A joint mission of China and the European Space Agency to study the Earth's magnetosphere. Double Star involves two satellites, Tan Ce 1 and Tan Ce 2, launched on 29 December 2003 and 25 July 2004 respectively. ‘Tan Ce’ means ‘Explorer’ in Chinese. Tan Ce 1 is in a highly elliptical near-equatorial orbit from which it investigates the Earth's magnetic tail, while Tan Ce 2 orbits over the Earth's poles from where it can monitor the development of aurorae. The two satellites work in conjunction with ESA's Cluster spacecraft.