Demystifying my first "UFO"

If you ever wondered why “UFOs” are rarely reported by those who regularly have their eye on the night sky, especially amateur astronomers, here’s why: Most of these “unidentified objects” are well known to us – bright planets, satellites or airplanes, or the infamous “weather balloon”. But when, in very rare cases, experienced observers get to see something “unidentified”, things get interesting. Here's the story of our first “UFO”, and of some able amateurs who managed to demystify it.

We know the island of La Palma pretty well. It’s one of the worlds best places for stargazing, it is even home to the Observatorio Roque Muchachos (ORM), Europe’s biggest astronomical observatory. No wonder we come here time and again for stargazing. After spending five nights at the ORM this year, we had our telescope already packed for our flight home. On our last evening we wanted to shoot a final time lapse, starring the Gran Telescopio Canarias (GTC), the (currently) largest single optical telescope in the world. The camera was already running for some hours when we went to the observatory's residence building for a coffee. The time was about 10 pm.

Camera is in position: The magnificent dome of the GranTeCan above atlantic clouds.
After an amicable chat with the observatory's caretaker (to whom we owe a big “thanks” for helping us with our stay), we left the building about half an hour later. The sky was as crystal-clear as before (in fact, we had two amazing weeks of sustained clear skies up there), but low on the western horizon we noticed a strange cloud. It was unusually shaped, and appeared more like a persistent train of a fireball than like a normal cloud. The camera was pointed in the right direction, so we hoped it might have recorded the fireball. Then we forgot about it.

A week later, home again, I processed the movie, and watched it, quite perplexed:

Excerpt of the La Palma time lapse, slowed down to 5 frames per second.
The short clip above is an excerpt from the complete time lapse showing the interesting part, recorded between 22:16 and 22:20 local time (21:16-21:20 UTC) and slowed down for convenience. Do you see the strange object appearing from the western horizon, moving in a south-easterly direction, “puffing” some sort of dust or liquid and showing some strange and sudden changes in form and brightness? Neither my colleague nor I had ever seen something like this before. Thanks to the internet, it reminded me to a rocket launch. Or could it be a satellite in orbit, venting fuel?

It should have been easy to find that out, but it wasn't: I couldn’t find any launches scheduled for that particular day, September 10, 2013. But I’m not an expert, so I mailed some friends who work at the European Space Agency’s Operations Centre (ESOC) in Darmstadt, Germany. They quickly confirmed that no satellite launch had been scheduled that day. Could I’ve been mistaken? Could it be that the denoted date or time were incorrect?

This stacked image shows two features that were important for time-verification: Venus and a flare of the Cosmos 1410 satellite.
As it turned out, they weren’t. First, there was no doubt about the date: The video was taken one night before our return flight, which was on September 11. If you board an airplane on this particular date, you do remember. The camera clock however turned out to be off exactly 6 minutes and 43 seconds. This is not negligible, but it's not that much either. Just to be sure: Look carefully, and you see Venus setting at the left just at the end of the video. The planet's set was calculated for 22:16 local time. Due to horizontal dip of about 1.5° at our site (2300m above sea level), Venus was visible a little longer, consistent with what you see on the video. Furthermore, if you look carefully again, you can see a brief flare appearing right at the centre of the frame. That’s Cosmos 1410, a well known satellite in polar orbit.

Both Cosmos 1410 and the setting of Venus provide an accurate timing, even if one doesn’t trust the camera’s internal clock. The only other artificial object known to pass over this section of the sky over La Palma on September 10 between 22:16 and 22:20 local time was a 1963 Atlas Centaur rocket stage – which certainly does not contain any residual fuel to explain the strange “puffing”.

I needed help – clearly I was not able to solve this mystery by myself. The ESOC guys had already alarmed Cees Bassa and Marco Langbroek, distinguished amateur satellite trackers and experts in the field. Marco quickly assured me that even if it had been a classified satellite, they would know of it. Amateur satellite trackers observe and track stuff in orbit world wide, and even if governments don’t inform the public about the orbits of their secret military satellites, they are visible even with the unaided eye, and therefore are no mystery to tracking amateurs. With backyard telescopes, some of them even spy back at classified spy satellites.

Marco Langbroek first suggested the "puffs" marked the ignition of the 2nd and 3rd stage of a rocket, respectively. The initial straight line trajectory deviated as the object left the fixed camera field of view, also indicating a rocket launch. (Credit: Marco Langbroek)  
So, no satellite. A launch then? Although Marco and Cees agreed that my “object” looked very much like a launch of some sort of rocket, they inquired if I could exclude the possibility of a lens flare. Had anyone played with a flash-light, or did a car passing by the camera while we were having our coffee? Since we weren’t there at that particular moment, it’s not possible to rule this out.

But it seems highly unlikely: The only road visible to the camera leads to the telescope dome, and there was no car going in or out during the first half of the night, a fact confirmed by the observatory staff. Furthermore, bright lights are per se prohibited inside the observatory area. Finally, there's an undoubted proof that the object was no lens flare: the “cloud” we saw when we left the residence. This cloud coincides perfectly in form and position with the residual “dust” visible in the video. So we did see the remains of the object in the sky – it was real.

Preliminary flightpath of the object (Credit: Marco Langbroek)
Based on our individual photos Marco did a rough analysis of the objects trajectory, assuming it was indeed a launched rocket. In his interpretation, the two sudden bursts of brightness and the subsequent expanding puffy clouds were the moments of jettison of the 1st stage and ignition of the 2nd stage and ejection of the 2nd stage and ignition of the 3rd stage of the supposed rocket, respectively. He also noted that the stacked images show that after the supposed 3rd stage ignition, the trajectory starts to deviate from the previous more or less straight line. Such a deviation fits a missile launch, but not a satellite trajectory in orbit, because such an object would move in a straight line when it's positions are plotted in a gnomonic projection. Since the stars do not show a similar distortion at the edges of the frames, a lens distortion effect can be excluded (we used an off-the-shelf zoom lens at f-stop 2.8, so some distortion is inevitable).

So it was a rocket launch. But what kind of rocket did we see, who launched it – and where? It clearly came from the west, crossing the Atlantic. Certainly, it was not a land-based launch. For a simple reason: such a launch would have been noticed. Marco therefore concluded the event was most probably a clandestine sea-borne launch, very likely a military launch by a US-, british or french submarine in the mid Atlantic. Those three nations are known to operate Submarine Launched Ballistic Missiles (SLBM), to be more precise, the Trident missile by the US and the British, the M45 or M51 by the French.

For a more accurate calculation of the trajectory, it was crucial to identify the two “puffy” events correctly. Marco suggested they marked the 1st and 2nd stage separations respectively, which give an indication of altitude and distance of those events. As Marco found out, the 2nd stage of a Trident missile ignites at about 70 km altitude, the 3rd at about 150km altitude. By measuring the astrometric positions of those events and calculating the corresponding azimuth and elevation in the sky, he derived a a rough estimate of the launch location. It would have taken place somewhere at latitude 23-25 degrees north and around 40° longitude west – right in the middle of the Atlantic, at least 2000-3000 km from any coast in any direction.

"Refinded" trajectory and altitude vs. ground range based on calculations by Cees Bassa. The two points in the altitute graph represent the two "puffs" visible in the timelapse, interpreted here as MIRV bus and MIRV separations (i.e. the warhead), at 1130 and 1330 km altitude in the ascending phase, at 6.5 and 8 minutes flight-time, 1860 and 2235 km from the launch location, respectively. (Credit: Marco Langbroek)
However, there was disagreement on that assessment. If the "puffs" instead represent the warheads separation, which happen at much higher altitudes than the rocket stage burns, they would have taken place at considerably larger distances from La Palma. The launch area would then have been much more to the northwest.

As it turned out, the clandestine launch hadn’t been totally unannounced after all. Somewhere in the depths of the internet Ted Molczan found a broadcast warning message for mariners, warning of “hazardous operations” with “rockets” in the South Atlantic between Sep. 9 and 14. The referred coordinates in this message provided a clue of the supposed impact area. 

With the available information and assuming the puffs were in fact the MIRV separations (which fits better to the data revealed in the warning message), Cees and Marco calculated and vizualized a more precise flight path, setting the launch at about 28 N, 68 W, more to the west than Marco’s first rough approximation (see above). The launch hence occurred at about 21:10:40 UT, and the projectile crashed into the Atlantic somewhere between St. Helena and the coast off Gabon and Congo after a 36 minute flight over a distance of 8660 km. According to Cees and Marco, it reached a maximum altitude of 1650 to 1900 km.

Trident II (D-5) missile underwater launch (Credit: Wikimedia Commons)
I was really impressed with how much these guys managed to pull out of our images – we now knew pretty well what the no-longer unidentified object most probably was, where it was launched and where it crashed back into the sea. The final question “who did it” finally resolved itself: Few days after I first posted the images, Lockheed and the US Navy announced they indeed tested a Trident II D5 missile that day, launched from a submerged Ohio-class submarine in the Atlantic.

So, another “UFO” turned out to be a mundane earthly gadget. By the way, although having it's minor flaws and glitches, the complete time-lapse tuned out quite nice, too: 

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