Starship Asterisk*

Comet CG Creates Its Dust Tail

Explanation: Where do comet tails come from? There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate. One of the best images of emerging jets is shown in the featured picture, taken in 2015 by ESA's robotic Rosetta spacecraft that orbited Comet 67P/Churyumov-Gerasimenko (Comet CG) from 2014 to 2016. The picture shows plumes of gas and dust escaping numerous places from Comet CG's nucleus as it neared the Sun and heated up. The comet has two prominent lobes, the larger one spanning about 4 kilometers, and a smaller 2.5-kilometer lobe connected by a narrow neck. Analyses indicate that evaporation must be taking place well inside the comet's surface to create the jets of dust and ice that we see emitted through the surface. Comet CG (also known as Comet 67P) loses in jets about a meter of radius during each of its 6.44-year orbits around the Sun, a rate at which will completely destroy the comet in only thousands of years. In 2016, Rosetta's mission ended with a controlled impact onto Comet CG's surface.

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Cg/computer graphics. Embrace the awesome.

Wow! To me, Comet CG looks looks some nasty sea creature!
I'm surprised that it will take 1000's of years to drain itself!

Reasons to believe ... in the existence of the Oort cloud, at the very least. If it wasn't for the Oort cloud occasionally dropping a comet into an orbit closer to the Sun our 4.6 billion year old system would have run out of short period comets by now.

The comet has two prominent lobes, the larger one spanning about 4 kilometers, and a smaller 2.5-kilometer lobe connected by a narrow neck. Analyses indicate that evaporation must be taking place well inside the comet's surface to create the jets of dust and ice that we see emitted through the surface. Comet CG (also known as Comet 67P) loses in jets about a meter of radius during each of its 6.44-year orbits around the Sun, a rate at which will completely destroy the comet in only thousands of years.

So doing da math, CG's larger lobe's radius is ~ 2000 meters and so it will totally disappear in about 2000m/1m/6.44 years, or about 12,880 years at maximum. But breakups will likely shorten that time considerably. In any event, it's but a tiny time in the age of our system.

Bruce

Do the jets impart a net thrust on the nucleus sufficient to change its trajectory or rate or axis of rotation over time? Has data been obtained on jet locations, composition and velocity sufficient to calculate mass flow rates and resulting thrust vectors? There seems to be some similarity to spacecraft RCS/OMS thrusters. I would expect the effect would be chaotic over an extended time, but observable and consistent over shorter times.

7UD4 wrote: ↑Sun Jul 12, 2020 2:24 pm Do the jets impart a net thrust on the nucleus sufficient to change its trajectory or rate or axis of rotation over time? Has data been obtained on jet locations, composition and velocity sufficient to calculate mass flow rates and resulting thrust vectors? There seems to be some similarity to spacecraft RCS/OMS thrusters. I would expect the effect would be chaotic over an extended time, but observable and consistent over shorter times.

Interesting question. I can't answer this definitively (and became even less sure while writing this!), but this - https://en.wikipedia.org/wiki/67P/Chury ... d_rotation - says that

Before Churyumov–Gerasimenko's perihelion passage in 2009, its rotational period was 12.76 hours. During this perihelion passage, it decreased to 12.4 hours, which likely happened because of sublimation-induced torque.[7]

So, even if there is occasionally some net thrust from the combination of all the differently directed jets, I would think they would be largely cancelled out by the rotation...unless there were jets directed predominantly along the axis of rotation...which actually seems to be through the bridge between the two lobes of the comet: there's a nice animation of this at the wikipedia link. So, yeah, all you would need is some part of the net thrust vector to be along the axis of rotation and you would have a sustained push over time that would not be cancelled out by the rotation. I do think this is likely.

Yesterday I commented on comets and I did not complete it so I will add and in reference to the image of the day that these are bodies of agglomeration of planetesimals in some area to the protection of the T-Tauri phase of the Sun in what today is known as the Oort cloud (more accurately Oort-Opik), composed mostly of water and fine dust with some larger rocks that can be as big as mountains, when it approaches the focus of its orbit (the Sun) and is about 5-9 AU away, not infrared heat but UV radiation heats it and small material such as water molecules or dust grains of 10-10,000 atoms become excited and begin to break down into steam or dust currents that are blown by the wind solar is driven in the opposite direction to the Sun generating the tails of dust and ions or plasma.
Kites with up to seven caudae have been seen (16th century) and as long as 10 AU. In order to maintain the movement, these "tails" take the same orbit of the nucleus throughout the entire trajectory and when the Earth's orbit (or any of the other objects with sufficient gravitational power) passes through it, "rain showers stars "that are nothing more than the remnants of dust from these" tails ", the brightest, are generally the size of a grain of sand and the rest one hundredth or thousandth smaller, the giants are called bolides.

Sa Ji Tario wrote: ↑Sun Jul 12, 2020 4:09 pm Kites with up to seven caudae have been seen (16th century) and as long as 10 AU. In order to maintain the movement, these "tails" take the same orbit of the nucleus throughout the entire trajectory and when the Earth's orbit (or any of the other objects with sufficient gravitational power) passes through it, "rain showers stars "that are nothing more than the remnants of dust from these" tails ", the brightest, are generally the size of a grain of sand and the rest one hundredth or thousandth smaller, the giants are called bolides.

"Fireball" is the preferred term, with "bolide" generally deprecated, in part because it is used differently in geology.

7UD4 wrote: ↑Sun Jul 12, 2020 2:24 pm
Do the jets impart a net thrust on the nucleus sufficient to change its trajectory....

With regard to a comet's trajectory due to jets:

There should be a delayed (Yarkovsky like) reaction force (especially near perihelion) either with or against the direction of motion such that there are (F dot v) changes in orbital energy.

https://en.wikipedia.org/wiki/Yarkovsky_effect wrote:
<<The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.>>

johnnydeep wrote: ↑Sun Jul 12, 2020 3:37 pm

7UD4 wrote: ↑Sun Jul 12, 2020 2:24 pm
Do the jets impart a net thrust on the nucleus sufficient to change its trajectory or rate or axis of rotation over time? Has data been obtained on jet locations, composition and velocity sufficient to calculate mass flow rates and resulting thrust vectors? There seems to be some similarity to spacecraft RCS/OMS thrusters. I would expect the effect would be chaotic over an extended time, but observable and consistent over shorter times.

Interesting question. I can't answer this definitively (and became even less sure while writing this!), but this - https://en.wikipedia.org/wiki/67P/Chury ... d_rotation - says that

Before Churyumov–Gerasimenko's perihelion passage in 2009, its rotational period was 12.76 hours. During this perihelion passage, it decreased to 12.4 hours, which likely happened because of sublimation-induced torque.[7]

So, even if there is occasionally some net thrust from the combination of all the differently directed jets, I would think they would be largely cancelled out by the rotation...unless there were jets directed predominantly along the axis of rotation...which actually seems to be through the bridge between the two lobes of the comet: there's a nice animation of this at the wikipedia link. So, yeah, all you would need is some part of the net thrust vector to be along the axis of rotation and you would have a sustained push over time that would not be cancelled out by the rotation. I do think this is likely.

In a few thousand years, after there is nothing left to create jets, Rosetta's remains and the non-watery bits will still follow CG's orbit?

Psnarf wrote: ↑Sun Jul 12, 2020 6:39 pm In a few thousand years, after there is nothing left to create jets, Rosetta's remains and the non-watery bits will still follow CG's orbit?

It's hard to say. Some comets appear to end as what are essentially rubble pile asteroids. How CG ends will ultimately depend on the ratio of volatiles to rock (is it a dirty snowball or a snowy dirtball?) as well as future interactions with the Sun. It seems certain that it will fragment into two bodies at some point in the future, but those bodies themselves could also fragment, with the non-volatiles dispersing. Yes, they'll still be substantially in CG's orbit (although the orbits of the individual bits will evolve over time until everything is scattered) but there will be no actual remaining core.

Chris Peterson wrote: ↑Sun Jul 12, 2020 6:54 pm

Psnarf wrote: ↑Sun Jul 12, 2020 6:39 pm
In a few thousand years, after there is nothing left to create jets, Rosetta's remains and the non-watery bits will still follow CG's orbit?

It's hard to say. Some comets appear to end as what are essentially rubble pile asteroids. How CG ends will ultimately depend on the ratio of volatiles to rock (is it a dirty snowball or a snowy dirtball?) as well as future interactions with the Sun. It seems certain that it will fragment into two bodies at some point in the future, but those bodies themselves could also fragment, with the non-volatiles dispersing. Yes, they'll still be substantially in CG's orbit (although the orbits of the individual bits will evolve over time until everything is scattered) but there will be no actual remaining core.

• Or does it shrink down to a 25143 Itokawa peanut

https://en.wikipedia.org/wiki/25143_Itokawa wrote:

Schematic of Itokawa's two lobes separated from each other. Their divergent densities suggest that these were stand-alone bodies that came into contact later on, making the rubble pile also a likely contact binary.

---

<<25143 Itokawa is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. The peanut-shaped S-type asteroid has a rotation period of 12.1 hours and measures approximately 330 meters in diameter. Due to its low density and high porosity, Itokawa is considered to be a rubble pile, consisting of numerous boulders of different sizes rather than of a single solid body.

The Hayabusa mission confirmed these findings and also suggested that Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together. The Hayabusa images show a surprising lack of impact craters and a very rough surface studded with boulders, described by the mission team as a rubble pile. Furthermore, the density of the asteroid is too low for it to be made from solid rock. This would mean that Itokawa is not a monolith but rather a rubble pile formed from fragments that have cohered over time.>>

Thanks for the responses Art and JohnnyD.

I'd not heard of the Yarkovsky effect, but that sounds like the mechanism that drives the Crookes radiometer (aka light mill) for a body not changing in mass. What I'm thinking about is more like a Hero turbine which would generate many orders of magnitude greater forces by expelling mass.

The net force would create translational delta-V from the component of force acting through the center of mass, and rotational from the component about the axis through the center of mass and normal to the vector from the center of mass to the vector summed point of action. ... I think. It has been way too many decades since working that kind of vector math so my description may be imprecise.

neufer wrote: ↑Sun Jul 12, 2020 5:26 pm

7UD4 wrote: ↑Sun Jul 12, 2020 2:24 pm
Do the jets impart a net thrust on the nucleus sufficient to change its trajectory....

With regard to a comet's trajectory due to jets:

There should be a delayed (Yarkovsky like) reaction force (especially near perihelion) either with or against the direction of motion such that there are (F dot v) changes in orbital energy.

https://en.wikipedia.org/wiki/Yarkovsky_effect wrote:
<<The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.>>

johnnydeep wrote: ↑Sun Jul 12, 2020 3:37 pm

7UD4 wrote: ↑Sun Jul 12, 2020 2:24 pm
Do the jets impart a net thrust on the nucleus sufficient to change its trajectory or rate or axis of rotation over time? Has data been obtained on jet locations, composition and velocity sufficient to calculate mass flow rates and resulting thrust vectors? There seems to be some similarity to spacecraft RCS/OMS thrusters. I would expect the effect would be chaotic over an extended time, but observable and consistent over shorter times.

Interesting question. I can't answer this definitively (and became even less sure while writing this!), but this - https://en.wikipedia.org/wiki/67P/Chury ... d_rotation - says that

Before Churyumov–Gerasimenko's perihelion passage in 2009, its rotational period was 12.76 hours. During this perihelion passage, it decreased to 12.4 hours, which likely happened because of sublimation-induced torque.[7]

So, even if there is occasionally some net thrust from the combination of all the differently directed jets, I would think they would be largely cancelled out by the rotation...unless there were jets directed predominantly along the axis of rotation...which actually seems to be through the bridge between the two lobes of the comet: there's a nice animation of this at the wikipedia link. So, yeah, all you would need is some part of the net thrust vector to be along the axis of rotation and you would have a sustained push over time that would not be cancelled out by the rotation. I do think this is likely.

7UD4 wrote: ↑Mon Jul 13, 2020 2:41 am
Thanks for the responses Art and JohnnyD.

I'd not heard of the Yarkovsky effect, but that sounds like the mechanism that drives the Crookes radiometer (aka light mill) for a body not changing in mass.

Replace the Crookes radiometer black/white paddles
with spinning black cylinders and you are about right.

Make the black cylinders also evaporate and, perhaps,
an "enhanced Yarkovsky effect" also simulates a comet changing in mass.