RADIO ASTRONOMY FROM THE LUNAR FAR SIDE: PRECURSOR STUDIES OF RADIO WAVE PROPAGATION AROUND THE MOON.

Y. D. Takahashi

Introduction: The far side of the Moon offers an ideal site for observations of the universe at the as-yet largely unexplored frequency window of 50kHz-30MHz. In this frequency range, effective observations are possible only from outer space because of absorption and reflection in the Earth’s ionosphere. The Lunar far side always faces away from the Earth, and so has the unique extra advantage that it is shielded from the terrestrial radio interference. Even though the idea of establishing a radio observatory on the Moon seems attractive (and therefore has been investigated a number of times over the past 15 years [1,2,3,4,5,6]), several important questions still need addressing before such a project could receive serious funding support. One of these is the exact degree of shielding offered by the Lunar limb. Here we develop a simulation of radio propagation around the Moon to answer some of the key questions.

Unanswered Questions: To predict the performance of a Moon-based radio observatory, we seek answers to the following questions:

1. How well is the radio frequency interference from Earth attenuated on the Lunar far side?
2. How do very-low-frequency radio waves reflect or scatter off the Lunar regolith to disturb the observation?
3. How do very-low-frequency radio waves propagate through the Lunar exosphere before reaching the receiver?
4. How are radio observations affected by interaction with the local plasma?
5. Where are the best sites for the observatory?

Required Measurements: These questions can be answered by making the following in situ measurements:

1. The level of terrestrial noise at various longitude on the far side of the Moon.
2. Electrical properties of the Lunar surface, including permittivity and conductivity; their variation with depth and wavelength.
3. The electron density profile above the Lunar surface, both during the day and the night.
4. Magnetic fields at candidate sites.
5. Accurate measurements of surface topology at candidate sites.

Precursor Studies: In advance of precursor missions able to make these measurements, some simulations are already possible. In this study, we simulate the propagation of radio waves to predict how it is affected by the electrical properties of the Moon. We solve the wave equation using the Lax Wendroff method on a uniform space-time grid. The Lunar surface (to a depth of several wavelengths) is constructed using Apollo data for electrical conductivity and electrical permittivity. Although this simulation is developed to be flexible, it is used here to help answer the first three questions above:

1. The influence of terrestrial interference is evaluated by examining how far to the backside of the Moon the waves from Earth reach by diffraction. The auroral kilometric radiation (AKR) from Earth is simulated as a superposition of incoming plane waves in a frequency range 100-750kHz. The interference from various telecommunication transmitters is simulated similarly.
2. Reflection and scattering of radio waves off the Lunar regolith are studied by introducing discontinuities in electrical properties under the surface.
3. Refractive and absorptive effects of the Lunar exosphere on radio waves from astronomical sources are studied.

Precursor Missions: Ultimately, the performance of a Moon-based observatory is best tested on-site through inexpensive precursor missions with significant scientific returns of their own. Some ideas of proposals for precursor missions to the Moon in the very near future will be suggested. Eventually, a very low frequency observatory on the Lunar far side could help us gain new views of the universe.

Acknowledgments: I would like to thank Dr Graham Woan for his advice. I would also like to thank the J William Fulbright Foreign Scholarship Board for the Fulbright Graduate Student Award.

References: [1] Woan G. (2000) GMS, 119, 267–276. [2] Kuiper T. B. H. and Jones D. (2000) GMS, 119, 351–357. [3] Bely P. Y. et al. (1997) ESA reportSCI(97)2, European Space Agency. [4] Bougeret J.-L. (1996) AdSpR 18, 11, 35–41. [5] Landecker P. B. et al. (1992) ASP Conf. Ser. 34, 335-365. [6] Douglas J. N. and Smith H. J. (1985) Lunar Bases and space Activities of the 21^st Century, LPI 301–306.