Lunar Gravitational Wave Antenna
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| Mission type | Gravitational-wave observatory |
|---|---|
| Operator | ESA (proposed) |
| Mission duration | 10 years (proposed) |
| Launch date | 2035 (proposed) |
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ESA Missions and Studies |
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The Lunar Gravitational Wave Antenna (LGWA) is a proposed lunar-based observatory designed to detect low-frequency gravitational waves through precision seismology on the surface of the Moon. The concept exploits the Moon’s exceptionally low seismic noise, lack of atmosphere, and stable thermal environment to sense gravitational-wave–induced deformations in the lunar crust. LGWA is intended to operate in the intermediate frequency band between the sensitivity ranges of terrestrial interferometers such as LIGO, Virgo and KAGRA, and space-based detectors such as LISA.
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The mission concept has been developed by an international collaboration of researchers in gravitational-wave physics, lunar geophysics, and planetary exploration engineering.
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Background
Gravitational waves produce tiny deformations in spacetime that can cause detectable elastic responses in planetary bodies. The Moon, with its low tectonic and oceanic noise and high mechanical quality factor, is considered an ideal natural detector for seismic gravitational-wave sensing. The LGWA concept builds on earlier research showing that the lunar body amplifies responses to gravitational waves in the millihertz-to-hertz band, providing a complementary detection window to interferometric missions.
History
The seminal idea of using an entire celestial body for detecting gravitational waves was formulated by Joseph Weber.[3] He proposed that the Earth itself could serve as a natural resonant antenna whose normal modes would respond to the passage of gravitational waves, with seismometers providing the corresponding readout. Although the sensitivity achievable with contemporary instruments proved insufficient, the concept introduced the broader idea of exploiting the elastic response of macroscopic celestial bodies for gravitational-wave detection.[3]
Following Weber’s proposal, several researchers in the 1960s and 1970s examined the theoretical coupling between gravitational waves and the normal modes of planets and stars. A landmark analysis by Freeman Dyson demonstrated that gravitational waves could excite low-order spheroidal modes of the Earth and developed estimates of their observable seismic signatures, although predicted amplitudes were far below instrumental noise at the time.[4] These early theoretical studies established the mathematical foundation for later work on gravitational-wave excitation of planetary normal modes.
Interest in planetary-scale gravitational-wave detection was renewed in the context of lunar seismology. The Apollo Lunar Surface Experiments Package (ALSEP) deployed highly sensitive seismometers on the Moon between 1969 and 1977, providing the first long-term record of seismic activity on another celestial body.[5][6] While these instruments, like the Lunar Surface Gravimeter, were not designed for gravitational-wave science, their exceptionally low seismic noise enabled early attempts to place empirical limits on gravitational-wave energy densities through analyses of lunar seismic data.[7] Subsequent work quantified the background noise conditions on the Moon and further highlighted its potential as a platform for precision seismology relevant to gravitational-wave studies.[8]
In 2009, the feasibility of implementing a lunar gravitational-wave detector inspired by Weber’s original concept was revisited in the context of emerging technological advances.[9] During the 2010s, advances in seismic instrumentation, computational modeling, and gravitational-wave astrophysics prompted renewed interest in planetary-scale gravitational-wave detection. Further investigations demonstrated that Earth’s normal modes could, in principle, be used to search for low-frequency gravitational waves through global seismometer networks.[10] Building on related techniques, further analyses later quantified the seismic response of the Moon to gravitational waves and outlined the scientific case for a lunar-based gravitational-wave observatory using modern broadband seismometers.[1]
More recent studies have refined sensitivity estimates and examined seismic-background limitations for future lunar gravitational-wave detectors.[11]
These developments laid the theoretical and technological groundwork for the concept of the Lunar Gravitational Wave Antenna.
Science Objectives
Lunar science
A central objective of LGWA is to advance the study of the Moon’s interior and seismic environment. Because the Moon acts as the test mass of the detector, its elastic properties and natural seismic background must be understood with high precision in order to extract gravitational-wave signals. The LGWA seismic array is therefore designed not only as a gravitational experiment but also as a next-generation lunar seismological observatory, with performance exceeding that of all previous lunar seismic investigations, including the Apollo missions.
Seismic sources
The Apollo Passive Seismic Experiment revealed that the Moon remains seismically active, recording more than 13,000 seismic events over seven years of operation.Cite error: The opening <ref> tag is malformed or has a bad name (see the help page). These events include deep moonquakes, shallow moonquakes, thermal quakes, and meteoroid impacts. However, the Apollo network was restricted to the nearside of the Moon and had relatively limited sensitivity, leaving farside seismicity poorly constrained.[8]
LGWA’s broadband accelerometers, with sub-picometer sensitivity above 0.1 Hz, are expected to detect seismic events significantly weaker than those observed by Apollo.[1] With multiple stations distributed inside permanently shadowed regions, LGWA would improve the detection and localization of moonquakes and enable characterization of seismicity on the lunar farside.
Internal structure
The Moon’s normal modes—global free oscillations excited by seismic events—provide a powerful probe of its interior structure. Apollo instruments recorded only a limited subset of these modes, and later studies showed that many candidate detections lay below instrument noise levels.[12]
LGWA is expected to resolve lunar normal modes with high signal-to-noise ratio due to its improved sensitivity and noise-cancellation strategies, enabling tighter constraints on crustal thickness, mantle layering, core size, and attenuation properties.[13] Techniques such as waveform cross-correlation, already demonstrated for Mars seismology, may be used to infer global internal structure even with a small number of seismometers.
Formation history
Improved knowledge of the Moon’s internal structure will inform models of its origin and early evolution. Competing formation scenarios, including giant-impact models and subsequent differentiation processes, make distinct predictions for core size, mantle composition, and thermal evolution. By resolving normal modes and small seismic events with unprecedented clarity, LGWA would contribute to refining these geological and geochemical models.[14]
Geological processes
LGWA’s seismic measurements can improve understanding of processes that shape the lunar surface. Continuous micrometeoroid bombardment generates a persistent “meteoritic seismic hum”, providing information about regolith production and surface gardening rates.[15]
LGWA data may also constrain tectonic patterns such as wrinkle ridges and graben systems, whose distribution reflects crustal thickness variations and thermal contraction processes.[16]
Local geological studies, including crater morphology and regolith thickness mapping, can be enhanced through co-located LGWA measurements and future lunar surface missions.
Low-frequency gravitational-wave detection
The Lunar Gravitational-Wave Antenna is designed to operate in the decihertz band between approximately 1 mHz and 1 Hz. This band contains sources that are inaccessible to existing instruments, including early inspiral phases of compact binaries, white-dwarf mergers, and a variety of transient astrophysical phenomena.[1]
Astrophysical explosions
A number of energetic astrophysical events produce gravitational waves with characteristic frequencies in the decihertz range. These include mergers of white-dwarf binaries generating Type Ia supernova explosions, and accretion-induced collapse of massive white dwarfs.
LGWA may also detect signatures from asymmetric core-collapse supernovae, including so-called “choked jets” that do not produce observable gamma-ray emission but generate low-frequency gravitational waves with memory-like components.[17]
Many of these sources produce waveforms that are difficult to model and lie below the sensitivity bands of terrestrial interferometers, making LGWA a complementary probe of explosive stellar phenomena.[2]
Compact-object populations and formation channels
The decihertz band contains large populations of compact binaries whose orbital frequencies evolve through the LGWA sensitivity range. These include double white-dwarf systems, white-dwarf–neutron-star binaries, neutron stars pairs, and mixed black-hole–neutron-star binaries.[18]
LGWA could also observe early inspiral phases of stellar-mass black-hole binaries and mergers of intermediate-mass black holes in the range of 103–106 solar masses.[1]
Systems with extreme or intermediate mass ratios (EMRIs, IMRIs), which provide precision tests of strong-field gravity, are also expected to emit gravitational waves in the decihertz band. Additional potential sources include tidal disruptions of white dwarfs by intermediate-mass black holes and continuous-wave emission from asymmetric, rapidly rotating neutron stars.[1][2]
Early warnings and sky localization
Because compact binaries can remain in the decihertz band for weeks to months before merger, LGWA could provide advance warnings for events that will later be observable by terrestrial detectors or electromagnetic observatories. The long residence time in band enables improved characterization of binary parameters and accurate sky localization, enabling coordinated multi-messenger studies.
Observations of double white-dwarf and white-dwarf–neutron-star binaries may also enable standard siren cosmology at low redshift, contributing to independent measurements of the Hubble constant.[1][2]
Fundamental physics
LGWA’s sensitivity in the decihertz band makes it suitable for probing fundamental aspects of gravitational physics. High-signal-to-noise ratio inspirals allow precision tests of general relativity through waveform phasing, constraints on dipole radiation, or deviations from the quadrupole formula. Potential signals from ultralight boson clouds around black holes, dark-matter overdensities, or modifications of gravity at astrophysical scales may also lie in this frequency band. In addition, LGWA could place bounds on stochastic gravitational-wave backgrounds from cosmic strings, first-order phase transitions in the early Universe, and other cosmological sources.[1][2]
Multi-band gravitational-wave astronomy
Operating in concert with observatories such as LISA and LIGO/Virgo/KAGRA, LGWA would enable observation of inspiral signals across multiple frequency bands, improved parameter estimation for coalescing binaries, and new tests of general relativity over broader dynamical regimes.[1][2]
Mission Architecture
The mission architecture proposed for the LGWA is based on a distributed network of seismic stations deployed on the lunar surface. Rather than relying on a single instrument, the concept envisions multiple, spatially separated stations, allowing correlated measurements that improve sensitivity to gravitational-wave signals and help distinguish them from local seismic disturbances.
The baseline configuration consists of four stations, a number chosen as a compromise between scientific performance, redundancy, and deployment complexity.[1][2] Each station would host an ultra-low-noise broadband seismometer optimized for operation in the lunar environment, together with supporting subsystems required for autonomous, long-duration measurements. Each station is proposed to be equipped with two horizontal Laser Interferometric Gravimeters, configured to measure surface displacements along two orthogonal directions.[19]
To reduce environmental noise, the stations are intended to incorporate thermal shielding and protective emplacement strategies, such as shallow trenches or partial burial in the regolith. These measures are designed to mitigate the effects of extreme temperature variations, surface disturbances, and lander-induced vibrations. Local electronics at each station would provide signal conditioning, time synchronization, and preliminary data processing prior to transmission.
The architecture also includes provisions for communication between the lunar surface and Earth. The concept assumes the use of relay-based communications, either through lunar orbiting assets or shared lunar communication infrastructure.[2]
Proposed site
The LGWA concept proposes deployment of the seismic array located within a permanently shadowed region (PSR) near one of the lunar poles.
PSRs occur primarily within polar impact craters and are defined as areas that never receive direct sunlight. Illumination at the surface is limited to sunlight reflected from Earth or scattered from illuminated portions of the surrounding crater walls. The existence of PSRs is a consequence of the Moon’s small axial tilt, which causes the lunar rotation axis to be nearly perpendicular to the direction toward the Sun. As a result, some polar craters remain in permanent shadow throughout the lunar year. Several PSRs exhibit surface temperatures persistently below approximately 40 K and show exceptional thermal stability.[20]
Although sites close to the north lunar pole are not excluded, the LGWA concept places particular emphasis on locations near the south lunar pole. This region hosts several extensive permanently shadowed regions, including areas in the vicinity of Shackleton crater, which combine exceptionally low and stable temperatures with favorable illumination conditions on nearby crater rims. These characteristics make the south polar region especially attractive for long-term seismic observations, power generation, and communications, while preserving the low environmental noise levels required for gravitational-wave detection.
Technology
LGWA does not employ long-baseline laser interferometers like those used in terrestrial gravitational-wave observatories. Instead, it aims to detect gravitational waves indirectly by measuring extremely small ground motions of the Moon, which acts as a natural resonant body. These motions are monitored using ultra-sensitive seismometers optimized for operation in the low-frequency seismic environment of the lunar surface.
LGWA seismometers
The LGWA payload is based on seismometers designed to measure minute ground displacements induced by both seismic activity and gravitational-wave excitation. Each station is planned to host two horizontal sensors oriented along orthogonal directions, enabling reconstruction of horizontal ground motion and polarization information. Correlated measurements between multiple stations distributed across the lunar surface allow discrimination between local seismic disturbances and global signals consistent with gravitational-wave excitation of the Moon.
The two proposed LGWA seismometer designs employ a Watt’s linkage mechanical suspension, which provides a low-frequency inertial reference by mechanically decoupling the test mass from ground motion above the resonance frequency. Two alternative sensing concepts are currently being investigated in parallel, differing in materials and readout technology.[19]
Laser-interferometric seismometers
One baseline concept combines a niobium Watt’s linkage with a laser-interferometric readout. In this configuration, the relative motion between the inertial test mass and reference points is measured using laser interferometry. Ground motion produces changes in optical path length, which are detected as phase shifts in the recombined laser light and converted into displacement signals.
Compared with conventional broadband seismometers that rely on mechanical transducers and resonant elements, laser-interferometric readout reduces mechanical dissipation and thermal noise while providing a linear response over a broad dynamic range. The technique also enables precise calibration and long-term stability, which are essential for identifying weak gravitational-wave signals embedded in the lunar seismic background.
Key elements of this concept, including kilogram-scale mechanical suspensions and superconducting actuators, have been developed and demonstrated in previous experiments, and further development is ongoing.[19] In addition, more sensitive laser-interferometric readout schemes with sub-femtometer displacement sensitivity are under investigation. Achieving such performance would require an ultra-stable laser frequency reference, which represents a significant technical challenge for this approach.[19]
SQUID-based seismometers
The second baseline concept employs a silicon Watt’s linkage together with a superconducting readout based on Superconducting Quantum Interference Devices (SQUIDs). In this approach, ground motion is sensed through superconducting displacement transducers whose signals are amplified by SQUID electronics.
This configuration has so far been developed at a conceptual level and requires additional research and development. However, it is expected to provide approximately an order-of-magnitude improvement in inertial displacement sensitivity relative to the laser-interferometric baseline, corresponding to a similar gain in gravitational-wave sensitivity. The increased performance potential of the SQUID-based approach is therefore balanced against its higher technological complexity and development risk.[19]
Noise mitigation
The lunar surface provides advantages, but LGWA still requires:
- Thermal stabilization to ~1 mK
- Isolation from lander vibrations
- Protection from dust deposition
- Mitigation of meteoroid impacts and electrostatic charging
- Correlated network sensing
By deploying several stations over large distances, LGWA can extract gravitational-wave signals using correlation analysis to distinguish them from local seismic events.[1]
Power generation
Power systems may consist of solar arrays combined with radioisotope thermoelectric generators or long-endurance batteries to survive the lunar night.
Comparison with Other Detectors
| Detector | Technique | Frequency band | Environment |
|---|---|---|---|
| LIGO/Virgo/KAGRA | Laser interferometry | ~10 Hz–1 kHz | Earth |
| Einstein Telescope | Underground laser interferometry | ~1 Hz–10 kHz | Earth (underground) |
| Cosmic Explorer | Laser interferometry | ~5 Hz–10 kHz | Earth |
| LISA | Interferometric spacecraft | ~0.1 mHz–0.1 Hz | Space |
| DECIGO | Interferometric spacecraft | ~0.1–10 Hz | Space |
| LGWA | Lunar seismometers | ~1 mHz–1 Hz | Moon |
Note: The quoted frequency ranges are approximate and represent the bands of highest sensitivity.
Substantial overlap exists between different detectors, particularly in the decihertz regime, where
LGWA, DECIGO, and future space-based interferometers complement each other, while next-generation
terrestrial detectors such as the Einstein Telescope and Cosmic Explorer extend sensitivity toward
lower frequencies compared to current ground-based observatories.
LGWA covers the intermediate low-frequency band, overlapping with but distinct from LISA and DECIGO, thereby providing a complementary approach to space-borne interferometers and a natural bridge between space-based and terrestrial gravitational-wave detectors.
Development Status
As of 2024, LGWA remains in the early study phase.
Further advancement depends on lunar exploration infrastructure from ESA, NASA Artemis missions, and international partners.
SoundCheck: The LGWA Pathfinder
The planning of the Lunar Gravitational-Wave Antenna (LGWA) requires detailed knowledge of the lunar geophysical environment inside permanently shadowed regions, including seismic, magnetic, and thermal conditions. At present, such information is not available at the level required for the design and optimization of a lunar gravitational-wave observatory. To address this gap, a dedicated pathfinder mission, known as Soundcheck, was proposed to the European Space Agency in 2022.[2]
Soundcheck is conceived as a combined science and technology-demonstration mission.
Its scientific objectives include measurements of seismic surface displacement, magnetic-field fluctuations, and temperature in a PSR environment. At the same time, the mission is intended to validate key technologies required for LGWA. The Soundcheck payload is designed to achieve sub-picometer-per-square-root-hertz inertial displacement sensitivity above approximately 0.1 Hz, enabling characterization of the lunar seismic background with unprecedented precision.
The technology validation activities foreseen for Soundcheck focus on several critical aspects of lunar operation. These include deployment strategies in permanently shadowed regions,
where illumination is absent; the mechanical performance of a Watt’s linkage inertial sensor for space applications, including release mechanisms; and the integration of interferometric position readout and associated electronics using existing technological heritage. Additional
development is required to demonstrate ambient cryogenic inertial sensing using a room-temperature laser system and battery-powered electronics. Platform leveling is also a key
element of the mission, building on space-qualified heritage while extending it to operation in a cryogenic lunar environment.
For Soundcheck, a relatively simple sensing approach without force feedback on the proof mass is adopted. Because the seismic background noise in PSRs is largely unknown, designing an optimal feedback loop is challenging. Instead, the mission relies on multi-fringe interferometric readout with sub-picometer-per-square-root-hertz sensitivity. Several readout techniques are considered, including homodyne quadrature interferometry and deep frequency modulation
interferometry. While these approaches are theoretically comparable in performance, homodyne quadrature interferometry is preferred for Soundcheck due to its lower computational requirements and compatibility with the mission’s power constraints.
Soundcheck was selected by the European Space Agency in 2023 for inclusion in the Reserve Pool of Science Activities for the Moon. This selection recognizes the scientific relevance of the mission concept and its potential role in enabling future lunar exploration and science activities, while not constituting a formal mission approval or commitment to flight.[2]
See Also
References
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{{cite journal}}: CS1 maint: unflagged free DOI (link) - ^ a b c d e f g h i j Ajith, P.; et al. (2025). “The Lunar Gravitational-Wave Antenna”. Journal of Cosmology and Astroparticle Physics. 2025 (01): 108. doi:10.1088/1475-7516/2025/01/108.
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{{cite journal}}: CS1 maint: article number as page number (link) - ^ Arca Sedda, M.; et al. (2020). “The missing link in gravitational-wave astronomy: discoveries waiting in the decihertz range”. Classical and Quantum Gravity. 37 (21): 215011. doi:10.1088/1361-6382/abb5c1.
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