LLO

Low lunar orbit is the operational altitude band where spacecraft circle the Moon at roughly 100 to 300 kilometers above the lunar surface. This altitude sits well above the Moon's highest peaks (which reach about 11 km) but low enough that orbital velocity and fuel requirements remain manageable for crewed and uncrewed missions. The term describes both the orbital regime itself and the specific trajectories used during lunar operations.

LLO velocity depends on altitude; at 100 km, orbital speed is approximately 1.68 km/s. Spacecraft in LLO complete one orbit in roughly two hours. This low altitude provides excellent resolution for lunar mapping instruments, proximity for landing operations, and a staging point for descent to the surface. It also minimizes the delta-v required to reach lunar orbit from Earth transfer, a significant constraint in mission design.

Variants and operational use

LLO encompasses a range of altitudes rather than a single fixed height. Polar LLOs, inclined at roughly 90 degrees, are preferred for global surface coverage and scientific observation. Equatorial orbits exist but offer less comprehensive mapping. Crewed lunar missions typically establish a command module in LLO while a lander descends to the surface; the LLO spacecraft serves as the abort-to-orbit destination and remains the crew's lifeline during the landing phase.

Perturbations in LLO are significant. Lunar mascons, regions of concentrated mass beneath the surface, create gravitational anomalies that destabilize orbits over time. Spacecraft in LLO may drift hundreds of kilometers over weeks without periodic station-keeping burns. Long-duration LLO missions therefore require fuel reserves for orbit maintenance or planned descent before drift becomes excessive.

The acronym LLO is standard in aerospace mission planning and spacecraft operations. It distinguishes lunar orbit from other regimes such as trans-lunar trajectory or surface operations. Ground controllers and flight dynamics teams refer to LLO insertion, LLO orbit, and LLO departure as discrete phases in lunar architecture, making the term essential shorthand in mission documentation and real-time operations.