The primary goal of the Deep Impact mission is to explore the interior of Comet Tempel 1 by using an impactor to excavate a crater in the comet's surface, after which the flyby spacecraft will take data on the newly exposed cometary interior. Scientists believe in-depth analysis of this new view of Tempel 1 will reveal a great deal not only about this comet but also the role of comets in the earliest history of the solar system.

In particular, the mission's scientific objectives are to:

• Dramatically improve the knowledge of key properties of a cometary nucleus and, for the first time, directly assess the interior of a cometary nucleus by means of a massive impactor hitting the surface of the nucleus at high velocity.
  
  
• Determine properties of the comet's surface layers such as density, porosity, strength and composition.
  
  
• Study the relationship between the surface layers of the comet's nucleus and the possibly pristine materials of the interior by comparing the interior of the crater with the pre-impact surface.
  
  
• Improve our understanding of the evolution of cometary nuclei, particularly their approach to dormancy, by comparing the interior and surface.

The main scientific investigation is to understand the differences between the interior of a cometary nucleus and its surface. Some of the questions that will be addressed are:

• If the crater depth reaches 20 meters (about 60 feet), does the material sud denly become carbon monoxide or carbon dioxide ice?
  
  
• Or, is the ice still predominantly water (H2O)? If water ice, is its structure crystalline or amorphous?
  
  
• Is the mantle devoid of volatile materials to depths of centimeters, or meters, or tens of meters?
  
  
• Is the comet's structure homogeneous from side to side on various scales?
  
  
• How does the ratio of ice to refractory (non-melting) material change?
  
  
• How old is the surface?
  
  
• Does the mantle seal off vaporization from certain areas? Or are certain areas just more devoid of volatile materials than others?
  
  
• Where will future missions have to go to really sample primordial material?

Though they know that the collision event will create a roughly circular crater on the comet nucleus' surface, scientists do not know what size and type of crater will form. There are three likely scenarios that the crater formation can take.

• In the first scenario, the crater formation is governed mostly by the gravity of the cometary nucleus (known as a "gravity-dominated" process). In this case, the cone of ejected material spreads outwards at an angle of around 45 to 50 degrees from the surface of the comet. The cone's base remains attached to the cometary nucleus. The majority (roughly 75 percent) of the material will fall back down onto the surface of the comet, forming a large-diameter ejecta blanket. In this model, the crater may be as large as a football stadium (around 200 meters or roughly 650 feet in diameter), and 30 to 50 meters (about 100 to 150 feet) deep.
  
  
• The second possibility is that the more dominant resisting force of the crater formation is the strength of the material (known as a "strength-dominated" process). In this case, the ejecta cone will be at a higher angle (around 60 degrees). The cone's base will detach from the crater, and may detach from the comet entirely. Less material (around 50 percent) will fall back to the surface of the comet in this scenario, leaving a smaller ejecta blanket. In this model, the crater will be much smaller, on the order of 10 meters (roughly 30 feet) or less. Predictions of the volume of ejecta produced differ by a factor of a thousand.
  
  
• A third possibility is that the cometary material is so porous that most of the impactor's energy and momentum are absorbed in the process of compression and heating (known as a "compression-dominated" process). Since so much energy is used in compression, there is less available for excavation, and the result is a much smaller diameter crater than expected. In this scenario, the crater will be deep, but produce a very small ejecta cone.

If, however, the crater turns out to be strength-dominated, then this suggests that the material of the nucleus is processed somehow, resulting in a comet that can hold together better under impact. This would mean that it is not the pristine, untouched material of accretion. It's also possible that the initial crater formation will be strengthdominated, suggesting a processed outer shell to the nucleus, but that the bulk of the crater is gravity-dominated, suggesting that the impactor has punched through this outer shell into the pristine material below.

Scientists also hope that observing the radius of the ejecta plume and the speed with which the plume changes over time will give them a better estimate of the nucleus material's density. Since the comet's volume will be known, as estimate of density allows for an estimate of the comet's mass.

Others in the audience
Along with the Deep Impact Flyby spacecraft, there will be numerous other "sets of eyes" watching the events unfold around comet Tempel 1. Assisting the Deep Impact team in their celestial pursuit of comet Tempel 1 are several teams of Earth-based astronomers. The Deep Impact team will use these ground-based observations to complement the data taken by the spacecraft.

In addition to large observatories such as the Hubble Space Telescope, the Spitzer Space Telescope and large instruments on Mauna Kea in Hawaii, the collision with the comet will be witnessed by a wide network of astronomers, both professional and amateur. The Deep Impact project has organized a small telescope science program, calling on technically proficient amateurs to fill in gaps of observations by large observatories. These observers are able to look at the comet on a repeated basis over a long period of time from many locations around the world, which helps to refine knowledge of how the cometary nucleus rotates. The first observing campaign ran from February 2000 through March 2001, after which the comet became too faint to observe. The program relaunched in October 2004.

Watching for the comet
Even those not participating in the formal scientific program may be able to get a look at comet Tempel 1 as it brightens in early 2005 and swings inward toward the Sun, if they have access to a small telescope or large set of binoculars. Early in the year the comet will be very dim, but it will begin to brighten after early April as it continues to approach the Sun and Earth. From that point forward until the collision event, it will appear in the evening sky in the constellation of Virgo.

If it weren't for the Deep Impact mission, the comet would only reach a magnitude of about 9.5. The limit of the unaided human eye is about magnitude 6 (larger numbers mean dimmer objects), so some form of telescope or powerful binoculars would be necessary. But the impact could make the comet 15 to 40 times brighter than normal -- perhaps as bright as 6th magnitude, around the limit of the human unaided eye. The comet's position and orbit are listed on NASA's Near-Earth Object website at neo.jpl.nasa.gov.