The selection process for Athena began in March 2013, when ESA issued a call to the European science community to suggest the scientific themes to be pursued by the Cosmic Vision programme's second and third Large missions of the ESA Science Program. In November 2013, the theme of "the Hot and energetic Universe" was selected for L2, with "the gravitational Universe" (now LISA) selected for L3. However, after 8 years of Phase A and Phase B1, the meeting of the ESA Science Program Commitee in June 2022 deliberated that Athena will not be adopted, because the estimated Cost-at-Completion to ESA exceeds the budget allocated for an L-class mission. The mission is therefore undergoing a design-to-cost exercise, aiming at defining a profile consistent with a strict cost cap while preserving its flagship nature.

Science Goals

Athena is a powerful, general-purpose open observatory, able to address a wide range of current astrophysical topics. Its scientific payload will allow unprecedented studies of a wide range of astronomical phenomena. These include distant gamma-ray bursts, the hot gas found in the space around clusters of galaxies, accreting compact objects such as black holes over their whole mass range and neutron stars, supernova explosion and remnants, stars, white dwarfs, exoplanets and their parent stars, Jupiter's auroras and comets in our own Solar System, and the interstellar medium (gas and dust). By combining a large X-ray telescope with state-of-the-art scientific instruments, Athena shall address key questions in astrophysics, among them:

• ascertain the nature of the primary source of high-energy radiation in stellar-mass and super-massive accreting black holes (Active Galactic Nuclei, AGN), and its connection with accretion and ejecton mechanisms close to the event horizon;

• determine the mechanism(s) regulating the cosmological co-evolution of accreting black holes and their host galaxies;

• measure the space density of the AGN that dominate the black hole growth;

• constrain the kinematics of hos gas and metals in massive halos (galaxy clusters and groups);

• map the properties of the most common baryonic reservoirs in the Universe, and probe their evolution and connection to the cosmic web;

• constrain supernova explosion mechamisms through the determination of the 3-dimensional kinematics, ionization state and abundances in young remnants;

• provide novel and unprecedented constraints on the Equation of State of Neutron Stars;

• study Solar-Planet Interactions through the stellar magnetic activity in exoplanet-hosting systems.

 Athena will also provide a key contribution to multi-messenger astrophysics, in synergy with gravitational wave arrays and neutrino telescopes.

Mission AND SCIENtific payload

The scientific payload of Athena comprises:

- the X-ray Integral Field Unit (X-IFU) a cryogenic imaging spectrometer covering the 0.2 to 12 keV energy range with unprecedented energy resolution of 2.5 eV at 7 keV over a field of view a few arcminutes across. Thanks to the telescope de-focusing capabilities, it will be able to perform spectroscopy of sources up to the sub-Crab regime (2-10 keV flux ~10^-8 erg cm^-2 s^-1) with only moderately degraded energy resolution;

and

- the Wide Field Imager (WFI) covering the 0.1 to 12 keV energy range, based on a silicon active pixel sensor. It features a large field of view, excellent spatial and energy resolution and count rate capabilities up to the Crab regime and beyond.

During each sky observation one of the two instruments will be placed in the focal plane of a single X-ray telescope with a focal length of 12 meter (limited by the launcher fairing). The Athena optics ensure a large collecting area exceeding 1 m² at 1 keV, an angular resolution of 5" (Half-Energy Width, requirement at 1 keV on-axis) over a field-of-view >40' side. Degradation of angular performance off-axis and vignetting are moderate by design. This combination of large area, high angular resolution and large field of view are made possible using the Silicon Pore Optics (SPO) technology developed by ESA and Cosine Measurement Systems over the last decade. SPO utilises commercially available Si wafers, which have surface figure and roughness quality ideally suited to X-ray optics applications, while ensuring an area-to-mass ratio largely exceeding any other X-ray optics technology flown in space so far.

Athena is currently baselined to be launched into a halo orbit at the Sun-Earth L1 (1^st Lagrangian) point, which provides for high observing efficiency, uninterrupted observations, and a benign thermal environment.