The Rumba and Tango satellites were sent into space on board a French-Russia launcher. Over the next week they will rendezvous with the Salsa and Samba satellites that were launched on July 16. The four satellites will undergo three months of tests before beginning their two-year scientific mission. Cluster is a joint mission between ESA and NASA, the European and American space agencies, and it will join two other joint missions - SOHO and Ulysses - that are studying the solar wind. Today's launch comes four years after the rocket carrying the original Cluster mission exploded shortly after take off in 1996.