The Space.com article Strange Sky Spiral May Come from Secretive SpaceX Zuma Launch shows images from this Tweet and this tweet, the second of which was posted by Dr Marco Langbroek (@Marco_Langbroek):

Palaeolithic archaeologist (PhD), spy satellite tracker, asteroid discoverer. Tweets about space, archaeology, astronomy & other interests. Asteroid (183294).

There is a lot more Zuma-related orbital mechanics Dr. Langbroek's twitter (and extensive blogposting) as well.

Is this unburned propellant, if so, why would this need to be done, and in any event, what gives it this particularly distinctive shape?

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1 Answer 1


All of the reports are that this was expending the unspent fuel from the second stage. There have been other sightings of unburnt fuel dumping, such as this one:

In the other instances, there appeared to be more of a cloud, in this instance, it appeared to spin.

Now, why the difference? The most likely culprits:

  1. Previous missions, the fuel dumping was more balanced. One of the valves might have been stuck.
  2. The weight distribution wasn't as expected. This could cause the satellite to spin while dumping fuel. This could be caused by a number of things, from the Northrup Grumman provided PAF being a different mass distribution then the SpaceX PAF, something still being stuck on to the stage.
  3. It could have been rotating for some reason. This could be caused by a separation that didn't occur quite as expected.
  4. SpaceX was trying something different for some reason
  5. Something could have bumped the second stage between the burn and the defuel.

I suspect it was one of these, but it's really hard to know for sure. I doubt SpaceX will release the reasoning behind this, so for now, one will have to live in the realm of guesses.

  • $\begingroup$ More time has passed, has an answer emerged? Five guesses is great, but do not an SE answer make. $\endgroup$
    – uhoh
    May 10, 2018 at 4:07

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