Science Goals (SG)

Keystone’s Science Goals not only address fundamental gaps in atmospheric science but will also deliver direct socio-economic benefits by improving satellite orbit prediction, climate projections, and space weather resilience.

SG.1: MTS Characterisation

To comprehensively characterise the density, composition, chemistry and thermal balance of the MTS, by pioneering observations of atomic oxygen – the missing keystone for understanding the critical transition region connecting the Earth’s atmosphere and space – together with key trace gases, thermal heat loss, and airglow.

SG.2: Multi-Scale Coupling

To study and assess coupling on multiple scales in the middle and upper atmosphere. This includes internal phenomena such as large-scale atmospheric waves, space weather effects, and MTS inter-hemispheric coupling.

SG.3: Whole-Atmosphere Modelling

To improve the representation, prediction and projection of the whole-atmosphere chemistry-climate system in next-generation whole-atmosphere models, with applications to space weather resilience, climate forcing, and satellite drag forecasting.

Societal Relevance: Resilience of Space Infrastructure

The proliferation of large satellite constellations has produced an urgent socio-economic need for accurate predictions of thermospheric drag and density variability. Keystone will help improve orbit forecasts, reduce collision risks, and lower costs for space operators. Furthermore, by enhancing climate projections and space weather resilience, Keystone will directly contribute to societal preparedness and the protection of infrastructure worth billions of euros. This will underpin the evolution of space weather models and forecasting capabilities.

Comparison of "true" atomic oxygen distribution vs. empirical model predictions — Atomic oxygen in the MTS according a more accurate physics model (WACCM) and a computationally cheap empirical model (MSIS) routinely used in the prediction of residual atmospheric drag and orbital trajectories. (Credit: WACCM-X data: Dan Marsh, Uni Leeds; Figure and MSIS data: Tom Dörffel, IAP Kühlungsborn)

Mission Objectives (MO)

Building on these Science Goals, Keystone’s four Mission Objectives are:

MO.1: MTS Composition Profiles

To observe vertical profiles of atomic oxygen and a wide range of important trace gases including O3, CO2, NO and OH as well as noctilucent cloud properties. These gases are necessary to determine the density, thermal balance and photochemistry in the MTS.

MO.2: Radiative Energy Balance

To observe coincident IR and UV-Vis emissions, vital for constraining radiative energy balance and validating models of the MTS.

MO.3: Temperature Profiles

To retrieve kinetic temperature profiles from THz, IR, and UV emissions to characterise atmospheric structure and support dynamic analyses.

MO.4: LOS Neutral Wind Profiles

To observe line-of-sight neutral wind profiles from Doppler shifts in THz emission lines to constrain constituent transport

Data Products

Altitude coverage envisaged for Keystone observations of atmospheric state variables, key constituents and emissions in the Mesosphere Thermosphere System:

User Communities

Keystone would be of high importance to many international programmes devoted to climate change, space weather, and space infrastructure and its resilience, under governmental and non-governmental organisations, e.g., the World Meteorological Organization (WMO), the International Science Council (ISC) and the International Union of Geodesy and Geophysics (IUGG).

In summary, Keystone would deliver the first direct, global observations of atomic oxygen together with co-located observations of composition, temperature and line-of-sight winds, enabling closure of energy and chemical budgets in the MTS. This would provide a new, observation-constrained basis for modelling the coupled atmosphere-space system, with many socio-economic benefits.