'Welease Wosetta!'

[Zyxzevn]

So the mass determination is in (1 x 10 ^13) and it's 3 times what they thought.

Note that the density of Earth's moon is 3.34 grams per cubic centimeter (from google)
and water ice is 0.9167 g/cm3 at 0 °C. So will 67P be more like the Moon or ice?

I suspect that they expected ice.
And with these numbers you are telling us:
They find 3 times as much mass as expected, so it is as heavy as the moon.

[viscount aero]

So the mass determination is in (1 x 10 ^13) and it's 3 times what they thought.

Note that the density of Earth's moon is 3.34 grams per cubic centimeter (from google)
and water ice is 0.9167 g/cm3 at 0 °C. So will 67P be more like the Moon or ice?

I suspect that they expected ice.
And with these numbers you are telling us:
They find 3 times as much mass as expected, so it is as heavy as the moon.

And since density= mass/volume it is going to be denser then they thought.

[paladin17]

But wait a minute, BBC says that the

Comet 67P/Churyumov-Gerasimenko has quite a low bulk density, something in the region of 300kg per cubic metre

, which means three times less than ice, and 11 times less than Moon.

Just a speculation: makes me wonder if the gravitational interaction between the comet and the spacecraft is altered somehow (maybe electrically?), for example there is a kind of repelling force present, which makes the mass appear to be less than it is.
Although that wouldn't explain why the preliminary mass determination was much lower than the new one, instead of being higher.

[Metryq]

Although that wouldn't explain why the preliminary mass determination was much lower than the new one, instead of being higher.

The spacecraft is slowly equalizing charge, thus any electrical repulsion is less.

[paladin17]

Although that wouldn't explain why the preliminary mass determination was much lower than the new one, instead of being higher.

The spacecraft is slowly equalizing charge, thus any electrical repulsion is less.

Good point.
But I meant these calculations, and they do not involve any spacecraft.
They could be wrong, of course. But the current ones (done via Rosetta trajectory alteration) could be wrong as well.

[Aardwolf]

Although that wouldn't explain why the preliminary mass determination was much lower than the new one, instead of being higher.

The spacecraft is slowly equalizing charge, thus any electrical repulsion is less.

Good point.
But I meant these calculations, and they do not involve any spacecraft.
They could be wrong, of course. But the current ones (done via Rosetta trajectory alteration) could be wrong as well.

Common sense should tell them that density cannot be correct.

[GaryN]

If the comet is just not going to fit into the presently accepted models, then as proponents of an electrical model, EUers should quit 'mocking the afflicted' and develop a new model that obsoletes the old one. I'll begin by offering that the hole in the top of the rubber duckies head is the main clue. They have not yetimaged, or not shown us at least, more detail on the hole. I am wondering how deep it is, and suspect it may go 1/3 of the way through.
The hole would, during formation, or maybe even when activity becomes high enough, be aligned with the centre of the flux tube, so facing downstream. The hole has been etched out, the material becoming the ionised contents of the flux tube.
It seems likely the neck would be formed similarly to how it appears to have been with 103P/Hartley.

The bright end of Hartley, in my model, would be the downstream facing end. What appears to be outgassing from the rims of the mid section could be a discharging, similar to what has happened on 67P, resulting in the grooves radiating out wards in places.
We also need to consider just what the NavCam is 'seeing'. The NavCam is a Star Tracker as far as I can determine, in which case it is probably showing us IR, so the image showing what appears to be a lighter patch at at the base of the neck could be a warmer area, not an ice patch.
Images from the WAC and NAC are really needed before we can offer a more detailed alternative explanation, so hopefully they will make those images available, and the filter/exposure data too, but thats unlikely to be any time soon.
And maybe my expectations are driving my perceptions, but I'm sure some of the boulders frow what NAC images are available, are looking rounded?
http://www.dlr.de/dlr/presse/en/Portald ... 6_A_xl.jpg

[viscount aero]

If the comet is just not going to fit into the presently accepted models, then as proponents of an electrical model, EUers should quit 'mocking the afflicted' and develop a new model that obsoletes the old one.

That has been proposed. Do you not follow the site?

[4realScience]

To their credit, ESA Rosetta mass determination page has been opened for comments. It looks like a bunch of EU guys are already there! They question the density value and so do I. I would like to see how they calculate the volume given the surprising new shape.



http://blogs.esa.int/rosetta/2014/08/21 ... et-67pc-g/

[GaryN]

@viscount aero

That has been proposed. Do you not follow the site?

Guilty of 'skimming' lately I admit. But, my recent excursions to one of our beaches gave me an idea about the possible properties of 67P, after examining a section of layered sandstone with a very open crystal lattice. If 67P is too dense to be water, or ice, or solid rock, then it may be a macroporous silicon with a plasma modified surface. Anyone running a 'book' on this?

[viscount aero]

@viscount aero

That has been proposed. Do you not follow the site?

Guilty of 'skimming' lately I admit. But, my recent excursions to one of our beaches gave me an idea about the possible properties of 67P, after examining a section of layered sandstone with a very open crystal lattice. If 67P is too dense to be water, or ice, or solid rock, then it may be a macroporous silicon with a plasma modified surface. Anyone running a 'book' on this?

That OH- has been detected on comets indicates they are at least, perhaps, a species of hydrated silica.

[paladin17]

If 67P is too dense to be water, or ice, or solid rock, then it may be a macroporous silicon with a plasma modified surface. Anyone running a 'book' on this?

Quite the contrary, the density of only 1/3 that of ice indicates that it's very light.
If it is more or less homogeneous, then probably its material resembles something like expanded clay aggregate, or aerated concrete. On the surface, of course, it could be a bit "molten" or something, so we would get these ice-cream like curls and wrinkles and areas of relatively smooth terrain.

[4realScience]

Watching the facts change is curious. The mass is now 3x but the density remains the same. Since the comet's rubber duck shape was a surprise and since their earlier ideas of it were roughly spherical how can this be true? I now more seriously doubt their currently reported density number. It, by their own posting, should be at least 3x dense.


Before Rosetta determined the mass here is what wikipedia had on it:

http://en.wikipedia.org/wiki/67P/Churyu ... erasimenko

Physical characteristics
Dimensions 3.5×4 km (2.2×2.5 mi)
Mass 3.14±0.21×1012 kg[2][3]
Mean density
102±9 kg/m³[2]
Escape velocity
0.46 m/s (1.5 ft/s)
Rotation period
12.7 hours
Temperature −90 °C (183 K; −130 °F)

And as of this post here is the updated Wikipedia:

Physical characteristics
Dimensions 3.5×4 km (2.2×2.5 mi)
Mass 1.0±0.1×1013 kg[2]
Mean density
102±9 kg/m³[3]
Escape velocity
0.46 m/s (1.5 ft/s)
Rotation period
12.7 hours
Temperature −90 °C (183 K; −130 °F)

Note density is unchanged from the value they had before the Rosetta arrived in sight of the true shape.

I predict the density is going to go WAY up, more than 3x.

[GaryN]

If 67P is too dense to be water, or ice, or solid rock

Ooops, me dense, comet not.

[paladin17]

If it is more or less homogeneous, then probably its material resembles something like expanded clay aggregate, or aerated concrete.

... or it could be a geode-type thing, hollow on the inside.