As NASA's moon rocket ignited in darkness, it triggered an avalanche of snowy white clouds around the launch pad.

The plumes billowed, clinging to the air long after the spacecraft vanished out of sight, like a magician's smoke bomb dashed against the stage for a disappearing act.

But to call these mysterious clouds "smoke" would be a bit of a misnomer, said Nate Perkins, an engineer at Aerojet Rocketdyne, which built the propulsion system mounted to the bottom of the rocket. Most of the plumes come from the four main engines, which don't kick out soot or carbon.

They're just the result of a chemical process that occurs when liquid hydrogen and liquid oxygen heat, meet, and explode. Remember from grade-school science class what happens when you combine oxygen with a couple of hydrogen atoms?

"You get steam. It's just water — water vapor," Perkins told Mashable. "That's all the byproducts of the RS-25 engines, and that's the majority of what you see."

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The Space Launch System, often referred to these days as NASA's mega moon rocket, is the most powerful rocket ever built. Its four main engines, the same as those used by the legendary Space Shuttle, consume 700,000 gallons of super-cold propellant. Those engines, combined with two side boosters, could keep eight Boeing 747s aloft.

Liquid hydrogen has been NASA's fuel of choice for decades because it has the lowest molecular weight in existence. That's ideal for traveling to space. The heavier the load, the more thrust a spacecraft requires to escape Earth's gravity. Hydrogen also burns with extreme ferocity.

When NASA filled the rocket with fuel before launch, the liquid oxygen and hydrogen were kept separate within the tank. At the last moment, the two ingredients mixed, causing a controlled explosion that hurled the spacecraft skyward.

The clouds are similar to what people see when an airplane streaks across the sky. Here, the rocket engines dump out steam at 13 times the speed of sound — fast enough to travel from New York to L.A. in 15 minutes — which condenses and hovers in the air like fog.

But there are other engines on this rocket. The solid rocket boosters, which produce their share of water vapor, also discharge carbon monoxide, hydrogen chloride, and nitrogen, among other gases. Tiny aluminum oxide particles and hydrochloric acid are also in their plumes and can look like a white vapor, said Kendra Kastelan, a Northrop Grumman spokeswoman, in an email. Much less of the rocket's huffing and puffing comes from these side boosters, which fire for two minutes, compared to the eight-minute romp from the four main engines.

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Clouds also form from the hundreds of thousands of gallons of water evaporating from the water spray system at the launch pad. The flood is intended to suppress the extreme heat that happens during ignition and liftoff.

Water keeps flames from spreading, but it serves another important purpose: preventing damage from loud noise. Without that gush, sound waves could burst pipes, crack walls, and even break parts of the rocket.

Depending on the conditions of the atmosphere, all that condensed vapor can create its own weather. Engineers see this happen during tests of the massive engines. '

"You get all that exhaust, the steam coming out," Perkins said. "You end up getting some localized rain and mist in the area."

And when it pours, they have a name for it: rocket rain.

（作者：汽车电瓶）