Very Low Earth Orbit mission concepts for Earth Observation: Benefits and challenges.

Very Low Earth Orbits (VLEO) can be defined as the orbits with a mean altitude below 450 km. Operating in these orbits can provide a number of benefits to Earth observation spacecraft as the spacecraft operates closer to the observation target. These benefits allow the performance of platforms in higher orbits be matched with simpler and smaller platforms in VLEO. This can result in smaller spacecraft and hence in lower costs. Flying at such low altitudes also means flying through a denser part of the atmosphere and thus increased aerodynamic forces. These higher aerodynamic forces can be seen as challenge, but they can also represent an opportunity. These forces can be used for orbit and attitude control, and to de-orbit spacecraft well below the 25-year IADC guideline. Potential shorter orbital lifetimes (due to increased drag) can also represent an opportunity for constellations to replenish their fleets of smaller spacecraft more frequently and thus become more responsive to technology and market changes. In this paper, the different benefits of VLEO with respect to traditional high altitude orbits are quantified considering both optical and Synthetic Aperture Radar (SAR) payloads. The challenges and opportunities emerging from the significant increase in aerodynamic forces are discussed, with some examples of aerodynamic orbit and attitude control provided. The debris resilient properties of these orbits are briefly quantified and discussed with different lifetimes scenarios analysed (from different combinations of altitudes and ballistic coefficients). Finally, several concept studies that highlight the main design drivers of platforms operating at such orbits are briefly presented.