Trevor Mahlmann is one of my favorite photographers. He does lots of different work, but is maybe most famous for his shots of stuff transiting in front of the sun or the moon. There are lots of great rocket shots, and in 2017 he and Destin Sandlin captured images of the international space station passing in front of a total solar eclipse. This particular photo, of the SpaceX IM-1 mission launch in February of 2024, is both strikingly beautiful and contains a wealth of interesting fluid flow. The bright orange streak lines (not the stars, the other ones) show the light emanating from the rocket exhaust shortly after it is expelled from the rocket; it’s hot, concentrated, and moving very fast. These lines trace out the actual rocket’s path through the sky. There’s the main one, coming in from the bottom of the image, as well as another which kind of curves out and away from the main flight path. This secondary path is that of the first stage of the Falcon 9 rocket during its boost-back maneuver. Because Trevor was capturing light for this image before, during, and after stage separation as well as during the boost-back burn, we get a glimpse at what the vehicle exhaust is doing long after it has left the rocket nozzles. I suspect that the sweeping, spreading exhaust plum is cause mostly by the first stage starting right after stage separation. The rocket uses reaction control thrusters to reorient itself in space, and those thrusters are exhausting a different gas than the main engines, and in directions largely orthogonal to the actual flight path. I’d guess that the short, sharp tendrils of gas are largely the result of the RCS system. I think that the exhaust of the main engines is also visible in the image, at least during the boost back stage: because the vehicle is turning sharply while burning (in order to reverse its velocity and return back to land), its exhaust is strewn out in the big fan shape we can see between the two clearer path/streak lines.

I am not sure how exactly Trevor captured this image; it’s possible he combined a bunch of separate ones into this. There’s definitely a long exposure involved, to get the streaked satellites and the rocket path lines; but I wouldn’t be surprised if there are also some short exposure stills that captured the gas plums at a specific point in time, after they had already started spreading but before they could dissipate. If it was just one long shot, I’d expect the plums to be much more blurry.

“IM-1”. Trever Mahlmann. 15 February 2024. https://www.tmahlmann.com/photos/Rockets/SpaceX/IM-1/i-GBn6DJc/.