The Lunar Gravitational-wave Antenna (LGWA) is a mission concept to measure the vibrations of the Moon caused by GWs. Its observation band reaches from 1mHz to several Hz, with peak sensitivity in the decihertz band. An explorer mission called LGWA Soundcheck was proposed for a geophysical study of a permanently shadowed region and as technology demonstrator.

Characteristic strains

Characteristic strain sensitivities for signals detectable with LGWA Soundcheck (dark gray) assuming 2 months of observation time. Its detection horizon will be vastly exceeded by LGWA (light gray) with detections of intermediate-mass black-hole binaries becoming possible beyond z=10. Signal traces were simulated with GWFish

LGWA will provide the ``missing link'' complementing the approved Laser Interferometer Space Antenna (LISA), which will observe below the decihertz band, and future ground-based detectors like the proposed Cosmic Explorer (CE) and Einstein Telescope (ET), whose observation bands lie above a few Hertz.

The high scientific revenue expected from observations in the decihertz band led to the proposals of technologically ambitious missions like Big Bang Observer and DECIGO. The LGWA mission concept is inherited from the Lunar Surface Gravimeter deployed in 1977 on the Moon with Apollo 17.

LGWA crater

Star-like deployment configuration of four LGWA stations in a kilometer-scale array equipped with cryogenic inertial sensors. The LGWA deployment site is one of the permanently shadowed regions (PSRs) inside a crater at the lunar north or south pole. A laser-power beaming system is shown as a possible power system for LGWA.

Two components of LGWA are crucial for its success as a ground-breaking decihertz GW observatory:

  • a cryogenic inertial sensor concept for the measurement of horizontal ground displacements reaching femtometer sensitivity at 1Hz;
  • deployment of a kilometer-scale array of at least four inertial sensors and using advanced noise-cancellation techniques for the reduction of seismic background noise in the decihertz band. At least four stations are required to create a configuration of one sensor being surrounded by other sensors in all directions for efficient noise cancellation. In fact, any lunar detector targeting the decihertz band requires an array of LGWA-grade inertial sensors to reduce the seismic background noise, since the background is random in nature and can only be analyzed with locally acquired array data.

Deployment and operation: LGWA sensors need to be deployed on the ground inside a PSR whose natural cryogenic environment leads to improved sensitivity of the inertial sensors and also eliminates thermal noise and seismic disturbances induced by solar radiation. The targeted lifetime of LGWA is 10 years, which benefits the science outcome especially with respect to the observation of rare events, and which is long enough to be able to extend the network beyond the baseline configuration proposed here. We propose a deployment of all LGWA stations with a single lander. A rover or astronauts lay out the array with fiber connections between LGWA stations and lander for powering and data transfer, where the power source and communication relay are located. 

GW science: As decihertz GW detector, LGWA has access to a unique band of masses and energies of GW sources, which cannot be studied by other GW detectors at high signal-to-noise ratio and out to high redshift. The decihertz band is of prime interest to cosmological studies, fundamental physics, and astrophysics.

LGWAScience
Graphical summary of the LGWA science case including multi-messenger studies with electromagnetic observatories and multiband observations with space-borne and ground-based GW detectors.

As link between space-borne and ground-based detectors, LGWA is an ideal partner for multiband studies where these detectors observe the same GW signals at different phases of their frequency evolution. Also, the wealth of astrophysical sources observable by LGWA containing binaries with white dwarfs and neutron stars, tidal disruption events, or massive and supermassive black holes with accretion disks leads to a rich multi-messenger science case.

Lunar science and exploration: As an array of seismic stations, LGWA would be able to provide unique contributions to a future lunar seismic network. The most important will be the recording of normal modes excited by the moonquakes. Moreover, it is conceivable that GWs can be used as probes to better understand the lunar interior structure by observing how the Moon responds to GWs. Meteoroid impacts, most of them weak seismic events by Earth's standards, can be used as probes to analyze the shallow Moon structure. LGWA will contribute to and improve the catalogue of moonquakes by reducing the event detection threshold and extending to lower magnitude events. These new data together with the use of modern machine-learning methods will contribute to the next generation of Moon interior models. 

LGWA can be seen as the result of a natural development from pure lunar geophysical missions to a mission that can see astrophysical and cosmological signals. The Farside Seismic Suite (FSS) will be deployed in the Schrödinger basin on the Moon with an approved CLPS-PRISM mission (expected to launch in 2024/2025) adopting the sensor design of the Mars InSight mission. Since the FSS seismometer will have greatly improved sensitivity over the Apollo seismometers, we can expect new insight into the distribution of weak seismic disturbances to the benefit of LGWA. The Lunar Geophysical Network (LGN) was proposed consisting of 4 separate deployments on the Moon. In addition to the invaluable information it would provide about the Moon, this mission is of great interest to LGWA since station deployment and lifetime have similar requirements to LGWA (albeit, LGN deployments are not proposed inside PSRs).