Active space experiments are performed for a variety of reasons. They allow basic plasma and gas dynamic experiments to be performed in a high vacuum without any unwanted wall effects other than interference by the space platform itself. If necessary, this interference can be minimized by making the interaction observations at a suitably large distance from the platform. Active experiments can also be used to study and simulate natural phenomena occurring in the space environment using controlled stimulus response experiments, rather than depending on highly variable and sporadic natural events. For example electron beams can be used in a radar mode to probe distant electromagnetic fields in the magnetosphere, gas releases can be used to carry out photochemical experiments in the upper atmosphere and electromagnetic stimulation of many kinds can be used for controlled studies of the variety of naturally occurring plasma wave in the Earth's environment. Finally active experiments obtain measurements basic to the development of practical applications involving new power sources for generating thrust on space platforms, or generating electrical power in space. Although the implementation of these systems is an engineering exercise, more basic knowledge of the physical mechanisms involved in the release of accelerated plasma from ion thrusters and on the current collection by large orbiting objects needs to be obtained by active experiments in space.
As we enter the era of space stations with increasing access to the Russian MIR (meaning world) space station, and the planned international space station, the facilities and resources to perform active experiments in the Low Earth Orbit (LEO) environment will increase dramatically. With the facilities on space station platforms, the concept of a vacuum laboratory in space could become a reality. Such a laboratory would have a distinct advantage over ground-based vacuum systems in that the experimental area is no longer a confined chamber, but is an essentially unbounded volume on scale sizes of 100 to 1000s of kilometers, depending if the to- or anti-earth direction predominates in the experimental interaction. The ambient residual gas and plasma composition in a space-based vacuum laboratory is not under the experimenter's control, but will naturally vary in a fairly predictable manner and with considerable range during the course of an orbit. In fact the space situation would be an inverted ground-based system with the atmospheric part of the operations in an enclosed chamber embedded in a vacuum rather than vice-versa as occurs on earth.
Active experiments on the space environment can also be performed from ground-based facilities, normally by transmitting intense electromagnetic waves into the upper atmosphere. The results of the interaction are observed either remotely from ground-based instruments or in situ from rocket or satellite based instruments. In addition, there continues to be an active, ongoing theoretical support for modeling and predicting the results of active experiments in space. In order to keep this section of the report to a reasonable size, I will focus only on the space-borne experimental part of the US program, primarily in the four years since the previous report.
An indication of the level of activity in active space experiments can be obtained by looking at the statistics of sessions and contributions on active experiments at recent national meetings of the American Geophysical Union (AGU). Table 1 below shows that since 1990, there has been consistent support for sessions on active experiments with a cyclic variation between the fall and spring meetings. The sessions encompass all aspects of active experiments; space-borne, ground-based and theoretical studies.