The Guardian's 'Out-of-control' Chinese rocket falling to Earth... Oops, I mean Out-of-control SpaceX rocket on collision course with moon says that Bill Gray's Project Pluto/DSCOVR says that the 2015 rocket body will hit the Moon on March 4th, and @Jonathan McDowell tweets so to, and adds:

...It's interesting, but not a big deal.

There's also Ars Technica's After 7 years, a spent Falcon 9 rocket stage is on course to hit the Moon

Question: Who's been tracking the DSCOVR rocket body object since 2015 such that we know thought it's gonna finally hit the Moon March 4th? Is the chaotic 3-body cis-lunar trajectory data coming from TLEs in SpaceTrack, from private citizens with telescopes + numerical integration? Something else?

See also:

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    $\begingroup$ Does your link projectpluto.com/temp/dscovr.htm answer your question? Bill Gray has been doing the predictions. The observers, at least for the latest observations Jan 15 through Jan 20, are listed. Presumably the same or similar observers have been providing periodic updates through the lifespan. (The observers who asked to not provide their observations will probably remain anonymous ;-) $\endgroup$
    – JohnHoltz
    Commented Jan 26, 2022 at 15:01
  • $\begingroup$ @JohnHoltz I think a big list is not a good Stack Exchange answer. Instead, someone familiar with the process will likely write a short but informative summary or "big picture" which future readers will find interesting. $\endgroup$
    – uhoh
    Commented Jan 26, 2022 at 17:33
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    $\begingroup$ I would alter the question like this: "Who's been tracking the <strike>DSCOVR</strike> rocket body since 2015 such that we know it's gonna finally..." There is a rocket body hitting the Moon after all. $\endgroup$
    – asdfex
    Commented Feb 14, 2022 at 11:24

1 Answer 1


Update 2022-02-14: It now turns out that it's not a Falcon 9 upper stage, it's a Chang'e 5-T1 booster ( 2014-065B).

The misidentification came from the coincidence that the tracked object passed by the moon a few days after the DSCOVR launch and thus was circumstantially assumed to be the DSCOVR second stage.

This is (almost) entirely based on Bill Gray's fantastic webpage covering this event (he is the only reason that this is an event). I have very little to add so I apologize if this is 'link-only-eqsue'

Bill Gray has been doing the tracking of 2015-007B (40391). There are TLE's, but there was a 5 year gap in data and the most recent TLE is from Sept-2021 (mean elements, use with caution):

2015-007B data

Bill Gray provides pseudo TLEs:

Can you provide TLEs for it?

Unfortunately, no. The orbit goes well past the moon and has an eccentricity of about 0.89. The 'usual' SGP4/SDP4 model for two-line elements (TLEs) fails in such cases. I have posted "TLEs" for this object; I use these to generate ephemerides and to identify the object from observations... and they work with my code and (probably) nobody else's.

As Bill Gray puts it his involvement is:

My "day work" is for the asteroid hunting community. The big surveys (Catalina Sky Survey, ATLAS, Pan-STARRS, and ZTF) observe on most clear, moonless nights, imaging the sky for slowly moving objects. [...] But there are about a dozen "high-flying" objects that can move slowly enough to look like a rock, at least briefly.

For about fifteen or twenty years now, I've taken these observations and computed orbits. Then, when the surveys find such objects, they can fairly quickly say "never mind; it's not a rock; it's just another nuisance artificial object", and go back to looking for actual rocks.

If it wasn't for that, these objects would go untracked. Objects in lower orbits are very carefully tracked, [...] High-altitude payloads are carefully tracked; these are all scientific missions, and you need to know where they're going. For example, NASA can tell you where the James Webb Space Telescope is quite precisely, but they lost interest in its booster once it separated from JWST. (I did not; the JWST booster is being tracked as it goes into orbit around the sun and its orbit computed.)

Generally speaking, high-altitude junk goes ignored. (Except, it appears, by me.)

A detailed rundown of the tracking timeline since 2015 is given on the website. Essentially, (professional?) telescopes reported observations for a provisional "WE0913A" near Earth object. Bill Gray used these observations (and subsequent ones) to refine the object's orbit and "back-predict" it to a lunar flyby in 2015 that matches the one the DSCOVR spacecraft (and thus rocket body) did. He has tracked it ever since.

Recent observations were specifically targeted (professional or amateur: I've no grounds to judge):


Bill Gray uses these observations with his custom software find_orb (on Github) to propagate the object's orbit and thus found the lunar impact date, time, and position.

The propagator appears to use a Runge-Kutta-Fehlberg 7(8) integrator.

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    $\begingroup$ Is there any information about the position and time error of the prediction? $\endgroup$
    – Uwe
    Commented Jan 31, 2022 at 21:49
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    $\begingroup$ @Uwe yes, see here $\endgroup$ Commented Jan 31, 2022 at 22:08
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    $\begingroup$ Many thanks for the link : "With all the data, we've got a certain impact at 2022 March 4 12:25:58 Universal Time, at latitude +5.18, east longitude 233.55, plus or minus a few seconds and a few kilometers." $\endgroup$
    – Uwe
    Commented Feb 1, 2022 at 1:02
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    $\begingroup$ @Uwe note this is also written in this answer $\endgroup$ Commented Feb 1, 2022 at 1:45
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    $\begingroup$ fyi I've just changed the title to "Why did we think that...?" arstechnica.com/science/2022/02/… links to projectpluto.com/temp/correct.htm $\endgroup$
    – uhoh
    Commented Feb 13, 2022 at 23:43

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