A dead comet for SOHO

By Phil Plait | September 26, 2007 9:35 am

Sometimes astronomy news isn’t spectacular, but it is still so way cool.

SOHO is an observatory that is parked about a million miles towards the Sun, where it can stare at the nearest star all the time. One instrument, LASCO (the Large Angle and Spectrometric Coronagraph), has a little paddle that blocks the fierce light of the Sun itself so that it can see the fainter hiccups and belches of billions of tons of plasma erupting from the surface.

But it can also see stars in the background, and planets, and, sometimes, comets. They are really amazing to watch in the animations as they streak in toward the Sun. Most — and over a thousand have been seen — evaporate away and are never seen again.

But a sharp-eyed observer named Sebastian Hoenig, a German PhD student, found one that came back! Although it’s not clear from the press release, I assume he was tracking comets and calculating the orbits of them. In 2005 he realized that two comets may be one and the same because the mathematics of their orbits (shape, size, period, etc.) were so similar. Assuming he had seen two apparitions of the same object, he predicted it would return in early September, 2007. Bang! There it was, right on schedule.

Now called, P/2007 R5 (SOHO), it’s a weird one. Comets usually get a tail, especially when close to the Sun. This one doesn’t. But it does brighten a lot when it gets close to the Sun, like comets do. It’s probably an extinct (well, almost extinct) comet nucleus. It used to be rock and ice, but now it’s almost all rock, with some ice still buried inside that only sublimates (turns into a gas) right when the comet is closest to the Sun.

It’s tiny, maybe 200 meters across, so it’s basically invisible to Earth-based telescopes, and only becomes visible when it’s near the Sun. Over time, any comet could become one of these weirdos, so it may not be all that weird– there could be thousands more like it on short orbits (this one has a four year orbit).

If you’ve never done it before, go poke around the SOHO website (linked above). It’s an amazing success story from NASA and ESA, with tons of incredible information, pictures, and animations. It’s time well spent.

CATEGORIZED UNDER: NASA, Pretty pictures, Science

Comments (17)

Links to this Post

  1. A Ler…-- Rastos de Luz | September 26, 2007
  1. Blame it on recently finishing Lucifer’s Hammer by Larry Niven & Jerry Pournelle, but I’m wondering what a 200m comet like this would look like if it hit Earth.

  2. Interesting. If there are quite a few dead comets, they’d contain material from the original accumulation of the Solar System. Failing that, they may exhibit new minerals due to heat-freeze-reheat cycles. Either way, they’d be scientifically valuable in one way or another.

    Perhaps once a dead comet whose orbital energy falls within the range of our available C3, we could send a probe out to go take a look at it and, if we’re really lucky, return a sample…?

  3. Arthur Maruyama

    Considering how close this comet gets to the sun (7.9 million km), I wonder if the brightness seen at its perihelion is due to vaporization of material other than water/ice? After all, it does get closer than 20% the orbital distance of Mercury at its perihelion (40 million km).

  4. Arthur Maruyama

    Oops! Sorry, missed a digit: Mercury’s perihelion is 46 million km, not 40.

  5. Laguna2

    If you are interested how he found it, you can ask him.
    Find his email address here:

  6. Donnie B.

    Give my people the lightning! :-)

    My question would be, does Mr. Hoenig get to put his name on his comet, or is it stuck forever with the (SOHO) label?

    Just how does a spacecraft discover a comet, anyway?

  7. Doc

    It’s not so much a cubic mile of hot fudge sundae, as is a cubic tenth-mile of rock candy.

  8. PK

    BA, do you know how much bigger the eye patch of LASCO is than the solid angle of the sun? In other words, how much do we miss?

    And also, I noticed quite a few big flares just as the comets pass through the corona. Is this a coincidence, or do the comets set them off?

  9. PK

    Actually, the sun is probably outlined by the white circle, right? Also notice the interference fringes around the edge of the eye patch, very cool.

  10. tacitus

    Come on guys, everybody knows this has to be an alien artifact, placed in a orbit where we would be unable to discover it until we reached the requisite level of advancement as a species. Obviously that’s why it’s not behaving like a normal comet…

    No… ?

    Well, a man can dream, can’t he?

  11. Tacitus:

    I’ve got a lunar magnetic anomaly whose origin point lies twenty meters under the surface of Tycho Crater that I’d like to show you…

  12. Just Al


    Beware The Centipede. He’s got a big bone held behind his back.

    [Cue superdramatic music]

  13. Butch: I just finished rereading that! A favorite. 😀

  14. OneHotJupiter

    That animation is some of the coolest stuff I’ve ever seen! Amazing to see the Sun , just bellow out all that stuff! Fascinating!!

  15. Hugh

    Wow, this is exactly why I read BA – this is something I’d never even pondered and from the animations it’s one of the more beautiful events our universe has to offer.

  16. Oz Engineer

    I agree with Hugh – This is exactly why I read the BA blog so eagerly. Now that I think of it, aren’t dark comets dangerous to Earth? They must be difficult to see, presumably their orbits take them out past Earth and back again, in this case every four years.

    As stated in the article, they are too small to see from Earth.

    What’s the chance of a thousand tonne bull’s-eye? That would take the enhanced greenhouse effect off the front page for a while… presuming that there are still front pages.


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