Slashdot Mirror
Atlas V's Sonic Boom Made Visible By Sundog
Ross-Shire Geek writes "Atlas V lifted off on Feb 11 from Kennedy. As it goes supersonic through a sundog (aka parhelion) you can see (video link) wonderful visible ripples of the shock wave in the sky."
To skip the first ~2minutes and cut to the... ripples: http://www.youtube.com/watch?v=SsDEfu8s1Lw#t=1m51s.
And for even more karma whoring: "A sun dog is a prismatic bright spot in the sky caused by sun shining through ice crystals. The Atlas V rocket exceeded the speed of sound in this layer of ice crystals, making the shock wave visible from the ground."
So I guess the normal compression wave by a sonic boom is not enough to alter the way light goes through it (think flickering air when looking across a heated highway), but these ice crystals do the trick. Right?
Closer to the pad, and less shaky:
http://www.youtube.com/watch?v=q9S0z1ofcIc
(it has the voiceover from NASA TV, but doesn't have the launch clock visible ... it might've been a camera angle that they didn't use live, as I don't remember seeing this on TV)
Build it, and they will come^Hplain.
Those look like regular acoustic waves to me.
I don't doubt the the rocket can go faster than the speed of sound (which gets lower as you get further from the surface), but those waves distinctly lead the rocket's motion, which means that they are the product of acoustic perturbations moving at the speed of sound in the medium.
If the rocket were moving faster than the speed of sound in that medium, then we would see the usual 'shock cone', where those waves would appear an a fairly narrow cone around the rocket as it passed though - certainly not before.
I qualify 'medium' since it is possible that the rocket is moving faster than the speed of sound (in the air) but not faster than the medium that the sundog constitutes. Liquids, for example, have much higher speed-of-sounds and it is (conceptually, not physically) fairly simple for something to be moving faster than the speed of sound in air at sea level but not be anywhere close to the speed of sound in a liquid that it is travelling - and thus producing the regular u-c, u+c acoustic waves.
However, as I understand them, sundogs are collections of ice crystals and probably don't have a higher speed of sound than the air around them. But anyway, waves preceding the motion of a body in a medium are certainly not shockwaves - if we could visualize the waves any object makes in the air, you would see acoustic waves arising from the object's motion before and after it.
Still neat-looking, though.
No, the compression wave always alters the way light travels through it. It's just that normally there's no light going straight through the wave to you, or if there is it's uniform in color and brightness (i.e. blue sky) so altering the direction of the light slightly doesn't produce a visible change.
My guess would be that the sundog by its very nature means sunlight is at the proper angle at that location to be reflected back to you. A compression wave at this location alters the angle of sunlight being reflected off ice crystals . So the large variability in brightness as a function of small changes in angle makes the ripples visible. Kinda like if someone were trying to signal you with a mirror. If you're seeing blue sky reflected in the mirror, shaking the mirror still yields blue sky so you don't notice any change. But if you're seeing the sun reflected in the mirror, shaking the mirror makes it alternately reflect sunlight and blue sky, causing a strobing effect which is easily visible.
Some debate here as to whether what we're seeing is a sonic boom, or just loud low-frequency sound waves. Let's do the math...
Basic question: is the rocket going at Mach 1 or greater when the phenomenon happens?
In the video, the launch happens at 0:38, and the ripples are seen at 1:53, 75 seconds later.
Here's a handy document showing the launch profile of an Atlas V. It doesn't show velocity vs time, but on page 19 there's an acceleration vs time graph for the Atlas V 401, the specific vehicle used in this launch. It shows the average thrust during the first 75 seconds is 1.4 +/- .05 g's (uncertain because I can't read the graph that accurately.)
Subtract out 1 g for gravity pulling the rocket down, to get a vehicle acceleration of 0.4 +/- 0.05 g, which over 75 seconds will lead to a final velocity of 294 +/- 36 m/s.
The speed of sound is 330 m/s. So at the time we see the ripples, the rocket is riiiiight about at the speed of sound, maybe a little over, maybe a little under, impossible to tell.
This transition to supersonic flow is often chaotic and irregular, which would explain the intense but complicated ripples seen. If the rocket was going at mach 2 or 3, we'd see a perfectly shaped set of concentric rings; if it was going at far less than mach 1, we'd see nothing at all.