This document is a work of the United States government, prepared by an officer or employee of the United States government as part of that person's official duties. It is not subject to copyright in the United States and there are no copyright restrictions on reproduction, derivative works, distribution, performance, or display of the work (derived from: http://www.usa.gov/copyright.shtml). NASA Web sites are not copyrighted, and may be linked to from other Web sites, including individuals' personal Web sites, without explicit permission from NASA. However, such links may not explicitly or implicitly convey NASA's endorsement of commercial goods or services.
