APOD: Structure in the Tail of Comet... (2024 Feb 16)

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APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by APOD Robot » Fri Feb 16, 2024 5:05 am

Image Structure in the Tail of Comet 12P/Pons-Brooks

Explanation: Heading for its next perihelion passage on April 21, Comet 12P/Pons-Brooks is growing brighter. The greenish coma of this periodic Halley-type comet has become relatively easy to observe in small telescopes. But the bluish ion tail now streaming from the active comet's coma and buffeted by the solar wind, is faint and difficult to follow. Still, in this image stacked exposures made on the night of February 11 reveal the fainter tail's detailed structures. The frame spans over two degrees across a background of faint stars and background galaxies toward the northern constellation Lacerta. Of course Comet 12P's April 21 perihelion passage will be only two weeks after the April 8 total solar eclipse, putting the comet in planet Earth's sky along with a totally eclipsed Sun.

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by rjansen » Fri Feb 16, 2024 8:56 am

Wonderful capture of this comet! An even fainter, broad dust tail seems to extend from the nucleus toward the upper right, roughly between 1 and 2 o'clock.

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by JimB » Fri Feb 16, 2024 9:17 am

I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by tracyjohnson » Fri Feb 16, 2024 10:02 am

How often does Comet 12P/Pons-Brooks visit the inner solar system?

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by Joe Stieber » Fri Feb 16, 2024 5:15 pm

tracyjohnson wrote: Fri Feb 16, 2024 10:02 am How often does Comet 12P/Pons-Brooks visit the inner solar system?
The orbital period (P) of comet 12P/Pons-Brooks is about 71.3 years. See 12P's light curve, orbital elements, an ephemeris and recent observations at COBS, the Comet OBservation database. For me, it's been a binocular object the past couple of weeks.

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by johnnydeep » Fri Feb 16, 2024 6:34 pm

JimB wrote: Fri Feb 16, 2024 9:17 am I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?
Probably due to a difference in the make up of the material in the coma (more gas than dust?) versus the tail (more dust than gas?), though the tail is definitely just former coma material. Hard to say exactly from the description of comet structure in Wikipedia though I'm sure others more knowledgeable can clarify. (Hmm, my Chris Peterson sense is remembering that the dust is affected more by the charged particles in the solar wind and photon pressure, whereas the ionic gas is directed by the magnetic field lines in the Sun's magnetosphere. Or something like that.)
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:Tail formation

In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[1][2] but these detections have been questioned,[3][4] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[5]
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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by Ann » Fri Feb 16, 2024 7:28 pm

Hmmm, I'm kinda sorta moderately interested in comets (although I definitely prefer them to asteroids). I prefer to just draw what I see here:

APOD 16 February 2024 annotated.png

So the comet coma in green, or rather a cyan shade of green. The reason for the color is a particular chemical compound that is formed as volatiles from the comet react with solar radiation. This chemical compound is actually quite quickly destroyed, which is exactly why it is only found in the coma of the comet, not in the tails.

There are indeed two tails. The ion tail is gaseous and ionized by solar radiation. It is bluish in color, and it points directly away from the Sun. The dust tail is yellow-white,it is made of small dust particles, and it basically reflects the color of the Sun. It could be that the (brownish) color of the dust particles themselves add some yellowness to the dust tail, which might otherwise perhaps be a pure shade of white. The direction of the dust tail is affected both by solar wind and by the motion of the comet itself.


And now I leave it to the rest of you to work out all the details of the comet anatomy here! :D

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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by johnnydeep » Fri Feb 16, 2024 7:54 pm

Thanks, Ann. So I was perhaps at least partially right about the dust in the coma making its way into the tail, but the coma is mainly composed
of short-lived water vapor. From Wikipedia:
https://en.wikipedia.org/wiki/Coma_(comet) wrote:The coma is generally made of ice and comet dust.[1] Water composes up to 90% of the volatiles that outflow from the nucleus when the comet is within 3–4 au (280–370 million mi; 450–600 million km) from the Sun.[1] The H2O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization.[1] The solar wind plays a minor role in the destruction of water compared to photochemistry.[1] Larger dust particles are left along the comet's orbital path while smaller particles are pushed away from the Sun into the comet's tail by light pressure.
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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by MarkBour » Sat Feb 17, 2024 2:49 am

Thanks johnnydeep and Ann. Very helpful info.
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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by Chris Peterson » Sat Feb 17, 2024 4:51 am

johnnydeep wrote: Fri Feb 16, 2024 6:34 pm
JimB wrote: Fri Feb 16, 2024 9:17 am I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?
Probably due to a difference in the make up of the material in the coma (more gas than dust?) versus the tail (more dust than gas?), though the tail is definitely just former coma material. Hard to say exactly from the description of comet structure in Wikipedia though I'm sure others more knowledgeable can clarify. (Hmm, my Chris Peterson sense is remembering that the dust is affected more by the charged particles in the solar wind and photon pressure, whereas the ionic gas is directed by the magnetic field lines in the Sun's magnetosphere. Or something like that.)
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:Tail formation

In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[1][2] but these detections have been questioned,[3][4] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[5]
Both the coma and the ion tail are primarily gas (although as the comet gets closer to the Sun, the amount of dust in the coma increases). But the two gas regions behave very differently in response to the solar wind (which is made up of charged particles), because the coma is composed of neutral gas, while the ion trail is (you guessed it) ionized. The solar wind is sweeping the gas that gets ionized out into the tail. Leaving neutral gas in place. The dust tail is driven primarily by radiation pressure.
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Re: APOD: Structure in the Tail of Comet... (2024 Feb 16)

Post by johnnydeep » Sat Feb 17, 2024 2:12 pm

Chris Peterson wrote: Sat Feb 17, 2024 4:51 am
johnnydeep wrote: Fri Feb 16, 2024 6:34 pm
JimB wrote: Fri Feb 16, 2024 9:17 am I wonder why the tail of the comet is streaming away from the sun, "buffeted by the solar wind" as in the description, but there seems to be a separate halo around the head of the comet which is more or less smoothly distributed and unaffected by the solar wind?
Probably due to a difference in the make up of the material in the coma (more gas than dust?) versus the tail (more dust than gas?), though the tail is definitely just former coma material. Hard to say exactly from the description of comet structure in Wikipedia though I'm sure others more knowledgeable can clarify. (Hmm, my Chris Peterson sense is remembering that the dust is affected more by the charged particles in the solar wind and photon pressure, whereas the ionic gas is directed by the magnetic field lines in the Sun's magnetosphere. Or something like that.)
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:Tail formation

In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations,[1][2] but these detections have been questioned,[3][4] and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

The streams of dust and gas each form their own distinct tails, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.[5]
Both the coma and the ion tail are primarily gas (although as the comet gets closer to the Sun, the amount of dust in the coma increases). But the two gas regions behave very differently in response to the solar wind (which is made up of charged particles), because the coma is composed of neutral gas, while the ion trail is (you guessed it) ionized. The solar wind is sweeping the gas that gets ionized out into the tail. Leaving neutral gas in place. The dust tail is driven primarily by radiation pressure.
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