The geomagnetic substorm is a process by which energy is stored and released in the magnetosphere resulting in serious disturbances in the Earth environment. The two TC spacecraft located near the Earth and Cluster located further down the tail will help to locate the starting point of the substorm and the physical mechanism responsible for it.

The Double Star orbits have been designed such as to get the best conjunctions with Cluster in the key scientific regions.

The orbit of  the DSP equatorial satellite (TC-1) has a perigee at 570 kilometres and apogee at 78 970 kilometres; the orbit of the DSP polar satellite (TC-2) is planned to have a perigee at 700 kilometres and apogee at 39 000 kilometres.

Scientific objectives

Double Star's scientific objectives are to:

• Study the magnetic reconnection at the magnetopause and in the magnetotail
• Understand and locate the trigger mechanism for magnetospheric storms and substorms
• Study physical processes such as particle acceleration, diffusion, injection, and up-flowing ions during storms
• Study temporal variations of field-aligned currents and the coupling between tail current and auroral current.

The equatorial satellite of DSP detects the physical processes of geospace storms in the near-Earth magnetotail and the energy transfer from the solar wind to the magnetosphere.

The polar satellite of DSP detects energy transfer from the solar wind to the magnetosphere via the dayside magnetopause. Furthermore it detects energy transfer from the solar wind and the near-Earth magnetotail to the polar ionosphere and upper atmosphere, as well as ionised-particle transfer from the ionosphere to the magnetosphere.

The two DSP satellites operate in the near-Earth equatorial region, the main active regions of magnetospheric storms, substorms, and particle events that are not covered by the current International Solar-Terrestrial Programme missions. The combination of two satellites forms an independent unique constellation-like exploring system that can explore the generation mechanism and evolving process of geospace storms. Cluster and DSP satellites will operate simultaneously and form for the first time a mini-constellation with 6 points in the magnetosphere.