Horizon 2000ESA's long-term space science research program covering the period from 1995 to the early 2000s. ESA originally introduced the program as early as 1984 and intended it to cover the years 1995-2007. There were four major ‘cornerstone’ missions. The first was ESA's contribution to the international Solar/Terrestrial Energy Programme (STEP). The second was the X-ray spectroscopy mission XMM. The other two were the Far Infrared and Submillimeter Space Telescope (FIRST) and the Rosetta cometary mission. There were also some less costly small and medium-sized projects. In 1992, ESA set up a survey committee to identify the main aims and challenges of future space missions and to design a new long-term program for the period up to 2016. This new program, Horizon 2000+, was approved in 1995. With the new project came a further four ‘cornerstone’ missions, including a Mercury orbiter (now under development as BepiColombo), an interferometry and astrometry mission (see GAIA), a project to observe gravity waves (now being developed as LISA), and a mission to join the search for extrasolar Earth-like planets (currently being considered under the name Darwin). The new name for the program, Horizon 2000+, was shortlived, however, and ESA decided to refer to both the original plan and its extension as Horizon 2000.
The Horizon 2000 program provided a basis for a number of projects not originally included, such as Giotto, Ulysses, and Hipparcos. It was also seen as the inspiration for the Huygens probe that formed ESA's contribution to the NASA Cassini-Huygens mission to Saturn, landing on Titan in January 2004. The first two ‘cornerstone’ missions of Horizon 2000 have achieved implementation: SOHO and Cluster (ESA's contributions to STEP) and the XMM satellite (launched as XMM-Newton in 1999); the fourth, Rosetta, was launched in 2004. a year behind its original schedule and with a new target, Comet Churyumov-Gerasimenko. The remaining ‘cornerstone’ mission, FIRST, has been renamed Herschel and is due for launch in 2007, alongside another ESA science mission, Planck. Horizon 2000 has been unanimously welcomed by the scientific community both within and outside Europe. Many non-European scientists collaborate on its missions. In 2004, ESA invited proposals for space projects to be considered covering the years 2016 to 2025.