Skimming Satellites: On the Edge of the Atmosphere
Building spacecraft is no simple feat, but designing a vehicle to operate outside Earth's atmosphere presents a unique challenge: managing aerodynamics. At the altitude where most satellites operate, drag is minimal, allowing for unconventional designs. However, this lack of drag has a trade-off. If a spacecraft orbits high enough, it might stay in space indefinitely without a way to reduce velocity, rendering it stranded.
In the past, this wasn't a concern, but today's crowded low Earth orbit (LEO) demands attention. With launch costs dropping, companies plan vast satellite constellations. A solution emerges: very low Earth orbit (VLEO). VLEO satellites experience enough drag to require constant thruster firing, ensuring self-cleaning if propulsion fails. Lower altitudes offer practical benefits like reduced latency.
The European Space Agency's (ESA) Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) exemplifies VLEO success. Operating at 255 km, it dropped to 229 km, far below traditional satellite altitudes. Its dart-like design, with solar panels mounted on the body and stubby tail fins, defied conventional satellite aesthetics.
GOCE's mission lasted four and a half years, ending due to propellant depletion. The Japan Aerospace Exploration Agency (JAXA)'s Super Low Altitude Test Satellite (SLATS) achieved an even lower orbit, reaching 167.4 km. SLATS, smaller and with less propellant, used chemical thrusters at lower altitudes. Its design, resembling an airplane, was more conventional, with solar arrays parallel to the body.
These missions showcase careful planning rather than technological breakthroughs. Ion propulsion and aerodynamics aren't new, but their novel application in VLEO is groundbreaking. The potential for a new breed of VLEO satellites, powered by air-breathing electric propulsion (ABEP) akin to jet engines, offers promise. DARPA and ESA studies, like Redwire's SabreSat, aim to revolutionize satellite design, with military and civilian applications. While test flights are years away, ABEP satellites may alleviate orbital congestion, ensuring space remains accessible.
