CometWatch 27 March

Last week, Rosetta embarked on an excursion around 1000 km in the anti-sunward direction to study the wider coma, tail and plasma environment of Comet 67P/Churyumov-Gerasimenko.

This week’s CometWatch is an image taken on the outward leg of the excursion, on 27 March, when the spacecraft was 329 km from the nucleus.


Enhanced NAVCAM image of Comet 67P/C-G taken on 27 March 2016, 329 km from the comet nucleus. The scale is 28 m/pixel and the image measures 28.7 km across. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

In February and March, Rosetta spent several weeks at very close distances from the comet nucleus, which overfilled the field of view of the NAVCAM, providing us with striking views of the surface. During the current excursion, instead, we can enjoy again a view of the full nucleus and the environment around it.

In this CometWatch image, the small comet lobe is on the left and the large one on the right. The image was taken at a very large phase angle of about 159 degrees, meaning that the comet lies between the spacecraft and the Sun, and that all three are very close to being on the same line.

In this configuration, the nucleus appears backlit, with only a few portions of the illuminated surface visible from this view – in the upper and upper right part of the nucleus.

Thanks to the combination of a long, four-second exposure, no attenuation filter and a low-gain setting on the analogue signal processor of NAVCAM (a setting that is used to image bright targets), the image reveals the bright environment of the comet, displaying beautiful outflows of activity streaming away from the nucleus in various directions.

It is interesting to note hints of the shadow cast by the nucleus on the coma below it, as well as a number of background stars sprinkled across the image.

Today, Rosetta is moving back below 600 km from the nucleus, having been at 1000 km on 30 March. The spacecraft will come back to about 200 km early next week and carry out a zero phase flyby on 9 April at around 30 km altitude.

Meanwhile, 984 new NAVCAM images were released yesterday, covering the weeks between 16 December 2015 and 9 February 2016. You can browse through them via the NAVCAM Image Browser tool.

The original NAVCAM image is provided below.




  • Kamal says:

    Lovely, maybe you should try an eclipse image while you are far enough!

  • masanori says:


    The original image looks white-y. Is this all because of The Sun’s rays or (partly) of something else??

  • logan says:

    Beautiful wave you got there, 4km below the core, kind of an advanced position, relative to its geometrical focus. Could it be other moments when the opposite is shown?

    • logan says:

      And being a total eclipse, silhouette shining should be coronal.

      Where does that shine at Her Top comes from?

      No other thing come close in luminosity, but tails themselves.

      • logan says:

        “…phase angle of about 159 degrees…”

        that leaves 21º that could leave Sun well up, out of the frame.

        Going to stop until being able to say some minimal sense.

    • logan says:

      “…Thanks to the combination of a long, four-second exposure, no attenuation filter and a low-gain setting on the analogue signal processor of NAVCAM…”

      Very ingenious. And precise work also at post. Enhancing waving and jetting.

  • Alec says:


    Can I get a copy of this image?

    Alec Hoon, MD

    Enhanced NAVCAM image of Comet 67P/C-G taken on 27 March 2016, 329 km from the comet nucleus. The scale is 28 m/pixel and the image measures 28.7 km across. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

  • Rod says:

    How close can Rosetta get to the surface with out being a kamikaze?
    Can Rosetta navigate to and maintain a super low altitude ( eg 100) meters before eventually slowly crashing?
    I assume most valuable information gathering is done encompassing the entire comet and its environs. I imagine the visuals from extreme close ups will be useful and certainly extremely exciting and fascinating.

    • Harvey says:

      I’ll leave the navigation question to the flight dynamics team, but 100’s of metres is for sure going to be hard.

      But the other limit is the cameras. They are designed mainly to image ‘infinity’, far from the spacecraft. As you get near, three problems occur.

      Firstly, camera focus. For the OSIRIS WAC there seems to be no focus adjustment. If you do a standard depth-of-field calculation, it remains ‘in focus’ (ie, not too badly out of focus!) down to ~~150m or so. For the NAC, in its standard mode, its in focus down to about 2km. It can move a plate in the filter wheel to shift that to a 2 to 1km range. Below 1km it will rapidly go out of focus.

      Secondly, movement. By the time Rosetta goes in to crash, it will be far from the sun & illumination dim, so it can’t use very short exposures. So the relative motion of Rosetta over the surface will be crucial, including the comet’s rotation. Whether is feasible to mitigate this with Rosetta’s motion matching the surface rotation I’ve no idea; very tricky navigation! Relative motion may be more serious than defocus I think.

      The third one is getting the data back. The bandwidth will reduce with earth-Rosetta distance, & these images are a lot of data. This might suggest a strategy of ‘increasingly dangerous’ low passes might make sense, to download the data after each pass until you dont get away with it.

      Its going to be a very interesting finale!

      • ianw16 says:

        Regarding the cameras, I’m not so sure. Have a look at this rather fetching view of Prof. Sierks!

        • Harvey says:

          That’s the WAC. I suspect it has actually got pretty degraded resolution, that distance is certainly way inside the standard depth of field calculation which gave about 140m. But given its very good resolution in good focus, you still get a recognisable image quite badly out of focus.

          For the NAC, they actually quote 1-2km with the plate in.

          Bigger problem I think will be motion blur and getting the data back.

        • Harvey says:

          The WAC picture at 15m intrigued me, and the camera performance ‘close in’ will be important. So stuck at home after knee surgery  I thought I’d look at it more carefully.
          Model. The WAC camera actually uses an off-axis reflective system which is astigmatic (140/131mm.) But for simple geometrical optics depth-of-field sums we can consider a simple equivalent thin lens system. I used the sagittal focal length of 140mm.
          Data. From a variety of web sources, including the detector manufacturer E2V:
          Pixel size 13.5um
          Focal length 140mm at F5.6
          Assumed fixed focus on infinity; it might be focussed very slightly closer, keeping infinity ‘in focus’, which would improve things very slightly, but an estimate suggests it’s a ‘small win’.
          Quoted angular resolution, 101uR, FOV 11.3*12.1 degrees.
          Firstly, as a check, note that the angular resolution, pixel size & focal length are consistent, 13.5um/0.14m=96uR, a good start. To briefly see where we are diffraction wise, using the usual rule of thumb at 600nm, of 1.22*F*lambda, the diffraction limited spot on the sensor plane is around 4um full width, a third of the pixel size, suggesting the system is a few times diffraction limit due to aberration, reasonable.
          Let f be the focal length, u the object distance, F the F number, & I will use (mgr) for much greater than, (gt) greater, (lt) less than, than and (mll) for much less than, the symbols break the blog!
          Using simple geometrical optics, just 1/u+1/v=1/f really, the focal shift is just shift=(1/(1/f-1/u))-f and the defocussed spot size on the detector shift/F. Then your target plane resolution is that defocussed spot size times u/f. Have I messed up somewhere?
          If u (gt) 259metres the out of focus blur is (lt) the pixel size; so around this range, you should start to degrade. If we add them in quadrature, you lose about 40% at 259m range. But the degradation is odd, it’s a regime I’ve never considered before.
          As you go closer, u (lt) 259m, *but* stay outside the condition u (mgr) f – which would be very close indeed, just a few metres – something quite odd occurs. In this range the angular resolution degrades as ~1/u due to defocus but the spatial resolution in the target plane is that times u – so it is constant! So with this model from just inside 259m, down to just a few metres, the spatial resolution on the target remains at about 2.5cm In this regime the resolution is f.u/((u-f)*F) so for u (mgr) f just u/F or 0.14/5.6 m=0.025m
          The picture taken at 15m is within this range; the resolution certainly appears better than 2.5cm (though resolution can be hard to judge visually) and I can’t figure why. But actually it should contain no more information than you would have got at (say) 150m, where the spatial resolution in the target plane in absolute, distance terms *is the same* as at 15m (but better in angle.)
          Note that due to all sorts of approximations & simplifications I would not claim the numbers to be exact. But they should give the general behaviour correctly. Assuming the residual whiffs of anaesthetic wandering round my system haven’t caused some horrid slip up! The value I get in the constant region doesn’t seem consistent with the web picture, which bugs me. Can someone check my geometrical optics/algebra please!

          • logan says:

            Hi Harvey.

            Filter Green (537 nm) / Empty (-)
            Exposure time 0.500 s

            Admitting never checked Tech Specs. Could Exposure time suggest some kind of aperture mechanism?

            Hopping You get better at your knee 🙂

          • logan says:

            Walk “repting” when in rehab. Like in 67P 🙂

          • logan says:

            Got it!
            ID WAC_2016-03-31T03.40.32.848Z_ID00_1397549300_F21

            Check again with _F21

          • Harvey says:

            Sorry, typo
            ‘…..lresolution is f.u/((u-f)*F) so for u (mgr) f just u/F or 0.14/5.6 m=0.025m’
            Should read f/F not u/F of course. Numbers are correct.

            It occurs to me that I need to do a little playing about with my DSLR. 140mm and F5.6 are well within medium telephoto zoom range territory.
            So focus on infinity, leave the focus fixed there, and look at nearby objects. It would be a very good simulation of the WAC.

      • Solon says:

        You seem to have knowledge on the cameras so was wondering if you could explain what it is that the NAVCAM is actually detecting, or how it works?

        • Harvey says:

          It is simply a ‘digital camera’.because it’s not considered a scientific instrument, but part of the spacecraft ‘infrastructure’, it’s harder to find details.searches get flooded by pictures from it.
          I did find that it 1024X1024, 5 degree field of view, 12 bit.
          But with no focal length or F number, can’t calculate depth of field; it will certainly be focussed on infinity.

          It simply images sunlight reflected from the comet (which is very dark, only reflecting a few percent – think coal!), sunlight scattered from dust, and background stars.

          There is yet another camera system, the star tracker, used to navigate the spacecraft by looking at the position of identified stars very accurately.

          • logan says:

            Considering that OSIRIS Public Status is just recently untangled, Visual neural ‘circuits’ around here are built with this ‘simple’ camera. Saluting honorable NAVCAM Team.

          • Solon says:

            “It simply images sunlight reflected from the comet (which is very dark, only reflecting a few percent – think coal!), sunlight scattered from dust, and background stars.”

            The CCD would seem to have UV to IR sensitivity, with a Lumogen coating to improve UV sensitivity. ,With no filters, how are we to know what wavelengths are being detected, if it is reflected sunlight in the visible range or perhaps only UV emissions from the Lyman series?
            Not knowing what we are actually seeing is very frustrating.

          • Gerald says:

            focal length, and f-number, and other camera parameters are described here:

            More detail here:

            NAC has two refocusing lenses, one for visible, one for IR. WAC cannot refocus, but optical performance is unaltered down to about 500m.

          • Gerald says:

            …no sorry, it’s only one NFP (visible). IR is FFP.

          • logan says:

            “…or perhaps only UV emissions from the Lyman series?”.

            Don’t believe NAVCAM to be very much short UV sensitive.

            Exactly because there is too much short UV Lyman emission. Perihelion Ducky ‘glows’ on short UV spectra.

            NAVCAM unprocessed shots would be too much ‘washed’ and lacking on contrast.

          • logan says:

            Of the shelf CCDs are quite IR sensitive. But being a navigational camera [always open] gambling IR should be strongly filtered.

          • Harvey says:

            Gerald. Thanks, but I had all the data on the OSIRIS cameras. It was the NAVCAM I couldn’t find optical data for.
            WAC unaltered down to 500m agrees with my sums; not much affected down to 250m or so. But I can’t quite reconcile the 15m picture of the PI with my sums; it seems better than I’d expect, and I don’t know why. It seems to be in this constant target spatial resolution regime, but looks better than I calculate. Maybe I lost a 2 or something!

          • Harvey says:

            Data on the NAVCAM seems sparse.
            Whether it has a filter in I don’t know. It seems quite possible it’s just the unfiltered CCD response, which will cover mainly the visible and near IR out to ~~900nm.
            Almost no sensitivity below about 350nm.

            Remember the role of the NAVCAM is navigation, not science. To locate the comet against the star background, check Rosetta’s position relative to the comet etc etc. So spectral response is much less of an issue.
            For OSIRIS there are full details of the filters on the web.

            ESA has been putting up NAVCAM pictures essentially for PR purposes, because people were frustrated by the slow release of the OSIRIS images. But it’s the latter which are the prime imaging scientific output, not the NAVCAM.

            OSIRIS filters etc here:

            NAVCAM sensor data sheet here, including spectral response:


          • Solon says:

            “Data on the NAVCAM seems sparse.
            Whether it has a filter in I don’t know. It seems quite possible it’s just the unfiltered CCD response, which will cover mainly the visible and near IR out to ~~900nm.
            Almost no sensitivity below about 350nm.”

            The data sheet says 200-1100nm, so UV and IR. And with no filters, so yes, they are collecting whatever light they can to get an image. Most of the space based instruments now seem to be concentrating on nearIR/IR,, the Mercury Messenger NAC for example.

          • Harvey says:

            Yes, the data sheet says 200-1100nm, but take a look at the sensitivity curve! That’s salesmen writing spec sheets.
            In reality the sensitivity falls off a cliff below 350nm, and is way down beyond 900 or so.
            These occur for opposite reasons; below 350nm the silicon is too highly absorbing, the photons don’t reach the active detector volume, and a lot is reflected. At the other end, the silicon is becoming transparent.
            For a NAVCAM, all rather less important than for the scientific cameras, which target specific spectral information. Outside the visible and near IR/UV they use more ‘exotic’ detectors not based on silicon.

          • logan says:

            Thanks a lot for the specs, Harvey. State of the Art for its time [Civilian Tech].

      • Rod says:

        Hi Harvey
        Excellent answer to my question. I am a complete non scientific follower of events, but the logical answer you gave makes a lot of sense. Thanks for going to the trouble to respond!!
        Dangerous low passes seems the way to go.
        From a PR point of view I think that some spectacular close ups, especially if they got a shot of Philea in her final resting place R.I.P. would be great to remind the general population of what an achievement the entire project is.(apart from analysing the surface features, is a picture still worth a thousand words?) Maybe the scientists would consider this too much of a trade off for the continuing info they are still gathering

        • Harvey says:



          Others have now pointed out that the WAC (wide angle camera) seems to hold focus better than I’d expect, there is a picture taken at 15m. I’m looking into that further. The motion and data download aspects are unaffected.

          Yes indeed, there is a PR aspect which needs to be considered. There will always be a tendency for the scientific team to prioritise data over everything.
          But we do need public support for expensive ‘big science’. My view would be that, whilst no important data should be lost just for PR purposes, it is legitimate to take PR aspects into account. If we can generate good PR shots at small data cost, we certainly should.

          If you have a DSLR camera, with a telephoto lens, you can simulate the WAC yourself! Set the lens to 140mm at F5.6, focus on infinity, then look at closer objects without refocusing. I shall try it tomorrow.

          • originalJohn says:

            Hmm, a lot of photographic detail discussed here, rather like an old fashioned camera club weekly meeting. Let us not forget however that we are actually referring to a NAVCAM image on this page, an excellent, informative and unique NAVCAM image, regardless of whether it was posted for publicity purposes or not.

            There is something about this image which has not been picked up by anyone else so I will have a go at it.

            We know that at a phase angle of 159 deg it is 90% backlit. This has been alluded to but the implications of it have not. In accordance with the ice sublimation hypothesis for the origin of the comet jets all the jet illumination we see is held to be reflected sunlight, reflected from the surface of dust particles suspended in the jets.
            Now, very helpfully, the comet nucleus shows us what a particle looks like when 90% backlit, at a 159 deg phase angle. It is about 90% in silhouette. So every particle within the jet halo should also be 90% in silhouette. This would be expected to reduce the jet intensity considerably compared with the lower phase angles we are familiar with, all other things being equal, from all other published NAVCAM images. The jets should look dark and shadowy.

            This is not though what we see in this image. The jets look the same, at least as illuminated and bright as in most other NAVCAM images. This is particularly evident in the sharply silhouetted lower valley region of the nucleus. We can only conclude from this image therefore that the jets are not illuminated by 10% of the incident sunlight grazing off the dust particle surfaces ie they are not illuminated in the way illustrated by the comet nucleus.
            in reality something completely different must be happening. I leave it to the scientists among you to deduce what that might be.
            Indeed this unique image continues to show its value.

          • Gerald says:

            Originaljohn: … So every particle within the jet halo should also be 90% in silhouette. This would be expected to reduce the jet intensity considerably compared with the lower phase angles we are familiar with, all other things being equal…

            This presumption is intuitive at first glance, but wrong.
            Dust grains are sufficiently small to be translucent.

            In more detail:
            On the surface of the comet we see anisotropic backscattering. For clouds of dust, we get either isotropic scattering or anisotropic forward scattering:

          • logan says:

            Hi OriginalJohn. As you, still speculating of some residual ‘fluorescence’, stimulated by UV produced, and later ACCELERATED ELECTRONS. [Accelerated should they be, as suggested by H2O splitting amount so near the core].

            ” So every particle within the jet halo should also be 90% in silhouette”. Linear deduction. Would agree on a gross ponderation, if not for missing to include the biggest [illuminating] particle in this scenario, core itself.

  • Linda says:


    Amazing article and image shown in last is really very beautiful.


  • Ramcomet says:

    Hi Claudia,

    Is it possible to get names of the background stars? Particularly the row of three on the right?

    Or, is the field of view so tiny that from Earth they are dim and inconsequental, therefore have no names, only numbers?

    Even so, we can still look them up in starguides. 🙂

    Thank you!

    • Claudia says:

      Hi Ramcomet,
      The three brightest stars in this image are in the Pisces constellation and are visible with the naked eye.

      The brightest one is Delta Psc (or 63 Psc), with V magnitude ~4.4. The other two stars in the row are 60 Psc and 62 Psc, with V magnitude ~6.

      Best wishes

  • Angelo says:

    beautiful image, but how exactly does Rosetta take pictures? Is there a kind of time interval in which Rosetta takes photos or how did it work? How does Rosetta focusing at the comet so that we can get great images. Or is this picture a lucky strike?

    Hope my english is not too bad 🙂
    Thank you!

    • Gerald says:

      Hi Angelo,
      the focus of the camera is fixed to infinity. This works well at distances of several kilometers.
      The location of Rosetta with respect to 67P is monitored all the time, and it can be extrapolated into the future. The attitude of Rosetta can be determined and extrapolated, too. So, the timing for the best photos can be calculated. These calculations are used as part of a plan for observations. This plan is then translated into a command sequence for Rosetta. The command sequence is then sent to Rosetta via large radio antennae. Rosetta has an own spacecraft clock. So it can execute each command at just the scheduled instant.

      • Harvey says:

        Gerald. The NAC does have an ability to shift focus using a plate in the filter wheel. With it in they quite 2-1km.
        A simple flat plate would introduce spherical aberration, but I guess either it’s negligible or the plate is figured to correct it.
        No mention of a focus shift for the WAC, but it retain good focus far closer.

    • Gerald says:

      …Follow Logan’s post for an example, that OSIRIS would be able to change the focal distance, if necessary:

  • Polar bear says:

    What distance is the Rosetta from the earth? What is the distance between the 67P 28 km nucleus from the Sun? Compared with our distance between our Sun? What is the behavior of 67P compared with Halley Comet and size? Since 67P is approx 18 miles across, in the photo our Sun isn’t large enough or close enough to 67P to dwarf this image taken so the question is; that 67P is very far from the Sun in comparison with your image. What is the approx speed of this object 67Por is it slowly drifting; towards the Sun or not? In comparison between the story told about Halley’s comet. 67P looking more like an astriod especially if drifting and lucky passing just by. Who’s names on 67P?

    • Gerald says:

      Hi Polar bear,
      if you follow this link, some of your questions will be answered:
      The image was taken from a distance of 2.687649 AU from the Sun, from a distance 1.744125 AU from the Earth, and 819.763 km away from 67P. One AU is the mean distance Earth – Sun.
      The diameter of 67P is about 4 km.
      Currently 67P is moving away from the Sun, together with Rosetta..

      • Angelo says:

        Thanks for the answear!
        So, Rosetta doesn´t take photos without a command sequence from earth?

        • Gerald says:

          Hi Angelo,
          I’m not entirely sure. Usually these space probes do some default activity, if they don’t get new commands from Earth. This might include a NavCam image, since knowing the location of the comet is essential for Rosetta’s security. Certainly the star trackers (including their cameras) remain active.
          OSIRIS is a science instrument, without usually providing essential engineering data. And it requires specific observation plans. Therefore it’s very likely, that it will be switched off without new commands.

          You’ll find some of the planning framework here:

        • Harvey says:

          Basically all the observations are commanded from Earth. A great deal of effort is put into sharing out the available time, choosing pointing directions etc to optimise the data obtained. There are other instruments besides the cameras. There are also complex limitations due to spacecraft engineering and flight dynamics issues.
          The craft does have a ‘safe mode’ Into which it will go to protect itself if it becomes ‘confused”. It will protect the instruments and spacecraft and seek instructions from Earth in such circumstances.
          To communicate pictures back it has to point it’s ‘high gain antenna’ (‘dish’) at earth. If this pointing is lost, it does have another, slower channel which does not require accurate pointing at earth. This enables control to be recovered if the craft gets disorientated.
          So it has some autonomous capability, mainly to look after itself. But observation sequences are uploaded from Earth.

    • ianw16 says:

      @Polar bear,
      Most of your questions can be answered from the ‘Where is Rosetta’ link to the right hand side of the page:
      67P isn’t 18 miles across, though; only ~4km at its widest point. It is nowhere near as large nor active as Halley.
      Halley is ~11km mean diameter, and outgassed roughly 100 x more volatiles per second than 67P at its peak.

      Have to disagree about looking like an asteroid though; most of them look a lot smoother, and their crater morphologies are typical of impacts into rock. 67P looks a lot different.

  • originalJohn says:

    Nice images Claudia, both the original and the enhanced. I take it that at the 159 deg phase angle the Sun is not in the image but as you say strongly backlighting the nucleus which shows up as the general haziness of the original, drowning out ( nice technical term there) some of the intensity of the jets. An intensity nevertheless which appears at 400.1 million km (2.68 AU) from the Sun as strong as anything we have seen, apart from a couple of days around perihelion. Looked at in the enhanced image it could almost qualify for the description flare up. Interestingly the comet as well as lining up ( almost) with the Rosetta craft and the Sun is also very close to conjunction with Jupiter and the Sun. This is good because it means that good jet measuring opportunities still exist and may do for a good while yet, provided nothing dramatic happens. I am not hoping for drama of course. Much better if it just went steadily on its way.
    There is actually an interesting aspect of the electrical hypothesis of comets which allows for the possibility of the ions discharged by the nucleus to be negative on the approach to the Sun and eventually, though maybe not yet , positive ions discharged as aphelion is approached. That would be another interesting jet characteristic to check. Actually anything to do with the ion content of the jets and their charge state would be interesting and valuable.

    • ianw16 says:

      You wrote:
      “There is actually an interesting aspect of the electrical hypothesis of comets which allows for the possibility of the ions discharged by the nucleus to be negative on the approach to the Sun…..”

      Which ions being discharged by the nucleus? The species sublimated from the nucleus are neutrals. Only over time will they become ionised. The lifetime against photoionisation at 1 AU is ~ 10^6 s. They will also be ionised by charge exchange with the solar wind, and associated electron impact ionisation. Close to the nucleus this is relatively minor, due to the neutrals far outnumbering the solar wind ions. And for considerable periods the solar wind is not getting near the nucleus.
      As Martin mentioned in another thread, the plasma density compared to the neutral density is about 1 ppm:

      I’m sure that if there was an unexplainable excess of ions, positive or negative, emerging from the nucleus, then we would have heard about it by now.
      And, of course, the ratio of ions to neutrals also tells us that the solar wind proton flux close to the nucleus is precisely that which is measured by Rosetta, wherever it happens to be. Were it to magically increase in density by some orders of magnitude between Rosetta and the nucleus, then we would be seeing a far higher ion count due to increased charge exchange.

    • Harvey says:

      Comparing their intensity visually is meaningless. The images are processed. The exposure times are different. Here the dust is back lit, we are looking at small angle forward scatter, & the angles & dust size distribution are critical too.
      Matt has told us absolutely explicityly that in *ALL* cases we are seeing scattered sunlight and no intrinsic emission. He has also told us there are no unexpected hot spots. The magnetic field long since ruled discharges out. Give up grasping at non-existent straws.

      • originalJohn says:

        Yeah, wrong Harvey. The original image in this post is not enhanced and shows the true intensity. In previous posts we have seen non enhanced images too, almost invariably posted as a comparison if there has been any enhancement, and I suspect those non enhanced images were all at similar exposure settings. We have also seen images that have not needed any enhancement.

        • Harvey says:

          Ok , wrong in that I didn’t notice one of the images is unprocessed. Apologies for that.
          However the principle is correct; *both* images must be unprocessed for a comparison to be meaningful. Processing involves several parameters, and is general nonlinear, so the comparison of processed images is generally extremely dubious regarding intensity.

          Generally one will expect longer exposures now. The sunlight intensity has dropped by what, three stops or so I guess since perihelion. Furthermore dust densities are far lower. You increase exposure to increase signal to noise, fill the dynamic range as best you can.

          Both these factors will generally tend to make images look more similar in intensity than they actually are, by their very nature. We do them to make dim processes easier to see, to avoid bright ones saturating the camera.

          For these pictures, we are looking at fairly small angle forward scatter. Although it all depends on the size, shape and material of the particle, and the wavelength of the light, in general this is the direction of strongest scatter.
          Link at the end shows, purely as an example, the sorts of polar diagrams you get.

          In summary, comparison of intensities between different images needs great care.
          Both images must be unprocessed.
          The exposure must be known and allowed for.
          The scattering geometry has a very strong effect.
          It’s perfectly possible with full access to the data; but a gee look at that reaction is hazardous.

          • ianw16 says:

            Agreed. I’m no expert in photography, but a quick flit through the navcam images shows exposure times from 0.01s to 4.00s (as in the above image). 4s is the longest exposure I could find in a quick, random search.
            Phil Plait, on his Bad Astronomy blog, summed it up quite well in laymen’s terms: “This image was taken specifically to study the material around the comet; it’s a relatively long four-second exposure, and the camera was set to be more sensitive to light to capture faint details. ”

            The blog entry from 4 April shows a much less spectacular view, despite being closer. My bet, without checking, is that that is due to a shorter exposure time.

      • logan says:

        Would like to think that Matt is not being absolutist 🙂

    • Gerald says:

      the ion tail doesn’t form directly at the nucleus, but further out, when the uv light of the Sun had sufficient time to ionize the emanated gas:
      In the images close to the nucleus, you just see the sunlight scattered off the dust grains.

      • Kamal says:

        Gerald: Interesting point. When Rosetta went behind the tail could it have detected a stream of gas/ions in the antisolar direction?

        • Gerald says:

          Kamal, the gas first streams away from the nucleus with the speed of sound or even faster.
          Until radiation pressure from the Sun or solar wind compensates for this, it takes some time. So we won’t see significant effects in the emanated gas.

          But some of the gas emanating towards the Sun might return later, moving in antisolar direction. This portion, however will be much sparser than the gas coming from the nucleus in a direct way , and it might be shielded, if the free path is too short.

          At large antisolar direction odds should be better for your suggested observation.
          Rosetta’s instruments are sensitive. So, at 1000km distance, might be.

  • originalJohn says:

    Claudia, I submitted a comment yesterday about this optical image and I see it is posted today. Thanks for that. On reflection and in
    connection with this image I have a further comment on VIRTIS imaging which I would like to address to Booth as we have previously had an exchange of views on this topic. I would appreciate it if you would draw Booth’s attention to it in case he should miss it in his
    perusal of the blog.

    Booth, I replied yesterday to your comment addressed to me on the ” magnetic field free bubble” post and it has yet to clear the
    moderators. Although my comment to you today is an extension of that reply it makes complete sense as it stands. So here it is.

    We have here on this post by Claudia an excellent optical image of the comet nucleus and one of a type I do not recall having seen
    previously; at a phase angle of 159 deg and therefore back lit by the Sun. It set me thinking further on the issue of temperature
    measurement of the jets and VIRTIS imaging. Looking from the dark side of the nucleus it acts as a mask for the sunlit side, apart from
    a narrow top band and what we see is the jets standing out in a sort of star form halo. It struck me that this would be an excellent
    angle from which to image the jets specifically, with VIRTIS, without the distraction of the sunlit side of the nucleus.

    If you were to precalibrate the VIRTIS M instrument with a black- orange- white scale and with a white threshold temperature of 1000 deg K and point it at the nucleus from this 159 deg phase angle (or so) and at a distance similar to this optical image ( 200-400km) you might expect on capturing the image to see one of two things:

    If the jets are essentially water vapour sublimating from ice the image would be more or less uniformly black, with the nucleus, the jet star type halo and the surrounding sky all fairly close and low in temperature.

    If on the other hand the jets are from a hydrocarbon combustion source you would expect to see the nucleus black, a thin orange band at the top of the nucleus profile, a white star form jet halo, more diffuse orange for the immediately surrounding sky and black again for the background sky.

    These are two completely distinct and conclusive images for either comet mechanism hypothesis. Have you the nerve to do it Booth. You really have nothing to lose. If the image is uniformly black you can give yourself a pat on the back for being right all along. If the image looks like a black orange and white target you can say blast that blog nobody was right after all. But the discovery will be yours and humanity’s comet knowledge will have taken a leap forward. I am seeking no glory but I would have the satisfaction of knowing that from what you have said up to now you would not have imaged the nucleus with that temperature calibration if I had not suggested it.

    Clearly you would know ways to refine the experiment and clearly you would not want to rely on a single measurement. Further imaging
    would be advantageous, perhaps of individual jets from close in and including the nucleus surface too.

    It would be definitive Booth. Over to you.

    • ianw16 says:

      In case nobody has mentioned it already, combustion is IMPOSSIBLE in a vacuum. Would like a link to anything that says otherwise.

    • logan says:

      “…[If] the jets are from a hydrocarbon combustion source you would expect to see the nucleus black, a thin orange band at the top of the nucleus profile, a white star form jet halo, more diffuse orange for the immediately surrounding sky…”

      Dare to read, all of it. The expectation is to catch a trace -a spark- [of combustion] somewhere around the eclipsed halo.

    • Gerald says:

      Originaljohn, it appears you don’t yet understand the way the instruments are calibrated.
      Usually the instruments send raw data to Earth. Those are essentially data numbers (voltages or counts). These raw data are then transformed into physically meaningful values using some reference data as a calibration.
      If the amount of collected data is too large, some instruments may be able to do some on-bord data reduction, e.g. integration over several data sets, and send only the sums.
      But most of the calibration is done on Earth. In order to be able to calibrate the data accurately, you need reliable reference data. So it would make more sense, if you would propose taking additional data for calibration purposes in the temperature-related spectral range you assess as interesting.
      But I presume, that these calibration runs are done routinely, if necessary.
      Usually things like dark currents or backrgound noise are more interesting to assess the performance of an instrument.
      Another way is in-flight cross-calibration between instruments.

      • originalJohn says:

        No good suggesting it to me Gerald. Suggest it to Booth.

        • Gerald says:

          Booth appears to be very skilled and well-informed regarding space exploration, and regarding the related science and technology.

    • Harvey says:

      As usual, the problem is a lack of understanding of the basic physics involved.

      The thermal radiation *of a black body* (NOT real objects) is given by Planck’s law:

      From this we can derive Wein’s displacement law:
      A useful rough form of this is
      Peak wavelength in microns =3000/T(K). So at 300K, the peak is close to 10um.
      VIRTIS only operates to about 5um. The peak of the emission would move into the VIRTIS range at around 600K; for all the colder, expected temperatures all the VIRTIS band is on the short wavelength falling side of the black body curve, so it would be particularly obvious if the temperature was above 600K; it would stare you in the face.

      Stefan-Boltzmann law:–Boltzmann_law
      Total emission=sigma.T^4. BUT ONLY FOR A BLACK BODY.
      For a real object, Total emission= emissivity*sigma*T^4
      Now that’s fine, if emissivity is a constant over the range of wavelength measured.
      But usually it isn’t, we need emissivity(lambda), and then integrate over the black body spectrum weighted by that.

      To understand, or start to understand, emissivity you need Kirchoff’s law of thermal radiation.
      From which a vital point is:
      “For an arbitrary body emitting and absorbing thermal radiation in thermodynamic equilibrium, the emissivity is equal to the absorptivity.”

      So it’s crucial to know if your object is ‘optically thick’, does it absorb strongly. If not you see a ‘mix’ of the object and what is behind it.

      So, to instruments like VIRTIS.
      Before an experiment, the only things you choose are the camera aperture (not sure VIRTIS can change that), exposure time and maybe some electronic gain settings. VIRTIS then gives you the object emission in many spectral bands. Because you have many bands, in the case of an opaque object, you can pretty much unscramble the effect of emissivity varying with wavelength. You will have ground calibration data on channel transmission, detector sensitivity as a function of wavelength, etc; some spacecraft have carried calibration black bodies for precision measurements, but I don’t think VIRTIS has, or needs them. You do all that analysis back here.
      Now, in the worst case, it’s much hotter than expected, some channels might saturate. That would almost certainly cause you to re measure that point with lower gain, exposure etc. But because the emission varies rapidly with wavelength, it’s not likely everything would saturate; and with a reasonable number of unsaturated channels you could still derive a temperature.

      Of course by 1000K, an object is visible to the eye as dark red, and OSIRIS and the NAVCAM would be seeing it far more strongly because of their near IR sensitivity. But they haven’t.

      Matt has told us the highest temperature he recalls was ‘something over 50C’ or some such phrase. It seems clear to me he meant 50, 60C ……And not 700. I also don’t believe he would be so disingenuous as to use such a phrase if there were violently saturated data lying around. The intent of the question was crystal clear, and so was the answer; no ‘hot anomalies’, just solar heating, as he said specifically.

      Why this ‘disinterest’ or some such phrase in the surface temperature? Combustion, or discharges, would clearly deposit a lot of power there; it should clearly get hot! My little MIG welder, just 150A which plugs into a house socket, happily melts steel at what 1250C or so; surely the ‘electric universe’ can do better than Machine Mart 🙂

      In summary, the VIRTIS data already reported gives no hint of anomalous high surface temperatures, and Matt has directly confirmed that. *THEY WOULD HAVE SEEN IT*; no special experiment is needed. No hot spots in NAVCAM, OSIRIS.
      As I’ve tried to explain, VIRTIS for jet temperatures is tricky; but MIRO, out at 3.4AU, clearly reported low water vapour temperature, 165K I think, and uses a different method not subject to those problems.

      The situation of the EU ‘theory’ reminds me of my favourite cartoon, Road Runner. He can of course run off cliffs and happily keep going – until he looks down, at which point there is a splat.
      Look at the data; look down; I’m afraid EU long since went splat. No intrinsic emission, no consistent magnetic fields, no hot spots. But most of all no credible physical mechanism, aside from a total misconception of how electrostatic induction and combustion work.

    • Harvey says:

      Incidentally, a high phase angle would almost certainly be a very bad choice for attempting to measure jet temperatures with VIRTIS.
      The comet blocks the sun; but you are right in the strong forward scattering lobe from the dust. So you will see strong scattered 6000K black body, weighted by the scattering as a function of wavelength. This will make seeing intrinsic IR emission far harder.
      A phase angle near 90 degrees is likely to be a better choice; the sun is out of the way, and scatter likely to be considerably lower. We’ve had lots of those already.

      • originalJohn says:

        Well, you say the comet blocks the Sun Harvey but the image here is not at 180 deg it is 159. I assumed at that angle the Sun is not in the frame, so nothing of its surface radiation. 159 deg is in any event an excellent angle for an optical image, and Booth doesn’t agree with you. He thinks it is also a good angle for VIRTIS imaging. Me, I would be happy with direct measurement of the jet temperature at any angle.

        • Gerald says:

          You need to take care not to point the sensitive instruments directly into the Sun.
          “VIRTIS and OSIRIS cameras are located at the top of the -X (anti-sun face) so that their radiator may view deep space. The top floor is extended over the top as a sunshield to prevent any direct solar illumination of these instruments, while the sun angle on the -Z side has to be limited to 80 degrees for the same reason.”

  • logan says:

    Well, Gerald, Harvey, something is going on…

    If Out-Reach Team is planing of publish, please delete.

    • Harvey says:

      Thanks for the link, interesting.
      Clathrates make a lot of sense.
      Biology is another matter. Although of course we don’t have direct evidence, the low internal temperatures would seem to preclude it. Remember these objects spend much of their life much further out in the solar system, and a lot colder than my freezer, where I rely on not much biology happening 🙂

      • logan says:

        “…the low internal temperatures would seem to preclude it. Remember these objects spend much of their life much further out in the solar system..”

        You are right, Harvey 🙂

        But if in a panspermia mood of play: Could We wander if Earth biology surrounding clathrates share more chemistry with 67P, THAN rest of terrestrial life, as an average?

    • Gerald says:

      The presence of clathrates makes much sense. Great to see progress in showing their actual presence.

  • logan says:

    Now really happy. Can we chat about clathrate surrounding biology?


  • logan says:

    This photo perspective and lighting highly suggestive of Ducky being a SINGLE crystal.

  • Booth says:


    Re. your comment on 2016/04/07

    You write, “We have here on this post by Claudia an excellent optical image of the comet nucleus and one of a type I do not recall having seen previously ….

    Tis true! Rosetta has not flown along the comet’s tail before.

    Throughout the mission, science and safety would dictate the flight path the spacecraft could follow. For example, rendezvous occurred from the sunward side to give OSIRIS the opportunity to map and characterize the debris field left over from the previous apparition. Upon arrival, an excursion along the “tail” was not as important as mapping the nucleus and selecting a landing site for Philea. Most of the mission, since then, has been flown in a terminator orbit to avoid upstream and downstream dust debris liberated by sublimation processes. This current “tailward” trip was made possible by reduced comet activity combined with a solid 20 months of science! That is to say, the inherent risks associated with possible spacecraft damage have now been mitigated sufficiently to allow a sojourn into the comet’s tail.

    You write, “It struck me that this would be an excellent angle from which to image the jets specifically, with VIRTIS, without the distraction of the sunlit side of the nucleus.

    I concur! This is a great angle and location to collect additional comet data from. Of course, VIRTIS is not the only instrument to benefit from this orientation. You can be sure that ALICE, COSIMA, GIADA, MIDAS, MIRO, OSIRIS, ROSINA, and RPC all acquired extensive data on the jets, the coma dust environment, the ionosphere, and magnetic field structures downstream of the nucleus. Unfortunately, it is not clear if Rosetta was properly aligned with the comet to allow RSI coma soundings to be conducted. We shall see.

    You write, “If you were to precalibrate the VIRTIS M instrument with a black-orange-white scale and with a white threshold temperature of 1000 deg K and point it at the nucleus from this 159 deg phase angle (or so) and at a distance similar to this optical image (200-400km) you might expect on capturing the image to see one of two things: …

    I was really hoping my previous commentary on the subject of temperature scales was finally starting to make sense! Let’s try this again …

    You do not pre-calibrate this instrument with a temperature scale.

    As I have tried to explain many times before (apparently without success), temperature scales are applied “after” the hyperspectral data has been acquired and massaged into a useful form. Contained within the massaged hyperspectral data, we will find specific pixels that possess the maximum temperature measured for that entire dataset. It should also be obvious that different pixels will be identified with that of the minimum temperature. At this point, we may apply an arbitrary, colour-coded temperature scale of our choosing. And if it makes sense, we can force the maximum temperature scale value to be greater than that of the maximum temperature measured. Likewise, it is possible to select a scale minimum that is less than the minimum measured value. For additional information regarding this “latest” concept, please refer to my previously posted Venus Express #3 example and Grassi et al (2008). Note the temperature scales applied in Figures 10 thru 13.

    Question – Are you familiar with temperature scales/maps based on isotherms?

    To reiterate (again), a temperature scale is used by humans to aid in our visualization of thermal data. For example, when we look at a surface temperature map of 67P, we may want to identify where the coldest areas are located. If our coldest temperatures are colour-coded as magenta, it will be obvious where these regions are on our map. In our hypothetical study, we can then ask, do these cold regions correlate with exposed water ice? Specific comet geomorphology? Topography? Outgassing due to sublimation? Or some as yet to be determined process? To answer these questions, we turn to data from other instruments (e.g., VIRTIS for exposed water ice and OSIRIS for topography) to aid in our investigation.

    Now, something has just occurred to me, and I am going to try one more variation on this theme! What follows is a simple thought experiment …

    VIRTIS cannot be compared to a mercury-in-glass thermometer.

    I think I understand where your confusion is coming from! VIRTIS is NOT a thermometer in the traditional sense! VIRTIS is a hyperspectral sensor (i.e., it records EM wavelengths between 250 nm and 5000 nm)! We have all seen dozens of VIRTIS-M temperature maps. Most utilize the “black-orange-white” colour scale. As noted in a previous post, published maximum temperatures coded as “white” range from a low of 188 K (OOPS! this maximum temperature is actually coded as “red”) to a high of 245 K. These are the maximum temperatures for different datasets! Note, a maximum temperature of 188 K (regardless of how it is colour-coded) is still a valid maximum for that one dataset!

    Next, we will apply your “pre-calibration” temperature scale to all VIRTIS data as if we were dealing with a “fixed point” mercury-in-glass style thermometer. Please note, VIRTIS has already determined that 67P is a cold celestial body with, for example, a maximum “surface” temperature of ~230 K at 3.0 AU. Thus, with a fixed minimum of ~130 K (i.e., the lowest temperature VIRTIS can reasonably measure wrt the SNR) and a fixed maximum of 1000 K, we can easily replot all VIRTIS temperature maps released to date. Without going into detail, it should be obvious that each and every VIRTIS temperature map that uses your “B-O-W” colour scale will be essentially black to dark red! There will be no orange or white pixels on any of the new temperature maps! Now, under normal circumstances, we should be able to dispense with the rest of this, but …

    Let’s get back to the two possible outcomes of your proposed imaging experiment. We will assume that we have acquired and processed hyperspectral data that includes the nucleus, dust halo and jets. We must also assume that we have gathered this data as close to the nucleus as possible to improve the spatial resolution – recall that on approach in August 2014, VIRTIS was only able to report an average temperature of 205 K for the whole nucleus due to a lack of resolution. Per your request, we will assign the “arbitrary” temperature scale as outlined above.

    You write, “If the jets are essentially water vapour sublimating from ice the image would be more or less uniformly black, with the nucleus, the jet star type halo and the surrounding sky all fairly close and low in temperature.

    The visible jets and halo surrounding 67P are composed of dust particles that scatter sunlight. These coma features are shaped by sublimating volatiles driven by insolation. For the given “fixed point” temperature scale in this scenario, most of the frame will be black (i.e., temperatures at or below ~130 K). One would also expect a thin dark red temperature band coincident with the illuminated nucleus surface given a maximum temperature value of ~240 K in that region.

    You write, “If on the other hand the jets are from a hydrocarbon combustion source you would expect to see the nucleus black, a thin orange band at the top of the nucleus profile, a white star form jet halo, more diffuse orange for the immediately surrounding sky and black again for the background sky.

    Peer Review Query – Please re-examine your anticipated experimental outcome in light of the constraints you yourself have placed on the experiment. 1) Anticipated combustion temperature of 1000 K. 2) Maximum “surface” temperature should be consistent with earlier reported values of 230 to 240 K at this distance. 3) Jets and halo are composed of non-volatile refractory dusts – i) It has been shown that these dusts do not produce intrinsic illumination (i.e., their albedo wrt scattered sunlight is typically less than 0.06). ii) Dust jets and halo become more diffuse with radial distance from the nucleus yielding coma features that are essentially transparent with respect to the CMB.

    Given these issues, one would expect the nucleus to be black, the illuminated surface band to be dark red (i.e., ~240 K), the surrounding “transparent” dust jets and halo to be very dark red (only near the surface of the nucleus) transitioning to the black CMB temperatures of space.

    Of course, your anticipated outcome is also missing one very important physical process. The comet combustion model, as outlined, is physically impossible. Simply put, there is insufficient atmospheric pressure to initiate and sustain a combustion reaction (i.e., chemical reactions require collisions to proceed).

    In reality, it does not matter how much fuel or oxidizer you have, if the molecules are too far apart, you will not get a combustion reaction. Period! Now, I know that this statement does not sit well with you, for obvious reasons, so let’s look at some data. The following table provides a short list of pressures for various “atmospheric” regimes and their associated mean free path (MFP) lengths.

    STANDARD ATM : 1E+05 Pa : 7E-09 m – Sea Level (obviously combustion occurs here)
    THERMOSPHERE : 1E+00 Pa : 1E-02 m – Base at ~90 km ASL (bound w. mesosphere below)
    THERMOSPHERE : 1E-07 Pa : 1E+05 m – Top at ~600 km ASL (bound w. exosphere above)
    LUNAR ATMOS. : 1E-09 Pa : 1E+07 m – At the lunar surface
    IPM (#Space) : 1E-10 Pa : 1E+08 m – Vacuum pressure at 67P **

    Note #1 – The mountaineering death zone for humans (a compact set of well regulated “combustion” reactions) is ~8000 m! Restated – There is insufficient oxygen and atmospheric pressure above 8 km to sustain human life!

    Note #2 – Naturally aspirated aircraft engines can sustain combustion up to ~5000 m. Above that altitude, a forced induction system (e.g., a turbo charger) is required to increase the ambient air pressure to a value sufficient to maintain combustion within the engine. It is worth noting that the current altitude record for an air breathing engine is only 37650 m. Restated – There is insufficient oxygen and atmospheric pressure above 40 km to sustain the “mechanically regulated” combustion of hydrocarbon fuels with oxygen!

    Note #3 – The International Space Station orbits within the Earth’s thermosphere at an altitude of ~400 km. In the event of an “uncontrollable” fire onboard the ISS, the fire fighting protocol of last-resort is to vent the cabin atmosphere to space. Restated – There is insufficient oxygen and atmospheric pressure above 400 km to sustain combustion of any kind! Period! Unless you wish to discuss rocketry which employs “mechanically pressurized” fuels and oxidizers …!

    Now, without belabouring the point … combustion is IMPOSSIBLE when there is insufficient atmospheric pressure to sustain the chemical reaction! I’ve been working on a problem with respect to the coma’s atmospheric pressure, and I don’t see it being any greater than 10 mPa at the nucleus surface. If you consult the table above, you will note that 10 mPa is consistent with pressures found in the lower thermosphere. No combustion!

    Question #1 – Gravity plays a critical roll in regulating combustion. Can you figure out how micro-gravity on 67P is going to prevent your combustion model from working? No?

    Question #2 – A protoplanetary nebula is far more chemically active/reactive than this comet is as it orbits the Sun. Any thoughts on why that might be? Thermodynamics perhaps? Nebular density perhaps?

    You write, “Have you the nerve to do it Booth.

    That’s an odd question, especially given the missing question mark! 😀 Remember, life is short and we need to be able to laugh at ourselves. I laugh at myself constantly! For a change of pace, see if you can find the comical punctuation error in this post!

    And what’s nerve got to do with conducting great science? Every scientist worth their salt, would be all over this tailward excursion. For some, it represents a golden opportunity to look back through the coma with a known and stable illumination source. For others, dynamic changes in the ionosphere and magnetic field structures need to be examined in situ.

    Regarding your query, by the time you posted your comment, Rosetta was already returning to 67P. If my “calculations” are correct, the spacecraft was ~120 km above the comet’s z-axis (i.e., approaching a 90 degree phase angle).

    You write, “If the image is uniformly black you can give yourself a pat on the back for being right all along.

    I think your missing a valuable “big picture” point here!

    Science is iterative! Individuals and teams build upon the works of others. It is a slow and sometimes tedious process. References cited in peer reviewed papers represent one or more of these iteration loops. And the world rolls on!

    Pseudoscience proponents are absolutely certain that they are always right. Any evidence that contradicts their expectations and/or world view is obviously flawed in some way. A bulk density of 533 kg/m^3 can’t be right because the EU/EC model states with absolute certainty that comets are made of rock. But what if comets are actually made of ices?

    You write, “Over to you.

    Originaljohn, I wish it was more obvious to you (and others) that the EU/ES/EC models violate basic laws of physics and chemistry, and thus, cannot possibly operate in the real world. I know that you want to see high temperatures consistent with combustion or electrical discharge, but there’s a reason that these temperatures don’t show up in any datasets … they don’t exist! And it’s not that scientist can’t or won’t measure these temperatures, … you can’t measure something that isn’t there! Temperature (i.e., thermal kinetic energy) is a fundamental property of matter! It is “measured” constantly around 67P. Why? Because it tells physicists and chemists a great deal about the processes operating within the system. There is no conspiracy to hide data that would prove your models correct. There is just science!

    • Harvey says:

      Broadly speaking, agreed.

      Two brief points.

      VIRTIS may have a problem getting a temperature in the jets. They are very diffuse, and probably not ‘optically thick’, is not highly absorbing and will not have high emissivity. Whether they are or not will vary strongly with wavelength due to gas phase lines, whereas the dust will be more a continuum, but with some structure. This considerably complicates the derivation of a temperature for the jet; you see a weighted average of the background and the jet which is hard to unscramble. The surface is far easier, it is optically opaque, high emissivity.

      MIRO in contrast derives temperatures from line profile and is not subject to this problem.

      Re combustion.
      Many months ago I posted a link to the flammable range of an H2/O2 mix as a function of pressure. This is one of the most easily ignited systems known, with an extremely wide flammable range of ~4% to 94% at one bar.
      For small reductions in pressure the lower limit can fall very slightly, but then the lower limit rises, the upper limit falls, until THERE IS NO MIXTURE THAT WILL IGNITE. I’ve lost the link, but that pressure was FAR above credible comet surface pressures.
      As you said, combustion is a collision mediated process; lower pressure, less collisions, eventually combustion stops.

      Oh, and I’ve climbed to 6100m, and camped around 5400m, and can confirm from personal experience that the ‘slow combustion’ in my body struggled considerably – and that’s at roughly half a bar. Also the single engine light aircraft I fly, with no turbo charger, basically give up at about 3000m, 10,000 feet, for the reasons you cite. I have a PPL; nothing like direct confirmation 🙂

      Whether a candle would burn in free fall was debated theoretically for years. Eventually they tried it on the Space a station I think. From memory it did just continue to burn, as a weak spherical diffusion limited flame, but someone needs to dig that up. As you say, combustion has a problem with free fall and no convection.

      • originalJohn says:

        Quite simply Harvey you are talking about ambient pressure with respect to combustion, and you know this because you referred to your little aeroplane being not turbocharged. If you think about it you will realise that combustion in the “vacuum” of space is a common phenomenon, in the rocket thruster motors of spacecraft. The fuel and oxidiser are fed into the combustion chamber under pressure where they ignite. The pressure to sustain combustion is maintained within the cavity of the chamber. The ambient pressure has no controlling influence.

        And in your aeroplane too if you were able to equip it with an oxygen tank and a pump the engine would buzz away happily at 3000 m and above.
        And up the mountain if you had the foresight to equip yourself with oxygen and a respirator the slow combustion in your body would struggle considerably less.

        So, clearly combustion gas pressure is a local factor. At the comet nucleus surface it is easy to conceive of such a local factor that might arise. Essentially pressurisation of freed oxygen at the interface beneath the the thick hydrocarbon layer that coats the nucleus surface. In other words forming a pressure cavity analogous to that of the rocket motor and rendering the combustion independent of the ambient pressure. I stress here that the combustion of hydrocarbons in oxygen gas is clearly not hypergolic, although exothermic and self sustaining once started. An ignition source is required and that is supplied by the energy of the impacting solar “wind” protons.
        The properties of the nucleus hydrocarbon coating have not been addressed so far but it would be reasonable to assume that it is not a continuous coating, so many points of access for protons are likely, if, that is, they had insufficient energy to penetrate the hydrocarbon layer.

        So the ambient oxygen pressure around the nucleus is irrelevant with regard to combustion in the comet situation.

        Once again i remind you that the reactants, hydrocarbons and oxygen, are there. The ignition source is there and these well known
        and ubiquitous combustion products have already been identified with this comet: Water, CO2, CO, atomic hydrogen, formaldehyde and elemental carbon.

        • Harvey says:

          Rockets have massive turbo pumps feeding high pressure fuel into the combustion chamber. In fact those pumps are some of the most challenging things to design and build. The pressure in the combustion chamber is huge – that’s why it works! For a sold fuel rocket (which cannot stop and start; it merely burns to completion) you have an intimate mix of oxidiser and fuel.

          Where, please, on 67P do I find large turbo pumps operating?

          Why do alpha active sources not ignite combusible materials in a one bar, 20% oxygen environment? A far more energetic particle, in hugely higher numbers, in a far more favourable oxygen rich high pressure environment. But no ignition.

          Why no hot spots, no emission?

          Why is the fire extinguishing method of last resort on a manned spacecraft to vent the craft to space?

          Sorry, ‘combustion’ is just a complete non starter.

          • originalJohn says:

            Look Harvey, whatever non happening combinations you come up with nothing changes.
            With the alpha particles/ stuff/ 20%oxygen (air?) perhaps the ratio of fuel to air is outside the required range. (It is definitely a risky one.) This applies to some combustions. For example methane in air requires a comparatively large volume of gas in an average sized room to reach an explosive ratio, which is why domestic gas explosions when they do happen are so powerful and destructive.

            The basic fact remains that when fuel, oxygen and energy come together in the right proportions combustion will, not may, occur. This includes of course at the right pressure. The dispute would be whether those conditions do or do not occur at the comet nucleus surface.

            There is no evidence they do not. There is a lack of evidence at all about those conditions. it is quite possible that hot spot and emission data exists and has yet to be processed, as well as proton density and energy data. They must have a huge volume of data.

            As for the possibly suicidal theoretical method of last resort obviously it gets rid of all the oxygen and the fire goes out.

            These are all valid questions but they prove nothing. In the absence of evidence to the contrary in the comet situation combustion is a very strong starter indeed. The products, the fuel, the oxygen and potentially the activation energy are all there.

          • Gerald says:

            Originaljohn, this part of your post is going to make sense:
            “There is a lack of evidence at all … that hot spot data exist….”
            Don’t try to construct the contrary by applying merely rhetoric but unlogical sophism.

            First point to the data showing thermal anomalies, before postulating their existence.
            Some instrument data in raw and calibrated form have already been, and more of them will be published in ESA’s PSA and NASA’s PDS.

            Here e.g. 5.9 GB MIRO data:

            Or browse the data, starting e.g. here:

            Overview and root of all data releases:

        • Harvey says:

          Ah, just to be completely fair.
          It’s mainly big boosters that have big turbo pumps.
          Hypergolic engines generally use fuel fed from a passively pressurised system.
          The liquid fuels are however still mi9xed at full density, and the hypergolic reaction establishes the pressure in the combustion chamber; the combustion is still occuring at high pressure.
          So the quiestion then is where do I find tanks of UDMH and N2O4 – some of the nastiest, most reactive compounds around – on 67P?

          Whether is reaction is self sustaining depends on the local conditionns. If the pressure is low enough, every reactioon I know ceases to self sustain, even H2/O2.

          You are defeating your own argument. *Of course* my plane would fly higher if equiopend with a turbi charger or oxygen supply (impractical, too heavy.) Of course if you are high enough, carrying oxygen helps physical exertion. (No lack of forthought; climbers rarely use O2 below about 7000m now; the cost/logistics are a big issue, & below that you can cope with training & acclimatisation.)

          But there *are* no high pressure, pure supplies of oxygen on 67P. Just dilute oxygen, at very low pressure.

          • originalJohn says:

            Read again Harvey my proposal for an oxygen pressurisation mechanism.

        • logan says:

          Think I got your Idea, OriginalJohn. And kind of agree on worth experimentation. Combustion the way it happens on Solid rockets?

          The great conundrum would be the lighting on. Then temp would be self sustained in those fortunate cracks containing the right mixes.

          [Then again, the instruments are able to catch it (even by accident). And should had been, by now].

          • logan says:

            Also remembering an Antarctic documentary where a torch had to be put near to the carburetor input.

            At this old memory example we have the right pressure, the right oxidant and oxidizer, THE SPARKS, and NOT YET the right threshold temperature.

    • logan says:

      I have found guidance here, Booth. Thanks for taking the time. 🙂

    • originalJohn says:

      Thanks Booth for you comprehensive and interesting reply. I like your style and I appreciate that you reply with seriousness and modesty and on the whole manage to avoid the irritated and insulting approach of others on this blog who defend the sublimation hypothesis. You clearly believe as I do that if anyone questions you in your area of expertise they deserve considered answer no matter who they are. In fact all the more so if you do not perceive them as having status or impressive labels.

      And one more ( look a sentence started with a conjunction) thing Booth before we address the serious stuff. You picked me up for not including a question mark. It may be important to you but it is trivial to me. Typing these blog comments is hard work. Punctuation is a minor issue if the meaning is clear. Or do you really think I do not know what a question mark is or how to use it. Actually I would probably only use it anyway if the question was not obvious, as in the Italian language where the question is conveyed by the voice inflexion. So enough of that.

      Now, I thank you for your unequivocal bold statement about the concept of precalibration. I did not understand this from what you have said already. In fact in one reply you said that VIRTIS M would require recalibration to measure temperatures above 700 K . I took this to mean precalibration. As you know I am no expert in spacecraft spectrometers, as you clearly are. I have had to rely on the the five line descriptions of the instruments which accompanied the start of this mission and frankly they were pathetic if you really wanted to know how anything worked. Tokenism I am afraid. I now realise thanks to your efforts that even VIRTIS M does not work like a camera at all and all the data is post processed and calibrated according to identified maximum and minimum wavelengths recorded. And it seems that you are now saying that VIRTIS M with any set up will record any wavelength characteristic of the temperature range zero to say 2000K. This is interesting to know. So now it is apparent that the data you acquire will be determined by where you point it and whether you have sufficient spatial resolution or sensitivity to record and separate regions of different temperature. Judging by the image we have referred to both of these factors should be good at around 300 km and 160 phase angle. So that data is still there to be acquired as it appears that we do not have it yet, just your speculation and mine.

      Moving on to the jets and what they consist of you say
      “Jets and halo are composed of non-volatile refractory dusts”
      well even according to your sublimation hypothesis this is wrong. You have overlooked the subliming water vapour ( neutral) which transports the dust and in addition other volatiles like CO and CO2, and numerous others already identified, as well as ions resulting from photoionisation of some of this neutral material or collisions with electrons ( also with a photon collision origin). These are the only ion sources permitted in the sublimation hypothesis. Now as you know I laugh at that hypothesis and have other ideas about the jet composition and ion sources in particular but let us not go into that detail here. Suffice to say that I would regard possible sources of elevated temperature in the jets as twofold: plasma discharge from the nucleus surface and combustion at the nucleus surface, and these could be separate or concurrent. Plasma discharge has been denied by some based on what they claim are the laws of physics and I claim are groundless assumptions. Combustion has been dismissed by others and now you based on the laws of both physics and chemistry. Strong stuff which I contest but first let us be clear on, with suspended disbelief, the characteristic temperatures we are talking about.
      A plasma discharge in dark mode could be cold and include the nice low temperatures you are so comfortable with. For glow mode temperatures of up to perhaps a few hundred K above the freezing point of water and for arc mode temperatures up to 5000 K or higher but perhaps more likely 2000 K in the comet situation. These would be widely acknowledged, not my ideas.

      Now hydrocarbon combustion, just from the temperature point of view, I have suggested 1000 K as a minimum and a nice round figure. It could probably be as high as 3000 K under some ( forced oxygen ie oxy-acetylene type) conditions. And these are maximum flame temperatures, not surface of origin temperatures.

      The plasma discharge hypothesis is not mine. It is long standing and comes from those more knowledgeable than me. I fully support it and its basis in sound physics and plasma physics. Those who deny it on the basis of physics do so once again because of ignorance and preferred assumptions.

      The combustion hypothesis, is to my knowledge entirely mine, with respect to comets, first proposed on this blog around January 2015. At first the naysayers argued no oxygen available. Then copious oxygen was identified around the nucleus. Then it was thermodynamically impossible, an absurd argument that seems to have faded away. Now it is too low a pressure for collision to occur between oxygen and fuel molecules. Combustion is cited as impossible in the “vacuum” of space.
      Lets look at that last one shall we, heavily relied on now it seems. We know of course countless examples of combustion occurring in space and those are the rocket thrusters on space craft, including Rosetta. Those tell us that the requirements including the pressure are a local phenomenon. The reaction is not bothered at all by the “ambient” pressure. And this is the problem with the arguments that have been advanced about pressure with mountains and high flying aircraft. They are talking about ambient air (one fifth oxygen) pressure. If Harvey, your fan, could take his own oxygen supply up the mountain ( as others had the foresight to do) his physiological combustion would proceed untroubled. And if he could take an oxygen supply and a pump with him in his little aeroplane his engine would continue buzzing away merrily. He would have trouble aerodynamically because of the air density but with a nice buzzy engine he could recover that at lower altitude.
      So these are irrelevant examples, to do with ambient air pressure. Clearly the ambient air pressure is extremely low to non existent around the comet nucleus but we now know that the oxygen pressure is a different story. I do not however recall any published figure for actual oxygen pressure so it would be my assumption that it could conceivably support combustion. But the combustion hypothesis does not need to rely on ambient oxygen pressure. Think back to the in space rocket situation. They are hypergolic. The craft carries both fuel and oxidiser and requires no ignition source. As long as the necessary pressure is achieved in the local environment of the combustion chamber the combustion proceeds.
      So I would surmise for my combustion hypothesis that a similar situation might prevail ( not hypergolic) at the surface of the comet nucleus. We know the nucleus surface is coated with a thick ( several metres? ( there’s one!)) layer of mixed hydrocarbons. I would assume in the absence of specific information that this is in the form of a dense tarry substance or a dry agglomerated powder, a solid, stuck hard to the surface anyway. I assume also that the oxygen comes from the silica content of the rock, released from the SiO2 bonds by the cascading release of energy of the impacting solar wind protons( 1-10 KeV, often higher), Si-O bond energy 4-6 eV. The horrified dogmatists argue that this freeing of rock oxygen is impossible ( mainly because they have never experienced it or dreamed of it). An example has even been cited that a fused silica pot does not cause spontaneous ignition of a hydrocarbon liquid in it or rocks in oil wells do not ignite the oil. These silly earthbound examples of course prove nothing and miss the point. At the comet surface we are observing a specifically unique situation. The hydrocarbon coated nucleus is sitting in a stream of high energy protons. This will of course be denied by some but on the basis of theory, model and interpretation. Even at this late stage we have yet to see a published figure for proton current density anywhere near( within a few km of) the nucleus surface and only one published figure more than a year ago anywhere in the coma. So I contend that protons are pounding densely into the nucleus surface nonstop. Prove me wrong with the data, not models.
      The protons are important as an energy source to free the oxygen and as an activation source for the exothermic combustion reaction. I hypothesise that as the oxygen is coming from the rock ( or being released from the ice as in your hypothesis) it is pressurised beneath the thick hydrocarbon layer, more than enough to facilitate the chemical reaction. It in fact forms its own little rocket motor type cavity. Nothing in this contravenes any laws of chemistry or physics. It does however contravene the preconceived ideas of the the dogmatists and their assumptions.

      So let us talk briefly about the chemistry. I have posted previously some typical combustion equations. There is no need. It was really just to assuage the sceptics. The hydrocarbon combustion equations are widely known and understood. They are also quite complex and even in the simplest case the intermediate reactions can run into hundreds. Nobody would even attempt to list them all in a practical situation. They can change from second to second. But some common, ubiquitous, typical ones are widely known as are their complete and intermediate products. And it is those that point strongly to a combustion discharge reaction. The most obvious is H2O itself, along with CO2 and CO, these three, in the variable ratios found, typical of variable oxygen hydrocarbon combustion. In addition however with this comet both atomic hydrogen and formaldehyde have been identified, both easy to understand as common incomplete combustion products. Difficult to understand as hanging around on a comet nucleus for billions of years or being produced by ice sublimation. Both highly reactive. And finally ( you can see I like this starting conjunction Booth. Call it my hallmark) elemental carbon has been identified, also a ubiquitous product of incomplete combustion.
      These “coincidences” are far too much to ignore and not to be further examined, whatever temperatures have been measured so far. Far from contravening any laws of physics or chemistry the combustion hypothesis makes complete sense, far more so than the extremely shakey sublimation story for which no proof has yet surfaced and for which no test has yet been applied. The test I proposed tests both and gives a conclusive answer.

      Finally Booth i single out your two questions to answer
      Question 1 : Gravity. Is that so. Then what role does gravity play in the hypergolic thruster motor. The fuel and oxidiser are fed under pressure and the flame direction is determined by the thrust nozzle. Gravity even tinier than with the comet.

      Question 2 : An unfortunate pick, the protoplanetary nebula Booth. It is an imaginary concept with no evidence that any planet ever formed in that way and not even a viable model. A handed down and peer approved consensus. Nothing more and it will not surprise you that I do not accept it. There are far more interesting and plausible ideas for the origin of planets.

      To close then Booth let me make it clear that there is no pseudoscience in anything I have proposed. We could argue for a long time about what makes science pseudo and I can assure you that I could and have found many pseudo markers in the “science” you are so confident in. I won’t list them here but perhaps you could have a little think about what they might be. i will give you a clue. For many of them the laws of physics have yet to be invented.

      And as for the so called EU models if you examine them seriously rather than dismissing them and then examine some of your consensus models with the same seriousness and objectivity you will soon realise which do and do not contravene the laws of physics. I see that outright dismissal of alternative explanations is the in thing to do amongst the established consensus but my advice to you is to stop writing off those who highlight the significance of interplanetary, interstellar and intergalactic plasma and the electricity it carries and ask yourself is gravity really as important as you believe it is. I am afraid your old gravity only model of isolated bodies in a vacuum is dead and the comet is demontrating that before your eyes.

      • ianw16 says:

        Crikey. Only when dealing with YECs have I seen someone so desperate to hang on to an evidence-free, faith based belief!
        I have no problem with faith based belief systems, as long as they leave me alone, and don’t try to pretend that what they believe has actually got anything to do with science.
        Please let us know when the Nobel Prize letter turns up re combustion, otherwise it remains firmly within the file marked “woo”.
        The whole post looks like someone has condensed the mad ideas off of a particular crank science website’s forum, and put them all in one post. And all these things would have been seen. And haven’t been.

        I can see how ~20 months of zero evidence to back up the faith, and mountains of evidence to show it is wrong, might make somebody desperate, and cause them to start to make up things to back up their beliefs, but this really takes it to levels that I’ve rarely seen before.

        There is a reason why EU ideas, and the electric comet in this particular case, is not taken seriously by anybody in the scientific community. This post is a perfect example of why that is.

      • logan says:

        This comment of mine is TOO BIG to deserve reading. Please skip, if short on time…

        “…The hydrocarbon coated nucleus is sitting in a stream of high energy protons…”

        No OriginalJohn, I can testify the huge amount of data backed contra-arguments presented here for several months now.

        Those surface hydrocarbons are quite radiation reduced.

        Solar Wind protons hardly could be termed ‘high energy’.

        And no, no relevant amount of SW at 67P perihelion surface.

        Neither can read those titanic paragraphs. Just commenting this your 13 word affirmation.

        IF you could think that by getting no reply you make some kind of point. THEN you could be wrong.

        • originalJohn says:

          I am not sure I understand your point logan. There is no dispute that the comet nucleus is indeed coated in a hydrocarbon layer. And by “radiation reduced” presumably you mean reduced in amount but by radiation? What radiation and how?

          The energy of the typical background solar wind radiation is not in dispute either. It is normally in the keV range and often MeV. I would class this as high energy when compared for example to Si-O bond energies of a few eV. However not high compared to cosmic rays commonly measured in GeV.

          What is definitely disputable is the solar wind proton energy and density at the comet nucleus surface. Hypothetically it can be increased from background levels by natural plasma processes. By plasma current filamentation, long range attraction and short range spiralling (repulsion) and plasma pinching to increase current density and by acceleration across double layers to increase energy. There is a very strong probability that a plasma double layer exists at the nucleus surface.

          However no data has yet been offered for any proton densities close to the nucleus and to my knowledge only one figure about a year ago for proton density at an indeterminate point in the coma. All the contra arguments you refer to are not backed by data at all but by models, interpretations and assumptions.

          As far as plasma properties go despite a suite of plasma related instruments no plasma data has been offered either. No ion compositions, no ion densities, no charge data and no data or even comment on likely boundary effects at the comet nucleus surface.

          Everything that is happening at the nucleus surface remains open to question logan, particularly in view of my own recent observation that the saturation vapour pressure of ice at -50 deg C is around 3 Pascals. To put that in perspective 1 old fashioned pound per sq in (psi) is 6894 Pascals. Even a primary school kid would see straight away that such a pressure could not possibly do the work we see in the comet jets, leaving the nucleus surface at the speed of sound and carrying tons/second of dust. It is clearly absurd to attribute that to sublimation of water vapour at 3 Pascals pressure.

          A plausible mechanism is needed for the comet jets and quickly. Any ideas logan?

          • sjastro says:

            You wrote,

            “Everything that is happening at the nucleus surface remains open to question logan, particularly in view of my own recent observation that the saturation vapour pressure of ice at -50 deg C is around 3 Pascals. To put that in perspective 1 old fashioned pound per sq in (psi) is 6894 Pascals. Even a primary school kid would see straight away that such a pressure could not possibly do the work we see in the comet jets, leaving the nucleus surface at the speed of sound and carrying tons/second of dust. It is clearly absurd to attribute that to sublimation of water vapour at 3 Pascals pressure.”

            ….. and a primary school kid would also note that the jets are working against gravity which at the comet’s surface is only about 1/10000 the Earth’s surface gravity.

      • sjastro says:


        I’m still waiting on your explanation on the failure of the researchers in detecting free oxygen in this experiment simulating the action of the solar wind on silicate rocks.

        In fact I have been waiting for months but like any inconvenient truth you simply choose to ignore it because it contradicts your beliefs.

        Similarly a thermodynamic and chemical kinetic explanation for the lack of free oxygen WHICH IS SUPPORTED BY THE EXPERIMENT also fell on deaf ears either because it clashes with your dogmatism or is beyond your grasp for comprehension.

        This is only part of the story and deals with the precursor stage (formation of oxygen), the combustion mechanism itself in a near vacuum environment is totally ludicrous.

        • originalJohn says:

          I have replied to your “crucial” questions several times. I will summarise my replies as follows. On your thermodynamic and kinetic argument I drew your attention to the fact that the huge difference between the solar wind (even background) proton energy and the Si-O bond energy made any thermodynamic argument irrelevant. And with respect to the work you linked to it was indeed modelled on the background energy when in reality it is likely that because of natural plasma effects the proton energy at the nucleus surface is much higher than background. It is theirs and your assumption that the work relates to the comet situation.

          In any event it is now confirmed that the comet nucleus sits in a cloud of oxygen. I hypothesise that it comes from the Si-O source.

          On the question of the feasibility of combustion in a vacuum this has already been discussed here. I repeat for your information that as in a rocket motor in space the the so called vacuum is irrelevant. With the rocket motor the local conditions are what counts, in the pressure chamber. A similar local pressurisation could occur beneath the hydrocarbon layer at the comet nucleus surface. Totally ludicrous? Only in your mistaken opinion.

          • sjastro says:

            You wrote “I have replied to your “crucial” questions several times. I will summarise my replies as follows. On your thermodynamic and kinetic argument I drew your attention to the fact that the huge difference between the solar wind (even background) proton energy and the Si-O bond energy made any thermodynamic argument irrelevant. And with respect to the work you linked to it was indeed modelled on the background energy when in reality it is likely that because of natural plasma effects the proton energy at the nucleus surface is much higher than background. It is theirs and your assumption that the work relates to the comet situation.”

            Once again you are ducking the question regarding the experiment. T
            The experiment is an ideal method of testing your hypothesis. The proton energies are at 5 keV well above the disassociation energy of Si-O yet there is no free oxygen formed, no oxygen depletion of the silicates and water is formed in situ rather than in the gas phase.
            The fact that water is formed in situ illustrates that proton capture is the preferred thermodynamic and kinetic option as I explained to you many posts ago.

            You wrote “In any event it is now confirmed that the comet nucleus sits in a cloud of oxygen. I hypothesise that it comes from the Si-O source.”

            Even if Si-O bond disassociation was possible which it isn’t, you run into another problem regarding the density of the comet. Another example of ducking an inconvenient truth or are you suggesting O comes from the silicate dust particles in which case it is up to you to show the stoichiometric relationship where copious amounts of O are released from Si dust particles.

            You wrote “On the question of the feasibility of combustion in a vacuum this has already been discussed here. I repeat for your information that as in a rocket motor in space the the so called vacuum is irrelevant. With the rocket motor the local conditions are what counts, in the pressure chamber. A similar local pressurisation could occur beneath the hydrocarbon layer at the comet nucleus surface. Totally ludicrous? Only in your mistaken opinion.”

            It is totally ludicrous to model the comet as an oversized rocket. Why doesn’t the comet behave like a rocket and follow Newton’s third law regarding thrust? How do comets maintain stable predictable orbits if subjected to thrust?
            Pardon the pun this going from the “sublime” to the ridiculous.

        • originalJohn says:

          When it comes to the force generated from high pressure combustion in an open ended cavity the gravitational force exerted by a minute comet sized nucleus at the origin has no practical significance. The force is generated in the combustion reaction and propels the combustion products and the dust, achieving in the case we are observing supersonic velocities.
          To support your beliefs you have to show how a saturation vapour pressure of 3 Pascals can generate supersonic vapour velocities and elevate tons per second of solid matter. I will allow you to assume zero gravitational opposition. It won’t help you though because obviously there is no gravity term in the thrust equation And here is a hint. Your exit pressure is 3 Pascals as you have no combustion reaction and no means of increasing the pressure beyond that. A difficult one for a primary school graduate, I know. Oh and another hint. You may not assume a supersonic exit velocity. You have to show how that velocity is generated.

          • sjastro says:

            A good example of the argument of personal incredulity at work.

            The key word of understanding what is happening is sublimation.
            In case you didn’t you know solids undergo sublimation directly into a gaseous state if the vapour pressure at its melting point exceeds the atmospheric pressure of the external environment. The atmospheric surface pressure at the comet is near enough zero due to the microgravity conditions.

            At -50C the mean thermal velocity of water vapour molecules is around 550 m/s which exceeds the speed of sound at sea level. The velocity can be further increased by nozzle or compression effects when sublimation occurs in cavities near the surface.

            Dust carried by gas flow can be modelled as particulate matter entrained in fluid involving parameters such as Reynolds number, viscosity and density.
            If the particle velocity is zero relative to the fluid, then it is being carried by the fluid.

            In the case of dust it reaches supersonic speeds if carried by the water vapour.
            There is nothing incredulous about it and doesn’t require exotic items such as internal combustion engines, only an understanding of the physics.



            IT IS A WELL KNOWN FACT that cometary tails and jets can result in thrust forces.
            The thrust forces are the result of sublimation but are insignificant compared to your internal combustion engine.
            To put this in perspective whereas thrust via sublimation can result in uncertainties in the orbital and rotational properties of the comet, your internal combustion engine would literally blow the comet out of its Keplerian orbit.
            Rather than saving your combustion hypothesis all you have achieved is to dig a bigger hole for yourself.

          • sjastro says:

            To expand on my previous post.

            When water vapour is produced, irrespective of whether it is through sublimation or your combustion process, the molecules move in a frame of reference away from the surface at a certain bulk velocity which is the mean of the individual molecular velocities.

            Due to thermalization caused by molecular collisions, a Maxwellian distribution is formed where the standard deviation is based on the individual thermal velocities of the molecules.
            In a stationary frame of reference at an equilibrium temperature of -50c, the mean thermal velocity for water molecules in a Maxwellian distribution is around 500m/s well above the speed of sound.
            Hence even for a “slow” bulk velocity, a large percentage of water molecules produced by sublimation travel at supersonic speeds at -50c.

            How fast the dust particles move depends on inertial and viscosity effects.
            Particles with small mass and high surface areas are more likely to be entrained and carried by the water vapour at supersonic velocities.

            Your internal combustion engine mechanism is a moot point anyway given that proton energies will not cause Si-O disassociation and combustion is impossible.

            There is another serious flaw in your hypothesis.
            A pressurized chamber is a necessary condition for ignition to occur so how does the coma form? Your subterranean chambers which are isolated pockets will only lead to the formation of localized jets. The coma on the other hand, is formed by water vapour production over the surface or just below it.
            Your mechanism prevents this as the conditions are not pressurized.

            On top of this as mentioned previously, the thrust forces for your internal combustion engine would ensure there would be no Keplerian orbit to speak of.

            With sublimation no such problems exist.

            There is absolutely nothing going for your hypothesis, every aspect of it has been thoroughly refuted.

  • Booth says:


    … and others

    Re. your comment on 2016/04/06

    The NAVCAM optics information ye seek can be found in the ROSETTA-NAVCAM Interface Control Document, V4.1 (ref. Table 4), and on the NAVCAM Instrument Kernal webpage at the JPL.

    For quick reference …

    * Wavelength range : 550 to 850 nm
    * Field of view : 5 deg
    * Number of pixels : 1024 x 1024
    * Pixel Size : 13 micron
    * Aperture
    DNA / FNA modes : 70 mm
    FA mode : 30 mm
    * Relative aperture
    DNA / FNA modes : F/2.2
    FA mode : F/5.1
    * Eff. Focal length : 152.5 mm

    Wavelength range is green to near IR
    DNA = Defocused, Not Attenuated – Used to determine position of point sources
    FNA = Focused, Not Attenuated – Used for close quarters imaging and navigation
    FA = Focused, Attenuated

    I would also like to thank you, Sir, for the link to the E2V CCD47-20 Data Sheet. I can’t remember the last time I had a good look at a product spec sheet. Brought back both good memories (of my younger days!) and some not so good memories (too much stress living on the bleeding edge of technology). 🙂 Thanks again!

    • ianw16 says:

      Thanks for that! Answered a couple of questions I had about a composite image I was putting together. I did it to show that the camera can obtain very different views of the comet, depending on settings. Here is the image:

      You’ll notice that the images were taken at ~ the same distance. The ‘darker’ image was taken ~32 mins after the ‘brighter’ one. Other differences are that the ‘darker’ image is a ‘navigation’ image, the ‘brighter’ is a ‘context’ image.
      The ‘brighter’ image has a much shorter exposure time (surprised me). It is also DEFOC_NATT, (defocused, not attenuated) as opposed to FOC_ATT. (focused, attenuated) for the ‘darker’ one. The ‘brighter’ image also is low gain as opposed to high gain for the ‘darker’ image.

      So, although the image in this thread has an unprocessed version that looks ‘bright’, that is purely down to the camera settings.

    • Gerald says:

      Thanks, Sir! These documents are useful. They remind me of what I ought to work on.

  • Harvey says:

    In turn, many thanks for hat link. I’d looked around but missed that.
    Really rather interesting.
    In both of the ‘NA’ modes the F number is a surprisingly low 2.2, so is depth of field will be considerably worse than the OSIRIS WAC, as it has a similar focal length.
    Also interesting that they *defocus* slightly to *improve* centroid location, the opposite of what one might expect. I guess this is because the de focussed PSFs (point spread functions) are probably smoother, and may vary less over the field, than in-focus, making it easier to define a point source location.
    At F2.2 I very much doubt it’s diffraction limited and the PSFs can be messy and asymmetric in focus.

    It also means that any comparison of NAVCAM image intensities must take into account NAVCAM mode, especially whether it’s NA or not, to be valid.

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