Dawn, a NASA deep space explorer launched in 2007, was built to learn more about the early solar system by studying Vesta and Ceres (objects in the asteroid belt). Using an ion propulsion system to reach its targets and to maneuver into low-altitude orbits, Dawn will be the first spacecraft to orbit two objects in the solar system. It successfully entered orbit around Vesta in 2011 and is scheduled to reach Ceres in 2015.

Ion engines work by accelerating ions to produce thrust. Following Newton's third law of motion, which states that for every action there is an equal and opposite reaction, thrust is the reaction force caused by accelerating mass in any one given direction. The accelerating ions moving toward the rear of the engine cause the spacecraft to move forward with an equal and opposite amount of force.

Ion engines cannot generate enough thrust to launch a rocket on Earth. However, in the low-friction environment of space, and given a long period of time to accelerate, the small thrust from an ion engine is very effective. Dawn's engines produce a gentle thrust of 90 mN (about the same force as a piece of paper pushing against your hand).

While Dawn accelerates slowly (it would take four days to go from 0 mph to 60 mph), given five years of thrusting, its speed could change by over 20,000 mph. Furthermore, ion engines are very efficient; they don't require much fuel, which can help keep down the mass and cost of a spacecraft.

Dawn uses xenon atoms as fuel; this noble gas was chosen because of its stability and relatively low ionization energy (the energy required to remove an electron). In the ionization chamber, electrons knock electrons free from xenon atoms to produce positively charged xenon ions. Two electrically charged grids at the rear of the engine accelerate the xenon ions. The first grid is more negatively charged than the walls of the engine but is more positively charged than the second grid. The first grid attracts the xenon ions, helping line them up so that they can pass through the grid; once the ions are near the first grid, the potential difference between the two grids accelerates them to very high speeds. This stream of ions is also known as a thrust beam.

This interactive activity features a model of an ion engine. Although the model does not portray all aspects of a real engine accurately—for example, the relative number and sizes of xenon ions is not accurate—it can be useful to help understand certain concepts. In particular, it illustrates how engine design relies on understanding the attractive and repulsive forces of electric charges: the space between the grids cannot be too large—too many ions in the space at the same time would create a shielding effect and the positive ions may stray as they repel each other. The model also shows how the time spent thrusting is as important to take into account as the strength of the thrust. Total impulse (measured in newton-seconds, Ns) is the product of the thrust and the time spent thrusting; because the thrust may change over time, total impulse can be found by multiplying time by the average thrust.

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