Now visible: Comet as big as the full moon

Hyperactive comet 46P/Wirtanen is approaching Earth for one of the closest Earth-comet encounters of the Space Age. Observers report that the comet’s gaseous green atmosphere now covers a patch of sky as large as the full Moon–and it is growing larger.

Above: Comet 46P/Wirtanen, photographed on Dec. 2nd by Mike Broussard of Perry, Louisiana. A picture of the Moon has been inserted for scale.

On Dec. 16th, Comet 46P/Wirtanenwill approach Earth less than 11.5 million km away–making it one of the 10 closest-approaching comets of the Space Age. It’s a small comet, with a nucleus barely 1 km wide, but such proximity makes even a small things appear large. The comet’s gaseous atmosphere is now as wide as a full Moon.

Despite its close approach, 46P/Wirtanen will never become a Great Comet like Comet Hayakutake in 1996 or Comet Hale-Bopp in 1997. Wirtanen’s relatively small core of dirty ice cannot produce enough gas and dust to create a really bright, flamboyant tail. The best case scenario is probably a big diffuse cloud of magnitude +3 or +4, barely visible to the unaided eye but an easy target for binoculars and small wide-field telescopes.

Comet Wirtanen passes through the inner solar system every 5.4 years. Right now it is just below the orbit of Earth, and the gap is narrowing. Click on the image above to explore the comet’s approach, courtesy of NASA/JPL.

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65 thoughts on “Now visible: Comet as big as the full moon

    • Reminds me of the “journalist” who some years ago asked an astronomer if climate change had anything to do with the approaching asteroid.
      How he kept from laughing in her face I’ll never know.

      • Hi Alaskasininfidel, I heard an Australian ABC presenter Sally Loane talking to Dr Karl Kruszelniki who was telling her about the latest in ideas about a solid state or expanding universe etc…and he used toe word “exploding universe” and she said, “Oh, is that what is causing Global warming?”
        Let that sink in for a moment….holy [SNIP]!

      • My favorite is the Hank Johnson Guam capsizing Congressional hearing with an Admiral. I watched that real time on CNN. I did spit out my food laughing. Till I realized this moron was a Congressman and now he has been reelected multiple times.

    • I bet it signals the dawn of Aquarius and passing into the native American next world and all good things to all good flower children, etc. ad nauseum. 🌅🌻🌈🌦🌼⛱…🤩

    • It seems it will be very easy to spot.
      On the 16th, which is the closest approach, it will be close to a direct line between the Pleiades and the Hyades clusters.
      The Pleiades are one of the easiest asterisms to spot for amateurs, and can be seen even in locations and on nights where the sky is not particularly dark.

      • Thank you. Was about to start digging to see if it would be visible in the NH. My luck, though, we are having an unusually rainy December here, so I may still not get to see it.

        • It is said to already be visible, and it is still getting closer to the Sun and to the Earth. Closest to the sun is the 12th, IIRC, and that is when it will be tend to be brightest in absolute magnitude, although since it will keep getting closer to Earth for four more days after that, it will be getting bigger and maybe even brighter until the 16th.
          So there will likely be most of the entire month when it will be at or more bright and at least as large as it is now.
          Hopefully it will not be cloudy every single night for you.
          This time of year, the Pleiades are in the sky pretty much the entire long December night, rising in the East around sunset (depending on your latitude), and close to straight overhead from about 9:00PM to midnight on the 15th for most people in the continental US and southern Canada., and not far from straight up for a few hours on each side of that.

          • Of course, it will only be passing the Pleiades on that night, the 16th, but they are a handy starting point to help anyone find the comet the closer it is to that date, before or after.

        • Correction, and sorry: After a closer look at some other charts, the comet will be passing the Pleiades on Dec. 15th.

  1. Let’s hope it doesn’t have a small companion trailing behind it, as some think Comet Hale-Bopp did. And some of those some were the nut-case Heaven’s Gate cult members, who committed mass suicide in order to get on board the trailing companion, which they believed to be a UFO. I’m not sure why one would want to “get on board” a UFO, since it is, by definition, “Unidentified”, i.e. something that may not have a board to be on. But I guess that people who named themselves after the biggest box-office catastrophe of all time had very little to lose. It is a pity that no one ever established whether there really was a companion body. Amateur astronomer Chuck Shramek was the first to claim to see it, taking a CCD image in November 1996. Astronomer Alan “Skipper” Hale maintained that it was the image of an 8.5 magnitude star (SAO141894). I prefer the Occam’s Razor explanation, which is that it was just a smaller comet along for the ride, and should always be known as Comet Teeny-Bopp.

  2. Graham Hancock and Randall Carlson have been claiming were working 30 years of passing through a dense cluster of the Taurid meteor belt, and they assert this increases our chances of an impact. Carlson has also listed the near Earth encounters by bolides and such, and it seems pretty frequent for comfort.

    If we start to cool slightly, have maybe one major volcanic eruption, and say a comet impacts… Could such a combination trigger another ice age? If so, that would suck. But I don’t think in that event the green religion could possibly survive

  3. Oh goody. A green flash in the sky.

    I asked an astronomer and “The chances of anything coming from Mars are a million to one” he said.

  4. See also for sensible, low-hype text (and warning that the waxing moon will greatly interfere with the diffuse comet):

    A number of different predictions have been made regarding the brightness of 46P as it passes closest to Earth in mid-December. The Smithsonian Astrophysical Observatory’s Minor Planet Center forecasts a peak magnitude of +8. However I believe, based on observations of 46P through late October, that the forecast of Japanese comet expert Seiichi Yoshida will be much closer to the truth,1 indicating a magnitude of no fainter than +4 during December and peaking near +3 at the December 16th perigee.

    Keep your expectations low

    But as compelling as this all may sound, I now must temper any excitement by providing a very important disclaimer.

    At the beginning of December, many people with binoculars and small telescopes will no doubt attempt to follow the path of 46P/Wirtanen across the night sky. But actually seeing it will strongly depend on your observing site. From locations that are plagued by light pollution, 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. This is because the comet will be unusually large in angular size, as well as appearing very diffuse . . . almost ghostly. Indeed, many with little observing experience will sharply question the predictions for a third or fourth magnitude object. But remember, you’re not looking for a sharp star-like object, but rather something which is spreading its light out over a relatively large area.

    • A good piece of info about magnitude of a diffuse object.
      Another problem with observation, I think, is that we will be seeing the comet head-on, with whatever tail it has behind, not to the side, of the head. shows that the comet will be between Aldebaran in Taurus and the Pleiades on Dec. 15.

  5. Boy that headline was like Deja vu for me! Back in May 1983 we had another ‘tail-less’ fuzzball comet come close to earth, 10,000,000 miles is close astronomically speaking. The comet I believe was the first ever detected by a satellite (IRAS the Infrared orbiting telescope/camera) as well as two earth based observers. It was known as Comet IRAS – Araki – Alcock from memory. We alerted the local newspaper that with expected fine weather the next night that the observatory would be open for viewing of the comet which at near magnitude +2 would be visible as a hazy patch “as big as the moon!” Much to our horror, the editors changed ‘big’ to ‘bright!’ In the days before decent laser pointers it wasn’t much fun trying to point with your finger at a dim patch to a thousand disappointed visitors. Needless to say we didn’t charge! After that we never allowed anything to be published without our final approval, similar with radio interviews as well.

    • Ian Cooper
      December 4, 2018 at 4:12 pm :

      Yes, Ian…it’s just as bad as the “Mars will be as big as the Moon” that comes around in the media every 2 years! Gives us headaches up here on the Coromandel with overhyped stargazers on the doorstep of the observatory. We’re going to try and spot it visually tonight now that the clouds are finally clearing.

      A bit like the AGW hype I guess…only that’s been non-stop for about 40 years now.

      • We have the New Zealand Astrophotography Weekend at Foxton Beach starting this Friday. The forecast is reasonable, especially considering how bad conditions are today. I have no doubt attendees will have a go at imaging the comet if conditions allow. A similar large, tail-less comet called Holmes graced northern skies over 10 years ago. I managed a photo of it with a 50mm lens from my rural home. For NZ it was low and near the horizon but still detectable. I will take my 25 x 100 bino’s along with a hope to getting a view of the comet.

        As for the Mars Hoax thing, well these things now have a way of coming back to haunt us unfortunately. people are always up to being sucked in though!

  6. There was a truly magnificent comet I saw when on the good ship “Kampala” in the Indian Ocean about Feb 1965. It had a visible tail about 20 to 40 degrees long, bright, possibly about -1.

    Can anyone identify this? I am fairly certain about date and location where I saw it, likely about 0400 or 0500 local time. Info would be much appreciated.

    Only sighted on the one night – possibly too cloudy for previous and following nights.

    • Are you sure it was February? The “brightest comet of the 20th century” appeared in October, 1965.
      October 15–31, 1965
      This showstopper was a member of the Kreutz family of sun-grazing comets. It was the brightest comet of the 20th century, visible in full daylight within a few degrees of the Sun. Japanese observers said it appeared “10 times brighter than the full Moon.” During the last week of October a long twisted tail, resembling a wispy searchlight beam above the eastern horizon, stretched for at least 25°. The nucleus split into three pieces.

      • Thanks Gregory. It appears I was mistaken. However, on 12/13 October my ship, the “Bombala” was in Calcutta. Prior to this the comet would have been near invisible. After leaving Calcutta we went to load in Chittagong and Chalna. This was just after the India Pakistan war, and relations between the two were rather strained – it is said the relations were so bad that the Indian police actually stopped the smugglers from moving raw jute across the border – jute was grown in East Bengal but not West Bengal, but the factories that processed it were in West Bengal, so it had to be smuggled across the border. For a short time Singapore became a major exporter of jute – never grown in Singapore!

        If follows then that I saw the comet after we left Chalna bound for Penang, in late October 1965, when the comet would have been at its brightest. Again thanks. It was a truly impressive sight.

  7. Green because the copper wiring has vapourized and is showing the typical green colour of copper in a flame. Or perhaps it is made of Copper Sulphate? Or Malachite? /sarc.

  8. Does no one read John Wyndham anymore?

    If you all go blind and get killed by Triffids don’t come crying to me!

    • “ … At midnight on the twelfth of August, a huge mass of luminous gas erupted from Mars, and sped towards Earth. Across two hundred million miles of void, invisibly hurtling towards us, were the first of the missiles that were to bring so much calamity to Earth. As I watched, there was another jet of gas, it was another missile starting on it’s way … and that’s how it was for the next ten nights. A flare, spurting out from Mars. Bright green, drawing a green mist behind it. A beautiful, but somehow disturbing sight. Ogilvy, the astronomer, assured me we were in no danger. He was convinced that there could be no living thing on that remote forbidding planet. …” – Jeff Wayne’s War of the Worlds – Eve of the War

  9. “ … the comet’s gaseous green atmosphere now covers a patch of sky as large as the full Moon …’

    “We are the Greenies, lower your shields, you will be assimilated, your biological and technological distinctiveness and bank-accounts will be added to our own, resistance is futile.”

  10. It’s interesting that something so small with such a low gravity has such a big atmosphere, bound spherically to the nucleus with a non existent tail?

    There must be more than just the forces of gravity holding that atmosphere in that configuration and dragging it along as a bound atmosphere in that particular geometry. There would have to be an plasma/electromagnetic component to that?

    • The coma is diffuse, and is formed of vapors out gassing from the head, being vaporized by solar radiation.
      It’s not bound to the head.

  11. Halley’s Comet first documented sighting was by the Chinese in 240 BC , although it is assumed to be an early remnant of a Solar system formed 4.5 billion years ago . It’s orbit period is 75 years and when most proximal to the Sun , “sublimates dirty Ice” from within it’s black exterior forming a tail(coma), the volume of which is millions, if not billions of times the volume of it’s nucleus , the coma, clearly visible from Earth for many days . I am no expert on reflectivity , still , small particles of ice and dust ,I think ,would not be visible from Earth unless the distance between particles were in the micrometer range , which would, I believe, entail a coma ,the mass of which is greater than the mass of the nucleus .If Halley’s Comet , or any other comet visible from earth , were billions of years old , and their orbits remained relatively unaltered , and were, for the most part nothing but dirty snowballs , then at some point in the not to distant past their nucleus must have been the size of Planets. Unless of course there exist continuous mass (hydrogen and oxygen), or energy entering the nucleus , capable of transmuting into frozen water .Then again , perhaps , our Solar system , or at least comets , are not early as old as is presently BELIEVED, and all mainstream theories discarded , and replaced with one more consistent with the abundance of new findings regarding these most mysterious objects , which , until recently , when sighted , caused much trepidation . I am no expert ,merely asking questions, but it seems to be that the history of humanity is being reinterpreted before our very eyes .

  12. Re Green lights – back in the early 1970s I was driving home one night, about 10pm., when the sky turned a bright green. As I was in the car I did not see what caused it and by the time I had stopped and got out it was all over. I lived then in Central Queensland and was about 15 miles from town so no other source of light was there to contaminate whatever I saw. No one mentioned a meteor in the media the next day so no idea what it was.

    • I’m not saying it’s aliens…..but it was aliens!

      You’re just lucky that they’d already met their quota of humans to probe the backsides of for that day 😉

  13. Wow – nonetheless

    “click to enlarge” yields no enlargement; anyway –

    Thanks for “warning in time! ” /sarc

  14. I saw it last night in my 10×80 binoculars. Very easy to see in a dark sky but not naked eye. A large fuzzball, a bit brighter than the Pinwheel Galaxy (M33) but not as bright as the Andromeda Galaxy (M31).

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