The space science community has narrowed down the shortlist for ESA’s next ‘medium’ mission to three finalists: M-Matisse, Plasma Observatory and Theseus. Following further study, one will be selected for implementation as the newest addition to ESA’s space science mission fleet.
It is expected that one candidate mission will be chosen by mid-2026.
M-Matisse would study Mars using two spacecraft, each carrying an identical set of instruments to observe Mars simultaneously from two different locations in space. In particular, M-Matisse would shed light on how the solar wind influences Mars’s atmosphere, ionosphere and magnetosphere. The mission aims to investigate the impact of these interactions on Mars's lower atmosphere and surface, which is a key aspect to understand the Red Planet’s habitability, as well as the evolution of its atmosphere and climate.Plasma Observatory is a seven-spacecraft mission to study the environment of electrically charged particles (called a plasma) around Earth. It would focus on two questions: how are particles energised in space plasmas? What processes dominate energy transport and drive interactions between the different regions of Earth’s magnetospheric system? Plasma Observatory would complement ESA’s current and planned missions looking at the Sun-Earth interaction to support our understanding of how the solar wind affects our planet, ultimately helping us keep life and technology safe from its effects.Theseus is a multi-instrument mission focusing on high-energy, short-lived events in the cosmos. In particular, Theseus would look at gamma-ray bursts near and far. Nearby, shorter-lived, gamma-ray bursts are likely counterparts to gravitational waves released by merging neutron stars. Distant, longer-lived gamma-ray bursts would help us understand more about the emergence of the first galaxies in the Universe. Overall, Theseus would cover a broad spectrum of science, from stellar astrophysics and the effects of stellar activity on exoplanets, to the physics of matter accretion and particle acceleration processes.