APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 3550
Joined: Fri Dec 04, 2009 3:27 am

APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by APOD Robot » Sun Feb 12, 2017 5:09 am

Image Comet 45P Passes Near the Earth

Explanation: A large snowball has just passed the Earth. Known as Comet 45P/Honda–Mrkos–Pajdušáková, or 45P for short, the comet came 10 times closer to Earth yesterday than the Earth ever gets to the Sun. During this passage, the comet was photographed sporting a thin ion tail and a faint but expansive green coma. The green color is caused mostly by energized molecules of carbon. Comet 45P became just bright enough to see with the unaided eye when it came closest to the Sun in December. Now, however, the comet is fading as it heads back out to near the orbit of Jupiter, where it spends most of its time. The kilometer-sized nucleus of ice and dirt will return to the inner Solar System in 2022.

<< Previous APOD This Day in APOD Next APOD >>
[/b]

User avatar
Ann
4725 Å
Posts: 9519
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Ann » Sun Feb 12, 2017 5:23 am

Is that really an ion tail? An ion tail should be bluish, but this one looks almost beige.

Ann
Color Commentator

Boomer12k
:---[===] *
Posts: 2486
Joined: Sun Apr 22, 2007 12:07 am

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Boomer12k » Sun Feb 12, 2017 6:40 am

Great image!!

:---[===] *

De58te
Science Officer
Posts: 247
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by De58te » Sun Feb 12, 2017 7:38 am

If this is a giant snowball how could snow be made out of molecules of carbon? I understand that snow is H2O or molecules of hydrogen and oxygen. No carbon in the formula.

User avatar
Ann
4725 Å
Posts: 9519
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Ann » Sun Feb 12, 2017 7:45 am

De58te wrote:If this is a giant snowball how could snow be made out of molecules of carbon? I understand that snow is H2O or molecules of hydrogen and oxygen. No carbon in the formula.
Astronomer Fred Whipple demonstrates his "dirty snowball" model
for comets with a five-hundred-pound snowball covered with dirt.
Source:http://www.scienceclarified.com/scitech ... Asteroids/
Vital-Statistics-of-Asteroids-and-Comets.html
Have you heard of dirty snowballs?

Nowadays, astronomers often think of comets as "icy dirtballs".

Or something like that. :wink:

Ann
Color Commentator

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1142
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by JohnD » Sun Feb 12, 2017 10:24 am

D58te,
Calling comets "snowballs" doesn't make them like the clean white stuff that may be falling outside your window, the product of the giant distillation vat that is weather, whose product is clean-ish water than has happened to freeze today.
The "formula" is, allow hydrogen to fuse at enormous temperature and pressure, into first helium and then all the other elements up to iron.
When the hydrogen runs out, your kettle (aka "star") explodes and covers the ceiling (aka "universe") with random specks of all those elements.
Gravity scrapes this mess up and starts star making again, leaving the solid bits to form planets and comets.

Now, how clean and tidy do you think those comets are? They'll contain bits of everything, just like a spilt casserole.
As Ann says, more icy dirtball than nice clean "snowball", that contains a randomish collection of the commoner products of "nucelosynthesis" (Which please look up!)

John

User avatar
rstevenson
Quis custodiet ipsos custodes?
Posts: 2572
Joined: Fri Mar 28, 2008 1:24 pm
Location: Dartmouth, NS, Canada

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by rstevenson » Sun Feb 12, 2017 2:38 pm

Ann wrote:Is that really an ion tail? An ion tail should be bluish, but this one looks almost beige.
I just ran my Digital Color Meter over it and it appears to be close to a neutral gray. I suppose it looks warmer than gray because of the green tone around it. Of course, that only tells me what it looks like in this picture, not what colour it 'really' is.

Rob

User avatar
JohnD
Tea Time, Guv! Cheerio!
Posts: 1142
Joined: Wed Feb 16, 2005 2:11 pm
Location: Lancaster, England

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by JohnD » Sun Feb 12, 2017 2:46 pm

Ann,
Have you looked at the link via "green colour" in the blurb to the pic?

It shows a spectrum that includes a peak, labelled "C2" for diatomic carbon in the blue range, confirming your expectation.
I expect that the bias towards grey is a product of reproduction or processing.

John

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Sun Feb 12, 2017 3:18 pm

Ann wrote:Is that really an ion tail? An ion tail should be bluish, but this one looks almost beige.
Said it before, will no doubt say it again.

Never, never, never assume that the colors seen in astronomical images are all that meaningful. Especially when those images are made with cameras designed for terrestrial imaging (such as with today's image). Between the filters in the sensor, the processing that occurs inside the camera, the processing that occurs after the image is taken from the camera, and all the different color spaces that exist between devices, there are plenty of things that can distort the color from what our eyes would see given a suitably bright source with the same spectral characteristics.
Chris

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

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Sun Feb 12, 2017 3:54 pm

Scott wrote:Why is everyone talking 'dirty snowballs' ? Probes and spacecraft have been landing on, crashing into and firing weapons at comets for quite some time now... and there is absolutely no sign of any ice or water (H2O) on any of them.
I don't know where you're getting that idea! Every comet we've examined, either directly (as with Rosetta at 67P) or indirectly via spectral measurements from Earth has shown substantial amounts of water. The jets observed from 67P are mostly water vapor. The interior of most comets appear to be dominated by water ice. Depending on how evolved the comet is, it ranges from mostly water with some rock to mostly rock with little remaining water (or other volatiles).
Chris

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

User avatar
geckzilla
Ocular Digitator
Posts: 8964
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by geckzilla » Sun Feb 12, 2017 4:00 pm

Weary of guest users posting erroneous information, I went ahead and removed guest user Scott. The pertinent part of his post remains in Chris's reply. If you must know what else Scott posted about it concerned comets as charged objects, which is part of EU theory, and is also a banned topic at the forum. Anyone reading this welcome to look up EU theory elsewhere on your own time, just don't talk about it here.
Just call me "geck" because "zilla" is like a last name.

ricky302
Asternaut
Posts: 3
Joined: Thu Mar 31, 2016 6:30 pm

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by ricky302 » Sun Feb 12, 2017 5:30 pm

geckzilla wrote:Weary of guest users posting erroneous information, I went ahead and removed guest user Scott. The pertinent part of his post remains in Chris's reply. If you must know what else Scott posted about it concerned comets as charged objects, which is part of EU theory, and is also a banned topic at the forum. Anyone reading this welcome to look up EU theory elsewhere on your own time, just don't talk about it here.
Censorship at it's finest.

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Sun Feb 12, 2017 6:01 pm

ricky302 wrote:
geckzilla wrote:Weary of guest users posting erroneous information, I went ahead and removed guest user Scott. The pertinent part of his post remains in Chris's reply. If you must know what else Scott posted about it concerned comets as charged objects, which is part of EU theory, and is also a banned topic at the forum. Anyone reading this welcome to look up EU theory elsewhere on your own time, just don't talk about it here.
Censorship at it's finest.
Or, as more reasonable people call it, moderation. Nothing ruins a science forum more quickly than pseudoscience.
Chris

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

User avatar
Ann
4725 Å
Posts: 9519
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Ann » Sun Feb 12, 2017 6:46 pm

JohnD wrote:Ann,
Have you looked at the link via "green colour" in the blurb to the pic?

It shows a spectrum that includes a peak, labelled "C2" for diatomic carbon in the blue range, confirming your expectation.
I expect that the bias towards grey is a product of reproduction or processing.

John
Comet ISON. Photo:
stardustobservatory.org/images.php?page=details&id=363
Thanks for the suggestion, John! :D I looked it up, and it contained some really interesting information about the bright, and indeed green, coma of a typical comet. But it said nothing about the color of the ion tail.

As you can see in the picture of comet ISON at left, the comet is seen to have a bright green coma and a very much fainter, but decidedly blue ion tail. This color is typical of the ion tails of comets. In today's APOD, the blue color of the ion tail can't be seen.

Ann
Color Commentator

User avatar
geckzilla
Ocular Digitator
Posts: 8964
Joined: Wed Sep 12, 2007 12:42 pm
Location: Modesto, CA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by geckzilla » Sun Feb 12, 2017 10:52 pm

Chris Peterson wrote:
ricky302 wrote:
geckzilla wrote:Weary of guest users posting erroneous information, I went ahead and removed guest user Scott. The pertinent part of his post remains in Chris's reply. If you must know what else Scott posted about it concerned comets as charged objects, which is part of EU theory, and is also a banned topic at the forum. Anyone reading this welcome to look up EU theory elsewhere on your own time, just don't talk about it here.
Censorship at it's finest.
Or, as more reasonable people call it, moderation. Nothing ruins a science forum more quickly than pseudoscience.
Call it whatever you want. It's a private message board run by people with rules stated up front, and if you don't like it, then this isn't the place for you. A whole lot of people appreciate and depend on the rules and moderation to keep the place sensible. Thanks for understanding.
Just call me "geck" because "zilla" is like a last name.

User avatar
DavidLeodis
Perceptatron
Posts: 1169
Joined: Mon May 01, 2006 1:00 pm

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by DavidLeodis » Mon Feb 13, 2017 1:44 pm

The "nucleus" link in the explanation http://www.cometcampaign.org/files/imag ... ecraft.jpg brings up a montage of images of comets. That for 67P/Churyumov-Gerasimenko taken in 2014 from Rosetta is very spooky having a face-like feature on its right. Pareidolia working well for at least me :wink:.

User avatar
bystander
Apathetic Retiree
Posts: 18360
Joined: Mon Aug 28, 2006 2:06 pm
Location: Oklahoma

Arecibo Captures Revealing Images of Comet 45P

Post by bystander » Wed Feb 15, 2017 4:25 pm

Arecibo Observatory Captures Revealing Images of Comet 45P/Honda-Mrkos-Pajdusakova
Universities Space Research Association (USRA) | 2017 Feb 15
Image
Only the seventh comet imaged using radar: comets rarely come close enough to Earth to capture detailed radar images

Though not visible to the naked eye or even with binoculars, the green-tailed Comet 45P/Honda-Mrkos-Pajdusakova (HMP) did not escape the gaze of the world-renowned Arecibo Observatory. Scientists from the University of Arizona's Lunar and Planetary Laboratory (LPL) and the Universities Space Research Association (USRA) at Arecibo Observatory have been studying the comet with radar to better understand its solid nucleus and the dusty coma that surrounds it. ...

Studying the comet with radar not only very precisely determines its orbit, allowing scientists to better predict its location in the future, but also gives a glimpse of the typically unseen part, the comet’s nucleus, which is usually hidden behind the cloud of gas and dust that makes up the its coma and tail. ...
Know the quiet place within your heart and touch the rainbow of possibility; be
alive to the gentle breeze of communication, and please stop being such a jerk.
— Garrison Keillor

Reward54
Asternaut
Posts: 2
Joined: Wed Feb 15, 2017 10:22 pm

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Reward54 » Wed Feb 15, 2017 10:31 pm

The text with the picture says the green color is from energized carbon and provides a hyperlink to Diatomic Carbon at Wikipedia. Wikipedia states that diatomic carbon is "..a gas that only exists above 3,642 °C."

I always think of the vacuum of space as cold, very cold. Comets are commonly characterized as "dirty snowballs," which also makes me think "very cold."

Could someone help me understand the presence of a glowing cloud of diatomic carbon gas of 3,000+ °C surrounding a cold dirty snowball? Is this due to the solar wind?

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Wed Feb 15, 2017 10:47 pm

Reward54 wrote:The text with the picture says the green color is from energized carbon and provides a hyperlink to Diatomic Carbon at Wikipedia. Wikipedia states that diatomic carbon is "..a gas that only exists above 3,642 °C."

I always think of the vacuum of space as cold, very cold. Comets are commonly characterized as "dirty snowballs," which also makes me think "very cold."
A vacuum can't really be hot or cold. In space, heat is transferred by radiation only. A rock in space near Earth's orbit might have a similar temperature to a rock in your backyard, both being heated by the Sun. It's actually a bit more complicated, because the final temperature depends on the balance between heat absorbed and heat radiated, but to a first order you can look at temperature this way. Comets are cold because they spend most of their time very far from the Sun, so they radiate more heat than they absorb. They are also pretty good thermal insulators, so their interiors don't heat up much when they are in the part of their orbit closest to the Sun. The surface boils or sublimates away, and the outer few meters heats up.
Could someone help me understand the presence of a glowing cloud of diatomic carbon gas of 3,000+ °C surrounding a cold dirty snowball? Is this due to the solar wind?
Yes, and to exposure to solar radiation to a lesser extent. Temperature is a measure of the energy that individual particles have, and the particles and molecules ejected from the comet can easily be energized when hit by charged particles or high energy photons. One way that energy can be measured is in terms of the equivalent heat that would be required. It's common for particles in space to have high speeds, and therefore high temperatures.

Where the ISS orbits is a region of Earth's upper atmosphere called the thermosphere. The particle temperature there is around 2500° C. Obviously, the ISS doesn't melt! And that's because it's nearly a vacuum. There simply aren't enough particles to transfer enough energy to the ISS surface to heat it significantly.

Temperatures in some nebulas can reach millions of degrees, even though their actual particle density qualifies them as hard vacuums.
Chris

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

User avatar
neufer
Vacationer at Tralfamadore
Posts: 15975
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by neufer » Thu Feb 16, 2017 1:14 am

Chris Peterson wrote:
A vacuum can't really be hot or cold. In space, heat is transferred by radiation only.
Reward54 wrote:
Could someone help me understand the presence of a glowing cloud of diatomic carbon gas of 3,000+ °C surrounding a cold dirty snowball? Is this due to the solar wind?
Yes, and to exposure to solar radiation to a lesser extent.
  • Well...that was confusing :!:
https://en.wikipedia.org/wiki/Coma_(cometary) wrote: <<On 2 June 2015, NASA reported that the ALICE spectrograph on the Rosetta space probe studying comet 67P/Churyumov–Gerasimenko determined that electrons (within 1 km) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma.

Comets were found to emit X-rays in late-March 1996. This surprised researchers, because X-ray emission is usually associated with very high-temperature bodies. The X-rays are thought to be generated by the interaction between comets and the solar wind: when highly charged ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "ripping off" one or more electrons from the comet. This ripping off leads to the emission of X-rays and far ultraviolet photons.>>
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Thu Feb 16, 2017 1:22 am

neufer wrote:
Chris Peterson wrote:
A vacuum can't really be hot or cold. In space, heat is transferred by radiation only.
Reward54 wrote:
Could someone help me understand the presence of a glowing cloud of diatomic carbon gas of 3,000+ °C surrounding a cold dirty snowball? Is this due to the solar wind?
Yes, and to exposure to solar radiation to a lesser extent.
  • Well...that was confusing :!:
Well, maybe. "Radiation" in this context generally refers to photons. A particle can absorb energy from a massive particle (as with solar wind), but that's not usually treated as radiative heat absorption in my experience. The conventional mechanisms of heat transfer are radiative, conductive, and convective. Transfer by solar wind is probably best described as convective. It does heat individual particles, but generally plays a small role in heating macroscopic bodies like asteroids or comets, where virtually all the heat transfer is radiative.
Chris

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

User avatar
Ann
4725 Å
Posts: 9519
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Ann » Wed Feb 22, 2017 8:03 am

Ann wrote:Is that really an ion tail? An ion tail should be bluish, but this one looks almost beige.

Ann
Comet 45P and the Whale and Hockeystick galaxies. Photo: Alan C Tough.
Okay, I give up!!!

I thought that the APOD, the portrait of Comet 45P by Fritz Helmut Hemmerich, had the wrong color balance, since the ion tail of the comet wasn't blue at all.

I take it back! Look at Alan C tough's picture of Comet 45P and the Whale and Hockeystick galaxies, NGC 4631 (the Whale) and NGC 4656 (the Hockeystick). Note that the galaxies (particularly NGC 4656) looks slightly bluer than the comet tail!

To me, this is quite incredible. We must bear in mind that even the bluest galaxy is made up of stars of many colors. Therefore no galaxy can be "all blue". So if the Whale and Hockeystick galaxies look bluer than the tail of Comet 45P, it can only mean that this comet doesn't have a blue ion tail.

Forgive me for doubting you, Fritz Helmut Hemmerich!

Ann
Color Commentator

User avatar
Chris Peterson
Abominable Snowman
Posts: 14362
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Comet 45P Passes Near the Earth (2017 Feb 12)

Post by Chris Peterson » Wed Feb 22, 2017 4:04 pm

Ann wrote:Is that really an ion tail? An ion tail should be bluish, but this one looks almost beige.
Perhaps a better term than "ion tail" is "gas tail". Only some of the gas will be ionized by solar UV radiation. What is the makeup of this gas? Well, most of the time it is dominated by CO+, which emits light at 420 nm - the blue color we commonly observe in these tails. But there are other gases present, as well, such as N2+ and CO2+, each with their own emission lines. Different comets certainly have different ratios of these different ionized gases, which will impact the apparent color of the tail.

Another important point to consider is that "color" isn't just hue, it is also intensity. So the brightness of the tail (and this comet certainly doesn't have a bright ion tail) impacts how we perceive its color. And instrumentally, a very dim tail may lack sufficient signal to allow an image to show much color.

So, there are many factors to consider looking at comet tail color.
Chris

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