  Traditional science searched for new objects and phenomena that led to
discoveries. Tomorrow's science will combine together the large pool of
information in scientific archives and make discoveries. Scienthists are
currently keen to federate together the existing scientific databases. The
major challenge in building a federation of these autonomous and heterogeneous
databases is system integration. Ineffective integration will result in defunct
federations and under utilized scientific data.
  Astronomy, in particular, has many autonomous archives spread over the
Internet. It is now seeking to federate these, with minimal effort, into a
Virtual Observatory that will solve complex distributed computing tasks such as
answering federated spatial join queries.
  In this paper, we present SkyQuery, a successful prototype of an evolving
federation of astronomy archives. It interoperates using the emerging Web
services standard. We describe the SkyQuery architecture and show how it
efficiently evaluates a probabilistic federated spatial join query.
