The two tails of Comet Garradd

By Phil Plait | February 22, 2012 6:55 am

CĂ©sar CantĂș is an astrophotographer in Mexico. I follow him on Twitter, and hardly a week goes by without him posting a link to some amazing picture he’s taken of a celestial object.

And this is no exception: here is his image of Comet Garradd, a chunk of ice and rock that’s currently about 200 million kilomertes (120 million miles) from Earth:

[Click to encomanate.]

Isn’t that lovely? The comet itself is a bit smeared out since it moved over the time as the picture was taken. But even so, wait a sec — you may have noticed something else odd about this picture. Comets have a tail, right? So why do you see two tails, a blue one pointing off to the left and the other reddish, pointing off to the right?

Aha! Oh, I love a chance to lecture a bit. Bear with me. This is cool.

As I said, comets have a lot of ice in them. As they near the Sun that ice warms, and turns directly into a gas (that process is called sublimation). This gas expands away from the solid nucleus, forming a fuzzy cloud called the coma (Latin for "hair").

Now this is where things get interesting. This coma has both gas in it as well as dust and grains of rock carried off as the ice goes away. The Sun blows out a wind of subatomic particles called the solar wind. This ionizes the gas — strips off one or more electrons — and that gas then gets dragged along with the solar wind. That wind is moving, traveling at several hundred kilometers per second, far faster than the comet moves. So that tail gets blown directly away from the Sun. It tends to be blue (or sometimes green), due to the ionized gas in it.

But the dust and rock isn’t affected as much. As it moves off the comet, it tends to lag behind a bit, following the comet in its orbit. This material reflects sunlight and also reddens it a bit, so that makes the dust tail look yellow or red.

And that’s why there are two differently colored tails pointing in different directions! You can read more about this here.

In fact, I can show you what’s going on even better. The JPL website has an orbit simulator for comets and asteroids, and I created a diagram for Comet Garradd for when CĂ©sar took his picture:

The Sun is in the center, and the planets are labeled; I deleted the orbits for all the planets except Earth and Jupiter so you can get a sense of the plane of the solar system. The comet is in blue, and as you can see its orbit is not at all aligned with the planets; it punches upward through the plane on the right, and then plunges back down on the left. It may be hard to get a 3D image of this in your head, but I added in the two tails: the blue ion tail pointing away from the Sun, and the redder dust tail lagging behind the comet itself. From the viewpoint of the Earth, "underneath" the comet, the tails appear to be on opposite sides of the comet and pointing in opposite directions! It’s just perspective making it look that way; at this point in the comet’s orbit the tails are actually closer to 90° apart.

Strange, isn’t it? I’ve found that three-dimensional thinking is one of the tougher barriers to people really understanding how objects move in space (that, and the vast physical scale of space that crushes our minds to dust). But perspective counts! In astronomy, as well as life itself. And when you get a little perspective, why, sometimes things are even cooler than you first thought.

Image credit: CĂ©sar CantĂș.

Related posts:

Ten Things You Don’t Know About Comets
One more Lovejoy time lapse
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The Sun fries a comet and we got to watch
The WISE family comets

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (14)

  1. Oh, I love a chance to lecture a bit. Bear with me. This is cool.

    Oh how I love to listen to (read) you lecture. I will always bear with you. :)

    As a pilot, I have always come by 3D thinking naturally. All that aerobatic training maybe? I know Kirk would never have tricked me like he did Khan.

    And yes, this is cool as usual!

  2. Robert

    I am currently taking Astronomy 201, and during one of my classes the professor mentioned something about comets and how they have two tails. But since that wasn’t the subject of the lecture, he said he would explain why when we get to that chapter. I intended to look it up on my own, but had forgotten about it. Thank you for the information.

    And like the previous poster, I too enjoy reading your lectures.

  3. Messier Tidy Upper

    The image there looks almost stellar with the rays artefact. Remarkable. :-)

    Without the write-up, going by just the photo I’d have first guessed it was a binary star – one hot the other cooler – with either jets or surrounding nebulosity.

    Just goes to show how first impressions can be misleading! 😉

    In astronomy, as well as life itself. And when you get a little perspective, why, sometimes things are even cooler than you first thought.

    So absolutely right.

    Also shows knowing something about what your seeing can make a huge difference in understanding and appreciation and spark sheer wonder of it.

    That’s a comet not what I’d guessed?

    That dot is a whole other world of methane ice & rock and nitrogen snow with a moon larger in its sky than ours and three other smaller moons and maybe faint rings too?

    That misty spot is a whole other distant galaxy two million light years away and yet on course for a close encounter that will merge with our own in billions of years time?

    That fuzzy spec that looks like a slightly blurred star is a cluster of hundreds of thousands of the most ancient metal-poor stars orbiting our galaxy and maybe with an intermediate black hole at its core?

    Thatstar at the heart of all that nebulosity is a hypergiant five million time s as bright as our Sun which may explode one night soon, maybe tonight, maybe in thousand years time or more and shine even brighter than than it did when it breifly became the second brightest in our skies a few hundred years ago?

    A little knowledge & the perspective it brings can create a lot of superluminous awe. 8)

  4. Zerodash

    Comet Garradd…at Tanagra.

  5. Chris A.

    I was under the impression that both tails (gas and dust) get swept away from the sun, but to differing degrees (the gas tail, mostly by the solar wind; and the dust tail, mostly by photon pressure). The fact that dust particles are a lot heavier than gas molecules, and photon pressure is pretty wimpy, is why the gas tail tends to be straight (the force pushing it away from the nucleus overwhelms the sun’s gravity) and dust tails tend to be curved (the dust particles follow slightly altered solar orbits, with gravity still playing a large role in their trajectories).

  6. Jan from Denmark

    A really cool example of : “It’s a matter of perspective”. It took me about 2 seconds from seeing the image till I had the “3D image”.

    Very good shot.

  7. When you look at better images of comet Garradd, e.g. those by Ronaldo Ligustri to which I’m often linking in my blog, you see that the dust tail is actually a wide fan as many finer particles *are* blown away by solar radiation pressure – the graphic here is misleading. That – regular – dust tail can be seen, in the better images, lying underneath the plasma tail. So technically Garradd has three tails: a gas tail, a dust tail and what’s called an anti-tail.

  8. Diederick

    “Aha! Oh, I love a chance to lecture a bit. ”

    I know. That’s why I visit this site every day.

  9. For  Zerodash-

    “Comet Garradd
at Tanagra, arms wide”

  10. Bob S.

    “He’s intelligent, but not experienced. His pattern indicates two-dimensional thinking.”

    Sorry, couldn’t resist…

  11. K. Nilsson

 you were referenced in Today’s APOD. Great Job.
    For 28 Feb 2012

    Way to go Phil !!!

    look under the intermediate viewing angle. link

  12. Matt B.

    Neat. I was hoping we’d see a perspective shot like this since the last time BA talked about comets having two tails. And it didn’t take long.


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