ground happens at about the speed you walk?

From the outside, a giant rocket moving at walking pace might seem anticlimactic. But up close? It’s a mind-boggling feat of engineering — and it says a lot about what rocket science really is. That slow crawl is where history, physics, and countless small engineering decisions all come together.

Why on earth do they assemble all of that together, miles away from the launch pad? And how did anyone ever decide that this was a good idea?

Let me tell you a little bit of the history — and a tiny bit of rocket science.

Rocket science isn’t just equations like force equals mass times acceleration, or velocity equals acceleration times time, or height equals one-half acceleration times time squared. That’s basic physics. Rocket science is what happens when thousands of little problems, constraints, and practical solutions all come together into one working system.

Nearly a century ago, when rocketry was still in its infancy, many of these problems had to be solved for the first time. Liquid-fueled rockets, for example, require both fuel — such as kerosene — and an oxidizer, usually liquid oxygen. You must load the correct amount of each into the rocket’s tanks. Any extra fuel or oxidizer is simply wasted weight, and weight was critical then just as it is now.

At a gas station, we measure fuel with a pump and don’t worry much about temperature. When it’s cold, you get slightly more fuel per gallon than when it’s hot — but it doesn’t matter much. In rocketry, it matters a great deal. Liquid oxygen is extremely cold and boils easily. Even if you know exactly how much you pumped into a tank, some of it will evaporate. So how do you know how much propellant you really have when loading is complete?

One reliable way is to weigh the entire rocket. You weigh it before fueling, then again after loading the fuel and again after loading the oxidizer. The difference tells you exactly how much propellant is onboard.

That sounds simple — until you realize that this requires a very large “bathroom scale” under the rocket.
But there are problems, a rocket can’t just sit loosely on a scale. It must be held firmly in place so it doesn’t tip over as engines ignite and thrust builds up, especially if there’s even a slight imbalance. That means a significant portion of the launch structure — the part that holds the rocket upright — must also sit on the scale.

Early American rocket launches took place in Florida, right next to the Atlantic Ocean. Assembly and testing takes time, and Florida weather can change quickly. Squalls can roll in with little warning, and the salty coastal air is excellent at corroding metal. Anyone who has lived near the ocean knows how quickly salt can attack exposed equipment.

The solution was to assemble and check out rockets indoors, under controlled conditions. But if part of the launch structure had to be on the scale with the rocket, then that structure needed to be assembled indoors as well. And once everything was stacked together — the rocket, the platform, and the hold-down equipment — the simplest solution was to move the entire assembly as a single unit to the launch site.

That’s why Artemis — and many rockets before it — rolls out to the pad on that crawler carrying so much more than just the rocket itself.

Just one small example of what “rocket science” really means.

All images here from NASA.

.:. © 2026 Ludwig Keck