Hubble peers into the weird heart of Comet Holmes

By Phil Plait | November 15, 2007 10:55 am

The European arm of the Hubble Space Telescope coordinating facility has just released a new image of the weird Comet 17/P Holmes, taken on November 4, almost two weeks after the comet suddenly expelled a huge quantity of dust (as usual, click the image to embiggen):

The color image is a deep ground-based observation, and the grayscale one is from Hubble (note the scale bar; Hubble could see details as small as 54 km across). Some things are obvious immediately from the picture. There is a lot more dust along the horizontal axis then the vertical; that’s why it looks like a funny vertical bow-tie. There is about twice as much dust horizontally (east/west) than vertically (north/south). That means the dust was expelled in a preferred direction, and not isotropically, that is, in all directions like an expanding spherical shell.

Even nearly two weeks after the outburst, the nucleus was still enshrouded in dust. A series of images taken over the course of seven days shows the central region of the comet dimming as the dust cloud expanded and cleared out:

Remember, all the action is coming from the nucleus of the comet, a chunk of ice and rock only a few kilometers across. If comets stayed that way, all tucked into a giant boulder, we’d never see them. But they heat up when they approach the Sun, and the ice sublimates (turns into a gas), expanding outward, carrying dust with it. This reflects sunlight, brightening the comet. In the case of Holmes, it was farther out from the Sun than Mars is when it suddenly brightened by a factor of a million. Some event on or under the crust of the comet nucleus caused a catastrophic release of material. The expanding cloud of debris is now bigger than the Sun itself (about 1.5 million km across).

Two factors compete to account for the brightness of the cloud. Initially, when the cloud is dense, it gets brighter as it gets bigger because it can reflect more sunlight. But eventually the cloud starts to thin out and becomes less efficient at reflection. It then gets fainter. For a while the comet cloud from Holmes got brighter, but has since been fading. I haven’t had a chance to see it for a week or so (weather, travel, and such) but I’m hoping to get a chance again soon. A few days after the outburst the cloud got so big it was obviously non-starlike to the naked eye, which was astonishing.

We understand a lot about the basics of comet behavior (despite some kooky claims), but each comet is an individual, and can be difficult to predict. Holmes has shown us precisely why we need to keep observing each comet as they present themselves to us. Not only can we learn more about these objects and satisfy our scientific hunger, but they also appeal to our sense of grace and beauty.


Comments (18)

  1. kim

    No orbital change?

  2. djcinsb

    I’ve been watching it nightly since first reading about the outburst here. It’s still (as of last night) naked eye visible — at least, outside of Tucson, with pretty dark skies — and has moved noticeably in the sky.

  3. Tom

    My kid’s eyes popped when I told them the cloud was bigger than the Earth; can’t wait to show ’em these pics and tell them it’s bigger than the Sun now!

  4. I’ve been watching this comet nearly every night. Too bad I don’t have a better camera or I could have taken some sweet pictures. It certainly has moved quite a bit and its “nebulosity” seems to get bigger all the time (even if it does get slightly dimmer). It actually showed up on some of my Perseus photographs before I actually knew what it was!

  5. Could the outburst simply mean the comet clobbered something smaller? Could an impact with a small asteroid, depending on relative velocities, have been the cause?

  6. I saw the comet last week on a cruise to Hawaii. The skies were as dark as can be out at sea. When I pointed my 10×50 binoculars where I thought it would be it took me about half a second to find it.

    When I saw it I shouted “HOLY HALEAKALA!!!” (Though, it may not have been Haleakala I shouted). I was taken aback. I was astounded! My jaw hit the tops of my shoes. It was probably the most spectacular celestial object I have ever seen with my binoculars. It was like a big fluffy snowball hanging in the sky; completely unambiguous, right there, big as life. WOW!

    BTW, Phil, my wife and I went to Haleakala and also to Keck. Thanks for the influence over the years. I’ll send you a few pics.

  7. Will.

    Jonathan beat me to it; perhaps an impact with another object caused the expulsion of material?

  8. Rumple

    The great Alan Dyer image was remarkably taken with a 4.1″ (105mm) A&M apochromatic refractor with an APM/LZOS (TMB designed) lens. Here is a picture of the scope:

    The camera was a Canon 20Da SLR (no longer made) which had an astronomical filter that let far more Ha light pass than a regular 20D. Of course, this has no impact on an image of a comet which does not emit Ha narrowband light!

    Here is a link to the image and some specs on how it was taken (scroll down). Total image integration time was only 7 minutes 52 seconds.

    I think it should be more widely know how well rather ordinary (if high end in this case) backyard equipment can do at astrophotography nowadays.


  9. Kevin


    It really is amazing what “amateur” astronomers can do these days.

  10. Crux Australis

    Wow, one percent of the distance from Earth to Sun! Just 100 of those bad boys would fit between them. That sound you hear is my mind, boggling.

  11. dave

    There is a new Comet Holmes website with info, links, and more

  12. Hey lookie, Dr. BA!

    Rob Roy Britt of is reporting that the coma has embiggened so much that it’s outsizing the sun. That seems like a lot of growth since the pictures you have above… or am I missing something?

    “It continues to expand and is now the largest single object in the solar system,” according to astronomers at the University of Hawaii.

    The coma’s diameter on Nov. 9 was 869,900 miles (1.4 million kilometers), based on measurements by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy. They used observations from the Canada-France-Hawaii Telescope. The sun’s diameter, stated differently by various sources and usually rounded to the nearest 100, is about 864,900 miles (1.392 million kilometers).

    That’s kinda on the plus-size side for a comet, ain’t it?

  13. Janie, I did say in the post it’s bigger than the Sun.

    I would disagree that it’s the biggest thing in the solar system. Jupiter’s magnetosphere is bigger, as is the magnetosphere of the Sun. Those aren’t solid, but then, neither is the comet’s dust cloud.

  14. Holy crap.

    I just had to read the whole post three more times before I found where you said that.

    My apologies for the brain cramp. (I don’t know what’s wrong with my reading comprehension skills, but that’s happened to me several times in the last few days.)

    But still, isn’t that abnormally large for a comet?

    (I get what you’re saying about the magnetosphere and lack of solidity – comparing apples to apples and all.)

  15. Mike

    I took a look at comet Holmes last night through binaculars. When I first saw the comet, I wondered if it was a cloud because were a few wispy clouds in the sky. But, low and behold, it just stayed in the same place. I imagine that if it weren’t for the interenet, some people would be claiming it was the Death Star or God preparing us for Armagedon.
    If it becomes much brighter, I suspect that we will hear a bunch fundies screaming that the end is near.

  16. doug

    Its about time this “dirty snowball” construct of comet composition be swept into the dustbin of bad astronomy, and yes, Phil you are engaged in this chrade of irrationality. First sign of bad science is theoretical concepts presented as irrefutable certainty.

    “Remember, all the action is coming from the nucleus of the comet, a chunk of ice and rock only a few kilometers across.”

    Its time the simplicity, intuitiveness, and elegance of electric universe theory be examined in ernest.


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