This blog was set up in December 2014 with the sole purpose of proving that the Comet 67P/Churyumov-Gerasimenko was once a single body that has since been stretched into two separate lobes. This could have happened via one of two different processes.

The first process is spin-up due to asymmetrical outgassing causing a torque on the comet. For this to happen, the rotation period of the comet would have been between 3.5 and 4.5 hours.

The second process is a ‘Roche pass’ at Jupiter where the comet would have been stretched due to the tidal effects below 135,000 km altitude. A Roche pass is also likely to induce spinning in such a way as to exaggerate the delta g forces of the tidal stretching. This depends on the pre-pass rotation period and axis and the altitude of the pass. If such spin exaggeration occurrs, the pass altitude could be higher than the non-rotating scenario for any given stretching force.

Both processes are well-known and documented in the astronomical literature. Even asteroids can be spun up over millions of years (via YORP effect) to a circa 2-hour rotation period before flying apart. 67P’s period has decreased by 20 minutes since 2009, due to asymmetrical outgassing (Sierks et al, September 2014). That is a blink of an eye on astronomical timescales. Similarly, comets have been observed to shred as they pass under the Roche limits of the Sun and Jupiter. Slightly higher passes will induce stretch without shredding.

Stretch theory has been completely overlooked as a means of explaining the shape of 67P. This is because comets are thought to be too brittle to stretch. This would indeed be the case if they were solid ice. However, it is becoming increasingly apparent that 67P is a loosely held together pile of ice pebbles with 70-80% porosity and a tensile resistance of 10 pascals in the ‘neck’ region between the two lobes (Thomas et al, January 2015). Spin-up at 3.5 to 4.5 hours would easily over come this tensile force and so stretching would occur as a result. The sub-135,000 km Roche pass would also overcome this tensile resistance.

None of the above is actually necessary to prove the stretching of 67P into two separate lobes. The two processes are simply cited as well-known mechanisms for anyone who may object to stretch theory at the outset and won’t therefore be inclined to look at the other evidence.

The evidence for such stretching is abundantly clear in the fact that there are a multitude of matching features between the two lobes along the ‘shear line’ where they separated. These matching features are documented in exhaustive detail in Parts 1-6 of this series. Parts 7-15 then go on to explain the multitude of strange features on 67P, which are baffling the Rosetta mission scientists. They are fully explicable when stretch theory is invoked. Part 16 will return to the matches between head and body and Parts 17 onwards will explain yet more of what we see on the comet as directly or indirectly the result of stretching.

February 2015






4 thoughts on “About

  1. Andy, Dr. Masaroni has engaged in the conversation! (He is right about one thing, we do need to condense the amount of words going forward.) To that end, if a picture is worth a thousand words, a video animation must be worth a million.
    My two sons may be interested in doing an animation to support Stretch Theory (to “reply” to that silly Contact Binary Sim they used!) as a donation to the cause. My 25 yr. Old is an accomplished CG animator working at Digital Brew in Orlando, FL. My 26 yr. Old is a Houston World Film Fest winner with a gold Remi award for best short film, also working at Digital Brew.
    This would be after hours, non-gratis, donation to the cause.
    Will confirm if it is a go, and would work with you on the storyboard which I would draw up, and you fine tune.
    Can you see my email here? If not please reply here, I will see it, and we need to figure out how to communicate.
    Rambo Benson


    • Zomby

      It’s difficult to model it exactly without having a good characterisation of the bilobed gravity field. Also, the tensile strengths of the crust and core are an issue but Thomas et al have made a stab at that. Those figures are used in the calculations below.

      These calcs could be considered as a single model run with specific general inputs for radius between centres of gravity for the two joined lobes and for the tensile strength of the neck. The radius between c of g’s is fairly accurate (+/-15% probably, based on measuring the shape model). The g accelerations and forces would be close real-world forces but for the bilobed gravity field and the radius between c of g’s. Also there’s the accuracy of the head lobe radius at the shear line (used for the initial shear force before stretch). That value is given on the ESA comet vital stats page. Only after calculating these forces do we need to introduce the tensile strength of the neck as an attenuating factor.

      The calculation is in a comment, linked below, which I made on the Rosetta blog. It assumes a non-bilobed gravity field i.e. treating the head and body lobes as point masses at a distance even when joined. The bilobed field would smudge this analysis in reality but not by a huge amount, like a 50% error, maybe 20-30% maximum. It means the result is somewhere near the correct figure if you assume Thomas et al’s tensile strength figures to be accurate. Of course, the mass of the comet is well characterised and the equations for gravitational acceleration and angular momentum conservation aren’t guesses at all. So the main uncertainty is the bilobed field. The inputs are quite conservative, implying a maximum spin-up necessary for all the stretch theory outcomes to happen as stated in this blog.


      Then, four comments below this one I address the issue of the tensile strength in the neck potentially stopping or attenuating the stretch:

      “As for the question of the neck stopping all this from happening, I’ve mentioned the low tensile stress of the neck material before in comparison with more modest spin-ups. For the neck, it’s 10-20pa (Thomas et al 2015). The highest estimate for the crust is 40pa.

      I calculated the negative g force on the shear line to be 273 pa at the time of shear. Inputs: r= 1095m; T=2.033 hours; head lobe radius at shear plane 1.25km; head lobe mass 2.5E12 kg.

      The 20 pa sapping of the upward tensile force by the neck is 20/273 of the whole, which is 7.5%. So the upward force that’s left for lifting the head is 92.5% of the amount used in the last comment. And due to linearity between terms, the negative g acceleration is 92.5% as well.

      This hardly affects the scenario in the last comment and even with a 40pa neck resistance it would be a comparable output- the head lobe would rise to about the same place, dragging up the neck with it.”

      /////end of quote from comment.

      This weak, stretching neck scenario implies the pebble pile theory for the comet’s core, either the 3-metre dinosaur eggs that are seen on 67P or the “marbles” that were predicted by Carey Lisse in models. This link quotes Holger Sierks of OSIRIS talking about the dinosaur eggs and Lisse referring to the marbles.


      This would explain the low tensile strength of the neck if it’s loosely sintered pebbles making it a ductile core.

      There’s a new paper out on the compressive/tensile strengths of 67P. It’s one of the 46 papers published on 30th October 2015. It will be interesting to see if they are comparable figures to those cited above. Another of these papers talks about CONSERT measurements showing the core to be different in some way. That wouldn’t surprise me or Marco Parigi, inventor of stretch for comets, at all. I haven’t got round to reading either of them yet.

      There will be a post with these figures updated with better inputs, in due course. I hope this preliminary info helps.


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