Background The European Space Agency (ESA) was established in May 1975 as the merger of the
European Space Research Organisation (ESRO) and the
European Launcher Development Organisation. In 1970, the governing Launch Programme Advisory Committee (LPAC) of ESRO made a decision not to execute astronomy or planetary missions, which were perceived as beyond the budget and capabilities of the organisation at the time. This meant that cooperation with other
government space agencies and institutions was necessary for large-scale scientific missions. In addition to the selection of the
International Ultraviolet Explorer observatory in March 1983, This, in addition to the lack of a long-term plan for scientific missions, along with budget setbacks from
NASA on the collaborative International Solar Polar Mission (later named
Ulysses), spurred the development of a long-term scientific programme through which ESA could sustainably plan missions independent of other agencies and institutions over lengthier periods. The leadership and advisory structure of ESA's Directorate of Science changed immediately prior to the programme's establishment. In the 1970s, ESA's Science Advisory Committee (SAC), which succeeded the LPAC, advised the
Director General on all scientific matters; the Astronomy Working Group (AWG) and the Solar System Working Group (SSWG) also reported directly to the Director General. In the early 1980s, the SAC was replaced with the Space Science Advisory Committee (SSAC), who were tasked to report to the Director of Science on developments in the AWG and SSWG. In addition, former SAC chair
Roger-Maurice Bonnet replaced Trendelenburg as Director of Science in May 1983.
Horizon 2000 Formulation In November and December 1983, ESA made the first open call for mission proposals to the European scientific community, based on an idea for a community-driven programme presented by Bonnet to the SPC in late 1983. The call yielded 68 proposals – 30 in the field of astronomy and 34 in the field of solar physics, with 4 miscellaneous concepts also submitted. An
ad hoc "survey committee" led by then-
SRON Director
Johan Bleeker was convened, consisting members of the SSAC,
CERN, the
European Science Foundation, the
European Southern Observatory, and the
International Astronomical Union, to examine the proposals submitted. Throughout early 1984, the survey committee formulated plans for a series of missions divided into three categories – "cornerstones" which would cost two annual budgets over a long implementation timeline, medium-size missions which would cost one annual budget, and small-size missions that would cost half an annual budget. The budget for the Science Programme was 130 million
accounting units (MAU) annually in 1984, and a 7% annual increase until 1991, when the budget would be fixed at 200 MAU per year onwards, was proposed. Medium-size and small-size categories would later be merged into a single medium-size category that would represent missions costing half a budget. This category was internally referred to as the "blue missions", named after their representation as blue boxes in a publicised diagram of the plan. The cornerstones selected were a
comet sample-return mission, an
X-ray spectroscopy mission, and a
submillimetre astronomy mission. Cornerstone objectives that were not selected due to financial and technical shortcomings, but mentioned by the survey committee as possibilities beyond Horizon 2000, included a solar probe, a
Mars rover, and a two-dimensional
interferometry mission. The survey committee's final meeting was held on
San Giorgio Maggiore,
Venice in June 1984, where the "Horizon 2000" plan was presented to ESA's then-Director General
Erik Quistgaard, and leading members of the European scientific community. The broad objectives of Horizon 2000 were to expand scientific knowledge, establish Europe as a developmental centre of space science, provide opportunities to the European scientific community, and spur innovation in the space technology industry. At the meeting, an additional fourth cornerstone presented by the SSWG was adopted – the Solar-Terrestrial Science Programme (STSP) consisting the
Solar and Heliospheric Observatory and
Cluster proposals, which became the first missions to be selected for launch under Horizon 2000. Quistgaard presented the Horizon 2000 plan at the 1985 Ministerial Council in
Rome, where it was approved with only a 5% annual increase of the budget through to 1989, instead of the requested 7% through to 1991. This was only enough to fund around half of Horizon 2000's objectives.
Implementation (pictured) and
Cluster was challenged early in development due to financial concerns|314x314px The
X-ray Multi-Mirror Mission (XMM) was conceived as the X-ray spectroscopy cornerstone mission at an ESA workshop in
Lyngby in June 1985, consisting a
space observatory with twelve low-energy and seven high-energy telescopes. Due to practical constraints, the mission's payload was reduced to seven telescopes overall by 1987, though the success of
EXOSAT inspired mission planners to improve the efficiency of the mission's observations by placing the spacecraft in a
highly eccentric orbit, allowing the payload of the spacecraft to be reduced to its final design of three large telescopes – each with a reflecting area of 1,500
cm2. By 1986, the cost of the STSP cornerstone was forecast to exceed its allocated 400 MAU budget, and in a February 1986 meeting, the SPC was presented with the possibility of cancelling the cornerstone in favour of a medium-size mission selection between SOHO, Cluster, and the
Kepler Mars orbiter proposal, which had gained popularity among members of the SSAC. The
Space Shuttle Challenger disaster, which occurred the month before, had an effect on proceedings, as SOHO was intended to be launched via the Shuttle. Despite this, the SSWG, SSAC, and SPC reaffirmed a commitment to the STSP cornerstone by descoping SOHO and limiting Cluster to three spacecraft, and reaching a collaboration agreement with NASA in October 1986 that would reduce the mission's cost – they would provide testing, launch services, and operations of SOHO and contribute various
scientific instruments, while cancelling their
Equator mission in favour of a fourth Cluster spacecraft on which American scientific instruments would be flown. The first medium-class mission was selected from proposals formulated by ESA in 1982, prior to Horizon 2000. A
Titan Probe that would piggyback on the American
Cassini spacecraft was proposed by a group of American and European scientists, The European-Soviet
Vesta multiple-flyby
minor planet mission and GRASP
gamma-ray observatory competed, but were rejected by the AWG and SSWG. After budget cuts resulting from the
Challenger disaster forced NASA to retract its support for LYMAN and QUASAT, the Titan Probe was selected by the SPC in November 1988, In the competition for the second medium-class mission in June 1989, a consortium of American and European institutions proposed
INTEGRAL, a gamma-ray observatory that combined GRASP with the American Nuclear Astrophysics Explorer (NAE), which had lost selection for NASA's
Explorers Programme that year. NASA supported the proposal, and the
Russian Academy of Sciences later offered launch aboard a
Proton launch vehicle in exchange for observation time on the mission. Despite concerns about NASA's commitment to the mission and their funding sources, INTEGRAL was selected by the SPC in June 1993, with NASA contributing
Deep Space Network services and a
spectrometer. In response, INTEGRAL was selected by NASA as an Explorers mission without competition. This, along with concerns about the sensitivity of the spectrometer designed for the mission, proved controversial among NASA's advisory bodies. In September 1994, ESA and NASA resolved to end NASA's involvement with the spectrometer, citing a lack of financial support.
CNES promptly assumed the financial burden, and led the design and manufacturing of a new spectrometer.
Rosetta and
FIRST were selected in November 1993 as the third and fourth cornerstone missions, with the latter mission eventually being rechristened the
Herschel Space Observatory.
COBRAS/SAMBA, later rechristened
Planck, was selected as the third medium-sized mission in July 1996. As of December 2016, four Horizon 2000 missions, including three cornerstone and one medium-sized mission, remain operational.
Horizon 2000 Plus ''
astrometry mission was launched as one of three missions in the Horizon 2000 Plus campaign.
Horizon 2000 Plus was an extension of Horizon 2000 programme prepared in the mid-1990s, planning missions in the 1995–2015 timeframe. This included two further cornerstone missions, the star-mapping
GAIA launched in 2013, and the
BepiColombo mission to Mercury launched in 2018; and also a technology demonstrator
LISA Pathfinder launched in 2015, to test technologies for the future
LISA. All of the Horizon 2000 and Plus missions were successful, except for the first
Cluster which was destroyed in 1996 when its launch rocket exploded. A replacement,
Cluster 2, was built and launched successfully in 2000.
Cosmic Vision Cosmic Vision 2015–2025 is the current programme of ESA's long-term planning for space science missions. The initial call of ideas and concepts was launched in 2004 with a subsequent workshop held in Paris to define more fully the themes of the Cosmic Vision under the broader subjects of
astronomy and
astrophysics,
Solar System exploration and fundamental
physics. By early 2006 the formulation for a 10-year plan based around 4 key questions emerged: • What are the conditions for
planet formation and the
emergence of life? • How does the
Solar System work? • What are the fundamental physical laws of the
Universe? • How did the Universe originate and what is it made of? In March 2007 a call for mission ideas was formally released, which yielded 19 astrophysics, 12 fundamental physics and 19 Solar System mission proposals.
Large class Large class (L-class) missions were originally intended to be carried out in collaboration with other partners with an ESA-specific cost not exceeding 900 million euros. However, in April 2011 it became clear that budget pressures in the US meant that an expected collaboration with
NASA on the L1 mission would not be practical. The down-selection was therefore delayed and the missions re-scoped on the assumption of ESA leadership with some limited international participation. Three L-class missions have been selected under Cosmic Vision:
JUICE, a
Jupiter and
Ganymede orbiter launched in April 2023;
Athena, an
X-ray observatory planned for launch in 2035; and
LISA, a space-based
gravitational-wave observatory planned for launch in 2035. In April 2024, ESA identified a mission to the Saturn system, including a tour of several Saturn moons, as the science case for the next large scale mission (
L4) and the first from ESA's "Voyage 2050" vision.
Medium class '' near-infrared observatory, launched as the second Cosmic Vision M-class mission. Medium class (M-class) projects are relatively stand-alone projects and have a price cap of approximately 500 million euros. The first two M-class missions, the
Solar Orbiter heliophysics mission to make close-up observations of the Sun, and the
Euclid visible to near-infrared space telescope, aimed at studying dark energy and dark matter, were selected in October 2011.
PLATO, a mission to search for
exoplanets and measure
stellar oscillations, was selected on 19 February 2014, against
EChO,
LOFT,
MarcoPolo-R and
STE-QUEST After a preliminary culling of proposals for the fourth M-class mission in March 2015, a short list of three mission proposals selected for further study was announced on 4 June 2015. The shortlist included the THOR
plasma observatory and the XIPE X-ray observatory. The competition for the fifth M-class mission concluded in June 2021, with the
EnVision Venus orbiter ultimately being selected for launch in 2031. The
SPICA far-infrared observatory and
THESEUS gamma-ray observatory were the other two proposals.
Small class Small class missions (S-class) are intended to have a cost to ESA not exceeding 50 million euros. A first call for mission proposals was issued in March 2012; the winning proposal would need to be ready for launch by 2017. Approximately 70 letters of Intent were received.
CHEOPS, a mission to search for exoplanets by
photometry, was selected as the first S-class mission in October 2012 and launched in December 2019.
SMILE, a joint mission between ESA and the
Chinese Academy of Sciences to study the interaction between Earth's magnetosphere and the solar wind, was selected as the second S-class mission from thirteen competing proposals in June 2015. As of September 2025, SMILE is scheduled for launch sometime in 2026.
Fast class At the ESA Science Programme Committee (SPC) Workshop on 16 May 2018, the creation of a series of special opportunity Fast class (F-class) missions was proposed. These F-class missions will be jointly launched alongside each M-class mission starting from M4, and would focus on "innovative implementation" in order to broaden the range of scientific topics covered by the mission. The inclusion of F-class missions into the Cosmic Vision program will require an increase of the science budget. F-class missions must take under a decade from selection to launch and weigh less than 1,000 kg. The first F-class mission,
Comet Interceptor, was selected in June 2019. On 2 November 2022, ESA announced the F-class mission
ARRAKIHS, to be launched in the early 2030s.
Missions of Opportunity Occasionally ESA makes contributions to space missions led by another space agency. Missions of opportunity allow the ESA science community to participate in partner-led missions at relatively low cost. The cost of a mission of opportunity is capped at €50 million. ESA missions of opportunity include contributions to
Hinode,
IRIS,
MICROSCOPE,
PROBA-3,
XRISM,
ExoMars,
Einstein Probe, and
MMX. though SPICA was one of the mission proposals being considered for M5.
Voyage 2050 mission concept by ESA (Voyage 2050 L4) The next campaign of the ESA science programme is Voyage 2050, which will cover space science missions operating from 2035 to 2050. Planning began with the appointment of a Senior Committee in December 2018 and a call for white papers in March 2019. Three Large class and six to seven Medium class missions are currently anticipated in this plan, as well as smaller missions and missions of opportunity. On 11 June 2021, the Senior Committee published the Voyage 2050 plan, and recommended the following science themes for the next three Large class missions: • Moons of the giant planets – a mission to an
ocean-bearing gas giant moon. • From temperate exoplanets to the Milky Way – a mission to either characterize exoplanets or investigate the Milky Way's formation history. • New physical probes of the early Universe – a mission to investigate the early universe through the
cosmic microwave background,
gravitational waves, or other fundamental astrophysical phenomena. On 25 March 2024, ESA announced that the first Large-class mission of Voyage 2050 (
L4) would focus on
Enceladus under the "Moons of the giant planets" theme. Three candidates for the first Medium-class of Voyage 2050 (M7) are being studied in preparation for selection expected in 2026 and adoption in 2028: •
M-Matisse: Mars Magnetosphere Atmosphere Ionosphere and Space weather science •
Plasma Observatory: Earth's plasma environment •
THESEUS: Transient High-Energy Sky and Early Universe Surveyor to study high-energy cosmic events == Missions ==