Determining the mass of comet 67P/C-G

Artist impression of  Rosetta approaching comet 67P/Churyumov-Gerasimenko. The comet image was taken on 2 August 2014 by the spacecraft's navigation camera at a distance of about 500 km. Credits: Spacecraft: ESA/ATG medialab; Comet: ESA/Rosetta/NAVCAM

Artist impression of Rosetta approaching comet 67P/Churyumov-Gerasimenko. The comet image was taken on 2 August 2014 by the spacecraft’s navigation camera at a distance of about 500 km.
Credits: Spacecraft: ESA/ATG medialab; Comet: ESA/Rosetta/NAVCAM

Rosetta is equipped with a range of instruments to analyse comet 67P/Churyumov-Gerasimenko, but the spacecraft itself can also be used to determine properties of the comet.

As the spacecraft passes close to comet 67P/C-G, it is pulled slightly off course by the comet’s gravity, resulting in tiny changes to the spacecraft’s velocity. These small deviations are recorded in the spacecraft’s radio signals as they are beamed back to Earth, and can subsequently be translated into measurements of the mass and density structure of the comet. The measurements can also be used to refine the details of the comet’s orbit along its trajectory around the Sun.

For the Rosetta mission, this task comes under the umbrella of the Radio Science Investigation (RSI). Using 80 hours worth of tracking data between 6 August, since arriving at the comet, and up to 9 August, the RSI team made a first estimate of the comet’s mass as approximately 1×10^13 kg +/-10%, or about 10 trillion kilograms.

The team caution that there is still work to do with analysing errors on the value, not to mention refining the value as the spacecraft gets closer in the coming weeks. Indeed, these early measurements were made from a distance of approximately 100 km; the spacecraft is now en route to a distance of 50 km.

At the same time, in order to be able to navigate the spacecraft close to the comet, ESA’s flight dynamics and operations teams have also independently determined the mass and gravity parameters. These values fall within the same range as those derived by the RSI team.

The values of both teams are constantly being refined as Rosetta moves ever closer to the comet. In time, once accurate dimensions of the comet have also been determined, the comet’s density will be revealed, and assumptions regarding the comet’s interior can be made.




  • Bill McDavitt says:

    Presumably a volume estimate has already been made, then a density calculation could easily be made. The question then becomes, do the different lobes have different densities?

    • Daavs says:

      Since the comet rotation pole is pointing roughly toward the sun, and Rosetta has also thus far been mostly in that direction relative to the comet, there probably isn’t enough data about the shape of the dark side of the comet to get a strong estimate of its volume yet.

    • Erich says:

      Seen the videos of rotation and the position of the rotation axis i can only say that the density is uniform within this kind of measure accuracy at least better the +/- 30%. Make a model of it and check. Use clay and stick an axis through it the way it is in the animations.

    • Erich says:

      The mass is 10.5 Giga ton plus minus 1% and its volume is 21 kubical kilometre making its density to be halve of water.
      It rotates in 14 hours and 24 minutes around its axis.

  • Ross says:

    Since the mass is Mt. Everest is about 3×10^15 kg and this comet is a bit smaller in size with 1×10^13 kg of mass, I think it’s safe to say we observing a dry rock rather than a dirty snowball.

  • Luke says:

    Interesting. This mass estimate is about 3 times the initial ‘guess’ of 3.2 X 10^12, mentioned in this earlier post The shape is better known now, but most of the difference must be in the density – much more solid ice (or rocks?) and less empty vacuum that was thought.

    • Ingo Althöfer says:

      Will the larger mass be helpful for the
      mission, in particular for landing Philae?

      • Rainald Koch says:

        If the mass were known beforehand, yes. As the lander had been designed for the smaller comet Wirtanen, no.

      • Ross says:

        Not if the cometary nucleus is a rock rather than snow or ice as it is assumed to be.

  • Lynn says:

    If the mass is three times the previous estimate, how will this affect the orbital period at 10 and 30 KM?

  • Matthias Rauls says:

    With no scale for the images at hand but the approximate diameter of 4 km of the comet I started a wild estimate: assuming that the comet consists of two spheres and assuming that ohe might have a diameter of 2.5km and the other one of 1.5km I end up with a volume of 10^10 m^3. From these figure one arrives at a density of 1000kg/m^3, i.e. close to the density of ice? What do you think?

    • Ross says:

      The density of ice is a bit less than 1g/cm^3, your value of 1000kg/cm^3 is 10^6 more dense than ice… but I’m not saying you’re wrong.

    • Rainald Koch says:

      That was exactly my estimate 10 minutes ago.

      If the earlier estimate of 3.14E12 kg was based on the much larger volume (3D model after Hubble pics), the assumed density was close to only 100 kg/m3. Was it a joke or a miscalculation?

  • Dave Higgins says:

    Such a low density? (300kg/cubic meter)

    Even a pile of course aggregate used as the base of road construction would have a far higher density than this, even though the pile would be full of air gaps.

    Are there any thoughts as to how the structure of the comet could be so full of holes, it should look like a sponge.
    When we look at large eroded areas like the striated side of the head, there is no clue to such a porous structure.
    Its also difficult to imagine it being hollow, the erosion at the neck has been so extensive, it surely, by now, would have broken through to reveal a hollow core.
    Also if the comet is built from interstellar dust, that is expected to be very fine, then you would expect it to pack very densely.
    Baffled by these results ??

    • Marco Parigi says:

      I think we have to stop making assumptions about comets being “pristine”, “primeval”, “icy”, “rubble piles”, “fragile”. First hand evidence shows a complex, evolved structure. Not only do we have to explain the low density despite a contiguous outer layer, there is also the discrete jets and the non-pristine compounds discovered in previous missions. I am thinking “chambers”, under the surface where pressure builds up, powering jets.

  • Luke says:

    The new estimated density is higher than expected, not lower. Uncompacted snow can be well under 100 kg/m^3, see and with no gravity or freeze/tham cycle to compact it, snow + dust at an average density of about 100 kg/m^3 was the guess. What Rosetta has found is either much dirtier (rock at density 3000-5000 vs ice at 1000) or much more compacted than expected.

    • Marco Parigi says:

      The assumption of no freeze/thaw cycles in comets has taken a battering from recent scientific discoveries. The discovery of a CAIs from stardust indicate the likelihood of melt zones early in the comet’s history. The discovery of cubanite also indicates that there was liquid water processing inside comets for the comet in question. The question is why a comet wouldn’t collapse down in on itself when a lot of its volatiles melt, evaporate and sublimate, increasing its density to close to that of ice.

    • Kamal Lodaya says:

      Suppose there are a lot of organic molecules, so that the surface has a much more squishy composition (I am thinking of something like tar). Would that match the density better? Would that explain sharp crater rims?

  • Roberto Nesci says:

    An ice block should be about 1 g/cm3, so your guess is not far from expectations, but surely the team has a much better estimate of the total volume, better than the mass, by now.
    It would be nice if they put out a first density measure, even at a 20% error level.

  • Benjamin says:

    How do you mesure theese tiny changes to the spacecraft’s velocity ? what are your FIX Points out there, that will help you determine this pulling/pushing Force ? (pushing, in sense of, how gravity maybe is a pushing from Outside force between 2 Bodies)

    • Daavs says:

      The main two measurement types are the doppler signal (the shift in radio frequency when Rosetta talks with ground stations on Earth, caused by relative motion) and optical data (identifying landmarks in pictures of the comet).

      Even though any individual measurement could be fairly incorrect, combining enough of them over a time span of several days makes it possible to determine the forces acting on Rosetta with good accuracy.

  • Dave Higgins says:

    I agree density the of 300kgs per cu meter is higher than first projected.
    However this and all the other comets we have seen look like solid rocks not icy balls, in which case this s.g. would look rather low.

    The comet pictures we have seen from this and other projects look like meteorites and as far as I know, we would not expect the debris of meteorites to be floating on the sea after coming to earth.

  • PaPe3814 says:

    I have thought it iwould be heavier.

    If the mass is 1×10^13 kg, then at 50 km distance, the orbiting period shall be 31.5 days and orbiting velocity 0.12 ms-1.

    At 30 km, period and velocity shall be 14.6 days and 0.15 ms-1.

    So measuring orbit time shall be enough to estimate the mass.

  • PaPe3814 says:

    The point is that it has not been orbiting yet, but “triangulating”.

    But amazing. Take care.

  • Paul W says:

    About as wrong as anyone could be Ross. If the mass of the comet is less than 1% of the mass of Everest, then the nucleus density comes out a very low number,, not rocky at all. Why don’t you wait until the scientists tell you the right answer.

  • What is the volume of 67P? How was it determined since its odd shape was a surprise?

    Is their a 3d model of it in some widely known format such as Googl’s SketchUp (.skp) or such?

    I know I could work with the density given by the newly discovered mass but it leads me to wonder whether the volume is also up to date.

  • Leo Vuyk says:

    About the extra mass I would suggest the possibility that there are two heavy nuclear reactors each inside both dumbbells? Then the lander could have a rerious problem?

  • Henk Smid says:

    Thanks once again. It is not ‘in orbit’ yet but I think you can give a reliable estimate of the density now (do !). Wikipedia among others earlier gave about 100 kg/m^3 (a bit, well, very low). I suppose it is now like known comets, 500 kg/m^3 or a tad more.

    You must have better numbers ! Groeten, Henk Smid

  • Viola says:

    This is a mineral density table for earth minerals. Compared to any estimate of the comet material density you soon realise that the comet is pretty fluffy. Its material actually floats on water better the most woods does. Its consistence i speculate to be like saw dust and absolutely not rocklike at all.

  • Erich says:

    This is my guess.
    The material is most likely sublimated ice of water mixed with ice of hydrocarbons and ice of carbon-oxides mixed with mineral dust. The nano- and micro-particles compressed during eons in very low gravity to a real softie. There are a few heavy gasses in situ but not much of it. Any liquids are rare as it is in vacuum with very low atmospheric pressure an almost vacuum as well. I estimate the core temperature to about 70 K so some gases are liquified and soaked up in its interior but so rare it is hardly making it wet. Whatever we will learn from it will be a surprise even for the scientist as they too have not much of a clue yet.

  • Leo Vuyk says:

    Comet 67P and Schoemaker Levy 9.
    What can the Schoemaker Levy 9 Comet destruction into 24 point sources, tell us about Comets in relation with the multi gravitating dark matter point source proposal for Comet 67P C-G by Quantum FFF Theory? Conclusion: Comets are aggregations of multiple dark matter point sources coined new paradigm interference Black holes. The different sizes of Comets (below) is an indication that Comets seem to grow by merging with other comets.


  • Kai Roesner says:

    Today’s blog post ( gives some details about Rosetta’s recent orbit: 28km x 29km at a period of 13d14h59m. This puts the mass at about ~9.9 x 10^12 kg which nicely fits the initial estimate of 10^13 kg +/-10%.

  • Kerri says:

    Do you think that NASA will actually show us an actual picture of the comet? Because I’m getting sick of seeing the same picture they showed last year from comet Ison. So i guess my question should be why is NASA showing us an old picture of comet Ison instead of an actual picture of the comet.

  • indio007 says:

    What deviation of radio signal was measured? Was it the doppler shift?

  • Stefaan says:


    I am not a scientist, so this question mail can be pretty embarrassing for me. I am just wondering about terminology such as the use of the word “orbit” in reports, media, etc…

    There is said that Rosetta is “orbiting” comet 67P. But can anyone explain how that is possible without serious engine force? Even if the comet is composed by the most dense and heavy materials, it is to small to generate enough gravity for a +1 ton weighting vehicle to stay in a natural orbit.

    I assume that we shouldn’t talk about orbiting? Probably Rosetta is just flying side by side? With a few corrections via electric engines?


  • Donald Gray says:

    What is the surface value of g where it landed?

  • Trudel says:

    it can orbit, The mass is small compared to the earth, but the distance of about 30km is so much closer than that of a sattellite around earth with more than 7000km.. the force increases with 1/r^2 The orbiting time is about 14d.

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