When Comet Halley approached its close encounter with the Sun in the winter of 1985-6, a total of five spacecraft from the world's four major space agencies had been launched to make observations of this rare event.
The Intercosmos of the then-Soviet Academy of Sciences had launched Vega-1 and Vega-2; the Japanese Institute of Space and Astronautical Science (ISAS) had launched the Sakigake and Suisei spacecraft; and ESA had launched the spacecraft Giotto. Together, these five spacecraft were able to accomplish unprecedented coordinated science during the comet flybys of March 6 to March 14, 1986, as a result of five years of cooperation between the four space agencies.
In 1981, realizing that many aspects of Halley Comet mission planning, spacecraft and experiment design, and data evaluation were common to these missions, and that the overall scientific return could be increased through cooperation, the four agencies then agreed then to form the Inter-Agency Consultative Group for Space Science (IACG). The IACG had the task of informally coordinating all matters related to the space missions to Halley's Comet and the observations of the comet from space.
The IACG thus became the focus of a global effort to explore Halley's Comet during its last manifestation. Halley became the most thoroughly studied comet ever. More data had been collected on it than on all other comets put together. The success of the Comet Halley activities led each agency to recognize the value of maintaining close international collaboration within the IACG framework. In addition, the scientific endeavors in each of the space agencies for the upcoming decade called out for interagency cooperation.
In the mid-eighties, each member agency of the IACG already had one or more solar-terrestrial science missions approved or near approval for launch and operation in the 1989-1996 time frame, many of these missions being bilateral arrangements between IACG members.
Solar-terrestrial science is of fundamental interest and importance to all nations since it aims to understand the physical environment of the Earth in space and the impact of the Sun on that environment. Hence, it is concerned with both basic space-physics processes and the habitability of the Earth, as well as the understanding of the structure and behavior of the Sun, particularly of its atmosphere, as it ultimately controls the solar-terrestrial environment.
Thus the IACG determined that its second major collaborative effort should focus on solar-terrestrial research. This unique form of informal global collaboration is now providing the means for optimally coordinated operations, and a mechanism for data sharing and joint data analysis by the science communities associated with the IACG members. Clearly, the scientific output of each of these missions will be significantly enhanced by the opportunity to achieve coordination with one or several other missions through the auspices of the IACG.
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