Every now and again something weird and wonderful happens in the sky, and for a few minutes I'm totally perplexed about what it is.
And then there's something that makes me literally gasp and say "WHAT THE FRAK WAS THAT?"
Yeah. Check out this amazing video:
Holy Haleakala! What was that?
The footage is from a webcam mounted outside the CFHT astronomical observatory in Hawaii (another view of it from a different webcam can be found here; sadly, both webcams are on Mauna Kea, not Haleakala). You see some stars and the horizon, then suddenly an ethereal pale arc pops into view. It rapidly expands into a thin circular shell, then fades away as it fills the view. The whole thing takes a few minutes to expand; you can see the stars moving during the event (some of the pixels on the webcam are very sensitive and make stationary "hot spots" in the field of view).
So what is it? Is it a trans-dimensional portal into the future, some wormhole from the Pegasus galaxy, or two alien spaceships battling it out?
In point of fact, we are seeing something related to space war...
I first saw this video on Starship Asterisk, the discussion forum for the wildly popular Astronomy Picture of the Day website. The conversation there about this event is going pretty well, and I think this whole thing has been nailed down to a reasonable series of events. First, let's look at a still frame from the video:
I blurred the image just a bit to reduce some of the noisy background, and what leaps out is that the expanding halo is limb-brightened, like a soap bubble, and fades with time. That strongly points toward something like a sudden impulse of energy and rapid expansion of material, like an explosion of some kind. Note that the ring itself appears to be moving, as if whatever caused it was moving rapidly as well.
It took me a minute after watching the video to remember the bizarre Norway spiral from a couple of years ago, a phenomenal light show caused by an out-of-control rocket booster jetting out fuel in space. I figured this was similar, but what was it?
As an aside, I think we can rule out a lens flare, which needs a bright object to be the source of internal reflections in a camera; besides, it was seen in two different cameras in the same location in the sky. We can also rule out such things as solar events (they don't create expanding halos like this) or meteors (again, no halos from those).
Asterisk board member calvin 737 was the first to suggest it might be related to a Minuteman III missile launch around that time. As more people on the forum dug into it, the timing was found to be right. The missile launched from Vandenberg Air Force Base (in California) at 03:35 Hawaii time, just minutes before the halo was seen. I noticed the stars of Cassiopeia are visible in the webcam, so the view was to the northeast, which is the right direction to see the missile as well.
OK, the timing and direction are perfect, so the rocket is clearly the culprit... but how, exactly?
One suggestion was that it's due to the shock wave as the supersonic missile blew through the tenuous upper atmosphere. I've heard some rumblings that this might be the cause, but I'm not convinced. I thought shock excitation has to be pretty strong to get air to glow (any experts out there willing to comment?) and the movement of the missile may not be enough for that. Also, in that case I'd expect to see more of a disk pattern than a thin ring, since the missile would be continuously blowing through the (very tenuous) atmosphere. The thin ring indicates to me this was most likely a single, short event. And if you posit the shock wave wasn't continuous as the missile moved, but instead was generated rapidly and ceased (maybe as the missile pierced some atmospheric layer) I don't see how a shock wave would create a ring that moves physically across the sky; it would expand away from a single point. Again, this idea doesn't convince me.
[Note: see update below; I realized there may be a little bit more to this idea.]
Another idea posted by board member neufer was that this was from a detonation charge in the missile's third stage. There are ports, openings in the sides of the third stage. Those ports are sealed for the flight until the right time, when they're blown open by explosive charges. This allows the fuel to escape very rapidly, extinguishing the thrust at a precise time to allow for accurate targeting of the warhead.
At this point, the missile is above most of the Earth's atmosphere, essentially in space. So when that gas is suddenly released from the stage expands, it blows away from the missile in a sphere. Not only that, the release is so rapid it would expand like a spherical shell -- which would look like a ring from the ground (the same way a soap bubble looks like a ring). And not only that, but the expanding gas would be moving very rapidly relative to the ground since the missile would've been moving rapidly at this point in the flight.
These are all exactly what was seen in the webcam footage. The timing of all this works out as well: as pointed out in that forum thread the third stage firing terminates about three minutes into flight, which is when the halo seems to appear.
So there you have it! I think this covers it: a missile launches from California, and three minutes or so later the third stage releases an explosive charge which blows fuel out into space. This fuel expands in a shell, fades as it gets bigger, and appears to move across the sky as it does so. And there's the other idea that this might be from a shock wave from the missile itself, which I cannot rule out.
[UPDATE: I literally woke up this morning realizing it may be a combination of both ideas: if the expanding fuel compresses the atmosphere as it expands, it might create the ring of light as the air gets excited. That would also explain all the characteristics we see. I'm hoping to hear from some physicists who can do the math.]
Man, I love stuff like this! A real mystery, something truly odd. For a moment your imagination runs wild, but then logic and reason kicks in. A little digging and an explanation turns up, several in fact, and more digging reveals more details that appear to fall right into place. Mystery solved!
And it takes away not one whit of the awe and sheer amazement such an event engenders. Oh, how I wish I could see something like this! How beautiful and astonishing it would be to stand under the open sky and witness such a thing. Of course, with such a rare event the odds of seeing it are low, so I suppose I'll do what I always do: spend as much time outside looking up as I can, maximizing the chances of seeing something.
And in the meantime, I get to see the sky, too. It's a pretty fair deal.
Video credit: Kanoa Withington/CFHT. Tip o' the nose cone to Josh Walawender for sending me the link, and thanks also to Kanoa for putting the video up on Vimeo so I could embed it!