  Space plasmas are three-dimensional dynamic entities. Except under very
special circumstances, their structure in space and their behavior in time are
not related in any simple way. Therefore, single spacecraft in situ
measurements cannot unambiguously unravel the full space-time structure of the
heliospheric plasmas of interest in the inner heliosphere, in the Geospace
environment, or the outer heliosphere. This shortcoming leaves numerous central
questions incompletely answered. Deficiencies remain in at least two important
subjects, Space Weather and fundamental plasma turbulence theory, due to a lack
of a more complete understanding of the space-time structure of dynamic
plasmas. Only with multispacecraft measurements over suitable spans of spatial
separation and temporal duration can these ambiguities be resolved. We note
that these characterizations apply to turbulence across a wide range of scales,
and also equally well to shocks, flux ropes, magnetic clouds, current sheets,
stream interactions, etc. In the following, we will describe the basic
requirements for resolving space-time structure in general, using turbulence'
as both an example and a principal target or study. Several types of missions
are suggested to resolve space-time structure throughout the Heliosphere.
