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How the International Space Station Works

ISS: Power, Propulsion and Communications

The ISS is basically a large spacecraft. As such, it must be able to move about in space, its crew must maintain communications with controllers on the ground and it needs power to accomplish all of this.

We take for granted having electrical power to operate our homes. For example, to use your coffee maker, you simply plug it into the wall without a second thought. Like in your home, all of the onboard systems of the ISS require electrical power. Eight large solar arrays provide electrical power from the sun. Each array is 239 feet (73 m) long and covers an area of 24,187 ft2 (approximately 2,247 m2), or about one-half acre. On each array are two blankets of solar cells. Each blanket is on one side of a telescoping mast that can extend and retract to fold or form the array. The mast turns on a gimbal so that it can keep the solar cells facing the sunlight. The Russian modules also have 80- to 97-foot (24- to 30-m) solar arrays that provide power [source: NASA ISS Facts and Figures].

Like a power grid on Earth, the arrays generate primary power -- approximately 160 volts of DC electricity. The primary power gets converted by a secondary transformer to provide a regulated 124-volt DC current to be used by the station's equipment. There are also power converters onboard to meet the different currents required by U.S. and Russian equipment. The primary power is also used to charge the ISS's three nickel-hydrogen battery stations, which provide power when the ISS passes through the Earth's shadow in each orbit.

The ISS orbits the Earth at an altitude of 217 to 285 miles (362 to 475 km). At this altitude, the Earth's atmosphere is extremely thin, but still thick enough to drag on the ISS and slow it down. As the ISS slows down, it loses altitude. In addition to atmospheric drag, solar flares also slow the station down and cause it to lose altitude. So, the ISS must be boosted periodically to maintain its proper altitude. The command and service modules have rocket engines that can be used to boost the ISS. However, the Progress supply ships will do most of the reboosting. Each reboosting event requires two rocket engine burns. During the burns, work on the ISS is suspended. After the burns, station life returns to normal.

The ISS must be able to know precisely where it is in space, where other objects are and how to go from one point in space to another, especially during reboosting. To know where it is and how fast it is moving, the ISS uses both U.S. and Russian global positioning systems (GPS). To know which way it is pointing, its attitude, the ISS has several gyroscopes. The combination of all this information helps the ISS move from one point to another in space. In addition, the Russian navigation system uses sighting on the stars, sun and Earth's horizon for navigation.

Now that you know how the ISS stays in space, let's see what it's like to live and work there.

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