The game plan for a first-of-its-kind space mission called Deep Impact unfolds like a cosmic billiards match. Early July 4, the NASA spacecraft will attempt to smack into the heart of a distant comet, Tempel 1. Scientists think the blow could open a crater as large as a football stadium and as deep as a 14-story building. With the observations by a companion spacecraft of the crater and the debris that is tossed out, astronomers expect to learn more about how the solar system formed 4.6 billion years ago. Tempel 1 is one of the millions of comets that swirl through the distant reaches of the solar system. Occasionally visible in the night sky of the Earth, comets are collections of the ancient ice and dust particles that provided brick and mortar for the assembly of the sun and planets, theorists think. What's more, the bombardment of the Earth by comets for hundreds of millions of years after the construction drew to a close may have delivered the abundant water that filled the oceans as well as the chemical precursors for life, some say. If the $333 million mission is successful, the collision will allow scientists to observe and characterize the chemical makeup of the primordial building blocks that have been preserved inside comets for billions of years.