The Double Star spacecraft, called also Tan Ce (TC) which in Chinese means ‘Probe‘, is a joint Chinese and ESA mission to study the effect of the Sun on the Earth's environment. The polar spacecraft (TC-2) will monitor the energy input from the solar wind into the polar ionosphere. The equatorial spacecraft (TC-1) will investigate the so-called substorm process, when it is in the Earth's magnetotail, and the entry of solar particle on the front side of the magnetosphere.