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Meet Comet Hartley 2!
Columnist: Dr. James Green

Meet Comet Hartley 2!
6 November 2010

On Thursday, our EPOXI spacecraft sent us breathtaking close up images of Comet Hartley 2. The images reveal a little world that's fascinatingly different from any of the other comets we have seen.

My first thought upon seeing the pictures was: What's going on? This comet cannot be characterized like any of the others we have seen up close! We all like to speculate and of course I am no different. While the real story will have to wait until the scientists are done analyzing the data, there are a number of clues we should look at carefully.

Scientists like to catalog things. It helps us make sense of our discoveries and see patterns. But Hartley 2 does not fit, so I thought, "Why not look differently at the five comets we have encountered over the last 25 years? Perhaps when we have flown by comets we have been viewing them at various stages in their evolution and demise?"

Hartley 2 is oddly shaped. It has two rough-hewn, longish lobes connected by a smooth shaft that appears stretched like taffy in a gravitational taffy pull. And we are watching gravity at work when we see this body. Are we observing Hartley 2 in its death throes - as it's being pulled apart?

Less than a month ago two meteorites entered our atmosphere along a trajectory that could be traced back to the comet - perhaps also signaling its death knell. It could be that these meteorites were pieces that broke off Hartley 2. If so, it would be reminiscent of when Jupiter's gravity broke Shoemaker-Levy 9 into a spectacular "string of pearls," which rained down on the giant planet in 1994.

Comets don't just sit there in a round ball a certain size and never change. They can get tugged into strange shapes and broken into chunks that then sublimate away. We're learning all about what happens to them as they go around the Sun in their elongated orbits, eroding and sometimes falling to pieces as the loosely packed "stuff" within them sublimates.

Could it be that we are seeing Hartley 2 in the in-between stage, before it finishes fizzing and crumbling away?

Think of that the next time you see the Perseid meteor shower. Remember that we enjoy this spectacle because we cross the orbit of an old, dissipated comet. The left over bits and pieces of that comet treat us to the periodic sky show every year when, still following the parent comet's orbit, they burn up into Earth's atmosphere.

I think these kinds of evolutionary processes are universal and have broad implications for life in the solar system.

We used to think of Earth as the place with all the solar system's water. But now we know that water is everywhere, including in comets, which also contain volatiles like ammonia, cyanide, and methane. As comets sublimate, they're spraying water and volatiles all over the place - try not to get wet! One colleague, after Phoenix tasted the water on Mars, stated: "well, the solar system is just one big soggy place." The truth of this statement really struck me.

The known elements that constitute matter are spread throughout the solar system. They are being mixed and spread about by comets, meteorites, and other mechanisms. Objects in the solar system have interacted for eons, and continue to interact - collide, accrete, disintegrate and spew their contents onto one another, etc., so this dispersal is fundamental. And it follows that life could also be spread around.

Because of that, I'm tremendously excited in the belief that we'll be able to find life beyond Earth - maybe not complex life, but life. Although like in real estate, it's all about location. You have to have the right materials in just the right spot.

With hold-your-breath flybys like Hartley 2, and with all of our other flybys and missions - to comets, asteroids, moons, and planets - we are taking snapshots in time. Snapshots of the bodies that make up our solar system and of the processes going on within, around, and among them. Snapshots revealing steps in the evolution of the comets, the solar system, and of life itself. It's just that comets, by their nature, are evolving right before our eyes. All we have to do is look.

And when we assemble all of these snapshots into a comprehensive album and consider the big picture, they speak volumes. A picture like this one is truly worth a thousand words.

For more information on the EPOXI mission, see and

See more comments on comets:

Look for the O/OREOS launch on November 19th, from Kodiak, Alaska.

Up next month: AGU, and Science Isn't Done until It's Shared

Read More by Dr. James Green

About: Dr. James Green
Photo of Dr. James Green
Dr. Green is the Director of Planetary Science at NASA Headquarters. He views his job as the keeper of NASA's planetary program and the top advocate for that program to flourish and grow.
Read More by Dr. James Green
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Last Updated: 16 Feb 2011