An international collaboration of scientists has developed a blueprint for moon-based seismometers that detect gravitational waves reverberating throughout the celestial body, a design that would pick up frequencies that are out of reach for detectors on Earth.
The detailed proposal, published March 22 in The Astrophysical Journal, expands on a 2020 submission to the European Space Agency as a potential project for future lunar missions, and the authors believe it could be deployed in the next 15 years.
Gravitational waves are disturbances in the fabric of spacetime that are generated by colliding black holes and other extreme cosmological events. First predicted by Albert Einstein's theory of general relativity, they were first detected in 2015 by the Laser Interferometer Gravitational-Wave Observatory using highly precise lasers spanning thousands of miles across the U.S.
The moon's quietness relative to the Earth makes it a more advantageous environment for many kinds of scientific observations, including the search for more gravitational waves. Jan Harms, an associate professor of physics at the Gran Sasso Science Institute in Italy and the new paper's lead author, said he thought there is a more feasible way to look for gravitational waves than a LIGO setup that needs intense monitoring and adjustments.
Instead, he revisited the very first experimental attempt to detect gravitational waves, devised in 1961 by Joseph Weber, a physicist who also made contributions to the development of lasers. Weber designed a device that would search for gravitational waves that resonated throughout the moon, and it was deployed by the Apollo 17 mission in 1972.
The device contained a manufacturing error that severely reduced its efficacy — it wasn't sensitive enough to pick up gravitational waves even if fully functional, Harms said — but advances in technology have persuaded some scientists to continue its legacy.
The lunar gravitational-wave detector described in the paper would be a configuration of four seismometers on the lunar surface that would monitor minute changes in the movement of the moon's surface. The devices could then parse which vibrations were caused by gravitational waves, and at what frequencies.
The moon's lack of an atmosphere and active plate tectonics makes it a more suitable environment for seismic detection methods than Earth. In addition, the moon-based detector could also pick up frequencies on the millihertz scale, unlike LIGO and other Earth-based systems, which opens the search to sources such as colliding supermassive black holes and spinning neutron stars. It could also test fundamental theories of gravity, the authors said.
"There are these massive and supermassive black holes that can form binaries, which we know to exist also in the center of some galaxies," said Harms, who is also involved with the Virgo detector that first picked up gravitational waves in 2017. "That would for sure be very exciting, to have a survey of these systems throughout the universe."
Among the several obstacles accounted for in the paper — including seismic tremors from moonquakes and meteoroids as well as potentially damaging cosmic rays — the biggest challenge is powering the seismometers, according to Harms. They are currently planned to be placed in a permanent shadow within a crater to avoid the wide swings of temperature between day and night on the moon, but the location rules out using solar panels as an energy source.
The energy generated by nuclear decay could be used, as it is on numerous space probes and Mars rovers, but the European Space Agency is not interested in nuclear power on spacecraft, Harms said. His current plan is to place another device outside the shadow that gathers solar energy and feeds it to the seismometers with a laser.
The initial version of the new design was one of 325 submissions to the ESA's recent call for ideas for scientific experiments on a planned lunar rover. A gravitational-wave detector was not mentioned in the agency's synthesis of its solicitation, which closed in March, but Harms said he and his co-authors are planning to submit an updated proposal for a future ESA call. If all goes well, he said an aggressive timeline would see it deployed by 2035, though, "Some people say that's quite optimistic."
The LIGO discovery has accelerated scientific interest in the field of gravitational waves, Harms said, and space expeditions are taking off at the world's leading space agencies; see NASA's ongoing Artemis program, which plans to revisit the moon. These two trends have converged in a "golden moment" that led to the lunar gravitational-wave detector receiving significant attention from scientists, according to Harms.
"People realize that we have real opportunity now which we haven't had since the last Apollo mission," he said. "So people are really very excited about this combination of this interesting science, packed together with the space exploration component that this has as well."
The proposal has also been noticed by the general public in Italy, where it was covered in several newspapers, according to Harms. Engaging the public is important to getting the project approved, he said.
"If you are not addressing the public with your project, if you are not convincing the public and getting interest in it, then it's just going to be more difficult for you later to find money," Harms said.
The study, "Lunar gravitational-wave Antenna," published March 22 in The Astrophysical Journal, was authored by Jan Harms, Eugenio Coccia, Samuele Ronchini, Francesca Badaracco, Simone Dall'Osso, Matteo Di Giovanni, Nandita Khetan, Gor Oganesyan and Ashish Sharma, Gran Sasso Science Institute and Laboratori Nazionali del Gran Sasso; Filippo Ambrosino, Astronomical Observatory of Rome, Institute of Space Astrophysics and Planetology and Sapienza University of Rome; Lorella Angelini, NASA Goddard Space Flight Center; Valentino Braito, Roberto Della Ceca, Roberto Serafinelli, Paola Severgnini, Marta Civitani, Stefano Covino, Paolo D'Avanzo, Giovanni Pareschi and Gianpiero Tagliaferri, Brera Astronomical Observatory; Enzo Brocato, Astronomical Observatory of Rome and Abruzzo Astronomical Observatory; Enrico Cappellaro and Claudio Pernechele, Padova Astronomical Observatory; Michael Coughlin, University of Minnesota; Massimo Della Valle, Capodimonte Astronomical Observatory; Cesare Dionisio, Space Boy Station; Costanzo Federico, Michelangelo Formisano and Alessandro Frigeri, Institute of Space Astrophysics and Planetology; Aniello Grado, Capodimonte Astronomical Observatory and National Institute for Nuclear Physics, Naples; Luca Izzo, University of Copenhagen; Augusto Marcelli, Frascati National Laboratory, CNR Institute of Structure of Matter and Rome International Center for Material Science Superstripes; Andrea Maselli, Sapienza University of Rome and National Institute for Nuclear Physics, Rome; Marco Olivieri, National Institute of Geophysics and Volcanology, Bologna; Andrea Possenti, Cagliari Astronomical Observatory and University of Cagliari; Maila Agostini, Emanuele Pace and Luca Naponiello, Polyfunctional Observatory of Chianti and University of Florence; Alessandro Bertolini, Nikhef; Christophe Collette, University of Liège and Free University of Brussels; Riccardo DeSalvo, University of Sannio at Benevento, University of Utah and RicLab; Mauro Focardi, Arcetri Astronomical Observatory; Carlo Giunchi and Daniele Melini, National Institute of Geophysics and Volcanology; Joris van Heijningen, Catholic University of Louvain; Gor Oganesyan, International Research School of Planetary Sciences and D'Annunzio University; Conor Mow-Lowry, VU Amsterdam; Ho Jung Paik, University of Maryland, College Park; Eliana Palazzi, Observatory of Astrophysics and Space Science of Bologna; Marco Pallavicini, National Institute for Nuclear Physics, Genoa and University of Genoa; Riccardo Pozzobon, University of Padua; Giorgio Spada, University of Bologna; Ruggero Stanga, Polyfunctional Observatory of Chianti; and Raffaele Votta, Italian Aerospace Research Center.