Something interesting apparently happened last Wednesday (7/7/2010) in the skies over Hangzhou, China. As regular Wonk-readers will quickly agree, trying to extract reliable information from regular media reports is difficult at best. Unfortunately, almost all the news reports of this incident that I have seen of it have something like “…UFO over Hangzhou…” in the title. That makes it even harder to believe the reports. It doesn’t help that some of the pictures seem inconsistent with each other. (Editors seem to believe that UFOs in the title means you can photoshop the photos as you like.)

Some of the most interesting photos remind me of the images of the reentry of the Japanese Hayabusa spacecraft. Here, the break-up of the object appears behind buildings reportedly in Hangzhou has a tremendously large apparent angle, making it very, very close to the city of Hangzhou. Too close, in my view, to be credible. ( See here for a discussion of “apparent angle.”) I believe these are photoshopped and I am ignoring them. (I freely admit it could be a mistake to dismiss these images but they just don’t seem credible even if they are extremely well done if photoshopped.)

Instead, let us consider some of the most obvious causes of what appears to have been seen. First, NASA does not list any satellite decays—a satellite reentering the Earth’s atmosphere after being in an established orbit—for that day, nor were there any predictions for satellites even near that date. So it is highly unlikely that the “UFO” was a reentering satellite. Furthermore, there were no satellites put into orbit that day so we can eliminate a lower stage reentering.

A DF-21?

The most credible image shows an arc streaking across the sky soon after (near?) local sunset. Assuming this is actually the “UFO” and that it really was taken at Hangzhou, then we can say some interesting things about its trajectory. First, it appears in the Northwest and the end of the trajectory (in this image) is roughly due North. Let’s make that a little more quantitative. At sunset (which happened at Hangzhou that day at 11:00 GMT), the Sun had an azimuth of approximately 300 degrees and remained in that approximate azimuth for a considerable time after setting; though, of course, its elevation continued to go negative. Therefore, the image—assuming it was not zoomed and that it was a typical cell phone camera—shows the end of the trajectory having an azimuth of at least 350 degrees. (That’s 10 degrees West of North.) The peak of the trajectory appears at an azimuth of about 335 degrees. I would say there is about a +/- 5 degree error in my heading estimates, depending mainly on where the Sun really is in the image. Of course, that does not have to be the actual point where the trajectory had its highest altitude. It could simply be the highest point on the apparent trajectory: the curve of the Earth’s surface and the angle the trajectory makes relative to the observer could make a different point appear to have the highest elevation in the image.

click on the image for a larger version

The red line represents the reconstructed line of sight for the “end” of the trajectory while the yellow line represents the reconstructed line of sight for the “peak” of the trajectory. The white line is the ground track for a hypothetical DF-21 trajectory.

The image above shows these reconstructed observation angles from Hangzhou; the red line represents the reconstructed line of sight for the “end” of the trajectory while the yellow line represents the reconstructed line of sight for the “peak” of the trajectory. I have added another line running roughly East to West from the Jiuquan Satellite Launch Center with a length of 1,800 km, roughly the range of a DF-21. Amazingly (perhaps an amazing coincidence) the halfway point of that trajectory corresponds roughly to the angle on which the apparent trajectory reaches its maximum. This peak appears to be about 25 degrees above the horizon in the image of the trajectory. Is that possible for a hypothetical trajectory so far away (~1,400 km) from the camera?

The apparent elevation of the peak of such a hypothetical DF-21 trajectory as seen from Hangzhou can be calculated from a simple geometric relationship as about 17 degrees. That is really amazingly close to the “observed” 25 degrees considering all the simplifications in the calculation I have introduced to make a quick calculation! So we certainly cannot rule out this possibility on geometric grounds alone.

Problems with a DF-21 Hypothesis

The major problem with a DF-21 hypothesis is that the peak of the trajectory occurs nearly 600 km above the Earth’s surface! So the arc we see in the image cannot be the trail produced by reentry heating. Perhaps we should rule a DF-21 launched from Jiuquan out on those grounds alone. But it is possible that continued discharge after burnout from the DF-21’s solid-propellant second-stage motor could be illuminated by the Sun if there was just the right geometry. Any other missile trajectory has to have the reentry much, much closer to Hangzhou. That creates lots of range safety issues and presumably are ruled out. (An errant missile trajectory would never have been allowed to get so close to such a large population center. All missiles, even SCUD combat missiles, have a self-destruct charge on board to prevent such large discrepancies from the planned flight path.)

So it seems to me that a DF-21 launch somewhere near Jiuquan and aimed at a point somewhere in the eastern Gobi desert is the most likely cause of this “UFO” even given the problem of illuminating the solid-motor discharge above the Earth’s atmosphere. Unfortunately, I doubt that we will learn anything new about the DF-21 from this image, given the uncertainty of where it took off, its heading, and its range.

A General Disclaimer

I have made a tremendous number of approximations and simplifications in this analysis in order to get a feeling for this event. Many of you will object to those and say more care should have been taken. I agree with you in principle but think such accuracy is unwarranted at this time considering the tremendous amount of uncertainty in judging which pictures are accurate and even what basic timing data to believe. Let me emphasize once again that this analysis assumes that the image at the top of this post is what caused the alarm in Hangzhou. I have no idea if that is the case. If some of the images I have discarded are really images of the phenomena, than this analysis’s assumptions are wrong at the start.

It is always hard to use media reports for quantitative analysis but particularly so for anything with UFO in the title. It seems that editors see that as a license for any kind of photoshopping they like. So, until better information is available, I think the most likely explanation is a DF-21.

Editor’s Note: Don’t even think about making a comment suggesting it was a real UFO, those comments will not get approved. This is a serious blog and is not interested in UFO theories. Get your own blog if you want to discuss that sort of thing.

Update (7/15/10)
The embedded video below shows both how difficult it is to get quantitative information off of the internet about “UFO” events and is a great example of a rocket plume in outer space. As an added bonus, there is a staging event at about 30 seconds. By the way, this is almost certainly a Progress-M launch from Biakonur.

Update (7/19/10) The video I posted above (which has at least temporarily disappeared during our move to this new format) has been picked up by <a href=,0,4283795.story > the LA Times with the wrong attribution: </a> The LA Times is attributing it to the Hangzhou incident.  Oh well, it just goes to show how hard it is to straighten anything out once it makes the internet.  However, I hope the editors of the LA Times look here (and perhaps try harder to check the provenance of the videos they find on Youtube.)