Comets are time capsules that hold clues about the formation and evolution of the solar system.
They are composed of ice and dust, which is the primitive debris from the solar system's earliest and coldest formation period - 4.5 billion years ago. Deep Impact, a NASA Discovery Mission, will be the first space mission to probe beneath the surface of a comet and reveal the secrets of its interior. Ice and dust debris will be ejected from the crater, revealing the fresh material beneath. Sunlight reflecting off the ejected material will provide a dramatic brightening that will fade slowly as the debris dissipates into space and falls back onto the comet. Cameras on the spacecraft will send images back to Earth of the approach, the impact and its aftermath.