APOD: Encke and the Tadpoles (2023 Oct 27)

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APOD: Encke and the Tadpoles (2023 Oct 27)

Post by APOD Robot » Fri Oct 27, 2023 4:06 am

Image Encke and the Tadpoles

Explanation: History's second known periodic comet is Comet Encke (2P/Encke). As it swings through the inner Solar System, Encke's orbit takes it from an aphelion, its greatest distance from the Sun, inside the orbit of Jupiter to a perihelion just inside the orbit of Mercury. Returning to its perihelion every 3.3 years, Encke has the shortest period of the Solar System's major comets. Comet Encke is also associated with (at least) two annual meteor showers on planet Earth, the North and South Taurids. Both showers are active in late October and early November. Their two separate radiants lie near bright star Aldebaran in the head-strong constellation Taurus. A faint comet, Encke was captured in this telescopic field of view imaged on the morning of August 24. Then, Encke's pretty greenish coma was close on the sky to the young, embedded star cluster and light-years long, tadpole-shaped star-forming clouds in emission nebula IC 410. Now near bright star Spica in Virgo Comet Encke passed its 2023 perihelion only five days ago, on October 22.

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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Ann » Fri Oct 27, 2023 6:48 am

Today's APOD shows a beautiful juxtaposition between green comet Encke and red nebula IC 410, featuring the famous Tadpoles.


The color contrast between comet Encke and nebula IC 410 is great, and the seeming juxtaposition between the comet and the nebula is indeed fascinating. Of course, since the comet is probably at most a few light-hours away, and the nebula may be 12,000 light-years away, these two objects are far from being neighbors!

Another attractive aspect of this cosmic traffic light show (where yellow is mostly missing) is the rich star field against which it is seen. But me being me, I'm not altogether happy. I have two complaints: One, the color blue seems pretty much completely missing here, and two, the nebula is "starless" even though the rest of the picture is full of stars.

Red emission nebulas, as well as blue reflection nebulas nebulas, typically form in the presence of (blue) stars. I know that starless nebula pictures are popular, but I want to see the stars.

This is what I want IC 410 to look like:

f1026Gm9pPiN_1824x0_nIwUaf-v[1].jpg
IC 410 in HαRGB. Note the tadpoles. Credit: Samuel at Astrobin.
IC 410 and cluster NGC 1893 Samuel.png
O-stars in IC 410.

As you can see, the picture by Samuel not only shows the stars of IC 410 (the cluster powering this nebula is NGC 1893), but Samuel's palette is also softer and richer in hues. That said, most color pictures of IC 410, particularly when IC 410 is shown along with its (apparent!!) neighbor IC 405, shows IC 410 as "all red". Unless, of course, the photographer used a narrowband OIII filter, because then IC 410 will be "almost all blue" from the OIII emission from the hot stars of NGC 1893!


So you could argue that I'm being unfair when I'm asking for more hues than red in a broadband picture of IC 410.

Back to the APOD. Did you notice the small white crescent-shaped nebula at upper center right? Because all the nebulosity is starless, I was racking my brains to try to figure out what this nebula is. I guess the only possible answer is IC 417.


As you can see, there is a bright yellow (foreground) star, phi Aurigae, next to IC 417. This star could have helped us identify IC 417 in the APOD, but that wasn't possible, since IC 417, just like the bright part of IC 410, is starless in the APOD.

Ann
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Christian G. » Fri Oct 27, 2023 2:08 pm

Two gems from the skies seen together, a comet and a nebula, with perfectly complementary colours on top of it! Well done!

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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by orin stepanek » Fri Oct 27, 2023 2:08 pm

th.jpg
Nice work Ann; of course I thought of Tadpole Galaxies!
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Fri Oct 27, 2023 8:56 pm

The Wikipedia link to comets says that comet 311P/PANSTARRS (3.24 years) has a slightly shorter period than 2P/Encke (3.30 years).

And again, how is it that a comet can manage to produce two (or more!) separate clouds of debris that the Earth can pass through and so cause meteor showers at different times of the same year? Is this a rare or common thing? This is the second APOD in a row featuring a comet that does this (1P/Halley was yesterday's example). I could see it happening if the comet's orbit precesses. What other mechanisms are there?
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Fri Oct 27, 2023 9:43 pm

johnnydeep wrote: Fri Oct 27, 2023 8:56 pm The Wikipedia link to comets says that comet 311P/PANSTARRS (3.24 years) has a slightly shorter period than 2P/Encke (3.30 years).

And again, how is it that a comet can manage to produce two (or more!) separate clouds of debris that the Earth can pass through and so cause meteor showers at different times of the same year? Is this a rare or common thing? This is the second APOD in a row featuring a comet that does this (1P/Halley was yesterday's example). I could see it happening if the comet's orbit precesses. What other mechanisms are there?
Comets don't produce "clouds of debris". The produce rings of debris, that over time fill their entire orbit (although "ring" slightly simplifies the reality of multiple filaments and varying densities, given that most material originates when a comet is near perihelion, and then slowly spreads out ahead of and behind the parent body). Any comet that crosses Earth's path twice can create two showers. (The comet crosses up across our orbit and then back down through it at another point.)
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Sat Oct 28, 2023 3:10 pm

Chris Peterson wrote: Fri Oct 27, 2023 9:43 pm
johnnydeep wrote: Fri Oct 27, 2023 8:56 pm The Wikipedia link to comets says that comet 311P/PANSTARRS (3.24 years) has a slightly shorter period than 2P/Encke (3.30 years).

And again, how is it that a comet can manage to produce two (or more!) separate clouds of debris that the Earth can pass through and so cause meteor showers at different times of the same year? Is this a rare or common thing? This is the second APOD in a row featuring a comet that does this (1P/Halley was yesterday's example). I could see it happening if the comet's orbit precesses. What other mechanisms are there?
Comets don't produce "clouds of debris". The produce rings of debris, that over time fill their entire orbit (although "ring" slightly simplifies the reality of multiple filaments and varying densities, given that most material originates when a comet is near perihelion, and then slowly spreads out ahead of and behind the parent body). Any comet that crosses Earth's path twice can create two showers. (The comet crosses up across our orbit and then back down through it at another point.)
Ok, rings of debris (eventually) not clouds. But how likely is it for Earth to cross a comet's debris ring more than once (the link implies Encke might be the cause of at least three showers)? And I would assume that even a single crossing would be the exception. After all, the Earth's orbit doesn't cross the path of any of the other planets at all!
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Sat Oct 28, 2023 3:36 pm

johnnydeep wrote: Sat Oct 28, 2023 3:10 pm
Chris Peterson wrote: Fri Oct 27, 2023 9:43 pm
johnnydeep wrote: Fri Oct 27, 2023 8:56 pm The Wikipedia link to comets says that comet 311P/PANSTARRS (3.24 years) has a slightly shorter period than 2P/Encke (3.30 years).

And again, how is it that a comet can manage to produce two (or more!) separate clouds of debris that the Earth can pass through and so cause meteor showers at different times of the same year? Is this a rare or common thing? This is the second APOD in a row featuring a comet that does this (1P/Halley was yesterday's example). I could see it happening if the comet's orbit precesses. What other mechanisms are there?
Comets don't produce "clouds of debris". The produce rings of debris, that over time fill their entire orbit (although "ring" slightly simplifies the reality of multiple filaments and varying densities, given that most material originates when a comet is near perihelion, and then slowly spreads out ahead of and behind the parent body). Any comet that crosses Earth's path twice can create two showers. (The comet crosses up across our orbit and then back down through it at another point.)
Ok, rings of debris (eventually) not clouds. But how likely is it for Earth to cross a comet's debris ring more than once (the link implies Encke might be the cause of at least three showers)? And I would assume that even a single crossing would be the exception. After all, the Earth's orbit doesn't cross the path of any of the other planets at all!
The planets are in near circular orbits. Comets are in very eccentric orbits so they can easily cross the orbits of inner planets. Every meteor you see at night represents a body with an orbit that crosses Earth's orbit! It would be odd indeed if no comets or asteroids didn't cross our orbit twice. And given that comets and asteroids can have their orbits perturbed, the parent can be moved out of the toroidal debris region and create a new one, meaning more than two meteor showers become possible. Likewise, debris streams can be perturbed over time, shifting when or if showers occur, or splitting into multiple streams that can create separate showers.
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Sat Oct 28, 2023 8:21 pm

Chris Peterson wrote: Sat Oct 28, 2023 3:36 pm
johnnydeep wrote: Sat Oct 28, 2023 3:10 pm
Chris Peterson wrote: Fri Oct 27, 2023 9:43 pm

Comets don't produce "clouds of debris". The produce rings of debris, that over time fill their entire orbit (although "ring" slightly simplifies the reality of multiple filaments and varying densities, given that most material originates when a comet is near perihelion, and then slowly spreads out ahead of and behind the parent body). Any comet that crosses Earth's path twice can create two showers. (The comet crosses up across our orbit and then back down through it at another point.)
Ok, rings of debris (eventually) not clouds. But how likely is it for Earth to cross a comet's debris ring more than once (the link implies Encke might be the cause of at least three showers)? And I would assume that even a single crossing would be the exception. After all, the Earth's orbit doesn't cross the path of any of the other planets at all!
The planets are in near circular orbits. Comets are in very eccentric orbits so they can easily cross the orbits of inner planets. Every meteor you see at night represents a body with an orbit that crosses Earth's orbit! It would be odd indeed if no comets or asteroids didn't cross our orbit twice. And given that comets and asteroids can have their orbits perturbed, the parent can be moved out of the toroidal debris region and create a new one, meaning more than two meteor showers become possible. Likewise, debris streams can be perturbed over time, shifting when or if showers occur, or splitting into multiple streams that can create separate showers.
I can see why a comet in an eccentric orbit orbit would be likely to cross the plane of Earth's orbit (and I suppose also any other planet), but why would it cross the orbit itself. That implies that it reaches the same distance as the Earth did - why would that be likely?

And why is crossing it twice the prevalent mode of interaction? Is it merely because if it happens to cross it once, it will necessarily cross it again at the same distance on the other side (due to orbits being symmetrical ellipses)?
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Sat Oct 28, 2023 8:38 pm

johnnydeep wrote: Sat Oct 28, 2023 8:21 pm
Chris Peterson wrote: Sat Oct 28, 2023 3:36 pm
johnnydeep wrote: Sat Oct 28, 2023 3:10 pm

Ok, rings of debris (eventually) not clouds. But how likely is it for Earth to cross a comet's debris ring more than once (the link implies Encke might be the cause of at least three showers)? And I would assume that even a single crossing would be the exception. After all, the Earth's orbit doesn't cross the path of any of the other planets at all!
The planets are in near circular orbits. Comets are in very eccentric orbits so they can easily cross the orbits of inner planets. Every meteor you see at night represents a body with an orbit that crosses Earth's orbit! It would be odd indeed if no comets or asteroids didn't cross our orbit twice. And given that comets and asteroids can have their orbits perturbed, the parent can be moved out of the toroidal debris region and create a new one, meaning more than two meteor showers become possible. Likewise, debris streams can be perturbed over time, shifting when or if showers occur, or splitting into multiple streams that can create separate showers.
I can see why a comet in an eccentric orbit orbit would be likely to cross the plane of Earth's orbit (and I suppose also any other planet), but why would it cross the orbit itself. That implies that it reaches the same distance as the Earth did - why would that be likely?

And why is crossing it twice the prevalent mode of interaction? Is it merely because if it happens to cross it once, it will necessarily cross it again at the same distance on the other side (due to orbits being symmetrical ellipses)?
Every comet and asteroid crosses the plane of Earth's orbit! Only a small percentage cross the orbit itself (meaning they are 1 AU from the Sun when they cross the plane). Which is why hardly any comets produce meteor showers. Nobody is suggesting it is "likely", just that it is inevitable a few will cross given the thousands that have visited the inner system over a few thousand years (the typical lifetime of a shower).
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Sun Oct 29, 2023 3:39 pm

Chris Peterson wrote: Sat Oct 28, 2023 8:38 pm
johnnydeep wrote: Sat Oct 28, 2023 8:21 pm
Chris Peterson wrote: Sat Oct 28, 2023 3:36 pm
The planets are in near circular orbits. Comets are in very eccentric orbits so they can easily cross the orbits of inner planets. Every meteor you see at night represents a body with an orbit that crosses Earth's orbit! It would be odd indeed if no comets or asteroids didn't cross our orbit twice. And given that comets and asteroids can have their orbits perturbed, the parent can be moved out of the toroidal debris region and create a new one, meaning more than two meteor showers become possible. Likewise, debris streams can be perturbed over time, shifting when or if showers occur, or splitting into multiple streams that can create separate showers.
I can see why a comet in an eccentric orbit orbit would be likely to cross the plane of Earth's orbit (and I suppose also any other planet), but why would it cross the orbit itself. That implies that it reaches the same distance as the Earth did - why would that be likely?

And why is crossing it twice the prevalent mode of interaction? Is it merely because if it happens to cross it once, it will necessarily cross it again at the same distance on the other side (due to orbits being symmetrical ellipses)?
Every comet and asteroid crosses the plane of Earth's orbit! Only a small percentage cross the orbit itself (meaning they are 1 AU from the Sun when they cross the plane). Which is why hardly any comets produce meteor showers. Nobody is suggesting it is "likely", just that it is inevitable a few will cross given the thousands that have visited the inner system over a few thousand years (the typical lifetime of a shower).
Thanks for clarifying. Yeah, I guess that blue part SHOULD have been obvious to me! (otherwise, the comet or asteroid wouldn't be in orbit around the Sun)

Now, about "crossing the orbit twice" (if it crosses it at all) - is that because the orbit has to be symmetrical as I posited?
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Sun Oct 29, 2023 5:34 pm

johnnydeep wrote: Sun Oct 29, 2023 3:39 pm
Chris Peterson wrote: Sat Oct 28, 2023 8:38 pm
johnnydeep wrote: Sat Oct 28, 2023 8:21 pm

I can see why a comet in an eccentric orbit orbit would be likely to cross the plane of Earth's orbit (and I suppose also any other planet), but why would it cross the orbit itself. That implies that it reaches the same distance as the Earth did - why would that be likely?

And why is crossing it twice the prevalent mode of interaction? Is it merely because if it happens to cross it once, it will necessarily cross it again at the same distance on the other side (due to orbits being symmetrical ellipses)?
Every comet and asteroid crosses the plane of Earth's orbit! Only a small percentage cross the orbit itself (meaning they are 1 AU from the Sun when they cross the plane). Which is why hardly any comets produce meteor showers. Nobody is suggesting it is "likely", just that it is inevitable a few will cross given the thousands that have visited the inner system over a few thousand years (the typical lifetime of a shower).
Thanks for clarifying. Yeah, I guess that blue part SHOULD have been obvious to me! (otherwise, the comet or asteroid wouldn't be in orbit around the Sun)

Now, about "crossing the orbit twice" (if it crosses it at all) - is that because the orbit has to be symmetrical as I posited?
It's a bit more complex. Consider that the Earth is not in a circular orbit, nor are comets. The "symmetry" you are talking about is with respect to the point of perihelion. Unless these lie in the same direction for the comet and the Earth, we won't get two crossings of the orbits.
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Sun Oct 29, 2023 7:03 pm

Chris Peterson wrote: Sun Oct 29, 2023 5:34 pm
johnnydeep wrote: Sun Oct 29, 2023 3:39 pm
Chris Peterson wrote: Sat Oct 28, 2023 8:38 pm

Every comet and asteroid crosses the plane of Earth's orbit! Only a small percentage cross the orbit itself (meaning they are 1 AU from the Sun when they cross the plane). Which is why hardly any comets produce meteor showers. Nobody is suggesting it is "likely", just that it is inevitable a few will cross given the thousands that have visited the inner system over a few thousand years (the typical lifetime of a shower).
Thanks for clarifying. Yeah, I guess that blue part SHOULD have been obvious to me! (otherwise, the comet or asteroid wouldn't be in orbit around the Sun)

Now, about "crossing the orbit twice" (if it crosses it at all) - is that because the orbit has to be symmetrical as I posited?
It's a bit more complex. Consider that the Earth is not in a circular orbit, nor are comets. The "symmetry" you are talking about is with respect to the point of perihelion. Unless these lie in the same direction for the comet and the Earth, we won't get two crossings of the orbits.
I think I need a picture - this is a top view:

comet crossing earths orbit.jpg

If the the two orbits lie in the same exact plane (or very close), then comet debris at both X and + could cause meteor showers. But if the orbit of the comet is inclined sufficiently out of the ecliptic (rotated out of the plane about the long axis), there will be no crossing. So how could there be only one crossing?
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Sun Oct 29, 2023 10:18 pm

johnnydeep wrote: Sun Oct 29, 2023 7:03 pm
Chris Peterson wrote: Sun Oct 29, 2023 5:34 pm
johnnydeep wrote: Sun Oct 29, 2023 3:39 pm

Thanks for clarifying. Yeah, I guess that blue part SHOULD have been obvious to me! (otherwise, the comet or asteroid wouldn't be in orbit around the Sun)

Now, about "crossing the orbit twice" (if it crosses it at all) - is that because the orbit has to be symmetrical as I posited?
It's a bit more complex. Consider that the Earth is not in a circular orbit, nor are comets. The "symmetry" you are talking about is with respect to the point of perihelion. Unless these lie in the same direction for the comet and the Earth, we won't get two crossings of the orbits.
I think I need a picture - this is a top view:


comet crossing earths orbit.jpg


If the the two orbits lie in the same exact plane (or very close), then comet debris at both X and + could cause meteor showers. But if the orbit of the comet is inclined sufficiently out of the ecliptic (rotated out of the plane about the long axis), there will be no crossing. So how could there be only one crossing?
Here we have the Earth's orbit in blue (eccentricity exaggerated), and a highly inclined comet. The comet's orbit is red where it is above the plane of Earth's orbit, and green below it. There is one point of intersection where the comet drops through Earth's orbital plane, and no others.
_
2orbits.jpg
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Mon Oct 30, 2023 1:23 pm

Chris Peterson wrote: Sun Oct 29, 2023 10:18 pm
johnnydeep wrote: Sun Oct 29, 2023 7:03 pm
Chris Peterson wrote: Sun Oct 29, 2023 5:34 pm

It's a bit more complex. Consider that the Earth is not in a circular orbit, nor are comets. The "symmetry" you are talking about is with respect to the point of perihelion. Unless these lie in the same direction for the comet and the Earth, we won't get two crossings of the orbits.
I think I need a picture - this is a top view:


comet crossing earths orbit.jpg


If the the two orbits lie in the same exact plane (or very close), then comet debris at both X and + could cause meteor showers. But if the orbit of the comet is inclined sufficiently out of the ecliptic (rotated out of the plane about the long axis), there will be no crossing. So how could there be only one crossing?
Here we have the Earth's orbit in blue (eccentricity exaggerated), and a highly inclined comet. The comet's orbit is red where it is above the plane of Earth's orbit, and green below it. There is one point of intersection where the comet drops through Earth's orbital plane, and no others.
_
2orbits.jpg
Thanks. Yeah, that works. I clearly suffer from a failure of 3D imagination. It's still not clear to me what percentage of comets that produce showers produce only one per year, versus those that produce two, or even more. Do you have an idea?
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Mon Oct 30, 2023 1:43 pm

johnnydeep wrote: Mon Oct 30, 2023 1:23 pm
Chris Peterson wrote: Sun Oct 29, 2023 10:18 pm
johnnydeep wrote: Sun Oct 29, 2023 7:03 pm

I think I need a picture - this is a top view:


comet crossing earths orbit.jpg


If the the two orbits lie in the same exact plane (or very close), then comet debris at both X and + could cause meteor showers. But if the orbit of the comet is inclined sufficiently out of the ecliptic (rotated out of the plane about the long axis), there will be no crossing. So how could there be only one crossing?
Here we have the Earth's orbit in blue (eccentricity exaggerated), and a highly inclined comet. The comet's orbit is red where it is above the plane of Earth's orbit, and green below it. There is one point of intersection where the comet drops through Earth's orbital plane, and no others.
_
2orbits.jpg
Thanks. Yeah, that works. I clearly suffer from a failure of 3D imagination. It's still not clear to me what percentage of comets that produce showers produce only one per year, versus those that produce two, or even more. Do you have an idea?
I think it is very rare for a comet to produce more than one shower.
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Mon Oct 30, 2023 1:48 pm

Chris Peterson wrote: Mon Oct 30, 2023 1:43 pm
johnnydeep wrote: Mon Oct 30, 2023 1:23 pm
Chris Peterson wrote: Sun Oct 29, 2023 10:18 pm
Here we have the Earth's orbit in blue (eccentricity exaggerated), and a highly inclined comet. The comet's orbit is red where it is above the plane of Earth's orbit, and green below it. There is one point of intersection where the comet drops through Earth's orbital plane, and no others.
_
2orbits.jpg
Thanks. Yeah, that works. I clearly suffer from a failure of 3D imagination. It's still not clear to me what percentage of comets that produce showers produce only one per year, versus those that produce two, or even more. Do you have an idea?
I think it is very rare for a comet to produce more than one shower.
Ok, and yet APOD has featured two such comets in the past few days! Somehow I thought you were implying otherwise above, but I think I was misreading your actual words.
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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by Chris Peterson » Mon Oct 30, 2023 1:58 pm

johnnydeep wrote: Mon Oct 30, 2023 1:48 pm
Chris Peterson wrote: Mon Oct 30, 2023 1:43 pm
johnnydeep wrote: Mon Oct 30, 2023 1:23 pm

Thanks. Yeah, that works. I clearly suffer from a failure of 3D imagination. It's still not clear to me what percentage of comets that produce showers produce only one per year, versus those that produce two, or even more. Do you have an idea?
I think it is very rare for a comet to produce more than one shower.
Ok, and yet APOD has featured two such comets in the past few days! Somehow I thought you were implying otherwise above, but I think I was misreading your actual words.
The Taurid showers originate from a very large and complex debris stream. The northern and southern components don't exactly originate from the same body (2P/Encke). The northern component's parent is an asteroid, which probably broke off of Encke. Indeed, the entire shower complex, with its large radiant and complex filaments, is the product of a disrupted comet. So this is not a case of a single body crossing Earth's orbit twice, but of a cloud of bodies in similar orbits crossing it. Which is why we see the Taurids over a short period of time with a nearly common radiant (as opposed to Halley's two distinct crossings, producing two showers at different times of the year with different radiants).
Chris

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Re: APOD: Encke and the Tadpoles (2023 Oct 27)

Post by johnnydeep » Mon Oct 30, 2023 2:52 pm

Chris Peterson wrote: Mon Oct 30, 2023 1:58 pm
johnnydeep wrote: Mon Oct 30, 2023 1:48 pm
Chris Peterson wrote: Mon Oct 30, 2023 1:43 pm
I think it is very rare for a comet to produce more than one shower.
Ok, and yet APOD has featured two such comets in the past few days! Somehow I thought you were implying otherwise above, but I think I was misreading your actual words.
The Taurid showers originate from a very large and complex debris stream. The northern and southern components don't exactly originate from the same body (2P/Encke). The northern component's parent is an asteroid, which probably broke off of Encke. Indeed, the entire shower complex, with its large radiant and complex filaments, is the product of a disrupted comet. So this is not a case of a single body crossing Earth's orbit twice, but of a cloud of bodies in similar orbits crossing it. Which is why we see the Taurids over a short period of time with a nearly common radiant (as opposed to Halley's two distinct crossings, producing two showers at different times of the year with different radiants).
Ok, got it - thanks.
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