9:31 PM: Don’t know yet what this was but we’ve received multiple reports, including the text that had the video attached, as seen from Alki Avenue SW. Help us sleuth it! We’ll add whatever we turn up.
9:57 PM: One commenter points to the possibility it was just a contrail. Haven’t found any meteor or rocket reports so that just might be it; still looking just in case.
11:10 PM: Our favorite expert skywatcher Alice Enevoldsen believes it’s a contrail. She writes:
tl;dr–In my expert opinion: contrail.
Longer answer: of course I don’t know for absolutely certain, who could possibly know that, besides the pilot? So let’s run through the main reasonable possibilities.
Space Junk Reentry
As mentioned by the viewers of the video (yay! thank you!) it could be space junk, right? You’re absolutely correct, there’s space junk burning up in our atmosphere all the time. That bright second half of the line sure looks like fire too. The speed is wrong though. Most reentries go a similar speed, since they’re stuff entering our atmosphere from orbit. Controlled reentries have a slightly different speed than uncontrolled ones, but they all tend to be a good bit faster than this is moving.
Here’s a gif of a satellite going about twice as fast as usual.
Again, the speed is wrong. Meteors go much faster, they’re also known as shooting stars. When your friend says “look! A shooting star!” by the time you look, it’s gone. The Chelyabinsk meteor seemed to go a bit slower, because it was huge (as average meteors go) and therefore got deeper into our atmosphere than most. Here’s a gif of a meteor, going about normal speed.
From our point of view looking at the sky on a given day, comets don’t move.
For something long, thin, and cloud-like the first direction to examine is a contrail. The speed of the object is correct for an airplane moving out of our field of view. Also, the direction works, because this “downward” path is consistent with a plane travelling west and eventually disappearing over the horizon. We still have some questions: why do we see it go so far down, and why is the contrail two colors: dark and “fire”?”
Both colors can be explained by the direction of travel and the time. This is around sunset. The end of the plane’s contrail is in the evening sky, in a position that is after sunset and therefore in the Earth’s shadow. The plane’s current position is in sunlight, but as it is sunset, the position is brightly lit with all the colors of the sunset. It is brighter than the clouds it appears near, because it is higher than them and therefore in brighter sunlight than them.
It is potentially possible that the lighting of this contrail makes it so we can see the plane and it’s contrail longer than usual, contributing to the “straight down” appearance of the trail. I’m not certain of that explanation for this part. It could also be as simple as observation bias: this one is so cool, we don’t remember other contrails going this direction.
P.S. Details soon on Alice’s summer-solstice sunset watch!