GIADA investigates comet’s “fluffy” dust grains

In a recent paper published in the Astrophysical Journal Letters, the GIADA team present their findings on the properties of dust particles from Comet 67P/Churyumov-Gerasimenko. This blog post has been prepared with inputs from lead author Marco Fulle, and GIADA principal investigator Alessandra Rotundi.

The location of GIADA marked on the Rosetta spacecraft.  Image courtesy Alessandra Rotundi

The location of GIADA marked on the Rosetta spacecraft.
Image courtesy Alessandra Rotundi

GIADA, the Grain Impact Analyser and Dust Accumulator, is designed to capture dust particles in the coma of Comet 67P/Churyumov–Gerasimenko as Rosetta flies around it. The characteristic properties of the dust grains can be used to infer the history of the material being ejected from comet.

The latest study focuses on the dust particles collected between 1 August 2014 and 14 January 2015. The GIADA team find that the dust particles impacting on their detectors can be separated into two families: ‘compact’ particles with sizes in the range 0.03–1 mm, and somewhat larger ‘fluffy aggregates’ with sizes between 0.2 and 2.5 mm.

The individual compact particles have a bulk density of 800–3000 kg/m3, consistent with a variety of minerals or mixtures of minerals. On the other hand, the larger aggregates are made up of many sub-micron sized grains with void spaces in between, resulting in fluffy, highly porous structures that are mostly empty space. These aggregates are associated with the fluffy particles seen by Rosetta’s COSIMA instrument.

Indeed, the fluffy particles have effective densities of less than 1 kg/m3, literally lighter than air (at sea-level), and which Marco likens to the equivalent density of a dandelion seed head in a vacuum.

During the study period, a total of 193 compact particles were detected, impacting the GIADA detectors at an average speed of 3 m/s. A total of 853 detections of fluffy particles were made, the great majority associated with 45 dust ‘showers’. Roughly 2–3 of these showers were seen by GIADA each week, lasting anywhere between 0.1 to 30 seconds. A typical shower might be broken up into a handful of sub-showers as well, lasting from 10 milliseconds up to a second.

The fluffy particles are likened to the density of dandelion seed heads. Image: Greg Hume - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

The fluffy particles are likened to the equivalent density of a dandelion seed head in a vacuum. Image: Greg Hume - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons

While more fluffy particles were detected than compact ones, their size distribution reveals that they only contribute a minor fraction of the total mass of dust being lost by the comet.

Also, importantly, most of the fluffy particles were detected hitting GIADA at less than 1 m/s. Because the escape speed of both types of dust particles from the surface of Comet 67P/C-G should be the same, the fluffy aggregates must be decelerated somehow. The scientists believe that this is happening due to Rosetta itself.

Measurements made by Rosetta’s RPC-LAP instrument show that the spacecraft is negatively charged at between –5 and –10 volts due to a variety of effects associated with the plasma environment of the comet and with solar UV photons hitting the spacecraft. This negative potential acts to decelerate approaching dust particles, which are also negatively charged.

Marco explains: “Both the spacecraft and the dust particles are negatively charged, so there is a repulsive force between them. The amount of deceleration experienced by any particle is related to its charge and mass, with the maximum amount of charge held by the dust particle determined by its geometry.

“For example, the fluffy aggregates can collect about 20 times more charge than compact particles of equivalent radius. These fluffy particles will be slowed down more, and could even be stopped or repelled by the spacecraft if their charge-to-mass ratio is large enough.

This image was taken by Rosetta’s NAVCAM at about 385 km from the comet centre and shows the comet's dust streaming out into space. The image measures 34 km across. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

This image was taken by Rosetta’s NAVCAM at about 385 km from the comet centre and shows the comet's dust streaming out into space. The image measures 34 km across. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

“In addition, the more charge collected by a fluffy dust particle, the larger the internal disruptive forces are, and so the greater chance it will become unstable closer to the spacecraft, leading to its fragmentation before arriving at GIADA.”

Thus, the denser compact particles are not greatly impacted by this effect, while the fluffy aggregates are slowed down and disrupted creating the showers and sub-showers seen by GIADA. The smaller fragments arrive at the spacecraft at speeds as low as a few centimetres per second and may further collapse into rubble piles, as seen in the COSIMA images.

Density is a critical parameter differentiating these two families of particles, and it is likely that they have quite different histories. By comparing the GIADA results with laboratory measurements, the scientists believe that the denser compact grains represent materials that underwent significant processing in the environment surrounding the new born Sun, before being accreted by Comet 67P/C-G as it formed in the outer Solar System.

The low-density fluffy particles, however, are thought to be primitive material linked to interstellar dust, material that pre-dates the birth of the Sun and escaped any processing before being accreted by the comet during its formation.

More detailed information can be found in the paper, which can be accessed here.



  • Thorsten Claus says:

    Wouldn't the repelling effect mean that there are actually many more fluffy particles around that could be much smaller but never make it to the collector?

  • Sovereign Slave says:

    Another riddle! This is fantastic. Would be interested to know if there are any other explanations being considered for the slowdown and what they are if so. Again, if dark energy/matter etc is pervasive throughout the universe, shouldn’t these be considered as a possible force for affecting something as insubstantial as this extremely light dust? Though I am very doubtful of these theories, seems like possible evidence for them shouldn’t be overlooked. But if a negative charge is indeed the reason for the slower speeds of the fluffy dust, would be nice to hear why the dust particles are negatively charged. I guess the likely culprit is the plasma environment and solar UV photons that have negatively charged Rosetta. By my calculations, at 30 kilometers, it would have taken the dust 2.78 hours to reach Rosetta at 3 meters/sec. Seems like plenty of time to negatively charge something like dust. But from what I’ve read about the solar wind, it’s pretty clear that there are still many questions and unknowns regarding it, and its many effects on the solar system’s environment. I could not tell why it would negatively charge dust, though others can no doubt explain this, or what the full extent of the plasma environment might be around P67 or how it is affecting it’s dynamics and the dust, but it is appearing to be very complex, with perhaps a good many unknowns still to be discovered. Am also wondering if the dust could have already been negatively charged before leaving the comet’s surface? And if so, could that be representative of the comet as a whole being negatively charged or not? And if so, was it already negatively charged before it’s re-approach to the sun, or is it, like Rosetta, being negatively charged by the plasma environment? Anyway, this report will no doubt raise many many more questions than it answers, which is always extremely exciting.

    • harvey says:

      Its not obvious why the dust would be negative; simple UV effects would drive it positive. It turns out to be the result of a fairly complex model, The secondary electron yield is the main free parameter; their Figure three shows the potential to be rather insensitive to this over a wide range dm>1.3 (which curiously does to include the lab observed values, dm~2, but seems to have asymptoted to a constant value; but I'm always cautious about extrapolations.) With dm>1 again its not obvious to me why it ends up negative! Can anyone come up with a simple argument?
      Everything is based on a 100eV, 2*10^8/m^3 plasma environment. The data is mainly late 2014, pre possible bowshock formation - if one has formed yet.

      By the look of this, the particle has 'long forgotten' whatever charge it had when it left 67P.
      Their model:
      "We assume that this value was
      measured in the expected plasma density of 2 ´ 10^8 m−3 and energy of 100 eV. Dust charging models were computed taking into account currents provided by cold solar wind ions and electrons, photoelectrons, coma electrons, and the secondary electrons induced by hot and cold plasma components. These simulations (Figure 3) show that the dust equilibrium potential
      is set by the electron collection current from the ambient plasma and by the secondary electron current emitted by the dust, which depends on the yield parameter dm (Mukai et al. 2001), 2.0 < dm < 2.3 for compact grains of olivine or graphite (Lin & Joy 2005; Balcon et al. 2012). The yield parameter decreases in time due to surface aging effects (Davies & Dennison 1997). In fluffy particles dm is lower than in compact grains because most secondary electrons with an energy of a few eV are ejected toward other parts of the same particle and reabsorbed. Here we consider dm as a free parameter, and we fix its value according to the assumed plasma parameters and the measured Usc value."

    • Gerald says:

      Spacecraft charging is a topic as old or even older than spaceflight, see e.g. this summary:
      or this paper of 1981:

  • Marco says:

    This is going to be fun for the moderators. All the right keywords are there - negative charge, porosity, plasma.

    Do we know anything about what elements and compounds are in the grains? Anything more specific than dust?

    • Kamal Lodaya says:

      Here are two pieces of data from the paper: "Fluffy particles
      are expected to have a low albedo due to the high number of
      internal reflections when illuminated by the GDS lasers." and
      "The GDS signal was converted into a geometric cross section using calibration curves based on compact amorphous carbon. These were the darkest laboratory analogs that could be used for GDS calibration."

      • harvey says:

        Last sentence of this section, omitted above, is important as it indicates the likely uncertainty if the particle is 'bright';
        "The GDS signal was converted into a
        geometric cross section using calibration curves based on
        compact amorphous carbon. These were the darkest laboratory analogs that could be used for GDS calibration. Fluffy particles are expected to have a low albedo due to the high number of internal reflections when illuminated by the GDS lasers. With calibration curves based on the brightest available silicates, the geometric cross section would become a factor of three lower and the related dust masses a factor of five lower."

        So the inferred density would be ~3^3/5 higher, ie a factor of 5.4 which is certainly much more dense - but still very low. (In using 3^3 I have obviously assumed 'uniform scaing', which could be wrong, but we have no data.)

    • Marco says:

      I knew it! So much fun. It would be so interesting in itself but hijacked by fringe (but popular) alternative theories, the comments become a confusing mess. I so want to ignore this thread, but it makes me laugh.

  • Lucas says:

    Is there any water in the dust? Electric charges... hmm no word "sublimation" used we have some prgogress....

    Can any one tell me why the jets, are oriented towards the sun all the way? Shaouldn't they be bended the other way(according to sublimation theory)?

    • harvey says:

      Jets have been observed in other directions; single frames often show jets at 90 degrees to each other from different regions. There is no reason sublimation diriven jets would only point towards the sun.
      The charges and voltages involved here are tiny, entirely within the range expected by conventional theory, and provide no support whatever for the EU so called 'theory'.

    • Gerald says:

      Any ice in the (small) grains would have sublimated in the near vacuum, before reaching the detector.
      Maybe bound water can survive the vacuum.

      Sublimation is more intense on the sun-illuminated side of the comet, hence the side pointing towards the Sun. Just sublimation from deeper layers continues on the night side.
      Ejected dust is later bent away from the Sun, mainly due to radiation pressure.
      Solar wind acts mainly on ionized gas.

      • Kamal Lodaya says:

        Gerald: Thanks for the correction, mainly radiation pressure for the dust to move away and mainly solar wind for the ionized gas to move away.

    • Kamal Lodaya says:

      Lucas: "All the way" is an exaggeration. We only see the jets for 20-30 kilometres and the Sun is more than 200 million kilometres away. A comet's tail is seen to point the other way from the Sun. At the distances Rosetta is watching 67p from, we are inside the head ("coma") of the comet. Eugene Parker around 1958 proposed that the solar "wind", composed of positively as well as negatively charged particles coming all the way from the Sun (the same thing that causes aurorae in our atmosphere) pushes the dust and gases in a comet's head away from the Sun to form a comet's tail.

      Sublimation does not talk about this. It is supposed to explain how the dust and gases get out from the comet into the head, not how a comet forms a tail.

      Some proponents of the electric theory argue that electrons are stripped from the comet's surface when it is in a negatively-charged environment far from the Sun (which is positively charged), to the point where one can see bright spots of current jetting out of the surface. The comet's surface below the jet is therefore burnt out black. These electrons form a charged head around the comet.

      According to them the comet's movement towards the Sun (I don't know what force this is attributed to) results in a lack of equilibrium of this head which forms the tail. I can't figure out why in this theory the tail should point away from the Sun.

  • THOMAS says:

    The article in question “published in the Astrophysical Journal Letters » is pay-walled, so citizen scientists like me only have access to the Abstract ( We have to infer things from its necessarily elliptical wording before asking the most obvious question: with respect to the statement: “These aggregates are charged, fragmented, and decelerated by the spacecraft negative potential”, how comes it that the observed negative charge both on the spacecraft and on the dust grains that GIADA was designed to capture had not been foreseen and allowed for in the planning stages? Was this finding such a total surprise?

    Or perhaps the standard theory knew all along that everything inside the plasma sheath of a comet coma quite logically carries a negative charge with respect to the positive charge of the “solar wind”...? Whatever, if this confirmation of ubiquitous negative charges on and around a comet is the statement of an actual discovery, it’s a huge one.

    It follows logically that if this negative charge is detectable in the “repulsive force” observed between such tiny entities as Rosetta itself and the dust grains it is attempting to capture, then it must necessarily be significantly larger at the surface of 67P itself. This presumably thus also confirms the reality of the repulsive negative force acting between the two canyon walls of Hapi Valley which are attested to by the observed “plumber’s pipes”: ( and other jets which are clearly avoiding the opposite wall and each other: (

    And come to think of it, it may also even cause a sufficient repulsive force between Rosetta itself and the comet to lead to an understandable misreading/misinterpretation of the gravitational attraction data, thus clearing up the mystery of what always seemed to be the strangely underestimated mean density of half that of water which had been announced …

    The implications of the GIADA findings are huge. It sounds very much like the thin end of a very big wedge.

    I think it’s now time to release all the plasma data…

    • Harvey says:

      "It follows logically that if this negative charge is detectable in the “repulsive force” observed between such tiny entities as Rosetta itself and the dust grains it is attempting to capture, then it must necessarily be significantly larger at the surface of 67P itself."
      No, it doesn't follow at all. The paper clearly states that the charge originates from the interaction of the dust particle with the local plasma and solar UV. It has nothing to do with any charge it may, or may not have had when it left the comet. Furthermore the charges and voltages involved are minute.
      it is a completely trivial calculation to show that any charge the comet may have had would have been dissipated long since by the supposed discharge. I've posted details of itsseveral times.

      • THOMAS says:

        @ Harvey

        "The paper clearly states that the charge originates from the interaction of the dust particle with the local plasma and solar UV."

        This curiously simple, assertive statement flatly contradicts what you wrote above, in answer to Sovereign Slave: “Its not obvious why the dust would be negative; simple UV effects would drive it positive. It turns out to be the result of a fairly complex model”, before going on, in the same post, to express what seem to be a certain number of doubts, uncertainties and surmises.

        So could you please clarify your position, please? IOW, in this response to me, are you simply quoting what the paper “clearly states”, while at the same time having serious *private* doubts about the whole issue?

        • Harvey says:

          My position is perfectly clear; I quoted what ESA's model says, and made it clear I can't see a simple explanation of that result.
          Maybe I've missed an obvious explanation; maybe there isn't a simple one unless you have the full model to hand.
          It's not a matter of 'private doubts'; some things have simple obvious explanations, some don't - unless of course you are an EU adherent, which infallibly explains everything but utterly ignores real physics to do so.

        • sjastro says:

          Stop engaging in stawman arguments.Thomas.
          The negatively charged dust particles is due to the higher mobility of electrons in plasma compared to positively charged ions.
          Simply stated objects embedded in plasma such as dust particles or Rosetta are being hit by more electrons and therefore pick a net negative charge.

          • THOMAS says:

            @ sjastro

            Your simple model is not shared by everyone, apparently (see Harvey's comments above, for example).

            I don't see what it explains anyway. You are simply describing mechanisms and behaviour, which we all know about, whereas the debate is actually about first causes. Stated simply, WHY, not HOW.

          • sjastro says:

            It's not my "simple model".
            It's the application of the astrophysics of dusty plasmas to comets.

            If instead I said the collision/scattering cross section of electrons in plasma is smaller than positive charged ions resulting in a longer collision free path length would that clarify the issue for you?

            Somehow I doubt it....

          • sjastro says:

            A further addition here.
            Apart from the longer collision free path length, since the plasma is produced by UV ionization of cometary material the resultant electrons carry off thermal energy as they are knocked out of the parent atom.
            As a result they travel at higher velocities then the positive ions that are also produced during ionization.
            Matter embedded in the plasma is therefore more likely to be negatively charged by striking electrons which are more mobile than positive ions.

      • originalJohn says:

        No you haven't Harvey. You have posted details and assumed the triviality of the calculation based on your speculative and simplistic model.
        We have no figure for what voltage existed at any time or what the rate of discharge is or was ( other than earthbound measurements of specific ion or molecule or atom densities and dust volume). Also the elliptical nature of the comet's orbit combined with its speed means it is always moving into a new charge region so there is no tendency to equalise or dissipate its charge. Equalisation is however conceivable as the comet follows an approximately circular path at perihelion in the highly positive region near the Sun and at the opposite extremity of the ellipse out in the strongly negative region beyond Jupiter in the case of this comet. Comets have been known to cease discharging at or near perihelion. Comet ISON did. And they certainly cease discharging as they move slowly through the negative extremity region. The intermediate parts of the elliptical path towards or away from the Sun are those of maximum observed discharge, including extreme increases in discharge in some notorious cases.

        • Harvey says:

          I have posted the full numbers several times; no response.
          The capacitance of 67P is a few tenths of a uF. Let's take 0.5uF, overestimate.
          If we charge it to a GV, the charge is 500C
          That will discharge in 500 seconds at just one amp, a trivial current - in 500 seconds, for a crazy voltage.
          You appear to be suggesting it is simultaneously running a discharge as a cathode and mysteriously collecting electrons as an anode at the same time. So which way does the electric field point?! If it's on different regions, why, and why don't I see these strikingly different processes - and how does the current get from one place to another, through rock and ice, when CONSERT data (such as it is) makes it clear it's not a conductor? And of course there is no source either, the solar wind is neutral, and far too low current density.
          Yet another problem is the claimed acceleration of the neutrals; if the comet is negative as we are repeatedly told, the ions move *towards* the comet. Discharges are very ineffective at accelerating neutrals; the major mechanisms in technological discharges are dependent on wall effects in narrow bores, not relevant here. But we have no explanation at all of this.
          Neither does it even look like a discharge; none of the typical axial structure near a cathode, no transverse instability highly typical of large area discharges.
          The so called theory is nonsense at the most basic, simple electrical level. Actually there is no theory, just a mishmash of assertions, frequently internally inconsistent, and completely devoid of any numbers.

        • sjastro says:


          Kepler came up with the laws of planetary motion AD HOC based on observations.
          Newton decades later DERIVED the same laws using gravity. Deriving the laws is a relatively simple problem for undergraduate students.

          Since comets follow Keplerian orbits please show the DERIVATION of Kepler's laws of planetary motion using the electric comet model.

          It is pointless referring to the rate of discharge of along a comet's orbit when you have absolutely no idea that such orbits can exist in the first place.

          The fact is originaljohn, such orbits cannot exist. When the comet is charged it is losing energy in the Sun's magnetic field through cyclotron radiation. What is preventing the comet spiralling into the Sun?
          Then there is the consideration of Lorentz forces on top of the Coulomb forces in forming an orbit.

          What do you think will happen to the orbit if the Sun's magnetic field flips?

          Things are a lot worse given that that Thomas has declared the solar wind is positive in which case the electric comet and the Sun are both negatively charged. How does one form an orbit when the Coulomb force is repulsive?

          • originalJohn says:

            You misunderstand the significance of Debye length Harvey, or let us call it Debye radius, that is clearer as the constraint applies in all directions with respect to a particular charged particle. Outside the Debye radius there is no interaction between a particular particle and any other particle. It is effectively screened. There is therefore no tendency towards neutrality. Each particle behaves as an individual charged particle and movement of these particles constitutes a current. The pseudo neutrality is an overall numerical equality only. That is what a plasma is. The solar wind plasma is not therefore neutral. It is charge separated, because of the Debye radius. And the size of that sphere beyond which there is no influence becomes smaller as the particle density increases., as a quick glance at the Debye equation demonstrates. The constraint will likely be different in the solar wind in general and within the comet coma and different again very close to the nucleus surface, and by several orders of magnitude. At high densities Debye radius gets down to very small fractions of a metre.

            As for the electrons, those near the surface of the nucleus rock are freed by the reaction initiated by the solar wind protons and as you observe discharged as a current.. We don't know the value of that current, in the jets, as no data has been released. Glow currents however span a range of approximately 10 power -5 to 1 amp.

          • originalJohn says:

            No problem with Kepler's laws sjastro. The first states that the orbit of a planet is an ellipse with the Sun at one of the two foci. Substitute comet and the same applies. Ellipses however can vary a great deal. The planetary orbits are approaching circular. Those of comets are stretched out with a very large distance between the two foci. In such a case it is also implied in Kepler's laws that the speed of a comet is very high near to and as it approaches the Sun and very low as it approaches and rounds the opposite focus very far from the Sun. So I have a certain knowledge that such orbits exist. It is accepted fact. Also certain knowledge is the existence of an electric field centered on the Sun and radiating to the boundary of the heliosphere. The comet is therefore moving within that field and encountering a constantly changing state of charge as it progresses..

            Comets can emit cyclotron radiation detected as x rays in some past cases. Throughout most of the discharge phase of the orbit however the photons emitted have much lower energy levels, typically dark or glow mode.

            What is preventing the comet from spiralling into the Sun is the gravitational balance of the orbit. But it is a good question because the gravitational force is only attractive so has no self correction possibility if any small perturbation occurs from an external force.
            Nobody knows what the electrical influence is on planetary motion because it is not considered. That does not mean it is not happening. The laws of motion are observational and effects not separately accounted for could still be taking place. Interestingly the Coulomb forces are both attractive and repulsive depending on the sign of the charge.
            I don't see the Lorentz force, the inward radial magnetic force on a conductor induced by the current flowing in it, as significant in the relative motion of bodies but I could be wrong.

            The Sun is the anodic element in the field that spans the radius of the heliosphere. The cathode is a virtual one imparted by the electron density streaming across the boundary with the interstellar medium.

          • sjastro says:


            As a first order approximation Newton used a central force inverse square law (gravity) from which he was to able derive Kepler's laws of planetary motion.

            In a previous post I mentioned Coulomb forces playing a role if electric comets exist. In reality this is not true due to Debye scaling.
            The same can't be said for Lorentz forces.
            If X-rays are emitted by comets due to cyclotron radiation it indicates that Lorentz forces do play a significant role.

            There lies the problem. It means the central force law is not a simple inverse square law, since Lorentz forces must be included in the central force.
            As a result Kepler's laws can no longer be derived.

            Incidentally the X-rays detected cannot be due cyclotron radiation as X rays are emitted radially.
            Instead x-rays were found to be emitted on the solar side of the comet due to interactions with the solar wind.

          • harvey says:

            The Debye length is a concept that not uncommonly confuses students; its of importance not only in plasma physics, but in semiconductor physics & ionic solutions etc. So it is rather widely taught & used. It certainly confuses some people here!

            This is a rather good introduction to Plasmas, but in particular derives the Debye length nicely from around slide 15 in a simple manner; or as simple as it gets.
            Now its clear a lot of people here cannot follow the math, so skip to slides 24, 25 which give an explicit value for the net charge within a sphere with a radius of one to ten Debye lengths.
            For a ten Debye length radius sphere, the net charge is ~10^-5e. It is 'quasi neutral'.
            This applies to a *uniform* (density & temperature) plasma, where Lorentz forces from any current are also negligible.
            For plasmas which violate this, double layers etc can develop, typically on a scale of a few tens of Debye lengths.

            But I'm afraid it is not me that 'misunderstands' Debye length; given my background it would be a bit surprising if I did 🙂

      • THOMAS says:

        @ Harvey

        Your assertions and calculations are also based merely on the simple standard theory *assumption* that there can be no charge separation in space, whereas numerous images acquired both by Hubble and ground-base facilities provide evidence of objects at every scale which can only be explained in terms of plasma sheaths/double layers. Hannes Alfven’s warning words in his 1970 Nobel Prize acceptance speech that “double layers in space should be classified as a new type of celestial object” have sadly gone unheeded since.

        The double layer separating the negative charge present on 67P and in its coma (as with every other comet) from the net positive charge of the enveloping “solar wind” will finally prove him to have been right. And all previous assumptions will need to be revised.

        • Harvey says:

          I have no idea what that means.
          You assert there is a discharge, and that the comet is negative.
          That means there is a current of electrons leaving the comet.
          What is the magnitude of the current and where do the electrons come from?
          The solar wind does not have a 'net positive charge' on scales beyond the Debye length of order tens of metres it's neutral. It a mix of (mainly) protons and electrons.

          • martin says:

            If there would be discharges we would SEE it in our magnetometer data as specific wave modes, and probably even more in the MIP data. But so far I have not seen anything.
            And asking EC proponents for numbers ... never seen anything.

          • originalJohn says:

            Harvey the explanation that you don't follow is that the whole surface of the nucleus is negatively charged, approximating to a spherical capacitor. Electrons gather to the discharge points by surface conduction with negligible resistance.. There is no flow of electrons required into the nucleus as you surmise elsewhere. The magnitude of the discharge current or voltage has yet to be measured, or yet to be published so no argument is based on them. The discharge occurs across the double layer at the surface of the nucleus to the positively charged solar wind plasma.
            You do not see this because you do not understand the concept of Debye length. It in fact means the opposite of what you believe it does. The Debye length is the radius of a sphere centered on any charged particle outside which that particle has no interaction with any other particle. The charge separation is therefore maintained. Because of the Debye length constraint the medium in general consists of separate charged particles ie a plasma. At the current densities likely to exist within the coma, and particularly near to the nucleus, the Debye length will be a great deal less than 10 metres. You cannot escape these average figures can you. They have no significance in the special comet situation.

    • Kamal Lodaya says:

      Thomas: There are plenty of papers at EGU 2015. Scientists prefer communicating to their peers, that does not mean there is a conspiracy.

      • THOMAS says:

        @ Kamal Lodaya

        Who's speaking of a conspiracy? For my part, I believe that, whatever the instrument concerned, the data is so long in being released simply because it doesn't fit in that well with the expectations of the standard theory. So it necessarily takes longer for mission scientists to attempt to understand it and then account for it. I'm not suggesting anything more sinister than that.

      • THOMAS says:

        As for the "papers at EGU 2015", I've just read the latest post by Emily on one such paper, "Rosetta and Philae find comet not magnetised" (

        Apparently, the findings again disappoint standard expectations in that, possessing no intrinsic magnetic field which it was hoped might give support to one part of the accretion theory, the magnetic field which HAS BEEN detected on and around 67P must, (as the only possible alternative cause) necessarily be *induced* by *electrical activity* in relation to the positively charged “solar wind” (which is being largely kept at bay by the plasma sheath/double layer which has formed to protect the negatively charged comet environment).

        Just one more puzzle for the standard theory but an actual expectation of the EU model…

        • Harvey says:

          But I thought those plumes were all discharges; discharges mean current flows; current flows generate magnetic fields........ easily far greater than the few nT measured.
          Explain please........

          • THOMAS says:

            Philae's landing site was quite naturally chosen precisely because it was on one of the least active areas of the nucleus in terms of jetting. Its other three touchdown points were in the same general area. As Originaljohn has pointed out elsewhere, the LOCAL magnetic fields readings are certainly *much* higher around the powerful jets emanating from the ACTIVE areas. Why should this need explaining? It's obvious. We eagerly await THOSE readings, which are not yet forthcoming....

        • Harvey says:

          Oh, and the solar wind is NOT 'positively charged'. The Debye length in the interplanetary medium is around 10m. On scales greater than this it is neutral, electrons plus (mainly) protons.
          Oh, and if it were positive, that would be net loss of positive charge from the sun, so the sun would become negative; but we are always told the sun is positive..........

          • originalJohn says:

            You have failed Harvey to take into account the current of electrons flowing into the Sun, as electrons do with respect to the anode element.

            And it is a plasma, not neutral but positive and negative ions flowing in opposite directions acting separately. It is quasi neutral but does not act as a neutral medium. That is the plasma state.

          • Harvey says:

            I find it somewhat hilarious to be told I've forgotten then electrons, when time and time again I've pointed out the solar wind is NOT positive but neutral, and has a Debye length of order tens of metres.

            Anyway, we seem to have progressed *slightly*; despite having been told 67P is a negative charged object and hence the discharge, there appears to be some understanding that just does not work. The voltages, charges etc are ridiculous. Progress! But of course this non-theory is just a muddled moving target; destroy one bit of nonsense and another pops up; maybe a Hydra is a better analogy.

            I already addressed the alternative previously.
            Somehow, if 67P is 'negative', and hence a 'cathode' emitting electrons, it needs a power supply, a supply of electrons.
            Now just how does this work? There is no trailing wire to the power supply I can see. The 'discharge, cathode' areas how the electric field pointing 'out', but where the electrons arrive it must point 'in', be an 'anode', and stay like that. Why and how does it do that?
            We must have 'anode' and 'cathode' regions on 67P; one might have expected them to look different, could you point them out?
            And we know, at least for the limited CONSERT paths, that 67P is non conducting, and every suggested material from any theory is a good insulator; so how do the electrons get from the 'anode region' to the 'cathode region'? Why don't we see resistive heating from that current flow?
            Then, those scarey number things again. The heliospheric current sheet (not even sure 67P is in it) has a current density of around 10^-4A/km^2
            Yes, km^2
            These huge discharges, moving massive amounts of material as neutrals by completely unexplained processes, are really rather starved for current aren't they?

            So this version works just as well as the 'discharging capactitor' version; ie, not at all.

          • THOMAS says:

            As you must know, Harvey, the working of the solar wind is extremely complex and is yet not fully understood, mainly because relatively little funding has been allocated over the past four decades to exploring its undoubted electrical properties, for reasons too obvious to need explaining here... There seems to be a general consensus among most authors, however, to the effect that it carries a net positive charge.

            It thus seems not unreasonable to assume that charge separation between the plasma making up the comet coma and the solar wind is a self-organizing phenomenon which takes the form of a double layer/plasma sheath, as described by the 1970 Nobel Prizewinner Hannes Alfven. In my humble opinion, it is this phenomenon which is euphemistically called a "magnetosphere" in a previous ESA release ( and which came as a considerable surprise to mission researchers. (It would be nice to have an update on the state of this structure, BTW: it seems to have been quietly forgotten since this initial disclosure). The electrical activity of the plasma sheath is presumably also the cause of the "mysterious song" detected by the RPC instrument, about which AFAIK we have also had no further news (

            I'm not claiming that the EU model has all the answers, but simply that many of the "surprising" aspects of 67P's appearance and behaviour tend to support it rather better than they do the standard model.

          • Kamal Lodaya says:

            Thomas: Certainly the EU model does not have all the answers, for example see on a parallel blog OriginalJohn saying that why comet tails point in any particular direction cannot be explained by the electric theory.

            As you have pointed out here, it is your assumption that "charge separation between the plasma making up the comet coma and the solar wind is a self-organizing phenomenon which takes the form of a double layer" and Alfvén never said anything of this kind. In a later post you have wrongly attributed this idea to Alfvén.

            You also claimed above that it was an expectation of the EU model that the comet nucleus should not be magnetized. But no such prediction was ever made. Does such misappropriation of ideas happen frequently in the electric theory?

            We are all surprised by many things to do with 67P. I am sure there are many more things which will surprise us later this year. It is to be surprised by new knowledge that Rosetta was sent there. The fact that the electric theorists see everything filling in what they had already thought of suggests that what they had in their theory regarding comets was vacuous.

        • sjastro says:

          The false dichotomy argument at work again.
          The failure to detect an intrinsic magnetic field around the comet somehow supports the EU model??? What about the experimental evidence itself?

          Well if the EU model relies on a positively charged "solar wind" there is no point for further discussion....

          Evidently you have never heard of Maxwell's equations . Go read up on the subject Thomas, you will find the magnetic field associated with the solar wind or plasma in general is caused by the motion of the charge carriers and the time varying electric fields.

          Why hasn't ROSINA detected the sheath surrounding the comet. The answer is simple because it doesn't exist.

          • originalJohn says:

            Maxwell's equations and the Lorentz force sjastro are precisely what determines the behaviour of the plasma electric current which is the solar wind and establishes the properties of the coma. No dichotomy argument is needed. The electrical nature of what we are observing is obvious.

            The evidence to confirm this has yet to be released.

          • sjastro says:


            The false dichotomy occurs where the perceived failures in mainstream theory somehow validates the opposing EU view. This is a straight out logical fallacy.
            Like any other theory, electric comets should stand or fall through observation and experiment.

            If comets are charging and discharging, the electric field around the comet is time varying and according to Maxwell's fourth equation, a magnetic field is induced.
            Yet the Rosetta and Philae data shows there is no intrinsic magnetic field associated with the comet.

            The irony is that while EU supporters celebrate according to the false dichotomy, the magnetic field data itself or rather the lack of it effectively closes the door on the electric comet.

            On top of this as pointed out in my previous post, why hasn't ROSINA found evidence of a sheath around the comet.

          • THOMAS says:

            @ sjastro

            "Why hasn't ROSINA detected the sheath surrounding the comet. The answer is simple because it doesn't exist."

            It does exist, because it has been detected. It is euphemistically called a "magnetosphere" in the ESA release which disclosed this discovery, in tones of considerable surprise:, where it is specifically described as "a region surrounding the comet that is shielded from the solar wind."

            Does that sound familiar? Or did you miss that one? So why should a comparatively tiny object such as 67P, which has now been unambiguously analyzed as being intrinsically "not magnetized"(, possess a growing "magnetosphere" unless it actually possesses INDUCED magnetic/electric fields?

          • sjastro says:


            I said,
            "Why hasn't ROSINA detected the sheath surrounding the comet. The answer is simple because it doesn't exist."

            Your response.
            "It does exist, because it has been detected. It is euphemistically called a "magnetosphere" in the ESA release which disclosed this discovery, in tones of considerable surprise:, where it is specifically described as "a region surrounding the comet that is shielded from the solar wind."
            Does that sound familiar? Or did you miss that one? So why should a comparatively tiny object such as 67P, which has now been unambiguously analyzed as being intrinsically "not magnetized"(, possess a growing "magnetosphere" unless it actually possesses INDUCED magnetic/electric fields?

            Why don't you try comprehending the links you post instead you own reading your own ideas into the topics or cherry picking quotes..

            A plasma sheath and a magnetosphere are not the same thing.

            ROSINA does not detect magnetic fields.

          • originalJohn says:

            There is no requirement Harvey for a current of electrons into the comet nucleus. It is, at this stage in its orbit, a negatively charged capacitor. The whole surface is therefore negatively charged and discharging because of the constantly changing potential ( caused by the comets motion) between it and the solar wind. This would still require some movement of electrons to the discharge points and such current flow would occur at the surface, with negligible resistance.

            Once again you invoke a uniform current density throughout the solar current sheet by quoting the average current density. I think even you would agree however that there is still a huge total current available. And once again you have ignored the natural increases in current density that occur in plasma current flows. You are able to convince yourself this way because of your poor understanding the plasma medium and because there are as yet no results of current density or temperature measurements within the coma and particularly in the discharge jets near the surface of the nucleus.

        • Gerald says:

          Which "the" standard theory?
          There are loads of models.
          The only "the" standard theory I'm kowing of is "the" standandard model of particle physics (together with one of several equivalent versions of quantum theory), and maybe general relativity.
          Where is the "puzzle" with respect to those "standard" theories?
          The new observations constrain the number of well-fitting models; that's an improvement of knowledge.

          Strong electric currents - as the electric universe belief needs - would have magnetized magnetite grains in an ordered way easiliy detectable for the magnetometers. Hence the measurements are one more strong evidence ruling out the electric universe approach.

        • Kamal Lodaya says:

          Gerald, Harvey, sjastro and colleagues: I think we all believe one standard theory, that the comet goes around the Sun because of gravity, that the apple that Newton saw fall did so because of gravity. We have a -shall we say standard- expectation of the scales at which gravitation operates and the scales at which electromagnetism operates.

          So first things first: which force makes a comet go around the Sun? Why do tails of comets point away from the Sun? Once we have some agreement on the basics, we can start talking about differences at lower scales.

          • Gerald says:

            The orbit is described in first order by Kepler/Newton, including gravitational effects from other planets. Some uncertainty is due to outgassing, Dynamical effects are described by Newton's mechanics.
            Tiny effects are added by radiation pressure, solar wind.
            Higher-order terms are added by relativistic effects (perihelion advance).

            The pointing of the tail(s) is an effect of solar radiation pressure and solar wind.

            I think, there is an agreement about these basics among scientists.

            Not yet settled are details about the early solar system. Some of the main science objectives of the Rosetta mission are providing constraints to those initial conditions.

          • Harvey says:

            The comet just continues in orbit under gravity; it can get perturbed by forces from emitted gas jets near perihelion.

            The comet tails are acted upon by the solar wind and photon pressure.

        • martin says:

          the solar wind is not positively charged, and or the measurements during the landing event was hardly slowed down from its undisturbed value

      • originalJohn says:

        Perhaps you should check to see if your magnetometer is working Martin. You should at least be detecting the fields associated with the solar wind current and the variation in them.
        As you are claiming to be involved perhaps you could enlighten us with information about what and how many magnetometer measurements have been done, covering what percentage of the coma volume , and how near to the nucleus. That would be very interesting.

        The "EC proponents" do not have access to the numbers as you do. Do you want them to make them up. Do you not understand a qualitative argument.

    • Gerald says:

      '“During Philae’s landing, Rosetta was about 17 km above the surface, and we could provide complementary magnetic field readings that rule out any local magnetic anomalies in the comet’s surface materials,” says Karl-Heinz Glassmeier, principal investigator of RPC-MAG on board the orbiter and a co-author of the Science paper.'

      Any electric discharge activity would have changed the local magnetic field, i.e. Rosetta and Philae woud have measured different magnetic fields.
      Since magnetic field measurements have been almost perfectly identical between Rosetta and Philae, any relevant plasma discharge activities during the parallel measurements are ruled out.

    • harvey says:

      So, for fun. CORRECTED VERSION.
      (In the earlier version I incorrectly remembered Coulomb’s law; a dimensional check today caught the error, something was niggling at me! Apologies for the mistake.)

      If the electrostatic repulsion is cancelling out some 80% of the gravitational attraction, we can just equate the two to get an idea of the charges etc.
      Gm1m2/r^2=ke q1q2/r^2
      So the r^2 cancels out, it does have the right functional form.
      So q1q2=Gm1m2/ke.
      Now we roughly know the mass of Rosetta, around 1500kg ish (1260 dry). ESA says 67P is 10^13kg, Thomas would have us believe it's five times heavier, that will make things worse below.
      You can look up G and ke.
      Which gives q1q2 just over 10^-4 coulombs squared.
      Now you can distribute this how you like between the two.
      1 on one and 10^-4 on the other, 10^-2 on both, 10^-8 and 10^4, whatever; both must be the same sign.

      But the capacitance of 67P is only about 0.3uF (spherical capacitor of 2.5km radius, an overestimate)
      That of Rosetta is a bit harder, but approximating it as two big 14*2m rectangles (Cui Y, Eur. J. Phys. 17 (1996) 363–364) and a 3m diameter sphere I get around 750pF, let’s call it 1nF, 10^-9F.

      Now we know from this paper Rosetta is at about -10V so the charge on it is 10^-8C from Q=CV
      (Well, at least the sign is as we are told it should be.)
      To maintain our q1q2 product of 10^-4 the charge on 67P must be about -10^4C
      Which gives a voltage on 67P from Q/C of around 35 GIGAVOLTS.

      Now I rather think we might have noticed that!
      Or, if you prefer 67P to be five times heavier, multiply by five………….

      (The only way to reduce that is to increase the Rosetta potential; rock & a hard place. And if you do reduce it, the problem below gets worse.)

      Also, despite this insane voltage, 67P only has a charge of 10^4 Coulombs.
      So a pathetically weak 1A discharge (recall, my kettle takes 13A…) would discharge it completely in just under three hours; so not only would the ‘discharges’ stop, the electrostatic force would go away.

      Now of course I still might have slipped up somewhere, if so please feel free to correct the above.

      • Gerald says:

        What would have happened to Philae after charge exchange due to contact with 67P/C-G?
        I'd think we would get a relevant repulsive electrostatic force (understatement). Not just for Philae, but also for boulders, dust etc.
        Is there any material, which wouldn't disrupt?

        • Gerald says:

          Tektites (and any material with less tinsile strength) smaller than 140 m diameter would disrupt by electrostatic repulsion at 35 GV.

          Derived from chapter "Electrostatic Disruption of Dust Grains" (p.103f), in
          "Accretion of Extraterrestrial Matter Throughout Earth's History" Edited by Bernhard Peucker-Ehrenbrink and Birger Schmitz, (c) 2001 Kluwer Academic / Plenum Publishers New York

          • ianw16 says:

            Don't forget, 35 GV is for the "mainstream" density measurement. EU says this is rock (cos it looks like rock!), and has a commensurate density. So we are talking about >140 GV. What might that disrupt?

        • martin says:

          It was a worry that was taken into account, as we know that surface charging can happen at comets, as on the moon. With a huge charge on the comet it could have fried the electronics of Philae.

          • harvey says:

            Modest charging of spacecraft is routine, has to be dealt with, & I wouldn't be the least surprised if 67P has some small charge; indeed as its probably a rather good insulator, you have to ask the potential/charge *locally*, it may be nonuniform.
            The calculation relates to whether 67P & Rosetta could realistically have charges which would cause the density of 67P to be miscalculated.
            The answer is very clearly, NOT possible, unless I've slipped up; no one has pointed out an error yet.

          • ianw16 says:

            Well, I'm no electric genius, but if the equations are the correct ones (which they would appear to be; and you would know better than most), then the figures check out using your assumptions for the capacitance, which, from what I've learned over the last few days, would also appear to be good ballpark figures, and well within an order of magnitude.
            So, as you say, to paraphrase, *THIS IS NOT ROCK*, and the density is as measured, i.e. *HALF THAT OF WATER*.

          • harvey says:

            I used the capacitance of a 5km diameter sphere & rounded it up. That will clearly overestimate the C, which *helps* the EU arguments.
            The capacitance of a sphere is a classic Physics 101 electrostatics problem.

    • martin says:

      thomas said: "The article in question “published in the Astrophysical Journal Letters » is pay-walled, so citizen scientists like me only have access to the Abstract ( We have to infer things from its necessarily elliptical wording before asking the most obvious question"

      OR you can just ask here if someone can send you the paper ...

  • "More detailed information can be found in the paper, which can be accessed here" - no, it cannot as it is behind a paywall! Please do as NASA does with the Mars rovers where the results papers (in Science) are made open access for the public which paid for the missions and/or ask the authors to put their manuscripts on ArXiv. We are in the year 2015, please!

    • martin says:

      I do not understand why this is behind a pay wall, because the first batch of science papers were all open access.

  • logan says:

    'Dandelion particulate' 0.001 density. That's what I expected from 'primigenial' material.

    And didn't expected that over 67P. Surprised, again. Newborn at actual cryo-volcanoes? Brought all the distance and time, unaltered?

  • Lee says:

    Excellent reading here! Let's see...the comet was impacted by the lander a few times, would the impacts of several foreign objects...change or give the polarities a start? Many porous particles had to have been altered or crushed. Or foreign substances, as from metal shavings from impacts from the lander, introduced. Hmmm! .? Very interesting!

  • Harvey says:

    Well as far as I can see, the highest DC potential ever produced was 25.5MV by a Van de Graaf; can't think of anything that's gone higher. So experimentally at least we probably don't know 🙂
    I've never seen a Cockroft-Walton that high.
    Pulsed from say a Marx generator might have gone higher?
    Insulation even in SF6 or oil is getting to be tricky!

    Anyone know of anything higher (as a DC or pulsed potential; RF accelerated beams of course go immensely higher.)

    Oh, of course you do have to knock 20% off your 140GV, it's only claimed to be cancelling 80%

  • Kamal Lodaya says:

    It would be interesting (may be far from easy) to find out how many revolutions around the comet an average dust grain (let us say graded by size) expelled from it makes, before falling back or escaping into space. If the number is less than 0.5 then the dust raised in one hemisphere, if it does not escape (presumably into the tail), falls back on the same hemisphere. Otherwise dust from one hemisphere can travel to the other hemisphere and this should result in a more uniform morphology. I read somewhere (I forget in which context) that Rosetta has detected many micro-satellites of 67P, which I assume are large chunks of dust. I wonder if these things are detected making more than 1 revolution.

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