Explanation: Periodic Comet 46P/Wirtanen is now the brightest comet in the night sky, but too faint to be seen by eye. From dark sky sites it could just become naked-eye visible though, as it's 5.4 year long looping orbit takes it closest to Earth and the Sun in mid December. Fluorescing in sunlight, its spherical coma is about half the angular size of a full moon in this southern hemisphere telescopic view from November 7. Then the comet was about 2 light-minutes away or 35 million kilometers from Earth-bound telescopes, so the pretty greenish coma seen here is around 150,000 kilometers across. That makes it about the size of Jupiter. The stack of digital images also reveals a very faint tail extending toward 4 o'clock with a distant background galaxy notable at the upper left. As a regular visitor to the inner Solar System, comet 46P/Wirtanen was once the favored rendezvous target for ESA's comet exploring Rosetta mission.
<<The compositional evolution of the coma of Rosetta target comet 46P/Wirtanen was studied along its pre-perihelion orbit. At 2.34 AU the first gaseous species, CN and C3, were detected. C2 and NH2 arose at 2.0 AU. The production rate curves of gas and dust show a steep increase between 1.8 AU and 1.6 AU indicating a distinct change of the outgassing conditions. The C2 production increases more rapidly with decreasing solar distance than CN. As one consequence the C2/CN ratio strongly varies with heliocentric distance. Due to this variation a classification of 46P/Wirtanen by the taxonomy introduced by A'Hearn et al. distinguishing comets with typical abundance ratios from carbon-chain depleted comets, is impossible. It would be classified as depleted beyond about 1.6 AU whereas it would be designated typical at smaller distances. The knowledge of the unexpected changes in the activity curves of 46P/Wirtanen will help to optimize the Rosetta mission scenario in particular with regard to science operations. based on observations obtained at ESO La Silla.>>
<<The composition of water vapor from Churyumov–Gerasimenko, as determined by the Rosetta spacecraft, is substantially different from that found on Earth. The ratio of deuterium to hydrogen in the water from the comet was determined to be three times that found for terrestrial water. This makes it unlikely that water found on Earth came from comets such as Churyumov–Gerasimenko. On 22 January 2015, NASA reported that, between June and August 2014, the comet released increasing amounts of water vapor, up to tenfold as much.
The ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.6 mi) 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.
Measurements by the COSAC and Ptolemy instruments on the Philae's lander revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate and propionaldehyde. The only amino acid detected thus far on the comet is glycine, along with precursor molecules methylamine and ethylamine.
Solid organic compounds were also found in the dust particles emitted by the comet; the carbon in this organic material is bound in "very large macromolecular compounds", analogous to the insoluble organic matter in carbonaceous chondrite meteorites. Scientists think that the observed cometary carbonaceous solid matter could have the same origin as the meteoritic insoluble organic matter, but suffered less modification before or after being incorporated into the comet.
One of the most outstanding discoveries of the mission so far is the detection of large amounts of free molecular oxygen (O2) gas surrounding the comet. Current solar system models suggest the molecular oxygen should have disappeared by the time 67P was created, about 4.6 billion years ago in a violent and hot process that would have caused the oxygen to react with hydrogen and form water. Molecular oxygen has never before been detected in cometary comas. In situ measurements indicate that the O2/H2O ratio is isotropic in the coma and does not change systematically with heliocentric distance, suggesting that primordial O2 was incorporated into the nucleus during the comet's formation. Detection of molecular nitrogen (N2) in the comet suggests that its cometary grains formed in low-temperature conditions below 30 K.
On 3 July 2018, researchers reported that molecular oxygen is not made on the surface of comet 67P, a finding that supports the notion that the oxygen comes from the body of the comet, and may be primordial.>>
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 15, 2018 4:50 pm
by starsurfer
What is that galaxy in the upper left corner?
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 15, 2018 4:51 pm
by neufer
Bellerophon wrote: ↑Thu Nov 15, 2018 4:08 pm
the second sentence should read "its 5.4 year long looping orbit."
"Loop-the-Loop" is the name of a new entertainment which goes further in the way of tempting Providence than anything yet invented. The "Loop" is an immense circle of track in the air. A car on a mimic railway shoots down a very steep incline, and is impelled around the inner side of this loop. ... The authorities at Coney Island are said to have prohibited "looping-the-loop" because women break their corset strings in their efforts to catch their breath as they sweep down the incline.
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 15, 2018 5:23 pm
by Ann
starsurfer wrote: ↑Thu Nov 15, 2018 4:50 pm
What is that galaxy in the upper left corner?
starsurfer wrote: ↑Thu Nov 15, 2018 4:50 pm
What is that galaxy in the upper left corner?
That's what I would like to know too! Ann
Assuming that it is ~ 100,000 light years across it must ~300,000,000 light years away.
Why would one be interested in knowing "the phone number" designation of an average galaxy
so very far away simply because it has temporarily photo-bombed Comet 46P/Wirtanen
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Fri Nov 16, 2018 3:17 am
by Fred the Cat
What would be truly fascinating would be two supernovas in the same galaxy.
Random Galaxy.jpg (27.21 KiB) Viewed 7437 times
More likely foreground stars but double supernovas would make it quite the catch!
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Fri Nov 16, 2018 4:10 am
by neufer
Fred the Cat wrote: ↑Fri Nov 16, 2018 3:17 am
What would be truly fascinating would be two supernovas in the same galaxy.
More likely foreground stars but double supernovas would make it quite the catch!
Indeed...especially two SN of identical brightness and so closely paired.
(Double stars in our own galaxy are relatively common in comparison.)
Fred the Cat wrote: ↑Fri Nov 16, 2018 3:17 am
What would be truly fascinating would be two supernovas in the same galaxy.
Random Galaxy.jpg
More likely foreground stars but double supernovas would make it quite the catch!
Wow, well spotted, Fred!
I, too, doubt that they are double supernovas, just for the sheer rarity of them, but they sure look like they may be just that!
I can definitely spot one of the two "supernovas" in the DSS picture provided by Simbad. Actually I can see the second one, too. So sorry Fred, no real supernovas here!
The innermost, bright spot is yellow in color and must be a foreground star. The outermost, fainter one is bluish and just might be a huge region of star formation in ESO 354-29. Or else that one too is just a foreground star!
Simbad also tells us that ESO 354-29 is an emission line galaxy. This site explains to us that emission line galaxies are still forming stars and are blue in color. Yes, that is fairly obvious from the galaxy's shape and colors, even in the APOD, but thanks for providing the explanation of the term!
Simbad also says that ESO 354-29 is an Sc D Hubble type galaxy. It does look very much like a typical Sc galaxy, like M74. But I don't get the "D". Another mystery!
But thank you so much, alter-ego and Fred, for your help in solving the mystery and pointing fascinating details about the galaxy!
starsurfer wrote: ↑Thu Nov 15, 2018 4:50 pm
What is that galaxy in the upper left corner?
Ann wrote: ↑Thu Nov 15, 2018 5:23 pm
That's what I would like to know too! Ann
Assuming that it is ~ 100,000 light years across it must ~300,000,000 light years away.
Why would one be interested in knowing "the phone number" designation of an average galaxy
so very far away simply because it has temporarily photo-bombed Comet 46P/Wirtanen
neufer wrote: ↑Thu Nov 15, 2018 7:19 pm
Assuming that it is ~ 100,000 light years across it must ~300,000,000 light years away.
Why would one be interested in knowing "the phone number" designation of an average galaxy so very far away simply because it has temporarily photo-bombed Comet 46P/Wirtanen
Well... ~500,000,000 light years away. (Close enough for government work.)
We are interested because we live in a galaxy an not in a mere comet
It's a very nice galaxy too, and the comet is a virtually tailless green blob.
Exactly! It's a nice galaxy and a foreground green blob!
Of course the galaxy is more interesting than the foreground blob! Have you considered how many little green men might live inside that galaxy? There could be billions! But how many little green men live in or on that green blob of a comet? The answer is zero - and not even this green zero-guy is going to be there!
Comet of the Year 46P/Wirtanen arriving December 2018
June 03, 2018
Comet 46P/Wirtanen is an icy small Solar System body that, when passing close to the Sun, will warm and begin to release gases, a process called outgassing. This produces a visible atmosphere or coma, and also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. However, this time around, the ion tail will be pointing away from the Earth and difficult to see due to the geometry. It will hopefully improve as the comet approaches perihelion (Dec 12) and perigee (Dec 16).
Click to play embedded YouTube video.
Ann wrote: ↑Fri Nov 16, 2018 6:54 pm
It's a nice galaxy and a foreground green blob!
Of course the galaxy is more interesting than the foreground blob! Have you considered how many little green men might live inside that galaxy? There could be billions! But how many little green men live in or on that green blob of a comet? The answer is zero - and not even this green zero-guy is going to be there!
It's not easy bein' green
It seems you blend in
With so many other ordinary things
And people tend to pass you over
Cause you're not standing out
Like flashy sparkles in the water
Or stars in the sky
starsurfer wrote: ↑Thu Nov 15, 2018 4:50 pm
What is that galaxy in the upper left corner?
That's what I would like to know too! Ann
Assuming that it is ~ 100,000 light years across it must ~300,000,000 light years away.
Why would one be interested in knowing "the phone number" designation of an average galaxy
so very far away simply because it has temporarily photo-bombed Comet 46P/Wirtanen
Thanks for that, it's so good to know! I can finally sleep at night!
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Sat Nov 17, 2018 5:05 pm
by alter-ego
starsurfer wrote: ↑Sat Nov 17, 2018 12:21 pm
Thanks for that, it's so good to know! I can finally sleep at night!
That's good! Now I can finally sleep.
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 29, 2018 6:30 pm
by JohnD
I read that this commet is unusually active and now has a coma as broad as the Moon.
Please help this poor observer and tell me whert o llok in the sky and when, from North Western UK?
JOhn
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 29, 2018 6:40 pm
by BDanielMayfield
JohnD wrote: ↑Thu Nov 29, 2018 6:30 pm
whert o llok
A little bit o the old Gallic John? I suggest looking on Sky and Telescopes observing page.
Re: APOD: Comet 46P/Wirtanen (2018 Nov 15)
Posted: Thu Nov 29, 2018 6:49 pm
by JohnD
Thank you, BD.
Rather not encouraging, "I bet that sighting this comet is going to prove to be a difficult to near-impossible task. And even for those who are blessed with dark and starry skies, finding the comet could prove to be a bit of a challenge"
And directions for looking, " on the evening of December 15th it can be conveniently found passing almost midway between the Hyades and Pleiades star clusters" will be like a signpost for regular star-gazers , but will somebody suggest a compass direction and approximate height at a time, perhaps after sunset, to avoid any time zone mixups?
Comet 46P/Wirtanen
too, and the comet is a virtually tailless green blob.
