International Space Station

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International Space Station

The International Space Station (ISS), formally designated International Space Station Alpha, is a habitable orbital facility that has been under construction since 1998 and is scheduled for completion in 2006. When finished, it will contain about four times as much working space as the Russian space station Mir (1986–2001), the former record holder, and will weigh about one million pounds (453,000 kg). The ISS orbits at an average altitude of 240 mi (387 km). A number of science experiments are to be conducted aboard the ISS in such fields as health effects of radiation , molecular and cell biology , earth science , fluid dynamics , astronomy , combustion physics , and crystal growth.

History and structure

The ISS was originally proposed by U.S. President Ronald Reagan in 1984, and was slated to cost $8 billion. Thirty-six U.S. shuttle flights plus nine Russian rocket launches will be required for ISS construction. Today there are 15 major partners in the ISS effort, including the United States, Russia, Japan, Canada, and 11 of the member states of the European Space Agency. The United States, through its National Aeronautics and Space Administration (NASA), is the largest single contributor, bearing approximately $25 billion of the total $50–100 billion cost of building, launching, and operating ISS for at least a decade.

Assembly of the ISS commenced in 1998 with launch of the Russian control module Zarya on a proton rocket from Kazahkstan. The U.S. module Unity Node, a connecting segment, was carried into space on the shuttle Endeavor later in 1998. This unit is primarily a docking hub to which other sections join. In 2000, another Proton rocket lofted the Russian service module Zvezda, the main Russian contribution to the ISS. Zvezda provided living quarters and life support during the early phases of the ISS's growth; it also provides steering rockets to control the ISS's attitude (orientation in space) and to reboost it to higher altitudes as its orbit decays due to friction with high-altitude traces of the earth's atmosphere.

The ISS is powered by photovoltaic electricity . The first of its four large solar arrays (112 by 39 ft [34.2 by 11.9 m]) was added in 2000. When completed, the ISS will receive about 260 kilowatts of power (peak) from an acre of sun-tracking solar panels. An energy-storage sub-system consisting of six large nickel-hydrogen (Ni-H2) batteries supplies electrical power to the ISS during its passage through the earth's shadow, which lasts about 45 out of every 90 minutes.

In 2001, the U.S. laboratory module Destiny, the largest and most elaborate of the ISS's components, was added using the robot arm of the space shuttle Atlantis. The U.S. lab module contains 13 equipment racks, on which various scientific experiments will be mounted, and a 20-in (0.5-m) window set in the Earthside wall.

Smaller components were added piecemeal in 2002 by several shuttle flights, and in 2001–2002 several Russian flights ferried passengers and supplies. The ISS's final configuration will contain a European laboratory module, a Japanese laboratory module, three Russian laboratory modules, a Canadian robot arm to assist in assembly and maintenance, exterior racks for experiments requiring direct exposure to space, and an emergency Crew Return Vehicle on standby. The shuttle has been continuously inhabited since November 2, 2000, and presently houses a full crew of seven.

Science

The ISS is intended to serve as a platform for the performance of scientific experiments that can only be carried out in space. The presence of a crew allows more complex experiments to be performed with simpler equipment than would be possible using purely robotic space vehicles; on the other hand, human beings require much complex gear to survive in space. Further, the ISS is not a particularly efficient platform for astronomical experiments, as it is vulnerable to by vibrations, and experiments that merely require a vacuum can be performed economically in vacuum chambers on Earth . Yet, the ISS does offer something that cannot be obtained for more than a few seconds at a time on Earth: weightlessness, or, more precisely (since the components of the ISS itself create a slight gravitational field), microgravity.

Unlike traditional science-fiction space stations, the ISS does not rotate in order to provide a centrifugal equivalent of gravity to its inhabitants. Such an arrangement would require a much more expensive structure due to the stresses imposed by rotation ; observational science experiments that need to point steadily at one part of the sky would be difficult to operate on a rapidly rotating platform; and rotation would destroy the very microgravitational conditions that make the ISS a unique place to conduct science.

Several of the experiments that have exploited (or will exploit) microgravity are the following:

Dendrite formation in solidifying metals. When metals solidify they tend, like snowflakes, to form branching or tree-like crystalline structures termed dendrites (from the Greek "dendrites," meaning "pertaining to a tree"). Observing the growth of metallic dendrites undeformed by the earth's gravity should help improve mathematical models of dendrite formation, which in turn may help in the design of stronger and more durable alloys.

Bone deterioration. As previous experience with long-term habitation of space has shown, persons living in weightlessness lose about 1% of their bone mass per month, even when performing bone-stressing exercises. Generations of small animals raised in space will enable biologists to study the effect of microgravity on genetic mechanisms of bone growth and resorbption. Understanding these mechanisms may someday make long space voyages (e.g., to Mars ) medically feasible.

Atomic clock. A French experiment will use microgravity to improve the accuracy of an atomic clock by a factor of 10 by observing oscillations of cesium atoms in free fall.

Commercial research. Between 30–40% of the U.S. lab module resources are reserved for use by private corporations, who will pay for access to microgravity research conditions. A slightly lower percentage of lab resources are reserved for commercial buyers in the European laboratory module. However, few corporations have yet purchased lab time on the ISS.

Controversy

The ISS is enthusiastically supported by many people who are interested in space travel for its own sake and by those scientists who hope to fly their own experiments on the platform. However, it has long been heavily criticized by a majority of the scientific community for delivering too little science for the dollar and thus, in effect, diverting money from more effective research. Some scientists argue that the bulk of the research planned for the ISS addresses technical questions that are peripheral, rather than fundamental. For example, Science, the journal of the American Association for the Advancement of Science, complained in 1998 that the ISS's "greatest impact will be felt in the small community already studying problems related to spaceflight—a vital research area only if we assume that increasing numbers of people will someday travel, or even live, outside of normal Earth gravity." In other words, the ISS is an ultimately romantic project that puts astronauts in space in order to figure out how to put more astronauts in space.

The claim that the ISS has little to offer science was boosted by Russia's conveyance to the ISS of two private space tourists—officially designated Space Flight Participants—in 2001 and 2002, over loud protests from other ISS participant nations. Two wealthy men, one American and the other South African, paid $20 million apiece to the cash-strapped Russian government in exchange for a trip to the ISS.

Even before the loss of the space shuttle Columbia in February 2003, funding for completion of the ISS was in doubt. Both the United States Congress and the governments of the European Union have long been skeptical of the ISS's costs, and NASA was under such political pressure that it admitted it could not guarantee that the station will ever be grown beyond the "core complete" stage, with long-term living quarters for only three astronauts. Three astronauts, however, are not enough to tend the scores of experiments for which the ISS's racks have room, so if the ISS is not expanded much of the science potential already constructed will go to waste. Critics of the ISS argued that continued support for the ISS amounted to throwing good money after bad; supporters of the ISS counter-argued that ISS research is essential for make an eventual trip to Mars and that human space-travel projects generate valuable technological spinoffs.

The Columbia disaster of early 2003 has, as of this writing, made the ISS's future murkier. Although Russian rockets can supply many of the ISS's needs and ferry astronauts back and forth to it, only the space shuttle's cargo hold is large enough to carry many of the components planned for the ISS. Another, more urgent factor is that the ISS loses orbital altitude steadily due to friction with the outer fringes of the earth's atmosphere. Small rockets attached to the station regularly restore its altitude. The fuel for these rockets has been delivered via space shuttle, but after the Columbia disaster, a long delay seemed certain before frequent shuttle flights could be resumed, and Russian spacecraft have not been designed to deliver sufficient fuel. Engineers in both Russia and the United States have proposed alternate solutions, but as of March 2003 no firm course of action had been approved.