Tag: dust tail

The two tails of Comet Garradd

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

César Cantú 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 César 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: César Cantú.

Related posts:

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

CATEGORIZED UNDER: Astronomy, Pretty pictures

A celestial visitor, seen from space

By Phil Plait | December 22, 2011 11:26 am

I know I post a lot of pictures I describe as amazing, lovely, breath-taking, jaw-dropping… but that’s only because it’s always true. In this case, though, I think those adjectives fall way, way short in describing the seriously paralyzing beauty of this photograph: Comet Lovejoy, as seen by an astronaut on board the International Space Station:

[Click to encomanate — and yes. you need to.]

Oh. My.

This stunning photo was taken by astronaut Dan Burbank as the ISS passed over Australia at 17:40 GMT on December 21, 2011 [update: more pix here]. It was early morning over Australia at the time, and you can see the dark limb of the Earth, the thin green line of airglow (atoms in the upper atmosphere slowly releasing the energy they accumulated over the day), some southern hemisphere stars… and of course, the incredible, ethereal, other-worldly beauty of Comet Lovejoy, its tails sweeping majestically into the sky.

Wait, what? "Tails", plural? Yup. Hang on a sec. I’ll get to that.

First, the comet was discovered by amateur astronomer Terry Lovejoy in November. It turned out to be a sungrazer, a comet whose orbit plunges it deep into the inner solar system and very close to the Sun’s surface. It screamed past our star last week, on December 15/16, and, amazingly, survived the encounter. Some sungrazers do and some don’t, but Lovejoy is bigger than usual for such a comet, and that may have helped it remain intact as it passed less than 200,000 km over the Sun’s inferno-like surface.

Now the comet is moving back out, away from the Sun and back to the frozen depths of deep space. But the Sun’s heat, even from its greater distance now, is not to be denied. Comets are composed of rock and ice — the ice being what we normally think of as liquid or gas, like ammonia, carbon dioxide, and even good ol’ water. The heat from the Sun turns that ice directly into a gas (in a process called sublimation), which expands around the solid nucleus of the comet, forming what’s called the coma. Pressure from sunlight as well as the solar wind blows this material away from the comet head, resulting in the lovely tail, which can sweep back for millions of kilometers.
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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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