Topic

Very low Earth orbits (VLEOs), defined as 80 – 400 km (ref.), are becoming of interest due to uncertainties in their composition, ionization levels, and movement patterns1. In particular, The altitude ranges from ~80 to ~160 km— that is, the D- and E-regions of the ionosphere—is a critical region where key aspects of the energy and momentum transport between the M-I-T (Mesosphere/Ionosphere/Thermosphere) system take place. While previous LEO missions have provided information on the magnetospheric input into the upper atmosphere and ionosphere—as well as preliminary insight into its response—these missions have been unable to probe the mechanisms of coupling. Concepts for orbital “dipper” missions that regularly dip into very low Earth orbit (VLEO; i.e., the Eregion ionosphere) have been circulating in the community for several decades, no such mission has yet been implemented to answer these questions. Orbits at low altitudes will be defined by high relative spacecraft speeds: circular orbits are on the order of 7-8 km/s, equivalent to an approximate Mach number of 25. This is therefore a regime of rarefied gases and hypersonic flow. In this regime, spacecraft will encounter aerodynamic forces, particularly at the lowest altitudes, that will influence spacecraft orbits through drag. Also, in the regimes between 80 and 200 km, propellant efficient electric propulsion will not be possible due to the magnitude of the drag and composition and density of the atmosphere at those altitudes. Extended missions passing through lowest altitudes will therefore need to address some combination of 1. minimizing drag, 2. countering drag with propulsion using either on board or ambient propellant, 3. and/or possibly adapting the spacecraft to take advantage of the available aerodynamic forces to maintain orbit and/or change orbits. One successful example of maneuvering in this regime is the USSF X37b spaceplane, which has demonstrated aerobraking to change orbits aerodynamically3. To date, civil satellite missions or designs in the VLEO regime (ASTRE2, SOAR4) have not used control surfaces on their spacecraft to maintain orbit. For example, the proposed ASTRE mission would use hydrazine propulsion to remain on orbit; SOAR started at ~400 km and reentered within 8 months. To date, open tests of satellites in the VLEO regime (ASTRE2, SOAR4) have not used control surfaces on their spacecraft. Additional lift/steering forces from such surfaces would allow longer satellite durations and greater data collection. The desired products of this subtopic are therefore the design of either aerodynamic components, or a combination of such components with a viable propulsion system, to maintain orbit, or perform orbital maneuvers, at orbital altitudes that are not just VLEO, but particularly in the 80 km – 120 km region.