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Artist's concept of exomoon Kepler-1625b I orbiting exoplanet Kepler-1625b. Kepler-1625b I could theoretically have a subsatellite itself.[1][2]

A subsatellite, also known as a submoon or informally a moonmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.[3]

It is inferred from the empirical study of natural satellites in the Solar System that subsatellites may be rare, albeit possible, elements of planetary systems. In the Solar System, the giant planets have large collections of natural satellites. The majority of detected exoplanets are giant planets; at least one, Kepler-1625b, may have a very large exomoon, named Kepler-1625b I, which could theoretically host a subsatellite.[1][2][4][5] Nonetheless, aside from human-launched satellites in temporary lunar orbit, no subsatellite is known in the Solar System or beyond. In most cases, the tidal effects of the planet would make such a system unstable on an astronomical timescale.

Terminology

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Terms used in scientific literature for subsatellites include "submoons" and "moon-moons". Colloquial terms that have been suggested include moonitos, moonettes, and moooons.[6]

Possible natural instances

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Rhea

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Artist's concept of rings around Rhea, a moon of Saturn

There is a possible detection[7] of a ring system around Saturn's natural satellite Rhea that led to calculations that indicated that satellites orbiting Rhea would have stable orbits. The rings suspected were thought to be narrow,[8] a phenomenon normally associated with shepherd moons; however, targeted images taken by the Cassini spacecraft failed to detect any subsatellites or rings associated with Rhea, at least no particles larger than a few millimeters, making the chance of a ring system around Rhea slim.[9]

Iapetus

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It has also been proposed that Saturn's satellite Iapetus possessed a subsatellite in the past; this is one of several hypotheses that have been put forward to account for its unusual equatorial ridge.[10] An ancient giant impact on Iapetus could have produced a subsatellite; as Saturn despun Iapetus, the subsatellite's orbit would then decay until it crossed Iapetus' Roche limit, forming a transient ring which then impacted Iapetus to form a ridge. Such a scenario could have happened on the other giant-planet satellites as well, but only for Iapetus and perhaps Oberon would the resulting ridge have formed after the Late Heavy Bombardment and thus survived to the present day.[11]

Irregular moons of Saturn

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Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon.[12] Other candidates among the Saturnian irregulars include Bestla, Erriapus, and Bebhionn.[13]

Artificial subsatellites

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Historical

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Stamp depicting the first extraterrestrial orbiter (Luna 10) other than in heliocentric orbit and its flightpath, the first artificial satellite around a natural satellite.

Many spacecraft have orbited the Moon since the first one in 1966 (Luna 10).

As of 2024, no spacecraft has successfully orbited any natural satellite other than the Moon. In 1988, the Soviet Union unsuccessfully attempted to put two robotic probes on quasi-orbits around the Martian moon Phobos.[14]

Current

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Launched June 18, 2009, the Lunar Reconnaissance Orbiter (LRO) is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. Data collected by LRO have been described as essential for planning NASA's future human and robotic missions to the Moon. Its detailed mapping program is identifying safe landing sites, locating potential resources on the Moon, characterizing the radiation environment, and demonstrating new technologies.

CAPSTONE is a project that successfully launched on June 28, 2022. Composed of a 12-unit collection of CubeSats which spent a few months in transit to the Moon to arrive on November 14, 2022. It has spent over 2 years in the Moon's Near-rectilinear halo orbit. CAPSTONE is testing and verifying the viability of the planned NRHO of planned future Lunar Gateway and its communication efficiency.[15]

Future planned artificial moon satellites

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The interplanetary spacecraft JUICE launched in 2023 will enter an orbit around Ganymede in 2032, becoming the first spacecraft to orbit a moon other than Earth's.

Additionally, the multi-agency supported Lunar Gateway human-rated space station began construction in April 2024 in a near-rectilinear halo orbit (NRHO), primarily in support of the later stage NASA Artemis program missions to the Moon. Lunar Gateway will also potentially support future missions to Mars and outlying asteroids.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Subsatellite

View on Grokipedia
from Grokipedia
A subsatellite, also known as a submoon in hypothetical natural contexts, is a smaller —either artificial or natural—that orbits a larger satellite. In space exploration, it most commonly refers to an artificial designed to be carried into orbit by a primary and then released to conduct independent operations, enabling efficient deployment of multiple payloads from a single launch. Natural subsatellites, which would be moons orbiting other moons, remain purely theoretical, with no confirmed examples in the Solar System despite studies suggesting that certain large moons like Earth's , Titan, Callisto, and could potentially host stable submoons under specific conditions. The deployment of artificial subsatellites began in the early 1970s during NASA's Apollo lunar missions, marking the first practical use of the concept to extend scientific capabilities beyond the primary spacecraft. The Particles and Fields Subsatellite 1 (PFS-1), released from Apollo 15 on August 4, 1971, orbited the Moon to measure plasma, energetic particles, and magnetic fields, providing data for about six months until its orbit decayed. Similarly, PFS-2 was deployed from Apollo 16 on April 24, 1972, focusing on lunar gravity mapping and environmental studies, though it operated for 34 days before crashing into the lunar surface due to rapid orbital decay. These early subsatellites, each weighing approximately 36 kilograms and hexagonal prisms roughly 78 cm long and 36 cm across, demonstrated the value of secondary payloads in enhancing mission returns without requiring additional launch vehicles. In contemporary space operations, subsatellites have proliferated through the deployment of small satellites, particularly CubeSats, from orbiting platforms like the (ISS). Hundreds of CubeSats—compact, standardized satellites typically measuring 10 cm per side—have been released from the ISS since 2012 using deployers such as NanoRacks CubeSat Deployer and JEM Small Satellite Orbital Deployer, supporting diverse applications including , technology demonstrations, and astrophysics experiments. Notable examples include the MinXSS CubeSat, deployed in 2016 to study solar flares via X-ray observations, and IceCube, released in 2017 to measure atmospheric ice using submillimeter radiometry. This approach has democratized access to , allowing universities, startups, and agencies to conduct low-cost missions while minimizing launch risks for primary .

Fundamentals

Definition

A subsatellite is either a (submoon) that orbits a larger of a or an artificial designed to be carried by and released from a primary to conduct independent operations, typically in orbit around the same central body ( or ). This configuration creates a hierarchical system in the natural case, where the subsatellite is bound primarily to the moon's gravitational influence within the broader , or a deployment sequence in the artificial case, enabling multiple payloads from a single launch. The concept encompasses both hypothetical natural bodies, such as small moonlets captured by a larger , and engineered intentionally placed into orbits for scientific purposes. In natural contexts, subsatellites maintain stable orbits around the parent within its , where the moon's dominates planetary perturbations. Artificial subsatellites, however, are generally not gravitationally bound to the deploying but are released into independent orbits, distinguishing them from co-orbiting . Key characteristics include stability requirements influenced by gravitational fields and tidal forces, with natural formation via capture or accretion and artificial via precise deployment. Subsatellites are distinguished from co-orbiting objects or orbital , which may temporarily share space near a satellite but do not achieve sustained, captured orbits or purposeful deployment; consists of non-functional remnants posing collision risks. The term "subsatellite" implies either natural gravitational capture or intentional for orbital retention, excluding transient flybys or uncontrolled fragments. The term "subsatellite" was first coined in astronomical and literature in the mid-20th century, around 1956, initially to describe artificial satellites released from larger , and subsequently applied to hypothetical moon-of-moon systems to explore multi-tiered orbital architectures.

Orbital Mechanics

For subsatellites intended to orbit a parent moon, such as in submoon systems or certain artificial lunar missions, the dynamics occur within a gravitational where the moon's dominates over the parent 's influence, though the latter introduces significant perturbations through tidal forces. This setup forms a restricted involving the , moon, and subsatellite, with the subsatellite's mass negligible compared to the others. The moon's defines the approximate region of gravitational dominance, limiting stable orbits to fractions of this volume to avoid ejection by the 's tidal field. The Hill sphere radius rHr_H for the moon is given by the approximation rH≈a(m3M)1/3,r_H \approx a \left( \frac{m}{3M} \right)^{1/3}, where aa is the moon's semi-major axis around the planet, mm is the moon's mass, and MM is the planet's mass. This formula arises from the restricted three-body problem, where stability conditions near the collinear Lagrangian points L1 and L2 determine the boundary. At these points, the gravitational acceleration from the moon balances the effective tidal acceleration from the planet in the rotating frame. For small mass ratios μ=m/M≪1\mu = m/M \ll 1, the distance from the moon to L1 or L2 is approximately rL1/L2≈rH(1−(1/3)μ1/3)r_{L1/L2} \approx r_H (1 - (1/3) \mu^{1/3}), but the full Hill sphere serves as a conservative estimate for the onset of instability. Orbits beyond roughly 0.33 rHr_H become unstable due to perturbations, as confirmed by N-body simulations assuming circular, coplanar configurations. Linear stability analysis around L1 and L2 reveals saddle-point equilibria, with unstable eigenvalues leading to exponential divergence for displaced test particles, thus confining long-term subsatellite orbits well inside rHr_H. Planetary tidal forces perturb subsatellite orbits by inducing torques that cause transfer, potentially leading to inward migration toward the moon's or outward ejection beyond the Hill sphere. These arise from the differential gravitational field across the moon-subsatellite , with the perturbation strength scaling as ∝(GM/a3)r\propto (GM / a^3) r, where rr is the subsatellite's distance from the . Mean-motion between the subsatellite and the moon-planet can further destabilize orbits, creating gaps similar to those in planetary rings; for instance, a resonance at Psub/Pmoon=3P_{\text{sub}} / P_{\text{moon}} = \sqrt{3}
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