Number 139 (Story #3), August 10, 1993 by Phillip F. Schewe and Ben Stein HORIZON 2000 , the ambitious long-term program of the European Space Agency (ESA), has attracted many non-ESA collaborators, particularly American astronomers who are worried about the stability of NASA funding for space science projects. Currently scheduled important missions include SOHO (launch date in 1995), which will measure the solar wind and corona; Cluster (1995), a set of four satellites for monitoring near-Earth plasma; Huygens (1997), a probe which will get a ride to Titan on board NASA's Cassini craft going to Saturn; XMM (1999), an orbiting x-ray observatory; Integral (2001), a gamma-ray observatory, and the first tripartite ESA- Russian-US mission; FIRST (after 2002), a far infrared telescope; and Rosetta (after 2002), which would return samples from a comet nucleus. (Science, 30 Jul. 1993.)