Low Lunar Orbit

Low Lunar Orbit, or LLO, is the close, fast lap a spacecraft makes just above the Moon’s surface. It is where landers leave from on their way down and return to on their way back up.

Quick facts

• Altitude: below about 100 km (62 mi); Apollo used a nominal ~110 km (60 nautical mile) circular orbit, and the Lunar Reconnaissance Orbiter has flown as low as ~50 km.
• Orbital period: about 2 hours (roughly 1.96–2.13 hours across the ~100–200 km range).
• Orbital speed: about 1.6 km/s (~1.63 km/s for a ~100 km circular orbit).
• Inclination: any angle is possible, but only four “frozen” inclinations — 27°, 50°, 76°, and 86° — allow long-term low-altitude survival; mapping orbiters usually fly near-polar (~90°).
• Staying power: stationkeeping at ~50 km needs a nudge roughly every ~28 days; an unmaintained low orbit can crash in a matter of weeks (Apollo 16’s PFS-2 subsatellite hit the Moon after only 35 days).

How it works

A spacecraft in Low Lunar Orbit is in constant free-fall around the Moon. Its forward speed of about 1.6 km/s exactly balances the Moon’s gravity, so its curved path matches the curve of the Moon itself — it keeps circling instead of falling in or flying off. Think of swinging a ball on a string: the speed of the swing is what keeps it going around rather than dropping.

Because the Moon has no atmosphere, there is no air resistance (aerodynamic drag) to slowly steal altitude the way there is in Low Earth Orbit. That means orbits can sit very low. The catch is that the Moon’s gravity is lumpy. Buried under large impact basins are dense “mass concentrations,” or mascons, that locally strengthen gravity by about half a percent. As a low orbiter passes over these lumps again and again, the repeated tugs stretch the orbit’s shape (its eccentricity) until the low point dips into the surface — an impact in a matter of weeks.

Spacecraft fight back in two ways: periodic stationkeeping burns (small engine firings to correct the orbit), or flying at one of the four “frozen” inclinations — 27°, 50°, 76°, or 86° — where the tugs largely cancel out and the orbit stays stable with little or no fuel.

Why it’s used

LLO is the natural staging orbit for reaching the lunar surface and getting back. A lander separates from a vehicle parked in LLO, descends, and then the ascent stage climbs back to meet it there. This was the Apollo approach and remains the model for Artemis-era landings.

Its low, quick pass over the ground also makes it ideal for science: close-range imaging, altimetry (measuring surface height), and mineral and gravity mapping. A near-polar LLO lets a mapper photograph the entire surface, including the poles, as the Moon slowly rotates beneath it, all while staying close to its targets. In short, LLO is chosen whenever being near the surface — for landing access or fine detail — matters more than the cost of fighting the orbit’s instability.

Notable missions

• Apollo Command/Service Modules (Apollo 8, 10–17): flew nominal ~110 km near-circular orbits with ~2-hour periods, serving as the parking orbit while the Lunar Module went down and came back.
• Lunar Reconnaissance Orbiter (LRO, 2009–present): a NASA mapping orbiter in a near-circular polar low orbit, operated as low as ~50 km and nudged back into place roughly every ~28 days.
• Apollo 15 subsatellite (PFS-1): released near a frozen inclination (~28°) and survived about 1.5 years — a demonstration of frozen-orbit stability.
• Apollo 16 subsatellite (PFS-2): released at an unstable ~11° inclination and crashed after only about 35 days — a textbook example of LLO instability.
• Lunar Prospector (1998–1999): mapped from a low polar orbit (nominally ~100 km, later lowered); its tracking data helped refine the lunar gravity and mascon field.

Low Lunar Orbit is a trade between closeness and stability: fly low for the best resolution and the cheapest, fastest access to the surface, but pay for it with constant correction or be limited to a frozen inclination. That instability is a big reason modern crewed plans like Artemis often keep an orbiting station in a more stable Near-Rectilinear Halo Orbit and treat LLO as a brief staging stop for the landing itself.