Mission Overview

The Keystone mission would explore the Mesosphere-Thermosphere System (MTS) between 50 and 250 km of altitude, which encompasses the Earth’s mesosphere, thermosphere, and the overlapping D-and E-regions of the ionosphere. The MTS is the least directly observed region of Earth’s atmosphere, yet it governs the exchange of composition, energy and momentum between the lower atmosphere and near-Earth space. This knowledge gap limits the ability to assess key processes including satellite drag, space weather impacts, and upper-atmosphere climate change.

Schematics of the Mesosphere Thermosphere System (MTS), the critical hub of chemical and radiative energy exchanges between space weather, the ionosphere, and the neutral atmosphere below. (Credit: Jörg Gumbel, Stockholm University; Figure: ESA Graphics Team)

The MTS is a transition region driven by both terrestrial and solar influences. Keystone would provide critical insights into how MTS composition and dynamics respond to these influences, improving whole-atmosphere understanding. Beyond this fundamental science value, Keystone would also deliver practical socio-economic benefits, including: improved modelling of satellite drag and orbital decay, to support mitigation solutions for satellites operations (such as collision avoidance); assessing the atmospheric impacts of spacecraft re-entry; better assessment of climate forcing by quantifying upper-atmosphere cooling; and improved physical understanding of ion-neutral coupling processes relevant to enable space weather resilience strategies for communications and navigation. Keystone would address this gap by providing simultaneous limb observations of the key MTS state variables, enabling a step change in whole-atmosphere modelling and prediction. Keystone would provide a wealth of data relating to the physics and chemistry of this interfacial region where the atmosphere transitions from the dense neutral regime of the lower atmosphere to the electrodynamically driven environment of near-Earth space.

Atomic oxygen (O), produced from the photodissociation of molecular oxygen (O2), is the major reservoir of chemical potential energy in the MTS: between 80 and 120 km, the chemical heating rate from exothermic chemical reactions involving O significantly exceeds direct heating from solar radiation. With a lifetime greater than a day, O transports chemical potential energy within the MTS (vertically and horizontally). This leads to heating in the polar night, even in the absence of solar radiation, and to a significant energy flux from the thermosphere into the mesosphere. Moreover, many reactions involving O lead to chemical products in excited states which emit light; these chemiluminescent reactions are an important energy loss process in the MTS (e.g. the recombination of O with NO is critical for thermal recovery of the thermosphere after geomagnetic storms).

Keystone would provide the first direct observations of O across the MTS, on a global and continuous basis. This would allow closure of the global MTS energy and chemical budgets, which currently cannot be achieved from indirect observations.

Although O is the most important atmospheric constituent in the MTS, controlling nearly all chemistry and playing a major role in thermal balance and dynamics, its concentration has only ever been inferred indirectly by observing emissions from various product species of exothermic reactions. Keystone would observe O directly by exploiting the very low energy spin-orbit transitions of the ground state of O, which occur in the Terahertz (THz) region of the electromagnetic spectrum.

Mission Concept

The Keystone mission is conceived as a single-satellite low Earth orbit mission with continuous limb viewing, where the atmospheric volume of interest is sampled through a combination of orbital motion and regular line-of-sight scanning. The mission is designed to operate as an observatory with stable and repeatable observation geometries, well-defined calibration opportunities and co-located measurements across instruments. A Sun-synchronous orbit provides the most favourable balance between global coverage, illumination stability, thermal control and operational simplicity.

Keystone would be combining three co-registered instruments with a common vertical scanning approach. These instruments are: 1) a THz spectrometer operating between 1.1 and 2.2 THz, the first sensor of this kind deployed in space; 2) an IR instrument measuring broadband radiance profiles in a set of channels between 2 and 15 μm; and 3) a UV-Vis spectrometer operating between 240 and 800 nm.

Artist's impression of the Keystone satellite — Artist’s impression of the Keystone satellite with three co-located, limb-scanning payloads (Figure: OHB Germany)

The limb-scanning method dissects the atmosphere to return stacked vertical profiles of observables at the tangent point. It offers unmatched vertical resolution, which is important in the MTS, a region defined by steep vertical gradients, intricate structures, and strong variability.

Limb-Scanning Geometry — The Keystone satellite scans the limb of the atmosphere for very high vertical resolution measurements of trace gas, temperature, and wind profiles (Figure: OHB Germany)