Comet jet in 3D

OSIRIS scientists have created a spectacular anaglyph view of a jet seen blasting from the nucleus of Comet 67P/Churyumov-Gerasimenko in August.

Rosetta observed increased levels of cometary activity in the weeks around perihelion as the comet made its closest approach to the Sun along its orbit, at a distance of about 186 million km – between the orbits of Earth and Mars. On the approach, the increasing solar radiation heated up the comet’s nucleus, causing its frozen ices to escape as gas and stream out into space at an ever-greater rate, dragging the comet’s dust with it.

A period of peak of activity, where distinct jets increase in number and intensity, is thus expected in the weeks around perihelion. The jet shown here is one such example of the high level of activity seen over the last months; it was witnessed on 12 August, just one day before perihelion.

A ‘3D’ anaglyph view of Comet 67P/Churyumov-Gerasimenko based on two images acquired by Rosetta’s OSIRIS narrow angle camera on 12 August 2015, capturing a spectacular jet event. The two images are separated by 2 minutes 28 seconds, which corresponds to a stereo angle of 1.2 degrees. The image scale is 3.9 metres per pixel. In this orientation the Babi and Aker regions are visible on the large lobe to the left, while Ma’at and the circular Hatmehit depression are seen on the small lobe to the right. Diffuse dust emission and other jets are visible all around the nucleus. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

A 3D anaglyph view of Comet 67P/Churyumov-Gerasimenko based on two images acquired by Rosetta’s OSIRIS narrow angle camera on 12 August 2015, capturing a spectacular jet event. The two images are separated by 2 minutes 28 seconds, which corresponds to a stereo angle of 1.2 degrees. The image scale is 3.9 metres per pixel. In this orientation the Babi and Aker regions are visible on the large lobe to the left, while Ma’at and the circular Hatmehit depression are seen on the small lobe to the right. Diffuse dust emission and other jets are visible all around the nucleus. Acknowledgement: D. Romeuf (University Claude Bernard Lyon 1, France; images: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

But creating a 3d anaglyph of dynamic events like this is notoriously difficult: often the jets are too faint or their duration is too short to find two high-quality images taken several minutes apart that are suitable to pair together to create this type of view. However, the OSIRIS team got lucky with this particular event, capturing two images separated by about two-and-a-half minutes.

The image shows a bright, collimated jet embedded in a broader emission structure. The three dimensional perspective also reveals the conical shape of the jet and that the collimated feature is emitted towards the observer.

Jets have been seen to originate from a large diversity of morphological features on the comet’s surface, such as pits, cliffs or icy boulder fields. The physical processes responsible for the different jet structures are subject to much discussion, but anaglyphs like these can certainly help in gaining an understanding of their three dimensional form and evolution.

The image is best enjoyed with red-blue/green ‘3D’ anaglyph glasses. The left- and right-eye views have been extracted and are provided below.

Additional images and a movie of the perihelion jets and general activity can also be viewed here and here.






  • MRW says:

    Suggestion (3D without red-blue glasses): Place the original pictures side-by-side on the web page rather than one above the other. Some of us do not have red-blue glasses, but can do the “magic-eye” trick on suitably positioned image pairs quite well. 6-8cm of image separation (feature-to-feature), right eye image on the right, will allow one to see the stereo view in the following manner: (1) Allow your eyes to relax, as if you are looking into the far distance (don’t look at the images, look through them as if looking at an imaginary horizon out a window); (2) You should experience double vision on each of the images, with one of each pair overlapping in the center. (3) Move your head (not your eyes) to merge the central pair properly so there are three images total (4) Now the most difficult part – allow your eyes to focus on the middle, merged image while still looking into the distance. When successful, the central image should be quite sharp, the 3D view should pop out (it’s not strong in this image set though), and your eyes should not feel overly strained as they are essentially looking into the distance. The technique works well with thumbnail sized images.
    The idea is that when you are looking into the distance, a ray-trace from each eye to the horizon where you would be looking (if the screen were not in the way) would pass through corresponding features on each separate image. Focusing of the eyes uses different muscles than the pointing of the eyes, and with a little practice (allowing the eyes to relax as if daydreaming helps) one can learn to focus without moving the eyes pointing as well. Once you’re successful a few times it soon becomes quite easy. It will also work by deliberately crossing your eyes, however that requires switching the left and right images and is considerably more stressful on the eyes.

    • Harvey says:

      Well I tried, thanks.
      But unfortunately for some reason trying to do this destroys my sense of balance, I felt instantaneously vaguely sea sick and swayed on my chair!
      (No, I don’t have any relevant medical problems 🙂 )

    • Chris Elliott says:

      Here you go:
      I loaded both full resolution images into GIMP and put them side-by-side. (Left on left, and Right on right. Viewable using SIRDS style “wall-eyed” convergence instead of cross-eyed.

      Yes, I know this full resolution image is way too big for eye separation unless you are a giant. Just load the image into your favorite windowed image viewer and scale the window down to a comfortable viewing size.

  • Hilary Wright says:

    I was wondering why these jets don’t alter the orbit of the comet, or do they?

    I presume that you are using jets to alter the position of Rosetta, changing its orbit and location on a regular basis.

    Similarly why don’t the comet’s jets change its course?

    An explication would be appreciated. Thanks

    • Harvey says:

      Hilary. In principle they do both change the orbit and change the rotation of 67P.
      But the net effects are rather small for two reasons.
      Firstly 67P has a mass (‘weighs’) around 10^13kg; if you are not used to scientific notation, that is 10,000,000,000 tonnes; its VERY heavy, so it takes a big ‘push’ to do anything to it. The jets etc look spectacular in scattered sunlight, but are actually pretty tenuous; they aren’t like big ‘rocket engines’, the pressure is very low.
      Secondly, since jets emerge in lots of different directions, they tend to average out; jets which ‘accelerate’ the comet get cancelled out by jets that ‘decelerate it’ crudely.
      Similar arguments apply to rotation.
      For rotation, small changes caused by the jets certainly have been measured many times. I don’t know if changes to the orbit caused by them have ever been observed directly; they will be very small typically, but we can measure the orbit very accurately, so maybe.

      • logan says:

        Thanks Harvey, Gerald 🙂

      • logan says:

        Just in a small foot-note Harvey. As jetting tend to occur at perihelion, and at perihelion face, wouldn’t it tend to ‘push’ a little the orbit out of the sun?

        • Harvey says:

          If you have a given force and a given mass, you get a certain acceleration. Force=mass*acceleration.
          So in that sense it doesn’t matter when it occurs.
          But the direction in which it occurs etc will alter what change it makes to the orbit.
          But the net effect on the orbit tends to be pretty small, for the two reasons given. We can rearrange it as acceleration=force/mass.
          That huge mass on the bottom line, the denominator, makes the accelerations are small – and in addition, the force is probably pretty small.
          The jets *look* impressive – but they are very low density and low velocity compared to rockets for example – but they do go on for a long time.
          Undoubtedly changes occur, but for a fairly low activity comet like67P, generally fairly small.

    • Gerald says:

      The outgassing does change the orbit of the comet.
      This makes comets kind of unpredictable to some degree.
      So Rosetta could just hope to find the comet near the predicted position after hybernation.
      …and Rosetta needs to adjust the trajectory regularly considering cometary activity.
      This all looks easy from outside, not quite so for the navigation team.

  • Keith Wansbrough says:

    Yes please Side by side images would mean I can see 3D without finding my coloured glasses. Thanks!

    • emily says:

      Hi Keith,
      Yes, this is why we have provided both images. If they don’t display side-by-side in your browser, then just download them and arrange them following the helpful advice of MRW above.
      Best wishes

      • Benjamin Hehle says:

        why dont you do it ? if someone downloads them seperate, how to open 2 Pictures with the same Programm ? than they are to big to make em cross eyed. its total a valid wish, and if it hadnt been asked, i would have told you the same. So instead of, telling individual people, what to do, just make it cool for everyone with a simple html line? just my 2 cent.

  • THOMAS says:

    This image of the August 12 event is much more informative than the vastly over-exposed image originally shown in the post.

    With the fine detail of the jets revealed in this 3D image, we can now see striking similarities with the even more spectacular August 22 outburst ( In both cases, multiple jets are fanning out upwards from a very large white area on the surface which clearly possesses intrinsic brightness, as in the August 22 outburst. It is indisputable here that the jets are emanating not only from the centre of this active area (with the several-hundred-metre-high flame in the dead centre) but also from all around the extensive rim: the brightest jet is emanating from the very edge of the left-hand rim, from what seems to be higher ground. This again can only be an arc-mode discharge phenomenon, with temperatures high up in the thousands of kelvins. The acquired temperature data should be able to confirm this unambiguously.

    In this connection, it should be noted that, as far as I am aware, there has so far been only a passing reference to extremely high temperatures on 67P in one of the peer-reviewed papers published to date, namely in the abstract to the article “In-situ investigations of the ionosphere of comet 67P” presented at the recent European Planetary Science Congress 2015 in Nantes, France ( The whole abstract is extremely interesting with its mouth-watering allusions to the highly active plasma activity within the coma and in particular close to the nucleus, but the highlight is surely the statement: “In contrast to the often modelled scenario for a very active comet, the Langmuir probe instrument (RPC-LAP) finds electron temperatures mainly in the range of tens of thousand kelvin around this less active comet. This can be attributed to the lower density of neutral gas, meaning little cooling of recently produced electrons.”

    Strangely, these plasma and temperature findings failed to make the headlines as they surely deserved to… I eagerly await publication of the full article and the attempts which will no doubt be made to account for these truly electrifying findings within the framework of the standard ‘dirty snowball’ model.

    • ianw16 says:

      And none of any of that has anything to do with electric rocky comets! If you bothered to check the literature, you would find that the electron temperatures are as expected from modelling. They are no doubt similar to what was found outside the diamagnetic cavity of Halley, nearly 30 years ago!…295..795E&letter=0&classic=YES&defaultprint=YES&whole_paper=YES&page=795&epage=795&send=Send+PDF&filetype=.pdf

      You may further note that the very active comet 103P Hartley 2 had temperature measurements made of it. The areas of the jet regions on the smaller lobe are no different (maybe slightly cooler) than the surrounding areas.

      The “intrinsic brightness” you claim to see is just another example of pareidolia.

      • ianw16 says:

        Right, that first link above doesn’t work so well. Try this:…295..795E

        Click on “Other Article Options”, scroll down and select “Send PDF”

        • ianw16 says:

          And neither did that. fhggiuewgfiuhddkjfgkjssaa!!!”!”!

          Google scholar; type in “The electron temperature in the inner coma of comet P/Halley”

          (We really need an edit button here!)

          • Booth says:


            Sir! I would like to thank you for the link to the Halley electron temperature paper. It is interesting to note the progression of temperatures from a low of 105 K at 1740 km to a maximum 26140 K at 10950 km!

            The authors also make mention of field reversals observed by the Giotto magnetometer (Raeder et al. 1987) that may have played a roll in the temperature spike. Of interest is the innermost discontinuity located at 13500 km, just outside the region where the maximum temperature was measured. Some kind of inner shock structure? Curious about Rosetta’s recent sojourn through the plasma and what discoveries may have been made.

            And one additional little gem plucked from the text – the photoionization frequency for H2O is 5.5 x 10^-7 s^-1, independent of distance from the nucleus. I expect this value is a function of heliocentric distance.

            I would also like to concur with you on the need for an edit, or recall button. I have a post awaiting moderation that I would really like to fix before it gets released to the community. (So if you are reading this Emily, could you please reject said “ice cycle” text so I may perform some surgery on it).

            And a style guide would be nice. For example, can we use old V1.0 HTML syntax to format tables and prevent lost links and whole blocks of text. My problem has always been the standard “greater than” and “less than” symbols.

          • Harvey says:

            “the photoionization frequency for H2O is 5.5 x 10^-7 s^-1, independent of distance from the nucleus. I expect this value is a function of heliocentric distance.”
            Yes it is.
            The photoionization frequency is the product of the cross section for the process (in m^2) and the photon density (say N) over the relevant energy range (in photons/(m^2.sec)) Strictly an integral but we can ignore that complication.
            The process cross section is constant, and N does not vary significantly over the scale of distances relative to the comet.
            However it will simply scale as 1/(R^2) where R is the distance to the sun.
            To get the actual rate you multiply the neutral density by this frequency.

      • THOMAS says:

        @ ianw16
        “And none of any of that has anything to do with electric rocky comets! If you bothered to check the literature, you would find that the electron temperatures are as expected from modelling. They are no doubt similar to what was found outside the diamagnetic cavity of Halley, nearly 30 years ago!”

        The notion of “similarity” you allege is certainly not shared by the authors of the abstract who take pains to stress the SINGULARITY of the results from 67P by their formulation “IN CONTRAST TO the often modelled scenario for a very active comet…” (my capitals, for emphasis). They clearly believe there is something significantly different about their detection of such active plasma around the nucleus of 67P which electron temperatures “in the range of tens of thousand Kelvin” attest to. Read the rest of the abstract too for other mouth-watering comments on the “dynamic” and varied nature of the inner coma plasma activity.

        @ ianw16
        “You may further note that the very active comet 103P Hartley 2 had temperature measurements made of it. The areas of the jet regions on the smaller lobe are no different (maybe slightly cooler) than the surrounding areas.”

        This interpretation is presumably based on the “spectral cubes” shown in Figure 1 of the article you link to. You omit to mention that given the large size of the pixels on which the readings are based, the thermal emission values you refer to have necessarily been averaged out, and are in no way specific to the visible jets (which, in any case, emanate from the limb of the nucleus, with their bases barely visible, if at all). It’s like claiming that a hot submarine spring which was thought to exist in a particular part of the ocean cannot do so since the average temperature of the surrounding km² of water shows no significant difference compared with the rest of the ocean. I’m not sure either to what extent certain statements in the article have a bearing on this issue, such as “In this paper, we are only interested in continuum from the nucleus, i.e., reflected light and thermal re-radiation of absorbed sunlight” and data reduction methodology article 6 “Removing bad pixels : All known bad pixels (saturated, dead, cold, hot, or with a highly non-linear response) are flagged, and their values were replaced by NaN (Not a Number) and thus ignored in our analysis. The bad pixels represent 2% of the total number of available pixels.” Both of these statements suggest that even if heat from the jets was detected, it was deliberately removed from, or ignored in, the analysis. The other obvious point to be made about the Hartley 2 temperature readings which would tend to reduce their scientific value on this particular point (temperature of violent outbursts) is that they were acquired during a 44,300 km/h flyby at a distance of 700 km, i.e. over a period of mere minutes, during which there were clearly no violent outbursts such as the July 29, August 12 and August 22 events observed on 67P. It is the temperatures associated with these types of violent outbursts, necessarily unobserved during any of the comet flybys, which need to be disclosed for any serious conclusions to be drawn about their being the manifestation of electric arc-discharge activity.

        As for my alleged pareidolia with respect to “intrinsic brightness”, I note that it is unambiguously shared by the mainstream author of the blog-post on the August 22 event (, who stated that “the activity is EXTRAORDINARILY bright”, a very striking use of apparent hyperbole in a piece of scientific writing. In fact, the use of “extraordinarily” in that blog-post should be understood in its original, literal, non-hyperbolic sense of “extra ordinary”, i.e. “that which is not in the normal order of things”. This tacit admission of intrinsic brightness in the August 22 event (for which the Rosetta teams were stated as being “busy analyzing the data to understand the nature of these events”…) logically confirms the same phenomenon as being present in the slightly earlier August 12 event which we are discussing here.

        Given that you make no comment on the subject, you presumably concede my point that the jets are emanating not only from the centre but also from all around the rim of the surface reaction zone during this outburst, as was also the case with the August 22 outburst. Can you provide an explanation for the mechanism at work here in terms of the standard “sublimating ice” theory?

        • ianw16 says:

          Sorry, again that is just wrong. These electron temperatures are nothing out of the ordinary in the vicinity of a comet. As noted, the modelling and observation at Halley, were for a VERY ACTIVE comet. This one isn’t! Despite that, electron temperatures similar to this were seen at Halley. As were higher ones and as were lower ones. Read the paper I linked (sort of). And your contention that the authors are “surprised” is also wrong. As they say………..” finds electron temperatures mainly in the range of tens of thousand kelvin around this less active comet. THIS CAN BE ATTRIBUTED TO THE LOWER DENSITY OF NEUTRAL GAS.”
          i.e what one would expect to see at a less active comet.
          If you’re not sure, contact the author.

          As for the jets at Hartley 2, perhaps you could let us know the expected temperature of an electric discharge, and then explain how H2O ice grains could not only be entrained within it, but survive long enough to be detected?
          The jets were neutral CO2, and entrained H2O ice grains. They were cold.

          Regarding the blog entry, Claudia merely makes the point that the image is extraordinarily bright. This has no implications for some sort of imagined “intrinsic” brightness. This was a blog entry, not a scientific paper!
          To extrapolate what you have from those comments is nothing more than grasping at straws, quite frankly, although I’m sure Claudia is quite capable of speaking for herself.
          We won’t even need temperature data to disprove this electric discharge nonsense. Electric discharges are noticeable in other ways. They certainly wouldn’t have done much for radio communication, and they also produce hard x-rays, which have never been seen at any comet, including Hartley 2.

          In short, I suggest you await the publication of the actual scientific data regarding these jets, and the disappointment that that will no doubt bring.

          • originalJohn says:

            Ian, let us remember that there may not be just a single source of electrons in the vicinity of the comet. The electrons which constitute part of the solar wind current would typically display plasma temperatures of 100, 000 deg K or more.

            However, with respect to the comet nucleus and the hypothesis that it could be discharging electrons we are interested in the electron plasma temperature within the discharge jets, or indeed the positive ion temperature. If the jets exhibited electron temperatures of the order of thousands of degrees K those are electrons discharged from the nucleus, not solar wind electrons.
            Discharges of sublimated neutral gas would not indicate electron temperatures of thousands of degrees K, in fact would not exhibit electron temperatures at all as all the electrons would be tied up in the neutral gas. The concept of electron temperature applies only to a plasma.

            If as some claim the neutral gas would be subject to instant photoionisation ( in a proportion of typically 1 ppm ) the photon energy is no more than a few tens of eV so the resulting electrons could have no more energy than that, making electron temperatures of thousands of deg K from the photoionisation source an impossibility. Only an electric field could generate those sort of electron temperatures, of the possibilities associated with a comet nucleus.

          • Harvey says:

            You *CANNOT* convert an electron energy to a temperature without further understanding of the electron motion. If you had 10eV electrons, *exactly*, the temperature would be *ZERO*.
            Only if the energy distribution is Maxwellian and the motion random can you use this conversion factor. The energy quoted is then the peak of the distribution.
            So, for example, a 10eV electron beam can have a temperature *far* lower than you calculate – & this is a commonplace situation.
            I’m not commenting on whether the conversion is, or isn’t valid in this case as exactly what electrons we are talking about isn’t very clear. Just that you absolutely cannot use this conversion ‘blindly’ in any circumstances.
            (Similarly you have to be careful about ‘plasma temperatures’; the ion, neutral & electron temperatures commonly differ widely.)

          • logan says:

            Having a Zillion electrons at 1000Peta_eV pressure blasted, all dancing happily and coherently towards me.

            Going to be ‘cold’ fried 🙂

            Fully agree, Harvey. Indeed, actual temp is the ‘noise’ distribution of this otherwise ‘perfect’ scenario.

            Is Temperature a measure of ‘noise-ness’, Harvey?

          • logan says:

            Does exist any way to detect a blast like this, apart from alterations in the fields [and those fields altering something more visible]?

            Or, We will know when it happens? 🙂

        • Gerald says:

          THOMAS, the temperature is usually determined by the shape of the IR spectrum. This doesn’t average out easily.

          • Harvey says:

            We need to be a bit careful about what temperature we are talking about.

            Depending on the resolution is available, IR can measure:
            – Translational temperatures if in the Doppler broadened regime, requiring very high resolution.
            – Rotational temperatures, requiring moderate resolution.
            – Vibrational temperatures, requiring rather lower resolution. Different vibrational modes may differ in temperature.
            Electron temperatures generally need direct probe measurements; remote measurements are possible at short range (Thompson scattering), in Tokamaks) but not feasible at long range.
            UV/Visible spectra can give ‘electronic’ temperatures of ions and neutrals if indeed the use of a temperature is sensible at all.

            These various temperatures can differ *WILDLY*.

            Note that ‘temperature’ implies a Maxwellian distribution; this is most commonly seen in highly collisional systems. In a comet coma, it’s quite possible species will not show that distribution, and the use of ‘temperature’ is invalid. Sometimes we see a ‘two temperature’ system, with two roughly Maxwellian distributions at differing Ts superimposed. But sometimes that doesn’t work either.

            Beware ‘temperature’; it’s not quite so simple around 67P.

            Higher neutral densities will generally lead to lower electron temperatures; they cool by collisions. It would be surprising if they didn’t!

          • originalJohn says:

            Ok Ian I was wrong above about the photoionised electrons. I have checked the conversion and 1 deg K = 8.621738 x 10 power -5 eV so, say, 10 eV is equivalent to roughly 116. 000 deg K.
            Difficult then to separate photoionised and solar wind electrons.

            Easier however to separate electrons with temperatures of 1000 – 10, 000 deg K as coming from another source. That source could be electrons discharged from the comet nucleus.

          • Gerald says:

            for planetary bodies, usually the brightness temperature is meant by “temperature”. That’s not really a temperature in the strict physical sense, since actual temperatures cannot be determined easily by remote measurements.
            But for almost black objects, probably including the dust, the brightness temperature should be close (within a few / less than 10 Kelvins) to the grey/black body temperature.

          • originalJohn says:

            Agreed Harvey the concept of temperature with regard to the ionised state of matter is a tricky one and does not facilitate deliberations on this topic.
            Perhaps we are used to talking about temperature in general and absolute terms because the plasma state is not widely considered.
            Plasma temperature, regardless of the degree of ionisation is measured in eV or deg K ( they are interchangeable) and is not a description of the random Maxwelian motion and collision of particles. It is however a description of the energy of the moving particles and the work they are capable of. The degree of ionisation will however describe the quantity of neutral matter which is part of the plasma and within which the Maxwellian consideration remains as well as energy dissipated by collision with ions.
            The fact that the electron temperature can differ widely from the positive ion temperature does not help either but in the thermal state the electron and ion temperatures are in equilibrium so in the case of the plasma (plasmas) in the comet coma it is a matter of knowing what category of plasma we are dealing with. We need to know the electron and ion temperatures and the degree of ionisation at least to asses this.
            It is my judgement that in the solar wind plasma the degree of ionisation is high and the ions and electrons are in thermal equilibrium so the plasma temperature, which is measured continuously in the vicinity of the Earth, applies to both electrons and positive ions. It is likely too that this background level persists in the vicinity of the comet, outside the coma boundary.

            Within the coma boundary it is a whole new kettle of fish and we rely completely on the Rosetta team for the data there. As I have suggested before it is likely that at some points the proton speed and therefore temperature is higher than the background level, considerably higher, because of double layer effects and, as we are dealing with the influence of a strong electric field there, electrons too are similarly accelerated. So once again we may be considering a thermal plasma in some regions.
            Without the relevant data however it is pure speculation so the sooner it is issued the better.

            When you say ” exactly what electrons we are talking about” I presume you mean the origin of those electrons and the electron temperature within particular plasma regions is probably the only way of deducing that. From the figures released so far it is apparent that the electrons measured at tens of thousands of degrees K in the vicinity of the nucleus are not solar wind electrons but have a different origin, in my view the nucleus itself as electrons released in photoionisation processes would have similar energies to the particular solar wind UV photons ie 100,000 deg K plus.

            An important issue clearly and the more data available from within the coma the better with regard to understanding what is going on, including the origin of ions and electrons and the source of their temperature.

            By the way any polite advice or correction in the realm of heat would be appreciated but beware the critical differences between the plasma and gas states. For example a 10 eV electron beam, a non neutral plasma, has energy and is capable of doing work and generating heat even though it might have a Maxwellian temperature of zero.

        • Harvey says:

          This continual desperate splitting of linguistic hairs and desperate search for some support for this ‘theory’ by misrepresenting what people say really becomes tedious.
          The author says it is ‘extraordinarily bright’; and that’s *ALL* they say. There is no ‘tacit admission of intrinsic brightness’ whatever. The words mean what they say; it was extraordinarily bright. The meaning hardly needs ‘dissection’ or explanation.

    • Harvey says:

      The paper *does not* refer to ‘an ‘extremely high temperatures on 67P’; it refers to a fairly high *electron temperature* in the plasma surrounding 67P..
      That is a completely different *place* and a completely different *material*; the eletrons in a tenuous plasma not solid material.

      I see no reason it would ‘make headlines’; 67P is not a very active comet (despite the visible fireworks) so neutral densities are low, electron cooling low, electron temperatures high. Whats the big surprise? If you tell me the first fact, the next two are immediate probable results. Scientists like to hype their results up a bit too 🙂

    • originalJohn says:

      Good observation THOMAS.

  • originalJohn says:

    Interesting and spectacular images and with short term discharges orders of magnitude brighter than observed up to that time. Did this not suggest the idea of making some physical measurements on these new bright jets, such as temperature, ion content and ions to neutrals ration. Also specific ion analysis.

    • Harvey says:

      What is the point in repeatedly suggesting measurements we know are not possible?
      Rosetta cannot safely fly anywhere near these jets.

      So the plasma experiments cannot operate there.

      The spectroscopic instruments, especially ALICE, might give some data. But they will have trouble with the scattered sunlight from the dust masking the line emission. Only *excited* species can be measured that way; ground state neutrals or ions could only be observed in absorption (like Fraunhofer lines from the sun ) but I doubt that’s feasible owing to the low pressure*path product & scattered sunlight noise; but maybe.

      Rosetta is a brilliant, successful spacecraft; but it has its limitations, like any instrument. It is pointless ‘demanding’ things it cannot do.

      • originalJohn says:

        Oh I thought, Harvey, that the objective of the mission was to study the comet and how it works and that the instruments were selected accordingly. Do the ESA investigators have the same fear of approaching the nucleus as you do, and the same lack of confidence in their instruments capabilities to measure temperature in the thousands of deg K range, ion content and ions to neutrals ratio. Rather important and significant variables I would have thought.

        • Harvey says:

          It has little to do with the instruments, though some might be damaged.
          The spacecraft backed off because it’s star sensors are blinded by the dust; it cannot navigate or orient its high gain antenna if it looses star tracker data. It can maintain orientation for a while using other systems, but they drift.
          Really close in, in the Jets themselves, the risk of impact damage would also become acute.
          Yes, it’s mission is to measure 67P and its environment; it won’t do that if it’s destroyed by an approach which would be certain to be fatal.
          It’s not a matter of ‘fear’; it’s a matter of understanding the capabilities and limitations of the spacecraft.
          Actually, for very different reasons, I’d love to have data in there; it would be fascinating; but it’s not feasible till the comet quiets down again. We don’t need that data to disprove the discharge ‘theories’; we already have *ample* data for that, and they don’t obey basic laws of physics.

          • originalJohn says:

            Well Harvey as I understood it the objectives of the mission included placing a craft in orbit around the comet for the purpose of studying it. In orbit navigation is not required so the star tracker and the default that flicks it to safe mode can be immobilised . As for the high gain antenna, in orbit the craft is not continually passing through dense jets so periods should exist frequently when the star tracker could be reactived. I accept that the communication time is great but as the orbit is known the star tracker reactivation could be preprogrammed.

            I am sure ESA appreciate your sympathy but it seems to me not a beneficial approach to avoid the vicinity of the nucleus or wait until the comet becomes inactive. The active state is clearly the interesting one. Forego that crucial information in favour of what. More photographs. I am sure they could figure out a way of minimising the risk.
            And anyway presumably much valuable data has already been gathered at this stage and the ultimate fate of the craft was always destruction. So the risk is small. So what if the mission ends sooner rather than later in the retreat from the Sun, if the reward is the accumulation of crucial knowledge that could not be acquired in any other way.

          • Harvey says:

            Originaljohn. Bluntly, you clearly have no understanding of the spacecraft engineering issues. The best case would be the craft entering ‘safe mode’ which can take a *long* time to recover from. The worst case would be loss of the craft. There would also be substantial risks of impact damage, although relative velocities would be low.
            It would be an insane risk; far more important to observe the nucleus at high resolution post-perihelion to understand how the morphology has changed in detail.
            There is absolutely no evidence of discharges, they do not obey the most basic physical laws, they could be detected remotely (& already would have been long ago) – why waste a fantastic spacecraft on red herrings.

          • logan says:

            Understanding your bold tech proposal, OriginalJohn. Clever 😉

            But clear was from early December that former flying logistics was well beyond comfort of Teams’ majorities. It’s their call [and like their approach (as for boldness, prefer a final satellite ‘binding’)].

    • logan says:

      Hi OriginalJohn. You were around when confronting reality of dangerous particle-o-sphere. Why do you suggest such a bold navigational operation?

    • logan says:

      There is data from before departure and is going to be data from after it. Few weeks as to be safely extrapolated. No missed data collection.

  • Dave says:

    Ian its difficult to follow your reasoning, why the referance to other comets? We are alongside 67p.
    If we had more than the extract available to us I maybe could follow.
    The extract looks unambiguous, so i dont understand your vigour or direction. If you have read the whole article from which the extract is taken, then enlighten us so that we can understand what you are trying to get across. at the moment I am baffled.

    • ianw16 says:

      And I don’t understand what “extract” you are on about. The Halley paper is available in full, just type in the article name at Google scholar. That will take you to adsabs.harvard. Click on “other article options”, scroll down on the resultant page and click on the “send pdf” button.
      And you are right; it is unambiguous! These were the electron temperatures measured at Halley 30 years ago. They are perfectly in line with what we would expect from the vicinity of a comet. That is, the temperature has been lowered due to its interactions with the cometary neutrals.
      At Halley the temperature of 10 000K was seen at 22 000 km. At 67P it will be closer, as it is nowhere near as active as Halley. None of the stuff Thomas refers to is news. It has been observed before and fits in just fine with expectations and modelling for a comet.
      FYI the electron temperature in the quiet solar wind, is ~ 100 000K.

  • logan says:

    Looks ‘plume’ like. Inter-layering outburst?

  • Solon says:

    The jets and what is happening to the comets surface seems to me to be an example of electron beam etching.

Comments are closed.