  The U.S. Virtual Astronomical Observatory was a software infrastructure and
development project designed both to begin the establishment of an operational
Virtual Observatory (VO) and to provide the U.S. coordination with the
international VO effort. The concept of the VO is to provide the means by which
an astronomer is able to discover, access, and process data seamlessly,
regardless of its physical location. This paper describes the origins of the
VAO, including the predecessor efforts within the U.S. National Virtual
Observatory, and summarizes its main accomplishments. These accomplishments
include the development of both scripting toolkits that allow scientists to
incorporate VO data directly into their reduction and analysis environments and
high-level science applications for data discovery, integration, analysis, and
catalog cross-comparison. Working with the international community, and based
on the experience from the software development, the VAO was a major
contributor to international standards within the International Virtual
Observatory Alliance. The VAO also demonstrated how an operational virtual
observatory could be deployed, providing a robust operational environment in
which VO services worldwide were routinely checked for aliveness and compliance
with international standards. Finally, the VAO engaged in community outreach,
developing a comprehensive web site with on-line tutorials, announcements,
links to both U.S. and internationally developed tools and services, and
exhibits and hands-on training .... All digital products of the VAO Project,
including software, documentation, and tutorials, are stored in a repository
for community access. The enduring legacy of the VAO is an increasing
expectation that new telescopes and facilities incorporate VO capabilities
during the design of their data management systems.
