Another meeting of the Acme Pizza and Science Society, at our usual big round table in Pizza Eddie’s place on the Acme Building’s second floor. (The table’s also used for after‑hours practical studies of applied statistics, “only don’t tell nobody, okay?“) It’s Eddie’s turn to announce the topic for the evening. “This one’s from my nephew, guys. How high up is the sky on Mars?”

General silence ensues, then Al throws in a chip. “Well, how high up is the sky on Earth?”

Being a pilot, Vinnie’s our aviation expert. “Depends on who’s defining ‘sky‘ and why they did that. I’m thinking ‘the sky’s the limit‘ and for me that’s the highest altitude I can get up to legal‑like. Private prop planes generally stay below 10,000 feet, commercial jets aren’t certified above 43,000 feet, private jets aren’t supposed to go above 51,000 feet.”

Eddie counters. “How about the Concorde? And those military high-flyers?”

“They’re special. The SST has, um, had unique engineering to let it go up to 60,000 feet ’cause they didn’t want sonic boom complaints from ground level. But it don’t fly no more anyhow. I’ve heard that the Air Force’s SR-71 could hit 85,000 feet but it got retired, too.”

Al’s not impressed. “All that’s legal stuff. There’s a helicopter flying on Mars but the FAA don’t make the rules there. What else we got?”

Geologist Kareem swallows his last bite of cheese melt. “How about the top of the troposphere? That’s the lowest layer of our atmosphere, the one where most of our weather and sunset colors happen. If you look at clouds in the sky, they’re inside the troposphere.”

“How high is that?”

“It expands with heating, so the top depends where you’re measuring. At the Equator it can be as high as 18½ kilometers; near a pole in local winter the top squeezes down to 6 kilometers or so. And to your next question — above the troposphere we’ve got the stratosphere that goes up to 50 kilometers. What’s that in feet, Sy?”

<drawing Old Reliable and screen-tapping…> “Says about 31.2 miles or 165,000 feet. Let’s keep things in kilometers from here on, okay?”

“Then you’ve got the mesosphere and the exosphere but the light scattering that gives us a blue sky happens below them so I’d say the sky stops at 50 kilometers.”

Al’s been rummaging through his astronomy magazines. “I read somewhere here that you’re not an astronaut unless you’ve gone past either 80 or 100 kilometers, which is weird with two cut‑offs. Who came up with those?”

Vinnie’s back in. “Who came up with the idea was a guy named von Kármán. One of the many Hungarians who came to the US in the 30s to get away from the Nazis. He did a bunch of advanced aircraft design work, helped found Aerojet and JPL. Anyway, he said the boundary between aeronautics and astronautics is how high you are when the atmosphere gets too thin for wings to keep you up with aerodynamic lift. Beyond that you need rockets or you’re in orbit or you fall down. He had equations and everything. For the Bell X‑2 he figured the threshold was around 52 miles up. What’s that in kilometers, Sy?”

“About 84.”

“So that’s where the 80 comes from. NASA liked that number for their astronauts but the Europeans rounded it up to 100. Politics, I suppose. Do von Kármán’s equations apply to Mars as well as Earth?”

“Now we’re getting somewhere, Vinnie. They do, sort of. It’s complicated, because there’s a four‑way tug‑of‑war going on. Your aircraft has gravity pulling you down, lift and centrifugal force pulling you up. Lift depends on the atmosphere’s density and your vehicle’s configuration. The fourth player is the kicker — frictional heat ruining the craft. Lift, centrifugal force and heating all get stronger with speed. Von Kármán based his calculations on the Bell X‑2’s configuration and heat‑management capabilities. Problem is, we’re not sending an X‑2 to Mars.”

“Can you re‑calibrate his equation to put a virtual X‑2 up there?”

“Hey, guys, I think someone did that. This magazine says the Karman line on Mars is 88 kilometers up.”

“Go tell your nephew, Eddie.”

~~ Rich Olcott