Wow, it’s been better than two years since the last ‘Too Cool’ post – at least, the last with that in the title, though as a category, there’s definitely been a few since then. This one comes courtesy of Astronomy Picture of the Day, though I’m a little lax, since this was yesterday’s.
Taken by John Winkopp for WAI Media, this shows the SpaceX Falcon 9 rocket passing in front of the sun during launch, shot through a solar filter – going to that APOD link and clicking on the image will give you a higher resolution version. Excellent positioning here, since lining this up is a lot harder than you might imagine. The multiple shock waves coming off of the rocket are plainly visible, as well as the distortion of the propellant gases, and some sunspots for seasoning.
There are multiple shock waves from the rocket because each increase in width/cross section forms a compression zone in the air, which at these speeds creates an expanding interference cone that we typically register as a ‘sonic boom,’ though they’re only tangentially related. More, this is closer to being an example of Schlieren imaging, a method of visualizing air compression, often used in evaluating aerodynamics. Curiously, there appears to be another on the opposite side of the sun, curving away from the exhaust turbulence, though this doesn’t make immediate sense to me.
Another curious detail: the exhaust right near the rocket, which would be a pillar of flame in normal viewing conditions, is just a silhouette here – it can’t hold a candle (ah ha ha) to the light of the sun.
I toyed with Schlieren imaging over twenty years ago, and actually achieved a trivial result with it using the heat distortion from a candle flame, seen at right, but have never tackled it since. This guide will give you the details you need to know to try it yourself, which really isn’t all that hard – the key part is a smaller parabolic mirror like that from a telescope (and then, lot’s of fiddling.) I may have to try this again someday…