Humans have always looked to space and asked questions like, "How big is the universe?", "How old is the universe?", "Is there life anywhere else in the universe?", and "How did our solar system form?". The job of the National Aeronautics and Space Administration (NASA) is to seek answers to these questions. One of NASA's missions is to advance and communicate scientific knowledge and understanding of Earth, the solar system, and the universe, and to use the environment of space for their research. For instance, NASA's Rover vehicle missions have given scientists a wealth of information regarding the probability of water and life on Mars. By studying Mars, scientists get clues about Earth and possible futures for the planet Earth. Dawn spacecraft
Figure 1. Testing an ion engine. (Courtesy of NASA, Glenn Research Center, 2008.)

On September 27, 2007 NASA launched mission Dawn. The purpose of Dawn is to study the asteroid Vesta and the dwarf planet Ceres. These bodies are believed to have each formed at the beginning of the solar system. By studying these bodies, scientists hope to learn more about the early solar system and how the solar system was formed. Dawn is expected to reach Vesta in August 2011 and reach Ceres in February 2015. The end of the primary mission is slated to be July 2015. This mission will last a little over 8 years!

One of the components of the spacecraft that is making this mission possible is the ion engine. The ion engine propulsion system is replacing the standard chemical propulsion system that has been used in past missions. Standard chemical propulsion systems use liquid oxygen and liquid hydrazine (a highly toxic, dangerously unstable, colorless liquid that is used in different applications, including rocket fuel) as fuel. The combination of hydrazine and oxygen is explosive and provides the spacecraft with the thrust it needs to move forward. An ion engine, however, uses electric fields instead of chemical reactions to move the spacecraft. See Figure 2, below. The ion engine propulsion system is made up of a discharge chamber and two plates, which are located at one end of the discharge chamber. In an ion engine, the gas xenon is ionized, or given an electric charge. The xenon gas enters the discharge chamber, where it is ionized by high-energy electron bombardment. A high-energy electron collides with a xenon atom and knocks off an electron from the xenon atom. This process yields two electrons (including the original electron) and one