I might as well dive into the other argument, too. Kármán Line vs. 50 miles up. Where does space begin?
Well let’s start by saying the Kármán line is based on a silly thought experiment so it provides no real basis to define the limit of space. It means nothing and is silly.
2/ The Kármán altitude is based on how fast a vehicle has to travel to produce enough aerodynamic lift on its wings to support its weight. It depends on how thin the atmosphere is. The higher you go, the thinner the air, so the faster you must move to use wings to stay up. But...
3/ If you go fast enough, then ignoring atmospheric drag you would not need wings at all because you’d be in orbit. So the height where the required speed to use wings is the same as the speed if there were no air — that is the Kármán altitude. But.../
4/ ...to calculate that you have to assume a particular vehicle mass and a wing size and shape. The Kármán line depends on the vehicle. It isn’t a limit in actual physics. And also nobody actually uses the Kármán line for anything because...
5/ ...nobody says, gee, we will be flying at that limit, so let’s design our wings to fall off in case the atmosphere suddenly vanishes then we will be in orbit. It is a thought experiment that has no basis in reality.
6/ it isn’t even an accurate number. Kármán proposed rounding it up to 100 km since it’s just a number and any number is as good as another. So they rounded it up.
Meanwhile, the USAF needed to know when to award astronaut wings to pilots so they picked a round number 50 miles.
7/ The rest of the world didn’t go along with 50 miles since they don’t use English units. So they went with the approximately similar value of 100 km, because that was a round number in the system that *they* were using. Neither number means anything. They’re both arbitrary.
8/ Classification systems are always theory-laden. The USAF theory was “we want a number that is high but not too high so the club is exclusive.” The Euro theory was the same, but with the added thought “Kármán’s thought experiment sounds sciencey.” Not deep theory for either.
9/ We often have multiple classification systems. “Fruit” in everyday life is something different than “fruit” in biology. We don’t insist on using the same classification in different spheres of life.
So the question is, in the sphere of space tourism how do we define “space”?
10/ Since classification is always theory laden, we ask what theory underlies space tourism?
IMO, the only relevant theory is about customer experience. So “space” begins where you can see stars in the daytime and feel zero g long enough to blow your mind.
• • •
Missing some Tweet in this thread? You can try to
force a refresh
A paper just came out analyzing rocket exhaust blowing lunar soil. It is important for at least 2 reasons. 1/n
Reference: Chinnapan et al., "Modeling of dusty gas flows due to plume impingement on a lunar surface," Physics of Fluids 33, 053307 (2021) aip.scitation.org/doi/full/10.10….
2/n First, there is great uncertainty in how *fast* the lunar dust goes. It is hard to model rocket exhaust physics on the Moon because fluid flow equations break down as the gas spreads into vacuum. The relevant equation is the Navier-Stokes equation. (screenshot from Wikipedia)
3/n In that equation, the constant μ is gas viscosity. It is not really a fundamental thing in nature. It was invented by averaging lots of molecules bouncing in a small volume of space. It tells us how much the momentum from one volume diffuses into nearby volumes.
“It used to be there weren’t many people bold enough to cross the ocean, but now it’s so easy even fearful and faint-hearted people can cross it. So maybe someone will invent a way to travel to the Moon, even though it seems like such a terrible voyage across vast, empty sky! /2
[I’m translating a mix of two versions of his book to get his entire thought.]
“And without a doubt there will be bold people to take that venture, just like crossing the sea!
You might be thinking we have no way to sail into the sky, unless the poet’s fantasies were true.
/3
What I find cool about successfully hearing this sound isn't just the low density of the atmosphere, but also the fact that the atmosphere is almost pure carbon dioxide. CO2 is unusual among gases for having very high bulk viscosity, which vastly reduces sound propagation.
2/ Viscosity tells how momentum diffuses through a gas. Honey has high shear viscosity, meaning if flows slowly. Water has low shear viscosity, so it flows easily. In addition to shear viscosity, there is the lesser-known "bulk viscosity" related to the rotation of molecules.
3/ In most gases, the bulk viscosity is ~ zero, but in carbon dioxide the bulk viscosity κ is 1000 times greater than the shear viscosity η. The attenuation of sound is described by this equation. So (order of magnitude) sound attenuates about 1000x more on Mars than on Earth.
From the talk I gave at the ASCE Earth & Space conference today. When you land on the Moon, your rocket exhaust is faster than lunar escape velocity and there is no atmosphere to slow down the dust you blow. We need to worry about damaging things in orbit.
Short thread... /1
We've done a lot of experimental work to understand how much lunar soil will blow because of the rocket exhaust during a landing. The work included reduced gravity flights measuring soil erosion in lunar gravity. /2
As you would expect, erosion rate is faster when gravity is lower. That part of the physics is easily understood, at least. Erosion of soil on another planet scales as 1-over-gravity. /3
Since this Guardian writer has recycled the super-uninformed claim "we ought to be spending the money on Earth instead," it is now time to recycle the informed responses. (1) Hardly *any* money is spent on space. E.g., the US spends 4X as much on tobacco as on space. 1/n
2/n US consumers spend 5X more on credit card interest and fees as we spend on NASA. Every year the US spends $1.77 Trillion on retail food, and about 30-40% is thrown away, which is equal to 31X the amount we spend on NASA.
3/n Every year the US spends 12.5X as much on alcoholic drinks as we do on NASA, and 77% of that is for binge drinking. In other words, the US spends 10X as much on binge drinking as we do on NASA. (NASA makes you feel better in the morning, by the way.) cdc.gov/alcohol/featur…
😟😟😟 This chart is SO untrue, though, as the published records show. Asteroids were NOT removed by scientists from being planets until the 1950s. Moons were classified by astronomers as planets until the 1920s. I’m so sad this nonsense chart is being published because... 1/
2/...it is going to amplify the presentism fallacy that has plagued astronomers over the past few decades. Presentism is when you take a view that developed in recent times and use it to interpret past events as if that view had existed back then. voicesandimages.com/presentism-don…
3/ The idea that planets only include the primaries (no moons) and doesn’t include minor planets (no asteroids) is a relatively recent view among astronomers. But the public has held that view since the mid-1800s because it came from 1800s astrology, not from scientists.