When Deep Impact crashed into the nucleus of Tempel 1 at 23,000 miles per hour on July 4, it sent a huge, bright cloud of stuff upward and outward from the comet, providing a spectacular image that is already assured a place in the space history books, and may well be seared into the brains of all those who watched the event.
Deep Impact "lookback" image of Tempel 1
This "lookback" image was captured by Deep Impact's high-resolution imager as it receded from comet Tempel 1 early today. The bright spot is not an incandescent flare; it represents dust in the ejecta curtain spraying out from the comet, which is backlit by the Sun.
That cloud, as it turns out, was not composed of water, ice, and dirt as one might presume given that comets have long been called 'dirty snowballs' or 'icy dirtballs.'* Instead, Deep Impact's instruments indicate that this huge cloud was made up of very fine, powdery material. And apparently, Tempel 1 -- which scientists believe to be a frozen chunk of rock and ice about half the size of Manhattan -- is covered in the stuff.
"The major surprise was the opacity of the plume the impactor created and the light it gave off," reported Deep Impact Principal Investigator Michael A'Hearn of the University of Maryland.
"That suggests the dust excavated from the comet's surface was extremely fine, more like talcum powder than beach sand," he explained. "And the surface is definitely not what most people think of when they think of comets -- an ice cube."
The team's early findings have been corroborated by Smithsonian astronomers who monitored the impact using the ground based Submillimeter Array (SMA) in Hawaii, and NASA's orbiting Submillimeter Wave Astronomy Satellite (SWAS). The Smithsonian scientists are reporting only "weak emission from water vapor and a host of other gases" that were expected to erupt from the impact site.
Although the team had hoped Deep Impact would penetrate deep enough to reveal the interior of the comet where pristine materials from the solar system's formation hide, it appears now that it may have barely scratched the surface.
"It's pretty clear that this event did not produce a gusher," said SWAS principal investigator Gary Melnick, of the Harvard-Smithsonian Center for Astrophysics (CfA). "The more optimistic predictions for water output from the impact haven't materialized, at least not yet." Results, however, are still coming in. Moreover, it is still possible the comet might become more active in coming days and weeks, allowing for hope that distinct new emissions or outgassing may emanate from Tempel 1's new human-made crater.
Despite the conspicuous absence of water, there is still much to be learned from the 'spot on' Deep Impact, and the data it has returned is already deepening to cometary knowledge. Short-period comets like the potato-shaped Tempel 1 have been baked repeatedly by the Sun during their passages through the inner solar system, and the effects of that heat have been estimated to extend more than three feet beneath the surface of the nucleus, according to CfA astronomer Charlie Qi.
Deep Impact indicates that these effects could be much deeper, meaning that "[t]heories about the volatile layers below the surface of short-period comets are going to have to be revised," Qi said. Or, in other words, comets may have much thicker 'skins' that we ever imagined.
If you're wondering how a comet hurtling through our solar system could possibly maintain a deep surface made up of a substance with less strength than snow or even talcum powder, consider the environment. "This city-sized object is floating around in a vacuum," Pete Schultz, Deep Impact scientist from Brown University, pointed out. "The only time it gets bothered is when the Sun cooks it a little or someone slams an 820-pound wakeup call at it at 23,000 miles per hour."
The Deep Impact science team, and all the other ground-based observation teams, will be spending the ensuing weeks and months examining and analyzing the gigabytes of data collected during the Independence Day encounter with Tempel 1.
The Deep Impact science team will be examining every single one of the approximately 4,500 images from the spacecraft's three imaging cameras taken during the encounter. "We are looking at everything from the last moments of the impactor to the final look-back images taken hours later, and everything in between," confirmed A'Hearn. "Watching the last moments of the impactor's life is remarkable. We can pick up such fine surface detail that objects that are only four meters in diameter can be made out. That is nearly a factor of 10 better than any previous comet mission."
Since the final moments of the impactor's life set the stage for all subsequent scientific findings, they are important to know. This is what they know so far. The impactor took two "not unexpected" coma particle hits prior to impact, which slewed the spacecraft's camera for a few moments before the attitude control system could get it back on track. It hit at an approximately 25 degree oblique angle relative to the comet's surface. That's when the fireworks began, with the fireball of vaporized impactor and comet material blasting outward from the comet, and spreading out above the impact site at approximately 3.1 miles per second [5 kilometers per second]. At this point, the crater was beginning to form.
Team members don't know the exact size of the crater yet, but they do say that it was at the large end of their original expectations, and so probably is about the size of a football stadium. Deep Impact's flyby spacecraft is now more than 2.2 million miles [3.5 million kilometers] from Tempel 1, and opening the distance at approximately 23,000 miles per hour [37,000 kilometers per hour]. The operations team is still in contact and the flyby spacecraft is undergoing a thorough checkout. So far, they report, all systems appear to be in excellent operating condition.
The flyby spacecraft will continue to transmit data and images through August, at which point the mission will end. The project -- and analysis of all the collected Deep Impact data -- will continue through March 2006.
* More than 50 years ago, Harvard astronomer Fred Whipple developed a model of comet nuclei as "dirty snowballs," hypothesizing that comets consist of mostly ice, with some dirt and rock mixed in. Modern astronomers often refer to comets as "icy dirtballs,", reflecting the current prevailing notion that comets contain more dust and less ice than previously believed.
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