In situ chemical composition and density measurements of Earth’s thermosphere, exosphere, and ionosphere with CHESS

The Constellation of High-performance Exosphere Science Satellites (CHESS) mission is a dedicated CubeSat mission designed to analyse the upper atmosphere of Earth in situ. The status of this dynamic region is driven by external and internal forces, causing variations in temperature, the chemical composition, total number density, and their altitude profiles. Although such measurements are key for our understanding of the origin and evolution of the habitable atmosphere of Earth, the community lacks updated, local, detailed measurements. Recent technological advancements in instrumentation enable sensitive in situ measurements of the exosphere–thermosphere–ionosphere region with small satellites. On one hand, the miniaturisation of a high-performance mass spectrometer enables measurements of the chemical composition and density. On the other hand, a new generation of dual-frequency Global Navigation Satellite System (GNSS) receivers enable precise orbit determination. From the knowledge of the evolution of the orbits, the atmospheric drag and hence, the density can be derived complementary. Additionally, the GNSS receivers provide the dispersive line-of-sight total electron content from the linear combination of dual-frequency carrier phase measurements that can be converted into total column density between the spacecraft in low Earth orbit and the satellites of the GNSS in higher orbits. Thanks to the elliptic orbit of the spacecraft, the altitude profiles of these number density measurements of species can be converted into exospheric temperatures. Measuring with a constellation will allow for overcoming the space-time degeneracy to analyse the drivers and mechanisms including basic physics in detail. The CHESS mission is scheduled for launch in early 2026. Once available, the collected data will also be compared with atmospheric escape data on, for example, Venus and Mars to provide insights into the different evolution of these rocky, initially comparable planets.