New Astronomy from the Moon: A Lunar Based Very Low Frequency Radio Array

This thesis aims to contribute toward a proposal to set up a very low frequency (VLF: . 30 MHz) radio observatory on the lunar surface. The primary motivation for this proposal is to learn about our universe through a completely new spectral window in astronomy by taking advantage of the unique lunar environment. The secondary motivation is to take on a challenge of building and operating a facility on the Moon, especially through international cooperation, and to inspire everyone who looks at the Moon. After explaining these motivations and reviewing foregoing efforts toward this cause, the thesis examines unsolved questions about the advantages of the lunar environment, proposes a preliminary observatory to be set up at the lunar south pole, and identifies desirable measurements to be made at the earliest opportunities. Chapter 1 describes the motivations for astronomy from the Moon, particularly at very low frequencies. The Moon offers a unique environment that enables astronomical observations that are otherwise impractical. In particular the Moon can be utilized as a shield against unwanted radiations and as a large stable platform. These advantages are crucial for VLF astronomy. Thus far the Earth’s ionosphere and interference have prevented any detailed observations at frequencies below ∼30 MHz, keeping this VLF window the only part of the electromagnetic spectrum yet to be explored in astronomy. Accordingly the potential for unexpected discoveries is significant. The lunar far side may well be the only accessible site that enables sensitive galactic and extragalactic VLF observations. To realize this idea, Chapter 2 reviews the extensive foregoing efforts toward a Moonbased VLF observatory and identifies the next steps. The idea began in the mid-1960s and was advanced significantly during the 1980s, especially at a workshop dedicated to a lunar far side VLF array. In the 1990s, serious design studies were conducted by the Hughes Aircraft Company, the International Space University, and the European Space Agency. Referring to all these work, this chapter presents a background on observational considerations and the observatory design. The current consensus seems to be that although an array on the far side of the Moon is scientifically ideal and technologically feasible, funding is unlikely until the far side access becomes inexpensive. To accelerate the pace for this proposal, the key is probably to raise people’s interest in this project and its significant discovery potential. Also, the necessity of the Moon should be reaffirmed (Chapter 3), an affordable preliminary VLF array should be proposed for an initial sky survey (Chapter 4), and necessary measurements should be made utilizing every opportunity presented by the upcoming lunar missions (Chapter 5). Chapter 3 examines questions that must be resolved to confirm the advantage of the lunar surface for a VLF observatory. We must verify that (1) the Moon can shield the interference sufficiently, (2) any lunar ionosphere does not limit the observations, and (3) the lunar surface and subsurface do not disturb the observations, To address various issues relevant to the VLF array project, a general tool was developed to simulate the propagation of radio waves in the lunar environment. This tool was used to investigate (1) how radio waves penetrate into the lunar surface for possible subsurface reflections back up to the antennas, and (2) how well the Moon shields long wavelength radio interference. On the far side locations over half way (45 degrees) from the limb, the simulations seem to show that radio waves would be attenuated by at least 10 orders of magnitude, even at a very low frequency of 50 kHz. Prior to a full-scale observatory on the lunar far side, a crucial step now is to propose a realistic preliminary version at a more accessible site on the Moon, examined in Chapter 4. It will be for conducting an initial sky survey and testing an array on the lunar surface. The most economical method of deploying such lightweight antennas could be as a piggyback payload on some funded lander, most likely to the lunar south pole. This way, the project can utilize the same transportation, power, and communication systems required for further lunar development. A study was conducted to explore the possibility of using the 5-km tall Malapert Mountain near the lunar south pole as a shield against terrestrial radio interference. Simulations seem to indicate a several orders of magnitude attenuation over a region spanning ∼50 km on the far side of Malapert Mountain. A preliminary concept is developed for the first lunar VLF array to be deployed in this shadowed region. To be able to choose the site and design the observatory, Chapter 5 makes recommendations for specific measurements to be proposed for upcoming missions including SMART-1, LunarSat, and SELENE. It is especially critical to obtain detailed topology at candidate sites and to determine the electron density profile above the lunar surface at various times and locations. The final chapter includes my vision for how an international effort can make this project happen. Suggestions are given for an orbiting precursor array by∼2010, a surface array near the lunar south pole by ∼2015, and ultimately a far side array after 2020. Many nations share similar ambitions toward the Moon, including the United States, European Union, Japan, China, India, Canada, and Russia. Let us begin seriously exploring how to turn the individual objectives into a united proposal. I believe the Moon offers unique and significant opportunities for inspiring and uniting everyone on Earth.

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