July 5, 2005

Crash Course in Comet Construction

Comet Tempel 1 was hit hard by a NASA spacecraft earlier this week. Deep Impact scientists provided more details about what the impact has taught us about the comet's structure.

Astrobiology Magazine -- Debris is still streaming out of Tempel 1, and the flyby spacecraft is still snapping pictures, but mission scientists paused in their data collection and analysis to share more details and images of the comet. Deep Impact co-investigator Pete Schultz says that Tempel 1 most likely has a soft outer layer that covers harder ice, based on what they saw when the impactor spacecraft collided with the comet's nucleus.

"First you see a small flash, and then there's a delay. Then there's a big flash, and then the whole thing breaks loose," Schultz notes. This explosion pattern does not precisely match any of the simulations that attempted to model how the comet would react when hit with the spacecraft. Schultz says they will use the data collected today to figure out how the structure of the comet led to the impact pattern they witnessed.

The ejecta - mostly composed of dust and some gas - exploded up and out of the nucleus at the moment of impact. It now extends at least several thousand kilometers out into space. Ejecta material could continue to stream out of the nucleus for days or even weeks. Some of the ejecta material became incandescent due to the heat of impact.

"At the moment of impact, we heat materials to extremely high temperatures," Schultz explains. "Some of that is heated vapor, some of it is hot melt droplets from dust from within the crater itself. It's kind of like firing your flashgun: it's material that is glowing so brightly, that, without a light, you can actually take your own picture. And it tells us a lot about the cratering process."

The crater has not been imaged yet due to the large amount of dust and vaporized materials surrounding the point of impact. Deep Impact principal investigator Mike A'Hearn says that to see the crater, they will need to learn how to subtract that dust from the images. The infrared spectrometer on the flyby spacecraft could help image the crater, picking out temperature differences in the crater versus the rest of the surface. At the moment, the general belief among the scientists is that the crater is quite big, larger than a house at least.

The latest images show that the overall shape of the nucleus is different than had been previously described. One end is rounded while the other is more concave.

"(The nucleus) does not look like a pickle or a cucumber or the various things we talked about before," says A'Hearn. "It looks closer to a loaf of bread or to a muffin."

A'Hearn says that Tempel 1 looks different from comets Wild 2 and Borrelly, and they're scratching their heads, trying to figure out why.

"When Wild 2 looked different from comet Borrelly, we thought it might be due to the different orbital histories of the two comets," says A'Hearn. "But this comet's had an orbital history that we think is pretty much similar to Borrelly, and yet it looks totally different. So there's something more going on here that we haven't understood yet."

There is an unusual flat region on comet Tempel 1 that curves around one side of the nucleus. There are also what appear to be old impact craters, layering of surface materials, and jets of gas streaming outwards. These gas jets are reminiscent of what the Stardust mission saw with comet Wild 2.

Earth observatories saw Tempel 1 brighten by a factor of 5 at the moment of impact. The brightness dimmed over the next few hours but has not yet returned to pre-impact levels. The Earth observatories also witnessed changes in the comet's thermal and gas emissions.

During its final two hours, the impactor spacecraft made three self-adjustments to its position to make sure it was in the path of the comet. The impactor spacecraft's camera took pictures up to 3 seconds before the moment of collision. Tempel 1 is orbiting the sun at 37,100 kilometers (23,000 miles) per hour - 10 times faster than a bullet - so the impactor spacecraft was completely vaporized when it hit.

The flyby mothership took pictures before, during and after impact, scooting underneath the comet and then turning back around to watch the fireworks. Once the spacecraft finishes taking pictures of the nucleus and sending data back to Earth, it will be put into "mothball mode" - only running systems vital to the spacecraft's health. Rick Grammier, Deep Impact project manager at NASA's Jet Propulsion Laboratory, estimates the spacecraft still has 160 kilograms of usable fuel left.

It is up to NASA to decide if it wants to fund the mission further, sending the spacecraft off to take photos of other objects in the solar system. The Deep Impact mission did go over its projected costs - the $333 million-dollar project was initially supposed to cost $279 million. The cost cap was set at $350 million, and A'Hearn says at one point the mission was close to being canceled. Part of the cost overrun was due to changes that needed to be made after the Genesis mission's mishap.

The Deep Impact mission blasted a crater in Tempel 1 in order to allow scientists to see the interior of a comet for the first time. Comets formed 4.5 billion years ago, around the same time as the planets, so learning about how comets are constructed could provide clues about solar system formation.

This information could also be used to divert any comets that might threaten Earth some day. Large comet and asteroid impacts could lead to major extinction events on our planet.

"The knowledge that comes out of this - after we finish analyzing the data - is important to understanding how to deflect a comet," says A'Hearn. "Any serious attempt to deflect a comet with an impact, however, requires a much larger impact than we delivered. The orbital change from our impact is predicted to be so small that it is unmeasurable. But we can measure things like the amount of material that comes out compared to the amount of material that we put in, and that gives us some idea of the efficiency of changing the momentum of the comet."


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