INTERNATIONAL SPACE STATION-2:
ASSEMBLY & STRUCTURE:
The assembly of the International Space Station, a major aerospace engineering endeavour, began in November 1998. As of May 2009 the station is 82.8% complete.
The first segment of the ISS, Zarya, was launched into orbit on November 20, 1998 on a Russian Proton rocket, followed two weeks later by the first of three node modules, Unity, launched aboard STS-88. This bare 2-module core of the ISS remained unmanned for the next one and a half years until the Russian module Zvezda was added in July 2000, allowing a maximum crew of three people to occupy the ISS continuously. The first resident crew, Expedition 1, was sent later that year in November. The year 2000 also saw the arrival of two segments of the station's Integrated Truss Structure, the Z1 and P6 trusses, providing the embryonic station with communications, guidance, electrical grounding (on Z1), and power via a pair of solar array wings, located on the P6 truss.
Over the next two years the station continued to expand with a Soyuz-U rocket delivering the Pirs docking compartment. Space Shuttles Discovery, Atlantis, and Endeavour delivered the Destiny laboratory and Quest airlock to orbit, in addition to the station's robot arm Canadarm2, and several more segments of the truss structure.
The expansion schedule was brought to an abrupt halt, however, following the destruction of the Space Shuttle Columbia on STS-107 in 2003. The resulting hiatus in the Space Shuttle programme halted station assembly until the launch of Discovery on STS-114 in 2005.
As of July 2009, the station consisted of ten pressurised modules and the complete Integrated Truss Structure. Awaiting launch is the third and final American node, Tranquillity, a Permanent Logistics Module, the European Robotic Arm, several Russian modules and the Alpha Magnetic Spectrometer (AMS). Assembly is expected to be completed by 2011, by which point the station will have a mass in excess of 400 metric tons (440 short tons).
POWER SUPPLY:
The source of electrical power for the ISS is the Sun. Light is converted into electricity through the use of solar arrays. Before assembly flight 4A (space shuttle mission STS-97, launched November 30, 2000) the only power sources were the Russian solar panels attached to the Zarya and Zvezda modules. The Russian segment of the station uses 28 volts DC, as does the space shuttle. In the remainder of the station, electricity is provided by the solar arrays attached to the truss at a voltage ranging from 130 to 180 volts DC. These arrays are arranged as four pairs of wings, and each pair is capable of generating nearly 32.8 kW of DC power.
Power is stabilised and distributed at 160 volts DC before being converted to the user-required 124 volts DC. This high-voltage distribution line allows for smaller power lines, thus reducing weight. Power can be shared between the two segments of the station using converters. This feature has become essential since the cancellation of the Russian Science Power Platform because the Russian segment now depends on the US-built solar arrays for power.
The solar arrays normally track the Sun to maximise the amount of solar power. Each array is about 375 m2 (450 yd2) in area and 58 metres (190 ft) long. In the complete configuration, the solar arrays track the sun in each orbit by rotating the alpha gimbal, while the beta gimbal adjusts for the angle of the sun from the orbital plane. Until the main truss structure arrived, the arrays were in a temporary position perpendicular to the final orientation. In this configuration, as shown in the image to the right, the beta gimbal was used for the main solar tracking. Another tracking option, the Night Glider mode, can be used to reduce the effects of drag produced by the tenuous upper atmosphere, through which the station flies, by orienting the solar arrays edgewise to the velocity vector.
ATTITUDE CONTROL:
The attitude (orientation) of the station is maintained by either of two mechanisms. Normally, a system using several control moment gyroscopes (CMGs) keeps the station oriented, with Destiny forward of Unity, the P truss on the port side, and Pirs on the earth-facing (nadir) side. When the CMG system becomes saturated—a situation whereby a CMG exceeds its operational range or cannot track a series of rapid movements—it can lose its ability to control station attitude. In this event, the Russian attitude control system is designed to take over automatically, using thrusters to maintain station attitude, allowing the CMG system to desaturate. This scenario has only occurred once, during Expedition 10. When a space shuttle is docked to the station, it can also be used to maintain station attitude. This procedure was used during STS-117 as the S3/S4 truss was being installed.
ALTITUDE CONTROL:
The ISS is maintained at an orbit from a minimum altitude of 278 km (173 mi) to a maximum of 460 km (286 mi). The normal maximum limit is 425 km (264 mi) to allow Soyuz rendezvous missions. As the ISS constantly loses altitude because of slight atmospheric drag, it needs to be boosted to a higher altitude several times each year.These effects vary from day to day, however, because of changes in the density of the outer atmosphere caused by changes in solar activity. This reboost can be performed by the station's two main engines on the Zvezda service module, a docked space shuttle, a Progress resupply vessel, or by ESA's ATV. It takes approximately two orbits (three hours) to be boosted several kilometres higher. In December 2008 NASA signed an agreement with the Ad Astra Rocket Company which may result in the testing on the ISS of a VASIMR plasma propulsion engine. This technology could, in the future, allow station-keeping to be done much more economically than at present
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