Why can some lunar orbits persist at all?
An orbit is stable when small nudges don't grow into large deviations. Around the Moon, the main disturbances are the Moon's lumpy mass distribution and the gravitational pull of the Earth and Sun [2][3]. At low altitudes these effects are strong, but at medium altitudes they can be weak enough that long-lived, repeating orbits exist. A survey of low-to-medium lunar orbits found families of periodic orbits with repeating geometries and identifiable stability characteristics, which means not every orbit is doomed to wander off [3].
The Moon's gravity is unusually uneven, so the exact shape of the field matters most close to the surface. One analysis showed that very low orbits need extremely high-resolution gravity models to predict motion accurately, because near-surface paths are highly sensitive to fine details of the Moon's mass distribution [3]. That sensitivity is a practical warning about low orbits, not proof that all lunar orbits are unstable.
Why do very low lunar orbits crash within months?
Very-low-altitude lunar orbits are strongly perturbed by the Moon's gravity field and by third- and fourth-body effects from the Earth and Sun, and several studies expect such orbits to impact the lunar surface within a few months [2]. The practical consequence is that a satellite there cannot simply coast; it needs a station-keeping strategy, meaning repeated small corrective maneuvers, to stay within acceptable margins [2].
One proposed solution is a feedback control law that uses a steerable, throttleable low-thrust engine and keeps the spacecraft within bounds without following a pre-set reference path [2]. That approach was tested with the relevant disturbances included, and it illustrates the key point: low lunar orbits are not automatically stable, but they can be maintained with active control.
Are there special lunar orbits that are stable or only weakly unstable?
Yes, in specific dynamical settings. Near a 1:1 retrograde resonance with the Moon, researchers have identified symmetric periodic orbits around the Earth and analyzed both stable and weakly unstable cases [1]. The existence of weakly unstable cases means instability can be mild rather than catastrophic, and stability is a property of a particular orbit family rather than a blanket rule for everything near the Moon.
Orbits near the Earth-Moon L2 libration point, a gravitational balance region, are described as unstable, which is why missions there require orbit determination and maintenance [4]. Similarly, short-period orbits around the equilateral libration points L4 and L5 were studied with a linear state-feedback control policy to stabilize them against the Sun's gravity, solar radiation pressure, and the Moon's orbital eccentricity [5]. In both cases, the need for control reflects the local dynamics, not a universal instability of all moon orbits.
What actually decides whether a lunar orbit lasts?
The deciding factors are altitude, orbit shape, and which gravitational influences dominate at that location. Low orbits sit close to the Moon's uneven mass and are strongly disturbed, while medium-altitude orbits can be modeled with averaged Earth gravity and still yield long-duration periodic families [3]. Orbits near resonance or near libration points have their own stability character, sometimes stable, sometimes weakly unstable, and sometimes unstable enough to demand control [1][4][5].
The evidence supports a practical answer rather than a sweeping one: instability is common in some lunar regimes, especially very low orbits, but it is not inherent to moon orbits in general. The right question is not whether moon orbits are unstable, but which orbit, at what altitude, under which perturbations, and with how much control.
Sources used in this answer
3D stable and weakly unstable periodic orbits around the Earth near the retrograde co-orbital resonance with the Moon
Symmetric periodic orbits near the 1:1 retrograde resonance with the Moon include both stable and weakly unstable cases.
Low-Thrust Nonlinear Orbit Control for Very Low Lunar Orbits
Very low lunar orbits are strongly perturbed by the Moon's gravity and by Earth and Sun effects and are expected to impact the surface within a few months without station-keeping control.
Gravity modeling for lunar orbits
Low-to-medium lunar orbits include long-duration periodic families with repeating geometries and stability characteristics, and very low orbits require high-resolution lunar gravity models.
Integrated orbit determination and maintenance in Earth-Moon unstable dynamics environment
Orbits near the Earth-Moon L2 libration point are unstable, motivating integrated orbit determination and maintenance.
Controlled short-period orbits around Earth-Moon equilateral libration points for Lunar Occultations
Short-period orbits around the Earth-Moon equilateral libration points can be stabilized with a linear state-feedback control policy under solar gravity, radiation pressure, and lunar eccentricity.