This injection method is preferred because it increases the time that the propellant remains in the chamber. The propellant is injected into the ionization chamber from the downstream end of the thruster and flows toward the upstream end. NASA's conventional method of producing ions is called electron bombardment. In ion thrusters, plasma is made up of positive ions and an equal amount of electrons. Plasma is the building block for all types of electric propulsion, where electric and/or magnetic fields are used to accelerate the electrically charged ions and electrons to provide thrust. It has some of the properties of a gas but is affected by electric and magnetic fields and is a good conductor of electricity. Plasma exists everywhere in nature (for example, lightning and fluorescent light bulbs), and it is designated as the fourth state of matter (the others are solid, liquid and gas). Plasma is an electrically neutral gas in which all positive and negative charges-from neutral atoms, negatively charged electrons and positively charged ions-add up to zero.
A gas is considered to be ionized when some or all the atoms or molecules contained in it are converted into ions. With ion propulsion, the ions have lost electrons, so they are positively charged. An ion is simply an atom or molecule that has an electrical charge because it has lost (positive ion) or gained (negative ion) an electron. Ion thrusters eject ions instead of combustion gases to create thrust: the force applied to the spacecraft that makes it move forward. For the Deep Space 1 probe, ions were shot out at 146,000 kilometers per hour (more than 88,000 mph). Only relatively small amounts of ions are ejected, but they are traveling at very high speeds. Other inert gases, such as krypton and argon, also can be used. The majority of thrusters use xenon, which is chemically inert, colorless, odorless, and tasteless. Modern ion thrusters use inert gases for propellant, so there is no risk of the explosions associated with chemical propulsion. Image left: Xenon ion discharge from the NSTAR ion thruster of Deep Space 1. Large amounts of the gas push out at relatively low speeds to propel the spacecraft. Conventional chemical rockets burn a fuel with an oxidizer to make a gas propellant. This is like air escaping from the end of a balloon and propelling it forward.
Sir Isaac Newton's third Law states that every action has an equal and opposite reaction. Since much less fuel must be carried into space, smaller, lower-cost launch vehicles can be used. Deep Space 1 used less than 159 pounds of fuel in over 16,000 hours of thrusting. Acceleration continues throughout the flight, however, so tiny, constant amounts of thrust over a long time add up to much shorter travel times and much less fuel used if the destination is far away. To compensate for low thrust, an ion thruster must be operated for a long time for the spacecraft to reach its top speed.
#USES OF KRYPTON FULL#
Image right: Artist's concept of Deep Space 1 probe with its ion thruster operating at full power. These thrusters must be used in a vacuum to operate at the available power levels, and they cannot be used to put spacecraft in space because large amounts of thrust are needed to escape Earth's gravity and atmosphere. Current ion thrusters can provide only 0.5 newtons (or 0.1 pounds) of thrust, which is equivalent to the force you would feel by holding 10 U.S. The trade-off for the high top speeds of ion thrusters is low thrust (or low acceleration). In comparison, the Space Shuttles can reach speeds around 18,000 mph. Spacecraft powered by these thrusters can reach speeds up to 90,000 meters per second (over 200,000 mph). Several thrusters can be used on a spacecraft, but they are often used just one at a time. Currently, ion thrusters are used to keep communication satellites in the proper position relative to Earth and for the main propulsion on deep space probes. Chemical rockets have demonstrated fuel efficiencies up to 35 percent, but ion thrusters have demonstrated fuel efficiencies over 90 percent. The ion propulsion system's efficient use of fuel and electrical power enable modern spacecraft to travel farther, faster and cheaper than any other propulsion technology currently available. Ion thrusters, the propulsion of choice for science fiction writers have become the propulsion of choice for scientists and engineers at NASA.
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