Summary:
Earth’s upper atmosphere is a dynamic system that is determined by external and internal forces. Understanding this system allows for insights into the evolution of a habitable world and, therefore, the origin of our Solar System. In addition, it is the environment for many satellites that are key to our modern world. The uppermost part of Earth’s atmosphere, the exosphere and ionosphere, couples the collision-dominated thermosphere with outer space, where other processes determine the trajectories of the particles. Whereas the thermosphere is mostly gravitationally bound to the planet, a fraction of the particles present in the exosphere leaves Earth into interplanetary space. Over geologic times, these loss processes are an important factor in the evolution of an atmosphere and are considered being the main reason why Earth currently has a habitable atmospheric surface composition in contrast to Venus and Mars, which were all very similar once. In the short term, the variability of the Sun’s radiation, including both photons (from XUV to IR) and energetic particles causes considerable variations in the chemical composition, the density, and the spatial extent of the exosphere. The exobase is located at about 300 km altitude and the exosphere extends to tens of thousands of kilometers. Detailed knowledge of the exosphere is not only interesting for our basic understanding but is also important for spacecraft in near-Earth orbits, like the International Space Station. Although density and chemical composition are closely related, our present knowledge depends on separate measurements of the chemical composition, performed in the early 1980s, and the density, mostly performed during the 1990s. To end this long data gap, the community requires new in-orbit composition measurements, and to overcome the time-space degeneracy, it is necessary to measure both the chemical composition and the density of the exosphere with a network of satellites at several locations simultaneously. The constellation of high-performance exospheric science satellites (CHESS) program comprises a constellation of two 3U CubeSats, and more units later, designed to create an inventory of chemical species present in the exosphere, measure the density, and record their variability over both space and time. Each satellite is equipped with a novel time-of-flight mass spectrometer for both density measurements and highly sensitive chemical composition analysis of major to trace amounts of species. The payload is complemented by a new generation of dual-frequency global navigation satellite system (GNSS) receivers for precise orbit determination. It allows for computing the total density from estimates of atmospheric drag and the dispersive line-of-sight total electron content from the linear combination of dual-frequency carrier phase measurements. This pathfinder mission is the first step towards a permanent observation of the Earth’s exosphere from several vantage points simultaneously. It is designed to provide scale heights of each chemical species, their altitude profiles and exospheric temperatures to determine atmospheric escape parameters. Furthermore, it allows for analyzing the spatial and temporal variability to infer the drivers of the exosphere including the impact of anthropogenic climate change, improve satellite orbital decay models, and test possibilities of capturing earthquake precursors.
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