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Message Subject Rosetta Comet Orbiter ** Three days to live **Prize Competition ** Landing site J 10km orbit ** 67P selfie ** GLP pics ** 46,000 Gvolts
Poster Handle K Hall
Post Content
Could 67P be three times heavier and six times denser than originally thought ? I would say yes.


Yesterday ESA released a very early, preliminary estimate of 67P's mass. You may remember Rosetta has been performing a triangular orbit of 67P ( more of that in a later post ) in order to estimate its mass. The first triangle was completed on the 17th August.

The early estimate for the mass of 67P is

10 trillion kg which is about 11 billion tons ( U.S. )
1E+13kg ( +/- 10% )

[link to blogs.esa.int]

Which is fantastic, but unfortunately there is no early estimate for density, which is quite annoying as that is significant for comet conspiracy theorists on GLP.

So my first though was to do a back of the the envelope type calculation to work out 67P's volume. We have been told that 67P is 4km long and 3.5km tall so taking it to be 2.5km wide we can come up with a number. Subtract a quarter for it being boot shaped and assume 50% of the material that would fill out an L shaped polyhedron is missing and you come up with

13 km^3 - Which is probably too low.

Then I thought I could try and make my own 67P using my kids abandoned plasticine and ESA's shape model, after all that is what the flight dynamics people have been using.

[link to blogs.esa.int]

So I made my own ( yes I know it's backwards and too fat, the final one was slimmed down )

[link to i.imgur.com]

And measured its volume ( displacement ) to represent

19 km^3 - Which is probably on the high side, my model was fat.

So finally I thought comets are basically big lumps or stuff that ablate away to leave a pebble sort of shape. So I took the shape model and pulled apart the frames in GIMP and measured carefully ( assuming it is 4km long ) and treated 67P as two ellipsoids joined by a short cylinder. This time I came up with a figure that I am most confident about.

17 km^3 - Still a little high, it doesn't take into account the concave "foot" of the duck and craters.

I think I will give the very unscientific estimate of 16 km^3 +/-5 ( maximum range based on all my estimates )

Combine this with the mass estimate and we come up with a density of

0.62 g cm-3 with a range of 0.43 to 1 g cm-3

Compare this to other bodies we know about

Earth 5.5 g cm-3
Mars 4
Vesta 3.4
Moon 3.3
[silicate rock] 3
Triton 2
Ceres 2
[ water ice ] 0.9

Seeing as we know 67P contains both silicate rock and water ice it looks probable that 67P contains something else, namely nothing. For 67P's density to be as low as 0.6 g cm-3 then it will also have to contain voids and empty spaces. These may between grains of dust or even be in the form of pockets and larger structures. This is one of the things Rosetta will be investigating over the coming year.

Finally some people on the ESA blog were excited because this mass estimate is three times the estimate that had previously been given ( 3.14E+12 kg ) Taking that as proof of 67P being a rocky electric comet. That estimate comes from this paper.

[link to www.lpi.usra.edu]

Which is a brilliant clever application of what information the authors had on 67P. They came up with an estimated density of 0.1 g cm-3, just one sixth of my estimate. To put that in perspective, the densest aerogels are five times denser, I imagine this is where the "cigarette ash" analogy comes from. I think the Philae landing team will be very happy If I am right and those authors are wrong.

K
 
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