Keystone: a novel terahertz limb-sounding mission advancing chemistry, dynamics, and vertical coupling in the MLT

The Mesosphere and Lower Thermosphere (MLT, ~70–120 km) is a key transition region governing the coupling between the lower atmosphere and near-Earth space. Despite its central role in atmospheric chemistry and dynamics, the MLT remains one of the least observed domains, leading to large uncertainties in composition, temperature, density, and winds, particularly near the mesopause and below the turbopause. A long-standing “holy grail” of MLT research is the direct, global, and time-resolved measurement of atomic oxygen, the dominant energy carrier controlling the chemistry and thermal balance of the region, which has remained inaccessible until recent advances in terahertz (THz) receiver technology.

Keystone is one of the four ESA Earth Explorer 12 candidate missions and is currently undergoing Phase-0 science and system studies. Its primary scientific objective is to provide comprehensive, global, and time-resolved measurements of MLT chemistry, temperature, and dynamics, enabling improved understanding of vertical coupling and wave–mean flow interactions involving gravity waves, tides, and planetary waves from diurnal to seasonal timescales. The mission’s core payload is a high-spectral-resolution supra-THz (1–5 THz) radiometer, complemented by infrared and UV–visible limb instruments. Keystone will retrieve vertical profiles of key neutral species, including direct global measurements of atomic oxygen, together with temperature profiles and mesospheric winds derived from Doppler-shift spectroscopy. These simultaneous observations of neutral dynamics and composition also support improved understanding of the drivers of ionospheric variability, including the neutral wind dynamo governing electrodynamics in the E-region.

Beyond its fundamental science goals, Keystone addresses an important societal challenge. Improved constraints on MLT density and temperature provide physically consistent lower-boundary conditions for thermospheric density models used in satellite drag prediction. By reducing uncertainties propagated upward into the thermosphere, such constraints are expected to yield order-tens-of-percent improvements in residual drag and orbit propagation accuracy, supporting safer and more sustainable operation of the increasingly congested low and very-low-Earth-orbit environment.