Remember the scene (or the couple of scenes) in The Right Stuff when Yeager is pushing the limits (the scene at the very end of the film comes to mind): Yeager is flying up up up, through the blue, higher and higher, he starts to have trouble breathing, etc., and then – for a flicker of a moment – it is as though the blue of our atmosphere dissolves – but just for a second – and then there is the vast blackness of space.
His plane can’t take it – he plummets into a tailspin – he ejects – there is a fiery mesh – and Yeager emerges from the smoke like a renegade hero. The credits roll. The End.
Here’s my question about where space begins:
Is it like that? When does “the blue” stop? Is it a definitive line? There must be some transformation that occurs, some visible transformation when you leave our atmosphere.
Talk to me about when the blue stops and the black begins. (Sounds like a Neil Young song.)
I make no claim to being a learned science type, but I have flown across the line between daylight and darkness in a plane, and I think the transition is similar. The blue sky gets gradually deeper and darker, you start seeing more and more stars, and eventually you only see blue daylight below you on the horizon. The blackness doesn’t come all at once, the blue just fades out to black.
The demarcation point is arbirtrary. The traditional U.S. point is 50 miles; in Europe it’s 100 kilometers (the X Prize folks are using the latter).
The plane Yeager was flying at the end of The Right Stuff, BTW, was an NF-104, which was an F-104 Starfighter with a rocket engine added to it. He was at an altitude of something over 100,000 feet when the incident occurred.
I’ve always been fond of the planes in the “Century Series” (F-100 SuperSabre, F-101 Voodoo, F-102 Delta Dagger, F-104, F-105 Thunderchief, F-106 Delta Dart)…well, not so much the Voodoo.
Okay, you two.
Responses like yours make me realize, yet again, why I love blogging so much.
Thank you both!
Warning: extremely geeky answer follows:
The primary (but not the only) reason that the sky is blue is that molecules in the air, (very) small dust particles, and transiently dense bits of the atmosphere scatter light elastically. Elastic is a fancy way of saying that when the photons get scattered (bounced off an object), they leave with exactly the same energy as before the collision.
It’s a property of matter that when the scattering object is close to the wavelength of light, lower (bluer) wavelengths scatter more. So we see the sky as blue because the atmosphere scatters more blue than red. Particles larger than the wavelength of light do not cause this wavelength dependent scattering, which is why fog is white, not blue.
See here for a good explanation with pictures:
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
To answer your question, the blueness does not suddenly disappear. As the atmosphere gets thinner and thinner, the blue color fades. The exact transition depends on a lot of factors such as temperature and the amount of dust ina given area. I suspect that one reason that the demarkation appears sudden in DoD films is that the aircraft travels trhough the nether zone (where the blue is slowly fading) so quickly we don’t notice the fade.
Your exact question was “when does the blue stop?”. I’ll let the experts here (http://history.nasa.gov/SP-402/ch3.htm) answer that:
At balloon altitudes of 25 to 30 km (six times higher than mountaintop observatories) the sky in the visible region of the spectrum is nearly 100 times darker, but even there the sky is so bright that most of the outer corona cannot be seen. At an altitude of about 50 km above sea level the daytime sky is as dark as that seen on Earth during a total eclipse, which still limits practical observation of the corona to 5 or 6 solar radii. At satellite altitudes of 200 to 400 km, so little air remains that the daytime sky near the Sun is truly black offering no limit to observation of the faintest details of the white light corona.
John:
When I said the words “learned science types” what I really meant was “geeks” – but I didn’t want to alienate anyone.
A question like this demands real geekiness in determining an answer – so I thank you!
I feel like the classic 4 year old:”Why is the sky blue?”
But I understand what you just wrote:
So we see the sky as blue because the atmosphere scatters more blue than red. Particles larger than the wavelength of light do not cause this wavelength dependent scattering, which is why fog is white, not blue
LOVE THAT.
Dammit, wish I had had some science teachers who could have infused me with a bit of EXCITEMENT, because I do love it.
Glad I could communicate some love for the discipline. I spent 2 years in grad school studying “Light Scattering by Small Particles” by van de Hulst. Dry as cardboard.
The funny thing is, children have been asking that question since man could talk, and we only got a real answer in the first decade of this century.
Makes me feel bad for Cro-Magnon parents.
“Look, kid, I DON’T KNOW WHY the sky is blue. You’re gonna have to wait 30,000 years to get an answer. So pipe down and eat your drumstick.”
Please don’t fact check that.
One other factor in the fade is shortening path length as you rise in the atmosphere: there’s less atmosphere for light to travel through as you climb, so less scattering by the time the sunlight hits your eye.
Here’s a fun way to demonstrate path length effects on apparent color, although this is absorption and not scattering: get a thin container like a test tube. Put some whisky in it, then hold it next to the bottle. Same stuff, but the liquid in the test tube looks lighter than the suff in the bottle, because more light is lost travelling through the wide bottle than the thin tube. The fun part is what you do with the whisky after the experiment…
The US standard is 50 statute miles, or 264,000 feet. The European standard is 62 statute miles (100 KM) or 327,260 feet. SpaceShip One went to just over 328,000 feet to meet the stricter European standard for the X-Prize.
In comparison, airliners fly at 30,000-40,000 feet most of the time. The SR-71, one of the highest-flying aircraft, was documented to go to 85,000 feet, although some people believe it could bust 100,000. (I don’t know.) The Space Shuttle routinely flies at 644,000 feet. The X-15 Program, possibly the closest comparison to Space Ship One, maxed out at 354,000 feet.
I’m told that SR71 pilots usually saw “space” above at their operating altitude; no more blue. Similar to the Yeager scene in the movie.
The footage I saw of the world’s highest parachute jump (somebody jumped from a pressurized gondola under a yuuuuuge balloon) had a dark sky.
Holy crap, Mitch – that is quite an image. So obviously (forgive me – dumb question) these people were leaping with oxygen tanks strapped to their backs, I’m assuming?
I can’t even imagine what that must be like. How long they must fall for …
There’s an exciting scene in the third (I think) Bob Lee Swagger book (by Stephen Hunter)where Bob Lee makes a high altitude jump and is so scared he almost ruins the dive and gets hisself kilt. Keep in mind Bob Lee is a trained assasin and Vietnam marine, so scared is not his usual state.
“Out of the blue, and into the black…”
This is weird. I thought I was the only person in the world who thought it would be great if that Niel Young line was about going into space.
If my kids ever asked me why the sky was blue, I was planning on answering “It isn’t. The sky is black. The Earth’s air is blue. Other planets have different-colored air. The sky behind the air is black.”
Unfortunately (or fortunately?) they never asked.
For cavemen, it really must have been strange that the sky was blue. Why was THAT color so favored?
For some reason, I always think it less weird for the universe to have made the sky BLACK. For the color BLACK to be so favored is not strange.
The layers of the atmosphere are compressed down similar to a bunch of matresses pressed down on one another at the bottom, they are condensed by the weight. The higher you are from the surface less light is refracted from the air molecules. Ive seen pictures of the sky at various altitudes from experimental balloon websites and the sky seems blue until around 60,000ft or around 12 miles high when it starts to really become dark blue/black.