Earth’s liquid core circulates electric currents like a dynamo, generating a magnetic field. Earth’s magnetic field penetrates the planet and extends into space. The magnetosphere is a huge area where the magnet field extends beyond the surface. The magnetosphere acts like a shield, protecting Earth from charged particles in the solar wind. Earth’s magnetosphere is distorted by the solar wind. It’s smaller on the side being hit by the solar wind and larger on the lee side. The particles in the solar wind follow the path of the magnet field and are funneled to the north and south poles.

The People’s Republic of China launched the second of two satellites on missions to probe Earth’s inner magnetosphere. The Double Star program combines Chinese satellite and rocket technology with European space-science instrumentation left over from the Cluster magnetosphere research project. The Chinese satellites will collaborate with the four European Cluster satellites: Tango, Rumba, Samba, and Salsa that are in high polar orbits.

The two Tan Ce (Explorer) satellites were launched six months apart. Tan Ce 1 was launched in January 2004 into an equatorial orbit. Tan Ce 2 was launched on July 25, 2004 into a polar orbit. The orbits are in low-altitudes with very elongated shapes. Tan Ce 1 has an apogee (farthest point of orbit) of almost 50,000 miles above the equator, similar to the Cluster satellites. Tan Ce 2 passes about 25,000 miles over the Arctic Circle. It carries proton and electron detectors, magnetometers, and long antennas to measure plasma-wave phenomena. It has an advanced neutral-atom imager that takes “pictures” of the magnetosphere by detecting atoms rather than light.