On July 4th 2005 the first attempt to study directly the interior of a comet will take place. The Deep Impact spacecraft will release a 370 kg projectile to impact the surface of Comet Tempel 1 at 10 kilometres per second, creating a crater that will expose fresh material from the interior, and allow study of the crater formation process and the composition of the interior.
Deep Impact, a NASA Discovery mission, conceived by a team of scientists at the University of Maryland, built by Ball Aerospace and controlled from the Jet Propulsion Laboratory, was launched on 12 January. The flyby spacecraft carries scientific instruments to study the comet and the effects of the impact as well as bringing the impactor to the proximity of the comet. The impactor is a battery-powered spacecraft that operates independently of the flyby spacecraft for just one day. It is called a "smart" impactor because, after its release, it takes over its own navigation into the path of the comet. It carries a camera to relay close up images of the comet to the flyby spacecraft before impact.
After release of the impactor, the flyby spacecraft will adjust its course to pass around 500 km from the comet (in fact, the spacecraft is being overtaken by the comet in its orbit about the Sun). It will use a camera and infrared spectrometer to observe and record the impact, ejected material blasted from the crater, and the structure and composition of the crater's interior, for 14 minutes before the comet nucleus passes by. After its shields protect it from the comet's dust tail passing overhead, it will turn to observe the comet from the other side. The encounter takes place at a time when the comet can be observed from the Earth, even with a small telescope, so that professional and amateur astronomers can study any large scale effects of the impact.
Although it is hoped that the crater will be sufficiently large that it will allow study of the comet’s sub-surface material that is unaffected by the repeated effects of solar heating during each passage of the comet, the force of the impact is not sufficient to make any appreciable change in the comet's orbital path around the Sun. Our knowledge of the structure of cometary surfaces is still not sufficient to be certain how the comet will react to the impact. The way in which the crater grows and its final shape will provide important clues for the nature of the upper surface of the comet. Laboratory experiments at hypervelocities reveal that the ejection angle depends on the porosity of the target: Highly porous materials result in higher angle trajectories (relative to the surface).
The images and infrared spectra from the orbiter are expected to provide information on how the crater forms, its depth and diameter, the composition of the interior of the crater and its ejecta and determine the changes in natural out-gassing produced by the impact. Sunlight reflecting off the ejected material provides a dramatic brightening that fades slowly as the debris dissipates into space or falls back onto the comet. As ejected icy material is vaporised by the Sun’s heat, its composition can be studied by using the spectrometer by characteristic spectral lines produced by different elements or molecules. The impactor spacecraft is composed mainly of copper, which is not expected to appear in data from a comet's composition.