International Space Station

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 James Newman, an astronaut on the space shuttle Endeavor preparing for mission, working to connect wires on the International Space Station. AP/Wide World Photos. Reproduced by permission. 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.