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Ariel
NamesAtmospheric Remote-sensing Infrared Exoplanet Large-survey
Mission typeSpace telescope
OperatorEuropean Space Agency
Websiteesa.int: Ariel
arielmission.space
Mission duration4 years (planned) [1]
Spacecraft properties
Launch mass1,300 kg (2,900 lb) [2]
Dry mass1,000 kg (2,200 lb)
Payload mass300 kg (660 lb)
Start of mission
Launch date2031 (planned)[3]
RocketAriane 62
Launch siteGuiana Space Centre,
Kourou, ELA-4
ContractorArianespace
Orbital parameters
Reference systemSun–Earth L2 orbit[1]
Main Cassegrain reflector
Diameter1.1 × 0.7 m
Focal lengthf/13.4
Collecting area0.64 m2
Wavelengthsvisible and near-infrared
Instruments
Telescope assembly (TA)
Ariel infrared spectrometer (AIRS)
Fine Guidance System (FGS)
A grey opaque circle with the word "ARIEL" written in white across the circle's bottom half. A series of concentric circles close in on the black-colored dot in the "I", with the last circle colored yellow, representing an exoplanet transiting in front of a star.
Ariel mission insignia
← PLATO
EnVision →
ESA's exoplanet missions

Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is a planned space telescope and the fourth medium-class mission of the European Space Agency's Cosmic Vision programme. The mission is aimed at observing at least 1,000 known exoplanets using the transit method, studying and characterising the planets' chemical composition and thermal structures. Compared to the James Webb Space Telescope, Ariel will be a much smaller telescope with more observing time available for planet characterisation. Ariel was expected to be launched in 2029 aboard an Arianespace Ariane 6 together with the Comet Interceptor into the Sun-Earth Lagrange point L2.[4][5][6] However as of January 2026, it is likely to be delayed until 2031.[3]

Background

[edit]

The Ariel mission is being developed by a consortium of various institutions from eleven member states of the European Space Agency (ESA),[a] and international contributors from four countries.[b] The project is led by principal investigator Giovanna Tinetti of the University College London,[8][9] who had previously led the unsuccessful Exoplanet Characterisation Observatory (EcHO) proposal for the M3 Cosmic Vision launch slot.[10][11]; alongside RAL Space, STFC for the PLM engineering consortium leadership.

Operations of the mission and the spacecraft will be handled jointly by ESA and the consortium behind the mission's development, through a coordinated Instrument Operations and Science Data Centre (IOSDC).[7] A Mission Operations Centre (MOC) will be set up at the European Space Operations Centre (ESOC) in Darmstadt, Germany, while a concurrent Ariel Science Operations Centre (SOC) will be set up at the European Space Astronomy Centre (ESAC) near Madrid, Spain.[7] The MOC will be responsible for the spacecraft itself, while the SOC will be responsible for archiving mission data and scientific data downlinked from the spacecraft. The IOSDC will help develop results from the mission based on data received by the SOC.[7]

Objective

[edit]

Ariel will observe 1,000 planets orbiting distant stars and make the first large-scale survey of the chemistry of exoplanet atmospheres.[12] The objective is to answer fundamental questions about how planetary systems form and evolve.[12] A spectrometer will spread the light into a spectrum and determine the chemical fingerprints of gases in the planets' atmospheres.[12] This will enable scientists to understand how the chemistry of a planet links to the environment in which it forms, and how its formation and evolution are affected by its parent star.[12] Ariel will study a diverse population of exoplanets in a wide variety of environments, but it will focus on warm and hot planets in orbits close to their star.[12]

Spacecraft

[edit]

The design of the Ariel spacecraft is based on that intended for the Exoplanet Characterisation Observatory (EChO) mission, and has heritage from the thermal design of the Planck space observatory.[7][13] The body of the spacecraft is split into two distinct modules known as the Service Module (SVM) and the Payload Module (PLM). The PLM will complete its Assembly, Integration and Test (AIT) at RAL Space, STFC. The PLM consists of three aluminium V-Grooves and three pairs of low conductivity fibreglass bipod struts supporting the PLM.[13] A basic horizontal telescope configuration is used for the PLM itself, housing all of the spacecraft's scientific instruments and its oval 1.1 m × 0.7 m (3 ft 7 in × 2 ft 4 in) primary mirror.[13][1] At launch, the spacecraft will have a fuelled mass of 1,300 kg (2,900 lb), and will have a dry mass of 1,000 kg (2,200 lb).[1] The PLM will account for around 300 kg (660 lb) of that mass.[1]

Telescope

[edit]

The Ariel telescope's assembly is an off-axis Cassegrain telescope followed by a third parabolic mirror to recollimate the beam. The telescope uses an oval 1.1 m × 0.7 m (3 ft 7 in × 2 ft 4 in) primary mirror; the imaging quality of the system is limited by diffraction for wavelengths longer than about 3 μm, and its focal ratio (f) is 13.4.[14] The system will acquire images in the visible and near-infrared spectrum.[14] The near-infrared sensor and its front-end driver board is the same as that of Euclid's NISP instrument. To operate its infrared spectroscope between 1.95–7.8 μm, the telescope will be cooled to a temperature of 55 K (−218.2 °C; −360.7 °F).[7][14]

Ariel's 1.1-m-diameter primary mirror will be the largest telescope mirror ever constructed completely from aluminium.[15][16] It will be built in a conventional way from a single large piece of metal.[17] However, for future space telescopes, ESA is developing techniques for joining together multiple aluminium segments to form a single large mirror.[18]

Timeline

[edit]

Development

[edit]

In August 2017, NASA conditionally selected Contribution to ARIEL Spectroscopy of Exoplanets (CASE) as a Partner Mission of Opportunity, pending the result of ESA's Cosmic Vision selection.[19] Under the proposal NASA provides two fine guidance sensors for the Ariel spacecraft in return for the participation of U.S. scientists in the mission.[20] CASE was officially selected in November 2019, with JPL astrophysicist Mark Swain as principal investigator.[21]

In March 2018, ESA officially selected Ariel as its fourth medium-class science mission. At that time, its launch was planned for 2028.[22][23][24] In November 2020, Ariel moved from study to implementation phase, with a launch scheduled for 2029.[25] On 7 December 2021, ESA announced that the €200 million contract to build Ariel's SVM had been awarded to Airbus Defence and Space.[26] In August 2023, Ariel passed its payload Preliminary Design Review.[27] On 6 December 2023, ESA approved the construction of Ariel while maintaining a targeted launch date of 2029.[28]

Construction and testing

[edit]

In October 2024, the mission's construction phase has started at Airbus in Toulouse, France with the assembly of Ariel's structural model.[29]

Launch and trajectory

[edit]

The Ariel spacecraft is expected to be launched in 2031[3] by Arianespace's Ariane 62 launch vehicle.[26][25][30][31] It will be launched from the Guiana Space Centre in Kourou, French Guiana,[30][31] from ELA-4 purpose-built for Ariane 6 launches.[32] Ariel will be launched to the Sun-Earth Lagrange point L2, in a position located at a distance of 1,500,000 km (930,000 mi) from Earth,[30] where it will encounter a very stable thermal environment that is required for its operations.[31]

See also

[edit]

Notes

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

ARIEL

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from Grokipedia
ARIEL, or Atmospheric Remote-sensing Infrared Exoplanet Large-survey, is a mission led by the (ESA) to conduct a large-scale survey of atmospheres, aiming to analyze the of approximately 1,000 s orbiting distant stars. The mission focuses on detecting atoms, molecules, clouds, and hazes in these atmospheres using and photometry, providing insights into planetary formation, evolution, and diversity from rocky worlds to gas giants. Selected as ESA's fourth medium-class mission in the program in 2018 and formally adopted in 2020, ARIEL builds on prior exoplanet observatories like CHEOPS and by prioritizing statistical surveys over individual deep studies. It will observe exoplanets during transits across their host stars, measuring light absorption to map atmospheric properties and interactions with stellar radiation. The spacecraft features a 1.1 m × 0.7 m off-axis Cassegrain feeding two main instruments: the Aerosol Infrared Remote-sensing (AIRS) spectrometer for mid-infrared wavelengths (1.95–7.80 μm) and the Fine Guidance System (FGS) with visible/near-infrared photometry and spectroscopy capabilities. contributes to the Fine Guidance System via the Contribution to ARIEL Spectroscopy of Exoplanets (CASE), providing additional photometry and spectroscopy to observe clouds and hazes. ARIEL is scheduled for launch in 2029 aboard an rocket from Europe's in , sharing the ride with ESA's mission. It will operate from a around the Sun-Earth L2 , approximately 1.5 million km from , for a nominal mission duration of four years, extendable to six. The is being built by a of over 50 institutes across 16 countries, with as the prime contractor for the spacecraft. By complementing telescopes like the , ARIEL aims to create a comprehensive dataset on exoplanetary systems, advancing our understanding of how planets form and whether conditions for life might exist beyond our solar system. As of 2025, the mission is in its construction and testing phases, with ongoing challenges to refine techniques.

Overview

Mission Concept

The ARIEL mission, formally known as Atmospheric Remote-sensing Infrared Exoplanet Large-survey, is the fourth medium-class (M-class) mission selected under the European Space Agency's (ESA) 2015–2025 programme. This ESA-led initiative focuses on conducting a large-scale survey of atmospheres to advance understanding of planetary formation and evolution. At its core, ARIEL operates through transit spectroscopy, observing approximately 1,000 known exoplanets as they pass in front of their host stars to detect and characterize atmospheric compositions via wavelengths. Positioned in a around the Sun-Earth L2, the spacecraft will survey the entire sky every three months, enabling repeated observations of targets over the mission's lifetime. The nominal mission duration is four years following launch, with a potential two-year extension to further expand the dataset. ARIEL's development involves a broad international collaboration, including contributions from NASA through the Contribution to ARIEL Spectroscopy of Exoplanets (CASE) project for instrument integration, participation by more than 50 institutes across 16 ESA member states, and Airbus Defence and Space as the prime contractor responsible for satellite design and construction.

Scientific Context

The discovery of exoplanets began in the mid-1990s with the method, which detected the first Jupiter-mass planet orbiting a Sun-like star, , by measuring the star's wobble induced by the planet's gravitational pull. This technique, followed by the transit method that infers planetary size from dips in stellar brightness during orbital passages, led to the identification of thousands of exoplanets, particularly through NASA's Kepler mission launched in 2009, which revealed a diverse population including super-Earths and mini-Neptunes. Subsequent missions like NASA's (TESS) have continued expanding the census, but the post-Kepler era has highlighted the need for atmospheric characterization to understand planetary formation and evolution, a gap that the (JWST) addresses through detailed but limited observations of individual targets. Despite these advances, significant knowledge gaps persist in the statistical properties of atmospheres, including their chemical compositions and thermal structures, which are crucial for deciphering formation processes from protoplanetary disks to mature systems. Current data show degeneracies in mass-radius relationships that obscure distinctions between worlds, water-rich planets, and gas envelopes, limiting our grasp of the full diversity spanning super-Earths to gas giants and their dependence on host star types and irradiation levels. Without a comprehensive planetary based on atmospheric inventories, theories of migration, accretion, and atmospheric retention remain untested against large samples. ARIEL addresses these gaps as the first space mission dedicated to a large-scale survey of atmospheres, targeting approximately 1000 preferentially warm and hot transiting planets to provide statistically robust data on their compositions. Operating in the 1.95–7.8 μm wavelength range, it will enable the detection of key molecules such as H₂O, CO₂, and CH₄ through transit spectroscopy, revealing elemental abundances like carbon, oxygen, and nitrogen that trace atmospheric origins and dynamics. This approach builds on the statistical detections from Kepler and TESS while complementing JWST's in-depth studies of select targets. By linking atmospheric signatures to planetary bulk properties and host star environments, ARIEL will offer insights into the of planetary systems, including how and influence and composition, ultimately informing indicators of potential in diverse populations. These findings will contextualize the Solar System's architecture within the broader galactic context, advancing our understanding of planet formation pathways.

Objectives and Science Goals

Primary Objectives

The primary objective of the ARIEL mission is to conduct a homogeneous survey of approximately 1000 atmospheres, enabling the determination of their chemical compositions and thermal structures through comparative planetology across a diverse sample. This large-scale statistical approach addresses fundamental questions about exoplanet formation and evolution by measuring molecular abundances, temperature profiles, and dynamical processes in a statistically significant number of targets. To achieve these goals, ARIEL will employ transit spectroscopy to derive transmission spectra that reveal atmospheric layers during planetary transits, eclipse photometry to capture emission spectra indicating dayside thermal emissions, and phase-curve analysis to map longitudinal variations and weather patterns across orbital phases. These techniques will facilitate simultaneous observations in multiple photometric and spectroscopic bands, providing constraints on temperature structures as well as chemical gradients. Expected outcomes include a comprehensive of atmospheric ingredients such as clouds and hazes, alongside monitoring of temporal changes like diurnal or seasonal variations through repeated observations. The mission will also explore correlations between atmospheric properties and fundamental planetary parameters, including and , to elucidate diversity in exoplanet systems. Secondary goals encompass investigating the impacts of host activity on exoplanet atmospheres, such as through corrections for stellar variability, and validating theoretical models of planet formation and migration by linking observed compositions to predicted interior and evolutionary pathways.

Target Selection

The ARIEL mission targets approximately 1000 transiting exoplanets, spanning a diverse range from hot Jupiters to super-Earths, to enable statistical studies of their atmospheres. These targets are selected based on criteria that ensure frequent transits and sufficient signal quality, including short orbital periods, generally less than 50 days. This focus allows for multiple observations per target during the mission's primary phase, facilitating the detection of atmospheric signals through transit spectroscopy. As of 2025, host stars are preferentially FGK dwarfs located within a few hundred parsecs to minimize interstellar extinction and maximize photometric precision, while avoiding highly active stars that could contaminate planetary spectra with stellar variability. Recent updates from TESS and other surveys have expanded the catalogue, incorporating additional multi-planet systems. The selection process relies on a dynamic Ariel Input Catalogue, maintained and updated by the Ariel Mission Consortium through contributions from ground-based and space-based surveys like TESS and . This catalog prioritizes diversity in planetary radii, equilibrium temperatures, and formation environments, incorporating around 100 multi-planet systems to investigate system architectures and potential planet-planet interactions. Observations follow a tiered strategy to balance breadth and depth. The initial survey phase targets about 500 planets with low-resolution to achieve a baseline , followed by a deeper follow-up phase on 100-200 selected targets for higher-resolution measurements of molecular abundances and thermal structures. This approach ensures comprehensive coverage while adapting to new discoveries, with community input guiding refinements to the target list throughout the mission.

Spacecraft

Design and Specifications

The ARIEL spacecraft features a modular architecture comprising a warm Service Module (SVM) and a cold Payload Module (PLM), thermally decoupled via low-conductivity struts and to enable cryogenic operation of the payload while isolating it from the warmer platform systems. The SVM accommodates essential subsystems such as , power distribution, , and attitude control, while the PLM integrates the optical and scientific components. This design supports deployment at the Sun-Earth L2 , where the spacecraft maintains a three-axis stabilized orientation for precise observations. The total launch mass of the is approximately 1400 kg, including about 165 kg of for orbit insertion and maintenance maneuvers. Power is generated by body-mounted solar arrays providing over 1 kW at end-of-life, sufficient to meet the spacecraft's operational demands in the L2 environment. The stowed dimensions measure roughly 3.3 m in height and 2.7 m in width, with solar panels contributing to the overall span when deployed. Cryogenic cooling for the achieves temperatures below 55 K through passive thermal rejection using a series of V-groove radiators, which sequentially operate at approximately 150 K, 100 K, and 60 K to radiate heat to deep space while minimizing solar input. The payload incorporates radiation-hardened components and shielding to withstand the high-radiation flux at L2 over the 4-year nominal mission. The off-axis Cassegrain design reduces interference, enhancing spectroscopic sensitivity. Attitude and orbit control rely on reaction wheels for primary stabilization, supplemented by star trackers and the Fine Guidance System for absolute pointing accuracy better than 1 arcsecond and relative stability down to 0.23 arcseconds over durations. This enables precise tracking of transits lasting up to several days. Compressed science data generation reaches up to 236 Gbit per week, managed via an on-board computer with solid-state mass memory for downlink to ground stations.

Telescope

The ARIEL telescope is a 1-m class off-axis designed to deliver high-quality imaging in the visible and near-infrared wavelengths for atmospheric . It features an elliptical primary mirror measuring 1.1 m by 0.7 m, providing an effective collecting area of 0.64 m², and supports a diffraction-limited of 30 arcseconds. The optical layout includes a primary parabolic mirror, a secondary hyperbolic mirror, and a tertiary off-axis paraboloidal mirror to recollimate the beam, ensuring unobscured light paths free from central obstruction for optimal performance in transit observations. The structure and mirrors are constructed from lightweight aluminum alloy to minimize thermo-elastic deformation under cryogenic conditions and maintain isothermal behavior across the . It is passively cooled to approximately 55 K via the spacecraft's multi-layer V-groove thermal shield assembly, which reduces thermal emission noise in the band. Internal baffles are integrated around the primary mirror and along the to suppress from out-of-field sources, such as the Sun, , and , enhancing contrast for faint signals. Performance specifications include operation across a wavelength range of 0.5–7.8 μm, with average end-of-life throughput exceeding 82% in the infrared (1.95–7.8 μm) due to protected silver coatings on the mirrors. The point spread function achieves an RMS spot radius below 3 arcseconds at 2 μm, ensuring diffraction-limited imaging over the full field of view. Light collected by the telescope is directed as a collimated beam to the common optics module, where dichroic beam splitters separate the wavelengths: longer wavelengths (1.95–7.8 μm) feed the Ariel IR Spectrometer (AIRS), while shorter wavelengths (0.5–1.95 μm) are routed to the Fine Guidance System (FGS) for precise pointing and photometry.

Instruments

Ariel IR Spectrometer (AIRS)

The Ariel IR Spectrometer (AIRS) serves as the core scientific instrument for the ARIEL mission, enabling medium-resolution to characterize the atmospheres of hundreds of exoplanets by detecting molecular signatures and thermal emissions. It operates across the near- to mid-infrared range, providing continuous spectral coverage essential for identifying key atmospheric constituents such as (H₂O) and (CO) in diverse planetary environments. AIRS receives light directly from the ARIEL telescope's off-axis Ritchey-Chrétien via a focal plane sharing mechanism. AIRS employs a prism-based optical design divided into two independent channels to achieve broad wavelength coverage without gaps. Channel 0 (CH0) spans 1.95–3.90 μm with a of approximately R ≈ 100, optimized for higher precision in the shorter-wavelength regime. Channel 1 (CH1) extends from 3.90–7.80 μm with a resolving power varying from R ≈ 30 to 200, allowing flexible adaptation to different molecular features and thermal profiles. This configuration uses dispersive prisms to disperse the incoming beam onto dedicated focal plane arrays, ensuring Nyquist sampling in both spatial and spectral dimensions for robust signal extraction. The instrument's detection system features two HgCdTe sensor chip assemblies, each comprising a × pixel Hawaii-1RG array manufactured by Teledyne Imaging Sensors, tailored for low-noise performance in cryogenic conditions. These detectors are actively cooled to below 42 K using a Joule-Thomson cooler to suppress thermal noise and dark current, enabling high-sensitivity observations over extended integrations. The focal plane assemblies incorporate precision thermal straps and radiation shields for stable operation, with read-out integrated circuits supporting multiple subarray windowing modes to accommodate varying target brightnesses. In terms of performance, AIRS delivers low- to medium-resolution spectra suitable for tracing atmospheric composition and temperature gradients, with a demonstrated sensitivity capable of detecting trace gases like H₂O at levels as low as 10 ppm in atmospheres under nominal observing conditions. The design provides a wide , handling targets from bright s to fainter sub-Neptunes by adjusting integration times and gain settings, while maintaining photometric stability better than 100 ppm per hour. This enables comprehensive thermal mapping and molecular abundance measurements across a statistical sample of exoplanets, supporting ARIEL's goals for population-level atmospheric studies.

Fine Guidance System (FGS)

The Fine Guidance System (FGS) on the ARIEL mission is a multi-functional instrument that combines precise pointing capabilities with scientific observations in the visible and near-infrared wavelengths. As of the 2022 Preliminary Design Review (PDR), the FGS employs two Teledyne HAWAII-1RG mercury-cadmium-telluride (HgCdTe) detector arrays, each 1024 × 1024 pixels. It features three photometric channels and one low-resolution spectrometer, designed to operate within the shared of the module, with NASA's Contribution to ARIEL Spectroscopy of Exoplanets (CASE) subsystem providing additional multiband photometry capabilities. The two visible photometer channels cover 0.5–0.6 μm and 0.6–0.8 μm with a spectral resolution of approximately R ≈ 5, while a third near-infrared photometer extends to 0.8–1.1 μm; the NIR spectrometer spans 1.1–1.95 μm at R ≈ 10, enabling complementary data to the mission's primary observations. The FGS employs advanced HgCdTe detectors to achieve high sensitivity and low noise across its channels. All detectors are passively cooled to cryogenic temperatures below 55 K to minimize thermal noise and enhance performance during long-duration observations. In terms of performance, the FGS delivers pointing stability better than 2 arcseconds over 10-hour observation periods, ensuring accurate alignment for transit spectroscopy. It achieves photometric precision of around 100 parts per million (ppm) for 10th-magnitude stars, supporting high-fidelity measurements essential for atmospheric characterization. This precision also facilitates scatter correction in synergy with the Ariel IR Spectrometer (AIRS), improving the overall data quality for studies. The primary function of the FGS is to provide fine guidance during science observations by computing centroids of guide stars at up to 10 Hz, feeding real-time data to the spacecraft's attitude control system for stable pointing. Additionally, its broadband photometry enables detection of clouds and hazes in atmospheres through integration with NASA's Contribution to ARIEL Spectroscopy of Exoplanets (CASE) subsystem, which enhances sensitivity to scattering effects. The NIR spectroscopy channel offers supplementary contextual data on stellar variability and planetary albedos, enriching the mission's multi-wavelength dataset.

Launch and Trajectory

Launch Details

As of November 2025, the ARIEL mission is scheduled for launch in 2029 from Europe's Spaceport in , , using an rocket in its 62 configuration. This launch vehicle will carry ARIEL as a co-passenger with ESA's mission, delivering a combined capability of approximately 3,300 kg to a Sun-Earth L2 transfer orbit. Following liftoff, the deployment sequence commences with fairing separation at approximately 120 km altitude to expose the payloads to . Payload separation occurs at 200 km altitude after the upper stage achieves the required transfer trajectory. Immediately after separation, ARIEL's solar arrays will deploy to provide power, enabling initial system activations. During the subsequent transfer orbit to L2, lasting about 3 months, ground teams will conduct initial checkout operations, verifying spacecraft subsystems, instrument functionality, and overall health prior to insertion into the operational . The spacecraft's total launch mass is around 1500 kg, including margins for these early maneuvers. To address launch environment challenges, ARIEL incorporates redundant systems designed to withstand vibrations and acoustic loads during ascent. These mitigations ensure reliability through the dynamic phases of fairing jettison and payload release.

Orbital Operations

Following insertion into its operational , the Ariel will maintain a large-amplitude, eclipse-free around the Sun-Earth L2, located approximately 1.5 million kilometers from Earth. This configuration, with a semi-major axis of roughly 1 AU and a period of about 6 months, provides thermal stability and uninterrupted deep-space viewing while ensuring the complete sky is surveyed every three months, allowing some targets to remain observable for over 30% of the mission lifetime. The is designed for long-term stability, supporting operations exceeding 6 years if extended beyond the nominal duration. Ariel's pointing strategy emphasizes efficient repointing to capture transits, with the spacecraft undergoing attitude adjustments between observation slots averaging 7.7 hours in duration to align the telescope with priority targets. This enables continuous monitoring of transit events across diverse planetary systems, briefly referencing target selection modes that prioritize high-yield spectroscopic opportunities. Data acquired during these sessions will be downlinked via X-band communications to ESA's 35-meter ground stations, achieving an average science data rate supporting 236 Gbit per week. In-flight operations will proceed in distinct phases: an initial commissioning period of up to 6 months post-insertion for orbit stabilization, instrument calibration, and performance verification; a primary survey phase lasting 3.5 years focused on the core exoplanet atmospheric dataset; and a potential extension of up to 2 years, which may include guest observer programs for targeted follow-up observations. Thermal management throughout these phases relies on passive radiative cooling, with the payload module's radiators oriented toward the cold deep-space vacuum to maintain the telescope at 50–55 K and minimize fluctuations from repointing maneuvers.

Development History

Mission Selection

The ARIEL mission proposal emerged as part of the European Space Agency's (ESA) Cosmic Vision 2015-2025 programme, which aims to advance understanding of the Universe through targeted scientific missions. In response to a call for medium-class (M-class) mission concepts in 2014, ARIEL was submitted by an international team focused on surveying exoplanet atmospheres using infrared spectroscopy. It was selected in June 2015 as one of three candidate missions for the M4 slot—alongside THOR (a plasma physics mission) and XIPE (an X-ray polarimetry mission)—following a competitive evaluation of 26 initial proposals based on scientific merit, feasibility, and alignment with programme goals. The assessment phase (Phase 0/A) from 2015 to involved detailed studies to refine the mission concept, including trade-offs in instrument design and orbital parameters to ensure achievability within M-class constraints. A Phase A industrial study commenced in late , evaluating technical readiness, cost estimates, and risk mitigation, which culminated in a down-selection review. In March 2018, ESA's Science Programme Committee selected ARIEL as the M4 mission, prioritizing its potential to characterize hundreds of atmospheres and address key questions in planetary formation and evolution over the competing proposals. Following selection, Phase B1 (preliminary definition) from 2018 to 2020 focused on solidifying the mission baseline. ESA member states formally adopted ARIEL in November 2020 during the Science Programme Committee meeting, approving a baseline budget of approximately €550 million for the ESA-provided elements, with additional national contributions for components. This adoption marked the transition to full implementation, contingent on industrial contracts and international partnerships. The ARIEL consortium was established early in the proposal phase, led by the University of College London (UCL) as the principal investigator institution, with Professor Giovanna Tinetti serving as the mission's science lead. It comprises over 50 institutes from 16 ESA member states, including key contributors from (payload elements), (spectrometer development), and (data processing), alongside non-ESA partners such as (providing detector technology) and (optical components for the infrared spectrometer). This collaborative structure ensured broad expertise in exoplanet , instrumentation, and mission operations. Early planning faced challenges in reconciling ambitious scientific objectives—such as observing a diverse sample of over 500 exoplanets—with the M-class cost cap and technical constraints. During Phase A, the design was iteratively refined, simplifying the off-axis architecture and optimizing spectrometer channels to reduce and power requirements while preserving core capabilities for atmospheric composition and structure. These adjustments, informed by feasibility studies and industrial input, maintained the mission's statistical power without exceeding budgetary limits.

Key Milestones

In November 2020, the (ESA) formally adopted the ARIEL mission following successful completion of its Phase A/B1 studies, transitioning it from to . This adoption enabled the awarding of key contracts, including NASA's Contribution to ARIEL Spectroscopy of Exoplanets (CASE) in 2020, which provides fine guidance sensors with multiband photometry capabilities to extend ARIEL's spectral coverage. In December 2021, ESA signed a €200 million contract with to design and build the spacecraft platform, marking the start of hardware development. The mission advanced through rigorous design reviews in 2023, with the payload Preliminary Design Review (PDR) successfully closed in May, confirming that the , Ariel IR Spectrometer (AIRS), and Fine Guidance System (FGS) meet scientific and technical requirements after scrutiny of 179 documents. Later that year, in December, the PDR was passed, validating the overall platform design and allowing progression to prototype manufacturing. These reviews established payload maturity, paving the way for the Critical Design Review planned for subsequent phases. By mid-2022, ESA member states signed an agreement confirming national roles and contributions to the payload consortium, involving over 50 institutes from 16 countries. International partnerships expanded with JAXA's confirmed contribution of optical elements and coatings for the AIRS instrument, alongside ongoing support from and the Canadian Space Agency. The mission has an ESA budget of approximately €550 million, with additional national and international contributions. In 2025, ARIEL reached further operational milestones, including the use of the CDP4-COMET platform as the mission parameter database, enhancing requirements management across development teams. The ESA Datalabs , held in January 2025, engaged early-career researchers in analyzing simulated lightcurve to address stellar contamination and instrument noise challenges. Looking ahead, payload integration is scheduled for 2026, with full mission readiness targeted for 2028 ahead of the 2029 launch on an rocket from , . These timelines account for development delays, including impacts from the and supply chain disruptions.

References

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  27. https://epubs.stfc.ac.uk/manifestation/53814481/STFC-APV-2022-079.pdf
  28. https://www.esa.int/Science_Exploration/Space_Science/Ariel/Ariel_s_instruments
  29. https://epubs.stfc.ac.uk/manifestation/53816780/STFC-APV-2023-036.pdf
  30. https://www.esa.int/Science_Exploration/Space_Science/Comet_Interceptor_factsheet
  31. https://ariane.group/app/uploads/sites/4/2024/10/Mua-6_Issue-2_Revision-0_March-2021.pdf
  32. https://www.esa.int/Enabling_Support/Space_Transportation/Launch_vehicles/Ariane_6
  33. https://sci.esa.int/web/ariel/-/ariel-moves-from-blueprint-to-reality
  34. https://oro.open.ac.uk/76344/1/ARIELgroundsegment.pdf
  35. https://www.technology.stfc.ac.uk/Pages/Project-Showcase/Neon-Joule-Thomson-Cryocooler-for-the-Ariel-Mission.aspx
  36. https://sci.esa.int/web/future-missions-department/-/56560-phase-0-studies-completed-for-the-m4-cosmic-vision-candidate-missions
  37. https://link.springer.com/article/10.1007/s10686-018-9604-3
  38. https://www.esa.int/Science_Exploration/Space_Science/ESA_s_next_science_mission_to_focus_on_nature_of_exoplanets
  39. https://www.esa.int/Space_in_Member_States/United_Kingdom/UK_leads_new_European_exoplanet_mission
  40. https://www.isas.jaxa.jp/en/missions/spacecraft/developing/ariel.html
  41. https://hal.science/hal-01880899
  42. https://www.esa.int/Science_Exploration/Space_Science/Ariel_moves_from_blueprint_to_reality
  43. https://science.nasa.gov/wp-content/uploads/2023/04/Swain_CASE_APAC_June2020_TAGGED.pdf
  44. https://www.esa.int/Science_Exploration/Space_Science/Exoplanet_surveyor_Ariel_passes_major_milestone
  45. https://www.esa.int/Science_Exploration/Space_Science/Ariel/Ariel_moves_from_drawing_board_to_construction_phase
  46. https://arielmission.space/wp-content/uploads/2022/06/Ariel-Press-Release_170622_NewCollaborationAgreement-1.pdf
  47. https://www.stariongroup.eu/how-cdp4-comet-serves-as-the-mission-parameter-database-for-esas-ariel/
  48. https://www.ariel-datachallenge.space/datalabs2025/documentation/about
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