Two objects 2.5 million lightyears apart... PanSTARRS & Andromeda. Credit and copyright: Brendan Alexander.
We warned you it was going to happen, and here’s visual proof! In this comet encounter of the extragalactic kind, Comet PanSTARRS and the Andromeda Galaxy met each other in the skies above Earth. This great image by Brendan Alexander in Ireland shows the spectacular view. He said it was “a difficult image to capture due to low cloud, the low altitude of the target and tracking issue. I hope to get the chance to improve on this!”
Here’s another image from UT reader Anna Morris:
Comet PANSTARRS and the Andromeda galaxy over Suffolk, England on April 2, 2013. This composite images shows the movement of the comet during the imaging session. Credit and copyright: Anna Morris.
Want to see this meetup for yourself? Tonight might be even better:
Comet PANSTARRS shown every three days as it moves across Andromeda, passing near the Andromeda Galaxy around April 3. You can use Cassiopeia to point you to Beta Andromedae and from there to the comet. The map shows the sky facing northwest about one hour after sunset. Comet and galaxy brightness are exaggerated for the sake of illustration. Stellarium
The northern lights or aurora borealis swirls over a fjord in the Norwegian Sea near Tromso, northern Norway. A mix of moonlight, aurora, and a nearby village illuminate the landscape. The Pleiades Star Cluster (the Seven Sisters) appears on the upper left. Credit and copyright: Babek A. Tafreshi.
I’m completely envious of this view! Not only is there a stunning winter vista of mountains and a fjord near Tromso, Norway, but in the peaceful evening twilight, Comet PanSTARRS (C/2011 L4) shows up. To top off the view, a sudden burst of aurora borealis makes an appearance! This view was captured by Babak Tafreshi on March 20, 2013.
See some of his stunning images below:
Click on each of the images for a larger view at Tafreshi’s website.
The northern lights or aurora borealis dances over a fjord mountains near Tromso, northern Norway. A mix of moonlight, aurora, and a nearby village illuminate the the landscape. Bright planet Jupiter and the Pleiades Star Cluster (the Seven Sisters) appear over the mountains. Credit and copyright: Babak A. Tafreshi.A fisheye view of the northern lights as it swirls over a fjord in the Norwegian Sea near Tromso, northern Norway. A mix of moonlight, aurora, and a nearby village illuminates the the landscape. Credit: Babak A. Tafreshi.
The Ultraviolet/Optical Telescope aboard NASA's Swift imaged comet ISON (center) on Jan. 30, when it was located about 3.3 degrees from the bright star Castor in the constellation Gemini. At the time of this 5.5-minute optical exposure, ISON was about 5,000 times fainter than the limit of human vision. Credit: NASA/Swift/D. Bodewits, UMCP
Will the comet that’s been billed as the “the comet of the century” live up to expectations? Astronomers are getting a better idea of the makeup of Comet C/2012 S1 (ISON), and have now taken a look at it with the Swift satellite. They’ve been able to make initial estimates of the size of the comet’s nucleus.
“Comet ISON has the potential to be among the brightest comets of the last 50 years, which gives us a rare opportunity to observe its changes in great detail and over an extended period,” said Lead Investigator Dennis Bodewits, an astronomer at the University of Maryland College Park (UMCP.)
Bodewits and his team used Swift’s Ultraviolet/Optical Telescope (UVOT) to make initial estimates of the comet’s water and dust production, and then infer the size of its icy nucleus. They observed the comet on January 30 and then again late in February.
The January observations revealed that ISON was shedding about 112,000 pounds (51,000 kg) of dust, or about two-thirds the mass of an unfueled space shuttle, every minute. By contrast, the comet was producing only about 130 pounds (60 kg) of water every minute, or about four times the amount flowing out of a residential sprinkler system.
Similar levels of activity were observed in February, and the team plans additional UVOT observations.
Using the water and dust production, the astronomers estimated the size of ISON’s icy nucleus as roughly 3 miles (5 km) across, a typical size for a comet. This assumes that only the fraction of the surface most directly exposed to the Sun, about 10 percent of the total, is actively producing jets. The astronomers noted that these rates of water and dust production are relatively uncertain because of the comet’s faintness.
“The mismatch we detect between the amount of dust and water produced tells us that ISON’s water sublimation is not yet powering its jets because the comet is still too far from the Sun,” Bodewits said. “Other more volatile materials, such as carbon dioxide or carbon monoxide ice, evaporate at greater distances and are now fueling ISON’s activity.”
At the time, the comet was 375 million miles (604 million km) from Earth and 460 million miles (740 million km) from the Sun. ISON was at magnitude 15.7 on the astronomical brightness scale, or about 5,000 times fainter than the threshold of human vision.
Like all comets, ISON is a clump of frozen gases mixed with dust. Often described as “dirty snowballs,” comets emit gas and dust whenever they venture near enough to the Sun that the icy material transforms from a solid to gas, a process called sublimation. Jets powered by sublimating ice also release dust, which reflects sunlight and brightens the comet.
Typically, a comet’s water content remains frozen until it comes within about three times Earth’s distance to the Sun. While Swift’s UVOT cannot detect water directly, the molecule quickly breaks into hydrogen atoms and hydroxyl (OH) molecules when exposed to ultraviolet sunlight. The UVOT detects light emitted by hydroxyl and other important molecular fragments as well as sunlight reflected from dust.
The Deep Impact spacecraft also imaged Comet ISON in mid-January, and NASA and ESA are planning an observing campaign with the rovers and orbiters at Mars around October 1 when the inbound comet passes about 6.7 million miles (10.8 million km) from Mars.
“During this close encounter, comet ISON may be observable to NASA and ESA spacecraft now working at Mars,” said Michael Kelley, an astronomer at UMCP and also a Swift and CIOC team member. “Personally, I’m hoping we’ll see a dramatic postcard image taken by NASA’s latest Mars explorer, the Curiosity rover.”
Fifty-eight days later, on Nov. 28, ISON will make a sweltering passage around the Sun. The comet will approach within about 730,000 miles (1.2 million km) of its visible surface, which classifies ISON as a sungrazing comet. In late November, its icy material will furiously sublimate and release torrents of dust as the surface erodes under the sun’s fierce heat, all as sun-monitoring satellites look on. Around this time, the comet may become bright enough to glimpse just by holding up a hand to block the sun’s glare.
An important question is whether ISON will continue to brighten at the same pace once water evaporation becomes the dominant source for its jets. Will the comet sizzle or fizzle?
“It looks promising, but that’s all we can say for sure now,” said Matthew Knight, an astronomer at Lowell Observatory in Flagstaff, Arizona, and a member of the Swift and CIOC teams. “Past comets have failed to live up to expectations once they reached the inner solar system, and only observations over the next few months will improve our knowledge of how ISON will perform.”
Based on ISON’s orbit, astronomers think the comet is making its first-ever trip through the inner solar system. Before beginning its long fall toward the Sun, the comet resided in the Oort comet cloud, a vast shell of perhaps a trillion icy bodies that extends from the outer reaches of the planetary system to about a third of the distance to the star nearest the Sun.
Formally designated C/2012 S1 (ISON), the comet was discovered on Sept. 21, 2012, by Russian astronomers Vitali Nevski and Artyom Novichonok using a telescope of the International Scientific Optical Network located near Kislovodsk. From now through October, Comet ISON tracks through the constellations Gemini, Cancer and Leo as it falls toward the sun. Credit: NASA’s Goddard Space Flight Center/Axel Mellinger, Central Michigan Univ.
Sungrazing comets often shed large fragments or even completely disrupt following close encounters with the Sun, but for ISON neither fate is a forgone conclusion.
“We estimate that as much as 10 percent of the comet’s diameter may erode away, but this probably won’t devastate it,” explained Knight. Nearly all of the energy reaching the comet acts to sublimate its ice, an evaporative process that cools the comet’s surface and keeps it from reaching extreme temperatures despite its proximity to the sun.
Following ISON’s solar encounter, the comet will depart the sun and move toward Earth, appearing in evening twilight through December. It will swing past Earth on Dec. 26, approaching within 39.9 million miles (64.2 million km) or about 167 times farther than the Moon.
The orbit diagram for Comet 2013 A1 as it will approach Mars in October, 2014.
There is a small but non-negligible chance that Comet 2013 A1 will hit Mars October of 2014. According the latest calculations from JPL, the comet is estimated to come within 120,000 kilometers (74,000 miles). In this video, various NASA scientists discuss the potential impact, and invariably view such an impact as scientifically very intriguing.
“I think of it as a giant climate experiment,” said Michael Meyer, lead scientist for the Mars Exploration Program. “An impact would loft a lot of stuff into the Martian atmosphere–dust, sand, water and other debris. The result could be a warmer, wetter Mars than we’re accustomed to today.”
An impact would likely have consequences for the current rovers on Mars. Meyer said the solar-powered Opportunity might have a hard time surviving if the atmosphere became opaque. Nuclear-powered Curiosity, though, would carry on just fine. He also notes that Mars orbiters might have trouble seeing the surface, for a while at least, until the debris begins to clear.
The trajectory for comet Siding Spring is being refined as more observations are made. Rob McNaught discovered this comet on Jan. 3, 2013, at Siding Spring Observatory in Australia, and looking back at archival observations has unearthed more images of the comet, extending the observation interval back to Oct. 4, 2012. Further refinement to its orbit is expected as more observational data is obtained.
Scientists estimate the nucleus of the comet is about 1 to 3 km in diameter. If it gets close to Mars and is grabbed by its gravity,and IF there was an impact it would be a substantial hit. “If it does hit Mars, it would deliver as much energy as 35 million megatons of TNT,” said Don Yeomans of NASA’s Near-Earth Object Program at JPL. According to what Yeomans says in this video, the current odds of it hitting Mars is about 1 in 2,000, but according to the parameters at JPL’s website, it appears the odds are about 1 in 10,000.
Comet C/2011 L4 Panstarrs, taken from New Mexico Skies between on March 25, 2013 using an FSQ 10.6 and STL11K camera. 65 frames over 18 min each with an exposure time of 2.0 sec. The stars have been enhanced for effect. Credit and copyright: Joseph Brimacombe.
Get ready for an comet encounter of the extragalactic kind. In less than a week, Comet PANSTARRS will slide by the Andromeda Galaxy, the brightest galaxy visible in northern hemisphere skies. On and around that date, you’ll be able to see them both glowing softly together in late evening and early morning twilight.
The Andromeda Galaxy is the closest large galaxy to our Milky Way. It’s easily visible in binoculars in the constellation Andromeda. Credit: Adam Evans
Their apparent proximity if of course pure sleight of hand; the comet will be a mere 121 million miles (195 million km) from Earth on that date compared to Andromeda’s 2.5 billion light years. For what it’s worth, 121 million miles (195 million km) equates to 0.00002 light years. Let’s just say they’re WAY far apart in reality. Their juxtaposition will make for enjoyable binocular viewing as well as offer astrophotographers an opportunity to create a classic image.
Comet PANSTARRS on March 22 photographed with a 200mm lens at dusk on a motorized tracking platform. Credit: Bob King
Last night under the clearest of skies I easily found Comet C/2011 L4 PANSTARRS in the constellation Andromeda about 15 degrees above the horizon an hour after sunset. Twilight was still a factor as was the rising full moon. That’s probably why the comet remained at the very limit of naked eye vision. Binoculars – I use 10x50s – clearly showed the comet’s bright parabolic head and two degrees (four full diameters) of tail streaming up and to the right.
Comet PANSTARRS shown every three days as it moves across Andromeda, passing near the Andromeda Galaxy around April 3. You can use Cassiopeia to point you to Beta Andromedae and from there to the comet. The map shows the sky facing northwest about one hour after sunset. Comet and galaxy brightness are exaggerated for the sake of illustration. Stellarium
The comet has faded considerably since it first emerged into the evening twilight three weeks ago. Its head now shines around magnitude 3.5 and is noticeably fainter than the stars of the Big Dipper. As compensation, PANSTARRS is now easier to find, since it’s both higher up in the sky and near a string of moderately bright stars in the constellation Andromeda.
PANSTARRS treks northward through Andromeda en route to the W of Cassiopeia in the next two weeks. It won’t be long before the comet becomes circumpolar and remains visible all night long. The term refers to celestial objects that circle around the pole star without setting. The Big Dipper is the most familiar circumpolar constellation for much of the U.S. and Canada.
Comet PANSTARRS in the early dawn sky during the first part of April. Once again, you can use Cassiopeia to help get you there. Don’t forget binoculars! They’re now essential to seeing the comet. The map shows the sky facing northeast about 75 minutes before sunrise. Stellarium
On its journey to all-night visibility, PANSTARRS started pulling a double-shift this week. You can now see it both at dusk and at dawn. Although a bright moon will compromise the dawn view for a few days, you can watch for the comet low in the northeastern sky starting about hour and 15 minutes before sunrise. For the moment, it’s about the same altitude above the horizon during both morning and evening hours. Evening is still preferred only because the bright moon has finally departed the sky during the hour or so the comet is visible.
Jose Chambo’s photo of Comet C/2011 L4 PANSTARRS from Spain on March 15 reveals a broad dust tail and narrower red-tinted tail. The red tail may be from sodium atoms released by materials colliding with each other as they jet off the comet’s nucleus. Click image to see more photos. Credit: José J. Chambó
Through my 15-inch telescope last night, PANSTARRS’ head held a brilliant topaz gem – the false nucleus. This tiny ball of bright, fuzzy light contains the icy comet itself, hidden behind a fury of its own dust and vapor boiled off by the sun’s heat.
Here’s some additional images and videos of PANSTARRS that Universe Today has received from readers:
Zlatan Merakov created this timelapse from images he took on March 20 from Smolyan, Bulgaria.
The view of Comet PANSTARRS L4 on 03-22-2013 over Warrenton, Virginia. Modified Canon Rebel Xsi DSLR 30 second exposure, ISO 1600, University Optics 80mm F6 Refractor (600mm). Credit and copyright: John Chumack.Comet C/2011 Pan-STARRS over Gradara Castle in Italy. Credit and copyright: Niki Giada.
Comet PANSTARRS above a farm near Alto, Michigan. Credit: Kevin's Stuff on Flickr.
Comet C/2011 L4 (PanSTARRS) keeps getting easier to see, and over the weekend, we were inundated with images and videos from astrophotographers around the world. NASA says that solar heating from the comet’s close pass of the Sun last week has caused the comet to glow brighter than a first magnitude star. Bright twilight sharply reduces visibility, but it is still an easy target for binoculars and small telescopes 1 and 2 hours after sunset. And as of March 15th, people reported they can see the comet with the unaided eye.
Photographer Fred Kamphues took this timelapse from the Leiden Observatory in The Netherlands, the oldest astronomical observatory in the world still active today. Kamphues notes that astronomer Jan Hendrik Oort of Leiden Observatory discovered the origin of comets in 1950. The observatory is used today by student astronomers to learn observing.
Comet C/2011 L4 (PANSTARRS) taken on March 16 from Mount Faito (Naples, Italy). Credit and copyright: Ernesto Guido & Antonio CatapanoSpecial filters and a negative image to try and ‘tease out the structure of the tail,’ says photographer David G. Strange.Comet PANSTARRS over Tallinn, Estonia on March 16, 2013. Credit and copyright: Karthikeyan VJComet PANSTARRS over the San Gabriel mountains on 3/12/2013 above Pasadena,CA, 3-4 miles from Mt.Wilson. Shot with a with Canon 60D. Credit and copyright: Henry Levenson.Comet PANSTARRS, shot from near Keene, Ontario, Canada, on March 16, 2013, using a Canon 50D (modified) with Canon 200mm lens; 4 sec. exp.; f/4.5; 640 ISO. Credit and copyright: Rick Stankiewicz, Peterborough Astronomical Association (PAA) [/caption
This video, above, from UT reader Brent (a.k.a. HelloBozos) in Florida shows this compilation of views of the Sun and the comet. “At 2:08 in the video, a bird flies in front of the camera,” Brent said via email, “This was all done off the side the road, on 3-16-13 8pm-8:30pm.”
[caption id="attachment_100801" align="aligncenter" width="580"] Comet PANSTARRS over Arizona on March 16, 2013. Credit and copyright: Chris Schur
This image is from Chris Schur in Arizona. He says, “Note the fan tail appearing! Also the tail is really starting to curve in the images. Very easy to see naked eye, and so was the yellow color in binoculars when it gets lower.”
Comet PANSTARRS on March 17, 2013. Credit and copyright: Andrei Juravle.Comet PanSTARRS (C/2011 L4) taken near Koprivnica (Koprivni?ki Bregi), Croatia. Credit and copyright: Vedran Matica.
Comet Pan-STARRS thrills Dutch observers of the Night Sky on March 15, 2013 shortly after sunset. Shot with a Canon 60D camera and Canon 100/400 mm lens, exposure time 15 seconds, ISO 300 Credit: Rob van Mackelenbergh
Comet Pan-STARRS thrills Dutch observers of the Night Sky on March 14, 2013 shortly after sunset- note the rich hues. Shot with a Canon 60D camera and Canon 100/400 mm lens, exposure time 2 seconds, ISO 800. Credit: Rob van Mackelenbergh See viewing guide and sky maps below Update – see readers photo below[/caption]
Comet Pan-STARRS (C/2011 L4) is exciting amateur astronomers observing the night sky worldwide as it becomes visible in the northern latitudes after sunset. And now it’s wowing crowds in Europe and all over Holland – north to south.
Check out the beautiful, richly hued new photos of Comet Pan-STARRS captured on March 14, 2013 by Dutch astrophotographer Rob van Mackelenbergh.
“I took these photos in the southern part of the Netherlands on Thursday evening, March 14, at around 7:45 pm Dutch time with my Canon 60 D camera.”
“I was observing from the grounds of our astronomy club – “Sterrenwacht Halley” – named in honor of Halley’s Comet.”
Comet Pan-STARRS is a non-periodic comet from the Oort Cloud that was discovered in June 2011 by the Pan-STARRS telescope located near the summit of the Hawaiian Island of Maui.
The comet just reached perihelion – closest approach to the Sun – on March 10, 2013. It passed closest to Earth on March 5 and has an orbital period of 106,000 years.
Comet Pan-STARRS from Holland on March 14, 2013 at about 7:45 PM, shortly after sunset – Canon 60D camera, Canon 100/400 mm lens, exposure time 2 seconds, ISO 800. Credit: Rob van Mackelenbergh
“Over 30 people were watching with me and they were all very excited, looking with binoculars and cameras. People were cheering. They were so excited to see the comet. But it was very cold, about minus 2 C,” said Mackelenbergh.
The “Sterrenwacht Halley” Observatory was built in 1987 and houses a Planetarium and a Celestron C14 Schmidt-Cassegrain telescope. It’s located about 50 km from the border with Belgium, near Den Bosch – the capitol city of southern Holland.
Comet Pan-STARRS was photographed from Sterrenwacht Halley – or ‘Halley Observatory” in Holland. Credit: Rob van Mackelenbergh
“It was hard to see the comet with the naked eye. But we were able to watch it for about 45 minutes altogether in the west, after the sun set.”
“The sky was completely clear except for a few scattered clouds near the horizon. After the comet set, we went inside the observatory for a general lecture about Comets and especially Comets Pan-STARRS and ISON because most of the people were not aware about this year’s pair of bright comets.”
“So everyone was lucky to see Comet Pan-STARRS because suddenly the sky cleared of thick clouds!”
Comet Pan-STARRS from Holland on March 14, 2013 at about 7:45 PM, shortly after sunset – Canon 60D camera, Canon 100/400 mm lens, exposure time 2 seconds, ISO 800. Credit: Rob van Mackelenbergh
“In the past I also saw Comet Halley and Comet Hale-Bopp, but these are my first ever comet photos and I’m really excited !”
“I hope to see Comet Pan-STARRS again in the coming days when the sky is clear,” Mackelenbergh told me.
Over the next 2 weeks or so the sunset comet may grow in brightness even as it recedes from Earth into darker skies. Right now it’s about magnitude 0.2.
So keep looking with your binoculars; look west for up to 1 to 2 hours after sunset – and keep your eyes peeled.
See a readers photo of sunset Comet Pan-STARRS below
Comet Pan-STARRS viewing graphic from NASAComet Pan-Starrs Sky Map. Viewing guide to find the comet low in the horizon after sunset.Credit: Spaceweather.com
C/2011 L4 (PANSTARRS on March 15, 2013. Taken in Hachimantai City Japan. Credit and copyright: Jason Hill.
You want images and videos of Comet C/2011 L4 (PANSTARRS)? We’ve got ’em! We’ll start with this stunning view from Japan, taken by Jason Hill. But there’s lots more below:
A first capture of Comet PANSTARRS on March 14, 2013. Credit and copyright: Adam Wipp.Another first view of Comet PANSTARRS from Valencia, Spain on March 14, 2013. Credit and copyright: Alejandro Garcia.
This timelapse comes from Andrew Takano, a graduate student at the University of Texas at Austin:
Comet PANSTARRS on March 14, 2013, as seen in the Arizona skies. Credit and copyright: Chris Schur.
Photographer Chris Schur said last night’s views were “the best and brightest comet yet in the western Arizona Sunset sky!” Schur said via email. “I was able to go much deeper tonight using an 80mm Zeiss refractor and Canon Xti. The head shows more fan like protrusions, and the tail is now really shaping up. … The comet here at our elevation of 5150 feet was very easy to see the entire time it was up, and I would rate it at first magnitude for sure.”
Comet PANSTARRS as seen from Aarhus, Denmark (56.2 N, 10.2 E). Credit and copyright: Jens Riggelsen.
Comet Panstarrs above Boulder, Colorado on the evening of March 13, 2013, courtesy of Patrick Cullis:
Comet PANSTARRS on March 13, 2013 as see from Newington, New Hampshire, USA. Credit and copyright: John Gianforte (theskyguy.org)Comet PANSTARRS seen from Oakland, California. The Port of Oakland and the Bay Bridge are in the foreground with the comet and crescent moon in the background. Taken on March 12, 2013. Credit and copyright: Jared Wilson.Comet C/2011 L4 (PANSTARRS) floats in the twilight sky over the lighthouses and pier at Grand Haven State Park in Grand Haven, Michigan on March 13, 2013. Credit and copyright: Kevin’s Stuff on Flickr.Comet PANSTARRS and a 5% illuminated Moon on March 13, 2013. Credit and copyright: Tavi Greiner.
You can see more at our Flickr page, and we’ll keep adding and posting! Thanks to everyone who has been so generous with sharing their great photos and videos.
Want to get your astrophoto featured on Universe Today? Join our Flickr group or send us your images by email (this means you’re giving us permission to post them). Please explain what’s in the picture, when you took it, the equipment you used, etc.
UPDATE: The webcast has been moved to March 16 at 17:00 UTC (1 pm EDT) due to bad weather in Italy.
Has it been cloudy where you live and you haven’t yet been able to see Comet PANSTARRS? The Virtual Telescope Project will have a live webcast of this comet, C/2011 L4 PANSTARRS, from Italy, March 15, on March 16 at 17:00 UTC, 1 p.m. EDT. “We have been waiting for it for over one year, and now the waiting is over,” said astrophysicist Gianluca Masi, who will host the webcast, which you can see at this link. Masi said they are keeping an eye on the skies, and will keep us updated on if they need to change the time of the webcast.
If you’re waiting for the weekend to see it with your own eyes, check out our detailed guides on how to see it here and here. Both are filled with graphics and great info on how to see this comet.
This comet has been a challenge to see, and was actually closest to the Sun on March 10, meaning that is when it was at its brightest. However, while Comet PANSTARRS will fade over the next few weeks, it will also rise higher into a darker sky and become – for a time – easier to see. So keep looking!
That red comet is pretty, but perhaps not so realistic. Credit: Game of Thrones Wiki/HBO (Screencap)
A blood-red comet appears in the sky. People quake in its wake.
This phenomenon, which happens in the second season of the medieval fantasy Game of Thrones, had us all wondering — can you ever actually see a red comet?
We talked to Matthew Knight, an astronomer at the Lowell Observatory in Arizona who observes comets. He gave us some answers just in time for the third season of Game of Thrones, which begins March 31.
At first blush, he said, the comet’s red color wouldn’t be possible because the strongest emissions from comets are in the blue and green regions, mostly from neutral gases such as hydroxide and cyanide.
There is a type of emission that is close to red, called “forbidden oxygen”, which occurs when atoms make a rare energy transition between states of “excitement”. But it’s very faint and short-lived, Knight wrote.
The visible light from a comet comes from a combination of reflected solar continuum (sunlight reflecting off of dust grains) and cometary emission (neutral and/or ionized molecules of gas that emit photons at a particular wavelength). The sunlight reflecting off of dust grains basically looks like sunlight and since the Sun appears yellow/white, this component cannot look red.
A small caveat is that due to the physical properties of dust grains, comet dust often actually does “redden” sunlight slightly when measured with sensitive equipment. However, this reddening is at a very low level and is not enough to cause the reflected sunlight to appear a deep red like in Game of Thrones. The strongest comet emissions in the region where human eyes can see are in the blue and green regions.
Comet Hale-Bopp. which displays the usual cometary colors. Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria
So what ingredients does a comet need to look like the one in Game of Thrones? According to Knight, it would have to meet these criteria:
Be visible in daylight, which really only happens about once a century;
Be close to the sun (he supposes this one is, given how straight the tail is);
Have a “strange composition” that is different from anything we know in the solar system. The composition could be that forbidden oxygen he talked about, coming from a comet whose ices are carbon monoxide and carbon dioxide. But that would be hard, because those types of ices would not survive long when exposed to sunlight.
If we really want to think in a science fiction vein, Knight suggests that maybe the comet could be made up unpredictably:
Alternatively it could be something else entirely unknown in cometary chemistry or dust, with really weird properties causing a much stronger reddening than is normally seen. In any event, the composition would be so anomalous that this comet would almost certainly have originated in another solar system. That would make comet scientists very interested in studying it!
But don’t despair yet. Comet ISON might be bright enough for daylight viewing when it swings by Earth late in 2013. Comets are unpredictable beasts, but we’re pretty sure of one thing: no matter how bright it is, it won’t look red.
