The Rosetta probe has woken up ...

davefoc said:
I understood this to some degree, but my idea was that the oxygen would not combine with the hydrogen as it was spewed from the star because it would be too hot initially and by the time it had cooled to a temperature where it might combine the concentration of hydrogen and oxygen would be too low. Water almost completely disassociates at around 3500K. So for water to form, the temperature that it is spewed out needs to drop below 3500K before the concentration of hydrogen and oxygen is to low for a significant amount of water formation to occur. Eventually the temperature of the hydrogen and oxygen will drop below 300K and there will not be enough energy available to initiate the combination of hydrogen and oxygen into water.

It seems that there is some water created during this initial cooling process but there is still a lot of elemental oxygen that makes its way into space. That's when the two other mechanisms mentioned in the article come into play. Combination facilitated by dust and combination facilitated by cosmic rays.

caveat:Obviously I'm not a chemist or physicist and I may not have everything exactly right above.
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There's another whole thread on the formation of water.
 
davefoc said:
This also explains what was going on with Hubble when it needed to have some of its reaction wheels changed although at the time I remember them being identified as gyroscopes. It sounds like reaction wheels are a kind of gyroscope where the spin rate is varied to control the spacecraft attitude.

I'm surprised reaction wheels are so unreliable. Electric motors are just about the most reliable electromechanical devices on the planet. Electric refrigerators run for generations and I doubt that people pour as much effort into making them reliable as they do spacecraft reaction wheels. Still most refrigerators aren't blasted into space and exposed to multiple g loads and then operated across the wide temperature variations that a spacecraft experiences and it may be that they need to operate in a vacuum as well. Maybe I should cut the reaction wheel engineers a little slack.
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Yes, Hubble had at least one of its reaction wheels replaced.

You are right, reaction wheels and gyroscopes are related. Gyroscopes are passive instruments that spin at all times. In case of, for example, Hubble they are used to stabilize the spacecraft (resist attitude change). The reaction wheels are used to change attitude (Hubble has both).

Reaction wheels are one of the very few moving parts on a spacecraft. I understand that the failure typically occurs in the bearing. Kepler, Dawn and other NASA missions that suffered from reaction wheel failures used mechanical bearings. One of the issues apparently is lubrication, which is a harder problem in zero gravity. It's easy to see how the reaction wheels would be among the systems more likely to fail earlier. On the other hand, there is also some discussion that maybe the parts NASA was using on these crafts are not as reliable as they could be:

http://www.nature.com/news/the-wheels-come-off-kepler-1.13032

There are also reaction wheels available that use magnetic bearings. They are used on some commercial spacecraft. I don't know why these scientific missions did not use them.
 
davefoc said:
However, I think there may have only been one on Philae so it didn't function as a reaction wheel. The reaction wheel's engineer said it was a momentum wheel which functions to make a spacecraft more resistant to changes in attitude but can't be used to adjust it like reaction wheels can. This makes it sound to me that it wouldn't have helped to keep the momentum wheel spinning after the first touch down because Philae would have some random attitude and the momentum wheel couldn't correct it.
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Probably true that it wouldn't have been useful for correcting the orientation, but the act of spinning down the wheel while the ship is free-floating causes the whole ship to spin instead of just the wheel. Momentum is conserved.
 
davefoc said:
Why would they expect CO2 and H2O frozen to -70 C to not be really hard?

The internal structure of the comet continues to interest me. Supposedly the density of the comet is about 40% that of water.
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Where did you get that number (40%). That is almost precisely the specific gravity of solid methane! If that comet with the 40% density were made out of methane, then it would have to be solid. So did your source use actual measurements (i.e., acceleration measurements of 'falling' comet debris) or a calculation based on the 'methane hypothesis'?

Most of the interstellar gases have solids with densities greater than that of solid methane. Solidified gases vary widely in density. Comet tails are mostly made out of water vapor, methane vapor, and ammonia vapor. One can hypothesize that the comet nucleus has a similar composition. However, the nucleus may be enriched in less volatile substances. They probably contain lots of minerals found in stony bolides. So I think the spongy structure is more plausible than the solid structure. However, we may know soon enough !-)

The comet could conceivably be solid if it was made entirely of solid methane. Solid methane has a low density (0.43 g/cc). A spongy blob of solid methane would not be dense enough to be the type of comets found swinging near the sun. However, many scientists conjecture that the comet is mostly solid water (0.98 g/cc). If the comet is mostly made of water, it is very likely to have a spongy structure.

Here is a little table of densities. I think it will help the discussion.

Liquid water has a specific gravity of 1.00, of course. Ice floats because it has a specific gravity close to 0.98.

Solid carbon dioxide has a specific gravity of 1.56.
http://chemistry.about.com/od/moleculescompounds/a/dryicefacts.htm
‘The specific gravity of dry ice is 1.56 (water = 1).’

Liquid nitrogen has a specific gravity of 0.808.
http://www.airproducts.com/~/media/Files/PDF/company/safetygram-7.pdf

Solid methane has a specific heat of 0.43.
Solid nitrogen has a specific heat of 0.83.
Jenam_Session4_4Csatorre-2.pdf


Solid ammonia has a specific gravity of 0.82.
https://answers.yahoo.com/question/index?qid…
the liquid NH3 has a density of 0.68 g/ml
the solid NH3 has a density of 0.82 g/ml

http://en.wikipedia.org/wiki/Solid_hydrogen
Solid hydrogen has a density of 0.086*g/cm3 making it one of the lowest density solids.


So unless this comet contains lots of methane and hydrogen, it will have a spongy structure.
 
pgwenthold said:
That's consistent with the claim that there are "multiple peaks" (fragmentation patterns). They are claiming "complex organic molecules" - anyone know the masses they are getting?
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I haven't been able to find anything except repeated discussion of isotope ratios for H, C, N, and O. The upper m/z limit of Ptolemy is about 150. It would be interesting to find out what "complex organic molecules" were seen.
 
This is a source for the comet density:
http://blogs.esa.int/rosetta/2014/10/03/measuring-comet-67pc-g/
Density 0.4 g/cm^3
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The density was originally estimated to be about .3 g/cm based on some indirect method that I don't remember and that I didn't understand. I was skeptical that it could be that low since we were told that the main constituents of the comet were CO2 and H2O and dust. If that were so the comet needed to be made up of some sort of matrix with most of the volume consisting of empty space.

But after thinking about it for awhile it seemed at least possible to me that the structure of the comet could be something like that. I thought about a bucket of frozen ice cubes that had partially melted and refrozen. That kind of thing makes a structure plenty strong enough to hold up in the very tiny gravity of the comet.

I also wondered about how that kind of structure could come about. Based on an article that I read it seems like most of the water in space is in the form of ice. So does the random accretion of ice (possibly attached to dust particles) on to a comet account for the nature of the interior of a comet? Or maybe something comes a long and randomly causes the surface to develop holes that are not destroyed as the materials coalesce on to the surface of the comet?
 
davefoc said:
This is a source for the comet density:
http://blogs.esa.int/rosetta/2014/10/03/measuring-comet-67pc-g/
The density was originally estimated to be about .3 g/cm based on some indirect method that I don't remember and that I didn't understand. I was skeptical that it could be that low since we were told that the main constituents of the comet were CO2 and H2O and dust. If that were so the comet needed to be made up of some sort of matrix with most of the volume consisting of empty space.

But after thinking about it for awhile it seemed at least possible to me that the structure of the comet could be something like that. I thought about a bucket of frozen ice cubes that had partially melted and refrozen. That kind of thing makes a structure plenty strong enough to hold up in the very tiny gravity of the comet.

I also wondered about how that kind of structure could come about. Based on an article that I read it seems like most of the water in space is in the form of ice. So does the random accretion of ice (possibly attached to dust particles) on to a comet account for the nature of the interior of a comet? Or maybe something comes a long and randomly causes the surface to develop holes that are not destroyed as the materials coalesce on to the surface of the comet?
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I pointed out that the specific gravity of frozen methane is about 0.43. So a comet made of solid methane would have a density of about 0.43. Given the irregular shape of the comet and the difficulty in determining total mass, the uncertainty in the established density must be large.

I conjecture that a comet could be made of solid methane with no porosity, and still have a density on the order of 0.4 g/cc.

Water ice would greatly increase the density. So, I go for the porous comet model. However, we may know soon enough.
 
Rosetta fuels debate on origin of Earth’s oceans / Rosetta / Space Science / Our Activities / ESA

Comet 67P has about 3 times as much deuterium (H-2) as the Earth does, and other comets are also enriched, typically by a factor of 2 or 3. Asteroids, however, have amounts of deuterium close to the Earth's, though some of them are enriched up to a factor of 2. Uranus and Neptune, however, have only 1/3 as much, and the Sun, Jupiter, and Saturn only 1/9 as much.

What likely did it: fractional evaporation and fractional condensation. Light isotopes evaporate more easily than heavy ones, and heavy ones condense more easily than light ones. One can measure this difference. At 1 atm pressure, ordinary water boils at 100 C, while heavy water boils at 101.4 C. Heavy water is (H-2)2O as opposed to ordinary water, which is mostly (H-1)2O.

The less massive objects could not accrete hydrogen gas, so they most likely got their hydrogen in water, and that required condensing out of the protoplanetary nebula. That may explain the increased concentration of deuterium: heavy water condensing out more easily than light water.

A good check may be provided by isotopes of oxygen, since they also may be fractionated.
 
It's extremely well-known that isotopic composition varies significantly throughout the solar system. I don't see a big deal. As noted, the deuterium fraction in this comment is in-line with what has been found for other comets. So it's not like this is news. I think it is on Venus where the Ar 38/40 ration is something like 1:1

Isotope fractionation mechanisms for various places are regularly discussed in the scientific literature. This is not something we haven't known before.
 
I'm bumping this with a technical question I don't wanna ask in the electric thread:
I know the insolation values on earth can be ballparked at a kilowatt hour / square meter. What's the value at the current distance, some where between Mars and Jupiter?
 
ApolloGnomon said:
I'm bumping this with a technical question I don't wanna ask in the electric thread:
I know the insolation values on earth can be ballparked at a kilowatt hour / square meter. What's the value at the current distance, some where between Mars and Jupiter?
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I think 1000W/m^2 is the standard value for earth's surface. Without an atmosphere it's a bit higher (more than 1300W/m^2). From here: http://www.livecometdata.com/comets/67p-churyumov-gerasimenko/ current solar distance is around 2.7 AU. Insolation/irradiation should fall off as the inverse square of the distance so:

I = (1/2.7)^2 Io
I = 1/7.29 * Io
I = 1300W/m^2 / 7.29
I = 180W/m^2
 
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