# Is this an IR or visible light image? What's illuminating the F9's grid fins?

I saw this image in Spaceflight Now's 30-Oct-2017 article SpaceX Falcon 9 successfully launches Koreasat 5A.

Pitch-black background and rocket body with extremely bright grid fins. Is this an infrared image showing how hot they are? I can't understand how they could be lit visibly from below, unless from below by the Sun perhaps?

Can someone figure out the orientation of the F9 booster and the source of the lighting for this image?

This is not Koreasat-5A, but SES-11/EchoStar 105, at about 21:11 in this video.

The first stage rocket body is nearly aligned with the sun setting behind the rocket. We're looking "down" the rocket body, toward the East. We're beginning reentry. The upper side of the rocket body and at least the base of the fins are in shadow, and the fins are glowing red hot.

The lens flare on the right-hand (second-stage) portion of the frame tells us it's late in the day and the sun is setting in the West behind the eastbound first and second stages. Koreasat-5A launched at about 2:30pm Eastern time (and nothing in its video looked like this), but the immediately previous EchoStar 105/SES-11 launched at 6:53 pm -- close to sunset (which led me to check its video).

The left side is the same camera angle we see in every broadcast; it's a color visible-light camera, and I haven't ever seen any indication of them switching to an IR camera in their webcasts.

• Ah, in the first photo the sun is roughly "over our shoulder" but slightly below, so this side of the rocket body is still shadowed. Since the grid fins are near the top, the length of the body on this side is short and it doesn't shadow the grid fins very much. OK it all makes sense now, thanks! – uhoh Nov 16 '17 at 22:35
• The geometric analysis is still mostly correct (though you can see the sun isn't directly behind, at around 20:00), but the fins are glowing hot rather than reflecting sunlight. – Russell Borogove Nov 16 '17 at 22:46
• I'm watching the changes in hue between 20:40 and 21:15 when the grid fins start to "glow". It's helpful to set the YouTube viewer to 2x speed to see the changes. Since the image is color/visible, if this is thermal radiation then it has to be really really hot! That's possible but remarkable that they would get red to white hot! Wow! – uhoh Nov 16 '17 at 23:25
• The camera auto-exposure could be blowing them out a little bit, but they switched from Al (660ºC melting point) to Ti (1668ºC) for a reason. Cherry red = 800ºC, orange = 900º, yellow 1000º, white 1200º. – Russell Borogove Nov 16 '17 at 23:38
• I see what you mean. Also Ti has a very low thermal conductivity, lower even than steel, so local areas and individual surfaces will get hot and glow quickly but conduct (heat-sink) back to the rocket body slowly. They may even be thermal insulation in the coupling to the actuators to prevent them from getting too hot and failing. This is really beautiful engineering! – uhoh Nov 16 '17 at 23:44