  Space weather refers to conditions on the Sun, in the interplanetary space
and in the Earth space environment that can influence the performance and
reliability of space-borne and ground-based technological systems and can
endanger human life or health. Adverse conditions in the space environment can
cause disruption of satellite operations, communications, navigation, and
electric power distribution grids, leading to a variety of socioeconomic
losses. The conditions in space are also linked to the Earth climate. The
activity of the Sun affects the total amount of heat and light reaching the
Earth and the amount of cosmic rays arriving in the atmosphere, a phenomenon
linked with the amount of cloud cover and precipitation. Given these great
impacts on society, space weather is attracting a growing attention and is the
subject of international efforts worldwide. We focus here on the steps
necessary for achieving a true physics-based ability to predict the arrival and
consequences of major space weather storms. Great disturbances in the space
environment are common but their precise arrival and impact on human activities
varies greatly. Simulating such a system is a grand- challenge, requiring
computing resources at the limit of what is possible not only with current
technology but also with the foreseeable future generations of super computers
