We have liftoff! Those three words are music to NASA ears as Artemis has faced a number of delays over the past several weeks. I think we can all understand mechanical concerns, high winds, thunderstorms and hurricanes as launch-blockers. What I didn’t realize until yesterday is that one good healthy cumulus cloud can stop a rocket launch in its tracks! A cumulus cloud? Those puffy, fair-weather innocent-looking clouds against a beautiful blue sky? The kind that don’t even produce rain? Yep. Those clouds.
How is that? Lightning! And before we get too far into this, let me explain how lightning naturally works: When cold and warm air meet inside a cloud, the warm air goes up creating thunderstorms. Ice crystals are found in the cold air while water droplets are in the warm air. During the storm, due to updrafts and downdrafts, those water droplets and ice crystals collide back and forth, like bumper cars. This produces static electricity with positive and negative charges from the top to the bottom of the cloud producing an energy release we call lightning. The key here is warm water droplets meeting cold ice crystals (or hail). Here’s a simple illustration:
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So how would a seemingly innocent cumulus cloud like the one below cause a problem for Artemis, or any other rocket?
First, the cumulus cloud has to be about a mile thick -- 4,000 to 6,000 feet -- and there must be a freezing layer of ice crystals (which is very common). Launching a hot rocket through that cold-topped cloud can actually create lightning! After all, the ice crystals are there and that rocket will supply an exhaust plume of 6,000 degrees -- that’s enough to replace those warm water droplets! So the same physics exists to create lightning and the last thing you want is lightning striking your rocket! While this is not ‘natural’ lightning as we know it, this “trigger” lightning, as it’s called, is just as dangerous.
Apollo 12
Trigger lightning was first discovered during the Apollo 12 launch and is well-documented. There had been rain in the area, but no thunderstorms were nearby for the launch, just clouds. Then just 35 seconds into launch, going through one of those innocent cumulus clouds, Apollo 12 took a lightning strike and then another at 52 seconds that knocked out much of the control system. Here’s a launchpad view:
Radio transmissions from Apollo 12 back to Houston:
000:00:56 Conrad (onboard): I just lost the platform.
Altitude, a mile and a half [nautical], now. Velocity, 1,592 feet per second.
000:01:00 Gordon: All we’ve got’s the GDC.
000:01:01 Conrad (onboard): Yes.
000:01:02 Conrad: Okay, we just lost the platform, gang. I don’t know what happened here; we had everything in the world drop out.
000:01:08 Carr: Roger.
Cmdr. Pete Conrad made the brave and quick decision to go on with the mission, repairing any problems during orbit rather than try to quick-fix the issues during launch. Aborting the mission was an option which would have sent Apollo 12 splashing quickly -- and maybe not safely -- into the Atlantic.
What changed?
Not much changed, except to make sure the following Apollo missions launched during pristine conditions! All went well for the subsequent Apollo missions. However, in 1987 an unmanned rocket was launched through clouds and that rocket, an Atlas/Centaur, triggered lightning shortly after takeoff. The strikes knocked out the onboard electronics and the rocket was destroyed in midair. Lesson learned.
So if you heard yesterday that NASA was concerned not about severe weather or hurricanes for launch, but just fair weather clouds, now you know why. You can read an excellent paper from NASA on the Apollo 12 lightning incident right here. It strikes me (pun intended) that we focus so much on the Apollo 11 mission when we landed on the moon (”One small step for man”) and the Apollo 13 mission (”Houston, we have a problem”) when tragedy almost struck, that Apollo 12 doesn’t get nearly the attention. But that one flight increased safety awareness beyond measure.
Here’s to smooth sailing and flying on all your missions!
Frank