APOD: Three Perseid Nights (2021 Aug 20)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4412
Joined: Fri Dec 04, 2009 3:27 am

APOD: Three Perseid Nights (2021 Aug 20)

Post by APOD Robot » Fri Aug 20, 2021 4:05 am

Image Three Perseid Nights

Explanation: Frames from a camera that spent three moonless nights under the stars create this composite night skyscape. They were recorded during August 11-13 while planet Earth was sweeping through the dusty trail of comet Swift-Tuttle. One long exposure, untracked for the foreground, and the many star tracking captures of Perseid shower meteors were taken from the village of Magyaregres, Hungary. Each aligned against the background stars, the meteor trails all point back to the annual shower's radiant in the constellation Perseus heroically standing above this rural horizon. Of course the comet dust particles are traveling along trajectories parallel to each other. The radiant effect is due only to perspective, as the parallel tracks appear to converge in the distance against the starry sky.

<< Previous APOD This Day in APOD Next APOD >>

User avatar
orin stepanek
Plutopian
Posts: 6826
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by orin stepanek » Fri Aug 20, 2021 11:04 am

ThreeNightsPerseids1024.jpg
Andromeda or another meteor flash?
You do not have the required permissions to view the files attached to this post.
Orin

Smile today; tomorrow's another day!

User avatar
Ann
4725 Å
Posts: 11527
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Ann » Fri Aug 20, 2021 11:33 am

orin stepanek wrote:
Fri Aug 20, 2021 11:04 am
Andromeda or another meteor flash?
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Have to say that I love the contrast between the muted green-brown-yellow colors of the vegetation of the Earth, and the icy blue pinpricks of stars against the stark blackness of space in the Milky Way.

Ann
Last edited by Ann on Fri Aug 20, 2021 4:39 pm, edited 1 time in total.
Color Commentator

User avatar
orin stepanek
Plutopian
Posts: 6826
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by orin stepanek » Fri Aug 20, 2021 12:08 pm

Thanks Ann; I was sure it was; but I figured I needed verification! :wink:
Orin

Smile today; tomorrow's another day!

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18374
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by neufer » Fri Aug 20, 2021 1:04 pm

Ann wrote:
Fri Aug 20, 2021 11:33 am

Can't find the "W" of Cassiopeia! :cry:
  • There is no "W" of Cassiopeia :!:
    (There are 2 "Σ"'s , however: ΚαΣΣιόπεια.
Art Neuendorffer

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18374
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by neufer » Fri Aug 20, 2021 1:17 pm

I did a back of the envelope calculation of the approximate brightness of a 1 mm (i.e., grain of sand) Perseid meteoroid (based upon a one second dissipation of its kinetic energy).

I got ~15,000 candela or half of an aerial flare.
Art Neuendorffer

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Fri Aug 20, 2021 4:16 pm

Ann wrote:
Fri Aug 20, 2021 11:33 am
orin stepanek wrote:
Fri Aug 20, 2021 11:04 am
Andromeda or another meteor flash?
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Have to day that I love the contrast between the muted green-brown-yellow colors of the vegetation of the Earth, and the icy blue pinpricks of stars against the stark blackness of space in the Milky Way.

Ann
Yup - a very nice color palatte.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Fri Aug 20, 2021 4:24 pm

I was going to post a question about what determines the Perseids' speeds, but found this very helpful diagram explanation instead. From https://sky-lights.org/2020/10/05/qa-in ... -a-meteor/

Earth moves around its orbit with a nearly-constant speed of 30 km/s. The Perseid debris stream reaches a top speed of 43 km/s at perihelion (just inside Earth’s orbit at 0.95 AU), but slows to around 40 km/s by the time it reaches Earth’s orbit (at 1 AU).

The debris approaches Earth on a 113° vector, so we can split that 40 km/s velocity into two components: one oriented toward Earth, and the other perpendicular to Earth’s motion, as follows:

speed in plane of Earth’s orbit = 40 km/s x cos(-113°) = 15 km/s
speed perpendicular to orbit = 40 km/s x sin(-113°) = 37 km/s

[Note: That negative sign on the angle is required because the Perseids are on a retrograde orbit. The default direction of motion in the solar system is “counterclockwise with north up”. So with an orbital inclination greater than 90°, the Perseids are orbiting “backwards”.]

To get the total Perseid approach speed (relative to Earth) we first need to add Earth’s 30 km/s speed to the component of Perseid speed in the plane of Earth’s orbit: 15 km/s + 30 km/s = 45 km/s. When we combine this component with the perpendicular component we get a resultant speed of:

v = √[(45 km/s)2 + (37 km/s)2] = 58 km/s

So... the Perseid debris stream contributes significantly to the collision speed during a meteor shower. A more accurate description would be “Earth and the Perseids run into each other.”
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18374
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by neufer » Fri Aug 20, 2021 4:35 pm

johnnydeep wrote:
Fri Aug 20, 2021 4:24 pm
https://sky-lights.org/2020/10/05/qa-incoming-speed-of-a-meteor/ wrote:Earth moves around its orbit with a nearly-constant speed of 30 km/s. The Perseid debris stream reaches a top speed of 43 km/s at perihelion (just inside Earth’s orbit at 0.95 AU), but slows to around 40 km/s by the time it reaches Earth’s orbit (at 1 AU).

The debris approaches Earth on a 113° vector, so we can split that 40 km/s velocity into two components: one oriented toward Earth, and the other perpendicular to Earth’s motion, as follows:

speed in plane of Earth’s orbit = 40 km/s x cos(-113°) = 15 km/s
speed perpendicular to orbit = 40 km/s x sin(-113°) = 37 km/s

[Note: That negative sign on the angle is required because the Perseids are on a retrograde orbit. The default direction of motion in the solar system is “counterclockwise with north up”. So with an orbital inclination greater than 90°, the Perseids are orbiting “backwards”.]

To get the total Perseid approach speed (relative to Earth) we first need to add Earth’s 30 km/s speed to the component of Perseid speed in the plane of Earth’s orbit: 15 km/s + 30 km/s = 45 km/s. When we combine this component with the perpendicular component we get a resultant speed of:

v = √[(45 km/s)2 + (37 km/s)2] = 58 km/s

So... the Perseid debris stream contributes significantly to the collision speed during a meteor shower. A more accurate description would be “Earth and the Perseids run into each other.”
v = √[(45 km/s)2 + (37 km/s)2 + (11 km/s escape velocity)2] = 59.3 km/s
Art Neuendorffer

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Fri Aug 20, 2021 6:46 pm

neufer wrote:
Fri Aug 20, 2021 4:35 pm
johnnydeep wrote:
Fri Aug 20, 2021 4:24 pm
https://sky-lights.org/2020/10/05/qa-incoming-speed-of-a-meteor/ wrote:Earth moves around its orbit with a nearly-constant speed of 30 km/s. The Perseid debris stream reaches a top speed of 43 km/s at perihelion (just inside Earth’s orbit at 0.95 AU), but slows to around 40 km/s by the time it reaches Earth’s orbit (at 1 AU).

The debris approaches Earth on a 113° vector, so we can split that 40 km/s velocity into two components: one oriented toward Earth, and the other perpendicular to Earth’s motion, as follows:

speed in plane of Earth’s orbit = 40 km/s x cos(-113°) = 15 km/s
speed perpendicular to orbit = 40 km/s x sin(-113°) = 37 km/s

[Note: That negative sign on the angle is required because the Perseids are on a retrograde orbit. The default direction of motion in the solar system is “counterclockwise with north up”. So with an orbital inclination greater than 90°, the Perseids are orbiting “backwards”.]

To get the total Perseid approach speed (relative to Earth) we first need to add Earth’s 30 km/s speed to the component of Perseid speed in the plane of Earth’s orbit: 15 km/s + 30 km/s = 45 km/s. When we combine this component with the perpendicular component we get a resultant speed of:

v = √[(45 km/s)2 + (37 km/s)2] = 58 km/s

So... the Perseid debris stream contributes significantly to the collision speed during a meteor shower. A more accurate description would be “Earth and the Perseids run into each other.”
v = √[(45 km/s)2 + (37 km/s)2 + (11 km/s escape velocity)2] = 59.3 km/s
And can I assume you're adding the escape velocity to account for the speed boost that would come from actually entering deep into the Earth's gravity well?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 16151
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Chris Peterson » Fri Aug 20, 2021 9:27 pm

johnnydeep wrote:
Fri Aug 20, 2021 6:46 pm
neufer wrote:
Fri Aug 20, 2021 4:35 pm
johnnydeep wrote:
Fri Aug 20, 2021 4:24 pm
v = √[(45 km/s)2 + (37 km/s)2 + (11 km/s escape velocity)2] = 59.3 km/s
And can I assume you're adding the escape velocity to account for the speed boost that would come from actually entering deep into the Earth's gravity well?
No. There is no significant difference between the speed of an object at the top of the atmosphere and what it would have at the surface (absent air resistance). No object that reaches the near vicinity of the Earth from a non-Earth orbit can have a speed less than Earth's escape velocity.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Joe Stieber
Science Officer
Posts: 190
Joined: Tue Aug 18, 2009 1:41 pm
Location: Maple Shade, NJ

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Joe Stieber » Fri Aug 20, 2021 10:16 pm

Ann wrote:
Fri Aug 20, 2021 11:33 am
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Ann
Here's a version with some highlights labeled, including the "W" of Cassiopeia.
ThreeNightsPerseidsLabel.jpg
Joe
You do not have the required permissions to view the files attached to this post.

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Fri Aug 20, 2021 10:47 pm

Joe Stieber wrote:
Fri Aug 20, 2021 10:16 pm
Ann wrote:
Fri Aug 20, 2021 11:33 am
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Ann
Here's a version with some highlights labeled, including the "W" of Cassiopeia.

ThreeNightsPerseidsLabel.jpg

Joe
Brilliant! Is that your own annotating work?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Fri Aug 20, 2021 10:53 pm

Chris Peterson wrote:
Fri Aug 20, 2021 9:27 pm
johnnydeep wrote:
Fri Aug 20, 2021 6:46 pm
neufer wrote:
Fri Aug 20, 2021 4:35 pm

v = √[(45 km/s)2 + (37 km/s)2 + (11 km/s escape velocity)2] = 59.3 km/s
And can I assume you're adding the escape velocity to account for the speed boost that would come from actually entering deep into the Earth's gravity well?
No. There is no significant difference between the speed of an object at the top of the atmosphere and what it would have at the surface (absent air resistance). No object that reaches the near vicinity of the Earth from a non-Earth orbit can have a speed less than Earth's escape velocity.
<sigh> Yet another thing I don’t understand. How is “near vicinity” defined? Would the same thing apply if the Earth was a neutron star instead?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 16151
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Chris Peterson » Fri Aug 20, 2021 11:11 pm

johnnydeep wrote:
Fri Aug 20, 2021 10:53 pm
Chris Peterson wrote:
Fri Aug 20, 2021 9:27 pm
johnnydeep wrote:
Fri Aug 20, 2021 6:46 pm


And can I assume you're adding the escape velocity to account for the speed boost that would come from actually entering deep into the Earth's gravity well?
No. There is no significant difference between the speed of an object at the top of the atmosphere and what it would have at the surface (absent air resistance). No object that reaches the near vicinity of the Earth from a non-Earth orbit can have a speed less than Earth's escape velocity.
<sigh> Yet another thing I don’t understand. How is “near vicinity” defined? Would the same thing apply if the Earth was a neutron star instead?
The force of gravity, of course, drops off as the square of the distance. At 100 km above the Earth, where meteors start to form, the acceleration of gravity is about 9.5 m/s2, compared with 9.8 m/s2 at the surface. So, not much different. What is escape velocity? It is the speed that you would need to launch a body off of the Earth in order for it to slow to zero speed at a distance of infinity. And that is symmetrical. A body that is infinitely far from the Earth (assuming no other objects in the Universe) will fall towards the Earth, reaching (after an infinite amount of time, of course), a speed of 11 km/s when it hits. The only way that a body can impact the Earth slower than that is if it's already in Earth orbit. It takes a (ballistic) speed of about 8 km/s to reach low Earth orbit (a few hundred km height), and a body falling from that height hits at that same speed. Which is, in fact, how I distinguish between space junk and meteors on my cameras.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
Joe Stieber
Science Officer
Posts: 190
Joined: Tue Aug 18, 2009 1:41 pm
Location: Maple Shade, NJ

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Joe Stieber » Fri Aug 20, 2021 11:46 pm

johnnydeep wrote:
Fri Aug 20, 2021 10:47 pm
Joe Stieber wrote:
Fri Aug 20, 2021 10:16 pm
Ann wrote:
Fri Aug 20, 2021 11:33 am
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Ann
Here's a version with some highlights labeled, including the "W" of Cassiopeia.

ThreeNightsPerseidsLabel.jpg

Joe
Brilliant! Is that your own annotating work?
Yes, I do it with FastStone Image Viewer, a freeware program I've been using for some years to label my own astronomical snapshots for mouseover on my web pages.

Joe

User avatar
Ann
4725 Å
Posts: 11527
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Ann » Sat Aug 21, 2021 4:53 am

Joe Stieber wrote:
Fri Aug 20, 2021 10:16 pm
Ann wrote:
Fri Aug 20, 2021 11:33 am
That's Andromeda! :D Thanks for annotating the image, because I'm not going to. Can't find the "W" of Cassiopeia! :cry:

Ann
Here's a version with some highlights labeled, including the "W" of Cassiopeia.
Joe
Wow, thanks a billion, Joe! :D And you make those razor-sharp straight lines, too...

Ann
Color Commentator

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Sat Aug 21, 2021 1:14 pm

Joe Stieber wrote:
Fri Aug 20, 2021 11:46 pm
johnnydeep wrote:
Fri Aug 20, 2021 10:47 pm
Joe Stieber wrote:
Fri Aug 20, 2021 10:16 pm

Here's a version with some highlights labeled, including the "W" of Cassiopeia.

ThreeNightsPerseidsLabel.jpg

Joe
Brilliant! Is that your own annotating work?
Yes, I do it with FastStone Image Viewer, a freeware program I've been using for some years to label my own astronomical snapshots for mouseover on my web pages.

Joe
Great job. I'll have to check out that image viewer. Windows Paint 3D is pretty limited and GIMP, though powerful, is not that easy to use.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Sat Aug 21, 2021 1:53 pm

Chris Peterson wrote:
Fri Aug 20, 2021 11:11 pm
johnnydeep wrote:
Fri Aug 20, 2021 10:53 pm
Chris Peterson wrote:
Fri Aug 20, 2021 9:27 pm


No. There is no significant difference between the speed of an object at the top of the atmosphere and what it would have at the surface (absent air resistance). No object that reaches the near vicinity of the Earth from a non-Earth orbit can have a speed less than Earth's escape velocity.
<sigh> Yet another thing I don’t understand. How is “near vicinity” defined? Would the same thing apply if the Earth was a neutron star instead?
The force of gravity, of course, drops off as the square of the distance. At 100 km above the Earth, where meteors start to form, the acceleration of gravity is about 9.5 m/s2, compared with 9.8 m/s2 at the surface. So, not much different. What is escape velocity? It is the speed that you would need to launch a body off of the Earth in order for it to slow to zero speed at a distance of infinity. And that is symmetrical. A body that is infinitely far from the Earth (assuming no other objects in the Universe) will fall towards the Earth, reaching (after an infinite amount of time, of course), a speed of 11 km/s when it hits. The only way that a body can impact the Earth slower than that is if it's already in Earth orbit. It takes a (ballistic) speed of about 8 km/s to reach low Earth orbit (a few hundred km height), and a body falling from that height hits at that same speed. Which is, in fact, how I distinguish between space junk and meteors on my cameras.
I think I had understood all that before, but there must be some subtlety I'm missing. Per WikiPedia, the escape velocity of a neutron star is north of 100000 km/s. If the Earth were replaced by a neutron star, why wouldn't a Perseid falling into it potentially reach that speed (assuming it could survive the tidal forces), and the resulting speed would have to be vastly greater than the combined effects of the orbit speeds of the neutron star and the Perseids around the Sun. I do realize that the orbits of the Perseids would likely be much different to begin with owing to the presence of a 1.4 solar mass body now at Earth's location...or is this perhaps the crux of the matter?

Oh, wait... If instead the Earth itself was compressed to the density of a neutron star, it would have a radius of about 150 m (from https://theastroholic.co.uk/2019/03/03/ ... tron-star/), and an escape velocity of 2300 km/s (from (2GM/r).5). So, a Perseid falling into it could potentially achieve that speed (assuming no atmosphere). Yet, the orbits of everything around the Sun would remain the same.

[ EDIT: fixed the neutron-Earth radius to be 150 m, not 150 km! ]
Last edited by johnnydeep on Sat Aug 21, 2021 3:42 pm, edited 1 time in total.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 16151
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by Chris Peterson » Sat Aug 21, 2021 2:08 pm

johnnydeep wrote:
Sat Aug 21, 2021 1:53 pm
Chris Peterson wrote:
Fri Aug 20, 2021 11:11 pm
johnnydeep wrote:
Fri Aug 20, 2021 10:53 pm


<sigh> Yet another thing I don’t understand. How is “near vicinity” defined? Would the same thing apply if the Earth was a neutron star instead?
The force of gravity, of course, drops off as the square of the distance. At 100 km above the Earth, where meteors start to form, the acceleration of gravity is about 9.5 m/s2, compared with 9.8 m/s2 at the surface. So, not much different. What is escape velocity? It is the speed that you would need to launch a body off of the Earth in order for it to slow to zero speed at a distance of infinity. And that is symmetrical. A body that is infinitely far from the Earth (assuming no other objects in the Universe) will fall towards the Earth, reaching (after an infinite amount of time, of course), a speed of 11 km/s when it hits. The only way that a body can impact the Earth slower than that is if it's already in Earth orbit. It takes a (ballistic) speed of about 8 km/s to reach low Earth orbit (a few hundred km height), and a body falling from that height hits at that same speed. Which is, in fact, how I distinguish between space junk and meteors on my cameras.
I think I had understood all that before, but there must be some subtlety I'm missing. Per WikiPedia, the escape velocity of a neutron star is north of 100000 km/s. If the Earth were replaced by a neutron star, why wouldn't a Perseid falling into it potentially reach that speed (assuming it could survive the tidal forces), and the resulting speed would have to be vastly greater than the combined effects of the orbit speeds of the neutron star and the Perseids around the Sun. I do realize that the orbits of the Perseids would likely be much different to begin with owing to the presence of a 1.4 solar mass body now at Earth's location...or is this perhaps the crux of the matter?

Oh, wait... If instead the Earth itself was compressed to the density of a neutron star, it would have a radius of about 150 km (from https://theastroholic.co.uk/2019/03/03/ ... tron-star/), and an escape velocity of 2300 km/s (from (2GM/r).5). So, a Perseid falling into it could potentially achieve that speed (assuming no atmosphere). Yet, the orbits of everything around the Sun would remain the same.
If the Earth were compressed to the density of a neutron star, its escape velocity profile would not change. It would still be 11 km/s at one Earth radius from the center. It's just that you can get a lot closer to the center. And as you surmise, an actual neutron star would radically change the orbits of other bodies in the Solar System. Right now, meteor streams orbit the Sun and are scarcely affected by the Earth, unless they get close to it- well inside the orbit of the Moon. If the Earth were a neutron star, meteoroids would likely be in orbit around it, or in chaotic orbits around both it and the Sun.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
johnnydeep
Commander
Posts: 833
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by johnnydeep » Sat Aug 21, 2021 3:40 pm

Chris Peterson wrote:
Sat Aug 21, 2021 2:08 pm
johnnydeep wrote:
Sat Aug 21, 2021 1:53 pm
Chris Peterson wrote:
Fri Aug 20, 2021 11:11 pm


The force of gravity, of course, drops off as the square of the distance. At 100 km above the Earth, where meteors start to form, the acceleration of gravity is about 9.5 m/s2, compared with 9.8 m/s2 at the surface. So, not much different. What is escape velocity? It is the speed that you would need to launch a body off of the Earth in order for it to slow to zero speed at a distance of infinity. And that is symmetrical. A body that is infinitely far from the Earth (assuming no other objects in the Universe) will fall towards the Earth, reaching (after an infinite amount of time, of course), a speed of 11 km/s when it hits. The only way that a body can impact the Earth slower than that is if it's already in Earth orbit. It takes a (ballistic) speed of about 8 km/s to reach low Earth orbit (a few hundred km height), and a body falling from that height hits at that same speed. Which is, in fact, how I distinguish between space junk and meteors on my cameras.
I think I had understood all that before, but there must be some subtlety I'm missing. Per WikiPedia, the escape velocity of a neutron star is north of 100000 km/s. If the Earth were replaced by a neutron star, why wouldn't a Perseid falling into it potentially reach that speed (assuming it could survive the tidal forces), and the resulting speed would have to be vastly greater than the combined effects of the orbit speeds of the neutron star and the Perseids around the Sun. I do realize that the orbits of the Perseids would likely be much different to begin with owing to the presence of a 1.4 solar mass body now at Earth's location...or is this perhaps the crux of the matter?

Oh, wait... If instead the Earth itself was compressed to the density of a neutron star, it would have a radius of about 150 m (from https://theastroholic.co.uk/2019/03/03/ ... tron-star/), and an escape velocity of 2300 km/s (from (2GM/r).5). So, a Perseid falling into it could potentially achieve that speed (assuming no atmosphere). Yet, the orbits of everything around the Sun would remain the same.
If the Earth were compressed to the density of a neutron star, its escape velocity profile would not change. It would still be 11 km/s at one Earth radius from the center. It's just that you can get a lot closer to the center. And as you surmise, an actual neutron star would radically change the orbits of other bodies in the Solar System. Right now, meteor streams orbit the Sun and are scarcely affected by the Earth, unless they get close to it- well inside the orbit of the Moon. If the Earth were a neutron star, meteoroids would likely be in orbit around it, or in chaotic orbits around both it and the Sun.
Ok, due to the extremely small* size of a neutron-Earth (I suppose?), Perseid impacts, would be rare, but for those that managed it, they could potentially be accelerated an additional 2300 km/s above what their solar orbital speeds alone would provide. Am I still wrong?

(*) I was off by a factor of 1000 in my post: a neutron Earth would be only 150 m in radius, not 150 km!
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18374
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by neufer » Sat Aug 21, 2021 4:13 pm

Chris Peterson wrote:
Sat Aug 21, 2021 2:08 pm

Right now, meteor streams orbit the Sun and are scarcely affected by the Earth, unless they get close to it- well inside the orbit of the Moon.
  • A passing 60 km/s Perseid gets deflected [gravitationally] by up to 2º.

    However, relatively slower objects can be affected somewhat more. (Tennis anyone :?: ) :
https://en.wikipedia.org/wiki/3753_Cruithne wrote: <<3753 Cruithne (kroo-EEN-yə) is a Q-type, 5 km Aten asteroid in orbit around the Sun in 1:1 orbital resonance with Earth, making it a co-orbital object. Cruithne is in a normal elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon". However, it does not orbit the Earth and is not a moon. In 2058, Cruithne will come within 0.09 AU of Mars. Relative to Earth, orbits the Sun in a bean-shaped orbit that effectively describes a horseshoe, and that can change into a quasi-satellite orbit. Cruithne does not orbit Earth and at times it is on the other side of the Sun, placing Cruithne well outside of Earth's Hill sphere. Its orbit takes it inside the orbit of Mercury and outside the orbit of Mars. Cruithne orbits the Sun in about 1 year but it takes 770 years for the series to complete a horseshoe-shaped movement around the Earth.

The name Cruithne is from Irish and refers to the early Picts (Old Irish: Cruthin) in the Annals of Ulster and their eponymous king ("Cruidne, son of Cinge") in the Pictish Chronicle. Cruithne was discovered on 10 October 1986 by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto, and Seppo Mikkola, working at the University of Turku in Finland.

Cruithne is approximately 5 kilometres in diameter, and its closest approach to Earth is 12 million kilometres (0.080 AU), approximately thirty times the separation between Earth and the Moon. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November. Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross, and its orbital plane is currently tilted to that of the Earth by 19.8°. Cruithne, having a maximum near-Earth magnitude of +15.8.
Due to a high orbital eccentricity, Cruithne's distance from the Sun and orbital speed vary a lot more than the Earth's, so from the Earth's point of view Cruithne actually follows a kidney-bean-shaped horseshoe orbit ahead of the Earth, taking slightly less than one year to complete a circuit of the "bean". Because it takes slightly less than a year, the Earth "falls behind" the bean a little more each year, and so from our point of view, the circuit is not quite closed, but rather like a spiral loop that moves slowly away from the Earth. After many years, the Earth will have fallen so far behind that Cruithne will then actually be "catching up" on the Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth and gravitationally exchange orbital energy with Earth; this will alter Cruithne's orbit by a little over half a million kilometres—while Earth's orbit is altered by about 1.3 centimetres—so that its period of revolution around the Sun will then become slightly more than a year. The kidney bean will then start to migrate away from the Earth again in the opposite direction – instead of the Earth "falling behind" the bean, the Earth is "pulling away from" the bean. The next such series of close approaches will be centred on the year 2292 – in July of that year, Cruithne will approach Earth to about 12.5 million kilometres. After 380 to 390 years or so, the kidney-bean-shaped orbit approaches Earth again from the other side, and the Earth, once more, alters the orbit of Cruithne so that its period of revolution around the Sun is again slightly less than a year (this last happened with a series of close approaches centred on 1902, and will next happen with a series centered on 2676). The pattern then repeats itself.>>
Art Neuendorffer

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18374
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Three Perseid Nights (2021 Aug 20)

Post by neufer » Tue Sep 14, 2021 6:01 pm

Click to play embedded YouTube video.
Art Neuendorffer