Prior to the Deep Impact experiment scientists had numerous questions and untested ideas about the structure and composition of the nucleus, or solid body of a comet, but we had almost no real knowledge.  The analysis of data produced by Deep Impact is revealing a great deal, much of it rather surprising.  For example, comet Tempel 1 has a very fluffy structure that is weaker than a bank of powdered snow.  The fine dust of the comet is held together by gravity.  However, that gravity is so weak that if you could stand on the bank and jump, you would launch yourself into space.  Another surprise is the evidence of what appears to be impact craters on the surface of the comet.  Previously, two other comets had their nuclei closely observed and neither showed evidence of impact craters.

The nucleus of Tempel 1 has distinct layers shown in topographic relief ranging from very smooth areas to areas with features that satisfy all the criteria for impact craters, including varying size.  The problem in stating with certainty that these are impact craters is that we don't know of a mechanism by which some comets would collide with the flotsam and jetsam in our solar system, while others would not, so this is an intriguing puzzle.

One of the more interesting findings may be the huge increase in carbon-containing molecules detected in spectral analysis of the ejection plume.  This finding indicates comets contain a substantial amount of organic material, so they could have brought such material to Earth early in the our planet's history when strikes by asteroids and meteors were common.  Another finding is that the comet interior is well shielded from the solar heating experienced by the surface of the comet nucleus. Mission data indicate the nucleus of Tempel 1 is extremely porous.  This porosity allows the surface of the nucleus to heat up and cool down almost instantly in response to sunlight, which suggests that heat is not easily conducted to the interior and the ice and other material deep inside the nucleus may be pristine and unchanged from the early days of the solar system, just as many scientists had suggested.  Researchers saw emission bands for water vaporised by the heat of the impact, followed a few seconds later by absorption bands from ice particles ejected from below the surface and not melted or vaporized.  In a couple of seconds the fast, hot moving plume containing water vapour left the view of the spectrometer, and scientists suddenly saw the excavation of sub-surface ice and dust.  It has been classified as the most dramatic spectral change some of the scientists had ever seen.

NASA Comment - 8th September

When Deep Impact smashed into comet Tempel 1 on July 4, 2005 , it released the ingredients of our solar system's primordial "soup." Now, astronomers using data from NASA's Spitzer Space Telescope and Deep Impact have analyzed that soup and begun to come up with a recipe for what makes planets, comets and other bodies in our solar system.  By observing the cloud of ejected material Spizer has spotted the signatures of a handful of ingredients, essentially the meat of comet soup.  These solid ingredients include many standard comet components, such as silicates, or sand, and like any good recipe, there are also surprise ingredients, such as clay and chemicals in seashells called carbonates.  These compounds were unexpected because they are thought to require liquid water to form.

Astronomers do not know how clay and carbonates can form in frozen comets, but their presence may imply that the primordial solar system was thoroughly mixed together, allowing material formed near the Sun where water is liquid, and frozen material from out by Uranus and Neptune, to be included in the same body.  Also found were chemicals never seen before in comets, such as iron-bearing compounds and aromatic hydrocarbons, found in barbecue pits and automobile exhaust on Earth.  The silicates spotted by Spitzer are crystallized grains even smaller than sand, like crushed gems. One of these silicates is a mineral called olivine, found on the glimmering shores of Hawaii 's Green Sands Beach .

Planets, comets and asteroids were all born out of a thick soup of chemicals that surrounded our young Sun about 4.5 billion years ago. Because comets formed in the outer, chilly regions of our solar system, some of this early planetary material is still frozen inside them.  Having this new grocery list of comet ingredients means theoreticians can begin testing their models of planet formation. By plugging the chemicals into their formulae, they can assess what kinds of planets come out the other end.

Bulls Eye!  The impact occurred exactly as predicted at 06:52 am UK time on 4th July.

Data from Deep Impact's instruments indicate an immense cloud of fine powdery material was released when the probe slammed into the nucleus of comet Tempel 1 at about 10 kilometers per second (6.3 miles per second or 23,000 miles per hour).  The cloud indicated the comet is covered in the powdery stuff. The Deep Impact science team continues to wade through gigabytes of data collected during the July 4 encounter with the comet measuring 5-kilometers-wide by 11-kilometers-long  (about 3-miles-wide by 7-miles-long).  One major surprise was the opacity of the plume the impactor created and the light it gave off, according to NASA.  This suggests the dust excavated from the comet's surface was extremely fine, more like talcum powder than beach sand. And the surface definitely seems not to be what most people think of when they think of comets -- an ice cube.

NASA scientists are now looking at everything from the last moments of the impactor to the final look-back images taken hours later, and everything in between.  Watching the last moments of the impactor's life shows fine surface detail, and 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.

The final moments of the impactor's life were important, because they set the stage for all subsequent scientific findings. Knowing the location and angle the impactor slammed into the comet's surface is the best place to start. Engineers have established the impactor took two not unexpected coma particle hits prior to impact. The impacts slewed the spacecraft's camera for a few moments before the attitude control system could get it back on track. The penetrator hit at an approximately 25 degree oblique angle relative to the comet's surface. That's when the fireworks began.

The fireball of vaporized impactor and comet material shot skyward. It expanded rapidly above the impact site at approximately 5 kilometers per second (3.1 miles per second). The crater was just beginning to form. Scientists are still analyzing the data to determine the exact size of the crater. Scientists say the crater was at the large end of original expectations, which was from 50 to 250 meters (165 to 820 feet) wide.

Expectations for Deep Impact's flyby spacecraft were exceeded during its close brush with the comet. The craft is now millions of kilometers from Tempel 1 and opening the distance at approximately 37,000 kilometers per hour (23,000 miles per hour). The flyby spacecraft is undergoing a thorough checkout, and all systems appear to be in excellent operating condition.

The Deep Impact mission was implemented to provide a glimpse beneath the surface of a comet, where material from the solar system's formation remains relatively unchanged. Mission scientists hoped the project would answer basic questions about the formation of the solar system by providing an in-depth picture of the nature and composition of comets.

The image below shows the comet's nucleus immediately after the probe impacted.