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Stardust-NExT: A Comet "Before and After"
Columnist: Dr. James Green

Stardust-NExT: A Comet "Before and After"
4 February 2011

Our planetary science missions continually surprise us by overturning what we think is true. We expect no less of the upcoming Stardust-NExT visit to comet Tempel 1.

With this mission, we'll be revisiting a comet for the first time. Why return to the same comet? After all, there have only been six flybys -- all to different comets -- and there are thousands of comets out there. And every flyby gives us an enormous amount of insight and information. It's never a case of "you've seen one you've seen 'em all" in this business. In the past we've seen only the "skin deep" beauty of comets since a flyby is in some ways just a tantalizing glimpse of what a comet is made of -- a look at its shape, form, and surface. With a one-time flyby, we see a comet at only one stage in its evolution.

Stardust-NExT is a mission that was extended, or reused, since the original mission, Stardust, had extra fuel left and was healthy. So we re-vectored it to another destination -- Comet Tempel 1. Stardust-NExT is a tremendously exciting mission because we will "look under the hood," opening our eyes to new depths of Tempel 1's beauty. Stardust-NExT gives us, for the first time, a unique opportunity to see the same comet at two different points in its life cycle.

Tempel 1's "before" occurred during the Deep Impact mission in 2005 when NASA placed a part of the spacecraft on a collision course with the comet -- and it worked. The Stardust-NExT "after" is all about going back and looking at Tempel 1 six years later, after its orbit around the sun passed perihelion -- or closest approach to the sun. So Stardust-NExT's encounter on Valentines Day will show us what happened to this comet since our first visit in 2005. We'll be looking for changes in a least three areas.

First, by comparing Deep Impact's Tempel 1 photos to new ones from Stardust-NExT, we'll learn something new about how solar heat vaporizes a comet, and how the coma -- or atmosphere -- and tail form.

The recent EPOXI mission raised new questions about these processes by revealing an intriguing new phenomenon on a different comet, Hartley 2. Flying away from the comet was a cloud of snowballs from tennis ball size to basketball size! This "blizzard" was a complete surprise.

Thanks to EPOXI, we now know that the sun's heat does more than sublimate material on a comet's surface. It can also cause gases to pop from deep within the core. The current theory is that on Hartley 2, carbon dioxide gas is formed in its interior and as it forces it way up to the surface creates the snow spectacle.

Is that a common occurrence on comets, or something that happens only in a comet's death throes? Or is it unique only to certain comets because of their composition? Stardust-NExT could help us answer these new questions.

Secondly, we'd also like a look at the crater Deep Impact's projectile excavated in Tempel 1's surface in 2005. The huge dust cloud from the impact and the position of the spacecraft after impact prevented Deep Impact's cameras from imaging inside the crater. If the crater is viewable during Stardust-NExT's flyby, it would be an incredible bonus. Here's why.

We've talked for a long time about landing on a comet, scooping up material from its surface, putting it in a special container, and bringing it home. That sample will have to stay above freezing -- but "freezing" would be a different temperature for each component in the sample. To select the right tools and containers for collecting, storing, and transporting the materials, we must find out what sorts of materials we'd be storing. And to land at all we have to characterize the surface we'd be landing on.

Comets are often described as "dirty snowballs", but our previous flybys show that they have dry, dusty or rocky surfaces. This suggests that the ices are hidden beneath a comet's crust. Viewing the crater would help us understand more about what a comet's surface is like. Is the hole the size of a car, a football field? How deep is it? Answers to these kinds of questions would indicate whether Tempel 1's comet's surface is hard or soft. And that's not all we would learn. The impact exposed the fresh undersurface of a comet, so there's no telling what we might see.

We aren't certain, however, that we'll see the crater side of Tempel 1. Like everything in the solar system, comets also spin, so it's hard to predict the side we'll see during a flyby when we flyby the comet at nearly 11 km/s and within 200 km at closest approach! We believe we understand comet rotation and how that rate changes with time well enough to know what to expect with Temple 1. It would be easy, however, to be wrong, and we could miss the crater side. If we're lucky, we'll see the impact region.

Finally, we're not going back to Tempel 1 just to try to see the crater. No matter which face Tempel 1 is showing during the flyby, we can view some part of the comet we've seen before and study the changes. We'll be looking at the history of a comet, discovering what happens to comets when they go by the sun -- how much material they lose, and where.

I'm delighted we're able to reuse our spacecraft in this way. It demonstrates what can be done through good engineering and proper planning with the setting of adequate margins for the initial mission. We can breathe new life into our planetary spacecraft and get a wealth of new data for new discoveries.

That's what happens when we go with different instruments or approaches to a place we've seen before. The place becomes a fabulous new world all over again allowing us to make new discoveries. Prepare to be surprised!

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About: Dr. James Green
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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.
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Last Updated: 19 Dec 2011