How a coffee can sized weather instrument kept SpaceX on the ground

Everything but the weather was ready Wednesday afternoon (May 27) for SpaceX to the maiden crewed flight of its Dragon capsule. The countdown was stopped with less than 17 minutes due when it became clear that weather concerns would not clear in time for the launch.

There was too much electricity in the atmosphere raising concerns about lightning. Central Florida is also known as "Lightning Alley”, the area of highest lightning activity in the U.S.

While there was no lightning in the immediate area as the most recent storm had passed sufficiently out of the area and cells moving eastward from Orlando had dissipated as expected, the rocket itself posed a lighting risk in the charged environment.

As ice crystals high within those thunderstorm clouds flow up and down in the turbulent air, they bump into each other, building up static electricity. Electricity looks for the easiest path. This sometimes comes in the form of cloud to cloud lightning and sometimes cloud to ground.

But an ascending rocket can act like a lighting rod, providing that path to the ground.

An ascending rocket can compresses an already strong electrical field to the point where it breaks down. This triggers lighting down the rocket, and down the electrically conductive exhaust plume.

Flights with astronauts aboard like this one have more stringent criteria not just because of the irreplaceable cargo, but also because aboard sceneries must be considered as well. More than 50 points across the Atlantic stretching from Florida to Northern Ireland are monitored for things like wave height to ensure a safe place to splash down should it be necessary.

Cape Canaveral is dotted with weather instruments at various heights. These measure what you’d might expect like temperature, wind, and humidity. But a series of more than 30 electric field mills measure that built up energy.

Each field mill is about the size of a coffee can and is mounted inverted on a sturdy tripod to ensure it continues to operate in heavy rain. Inside the cylinder, a four bladed shield rotates like the arms of a windmill, alternately exposing and coving sensors at the bottom. The network of field mills output readings in volts per meter.

1500 volts per meter is acceptable for Falcon 9 launches within 5 nautical miles. On Wednesday at least one field mill was showing greater than 2000.

While the threat of rocket generated lighting passed about 10 minutes after the scheduled launch time, launch windows for the Falcon 9 are "instantaneous" essentially a second wide, leaving no room for delays, weather or otherwise.

NASA and SpaceX will try again on Saturday for a launch at 3:22:45 p.m. EDT. An additional launch opportunity is availably at 3:00:11 p.m. Sunday. Weather is looking questionable both days however with the United States Air Force 45th Weather Squadron putting chances of favorable conditions only at 40% in their L-2 (2 days before launch) forecast.

We can blame orbital mechanics for having to wait until Saturday. The launch needs to occur when the station is in a favorable place, not long after it has passed over central Florida, for the capsule to catch up to the station.

According to the Kennedy Space Center press office, additional days beyond this weekend have not yet been selected by SpaceX and NASA. But some back of the envelope suggests that the station will in that favorable position mid next week.

As NASA Administrator Jim Bridenstine said after the scrub: "if we are not ready to go, we simply do not go. I'm proud, so proud that we made the right decision." He added "This was an important milestone just today, we did a wet dress rehearsal in full gear. We learn a lot every time we do these things."

Weather was much more cooperative on Saturday enabling. Field mill data from the two launch attempts shows conditions which might cause the rocket to generate lightning cleared well before the lunch. On Wednesday, these conditions cleared only briefly, after the available launch window.

(Visualization: Tony Rice, data: Kennedy Space Center Spaceport Weather Archive)