RADIO ASTRONOMY FROM THE LUNAR FAR SIDE:

PRECURSOR STUDIES OF RADIO WAVE PROPAGATION AROUND THE MOON

Y. D. Takahashi

Astronomy & Astrophysics Group, University of Glasgow, Glasgow, G12 8QQ, UK (yuki@astro.gla.ac.uk)

January 2002

Prologue

The Moon is the one object that virtually everyone throughout the history and future of Earth has or will have seen and stared at. Doing something inspiring on such a visible heavenly body could impact so many people around the world, giving them new views of our acquaintance in the sky. What could stimulate and expand our imagination more than observing the universe from the Moon? ...to find answers to what humans have always wondered about: How did it all begin... and evolve... to eventually create lives like us? The Moon as a site for astronomy has been proposed since at least the mid-1960s [1,2] when humans began to have access to outer space. The most seriously investigated concept for a Moon-based observatory has always been a very-low-frequency array on the far side for several well-grounded reasons. Setting up an observatory on the Moon could not only give us new views of the universe, but also inspire the billions of people who look at the Moon.

 

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 40 years), 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.

 

First Step: A Simple Radio Array

 

* Simplest & Least Expensive

A simple array of ~10 dipole antennas is probably the most technologically feasible observatory to be placed and operated on the Moon:

An extra reason for the radio observatory to be our 1st step: The unique radio quiet environment will likely not last long once humans begin development around the Moon! Especially important if we want to search for ET!

 

* Radio Observatory takes the Most Immediate Advantage of the Lunar Environment.

The Lunar far side is recognized as the best site of all for radio astronomy. Even compared to a free-flyer in space [3, 37]:

 

* Completely New Views of the Universe

Such an observatory will give us a completely new look at the universe by opening up the new frequency window of 50kHz-30MHz.

 

Taking the First Step

Since 1965, at least 40 pieces of work have been published specifically about radio observation from the Moon. The most comprehensive and recent has been ESA's "Very Low Frequency Array on the Lunar Far Side" (1997), 1-year design study by a team of nine experts [28]. "This study... showed the feasibility of the project within the framework of Phase III of the ESA Moon program: "science from the moon". Before the mission can be started, however, a number of in-situ measurements need to be performed to confirm certain environmental conditions."

 

Unanswered Questions & Required Measurements

To identify the best sites and to specify the observatory design, it is crucial to address the following questions by making the measurements below [28]:

  1. How far into the far side does the observatory site need to be for the terrestrial interference to be attenuated below the background level?
  2. => The terrestrial noise levels at various Lunar far-side locations.

  3. How might the Lunar regolith scatter radio waves to disturb the observation?
  4. => Electrical properties of the Lunar surface, including permittivity and conductivity, their variation with depth and wavelength.

  5. How much will the Lunar ionosphere affect the observation?
  6. => The electron concentration profile above the Lunar surface, both during the day and the night.

  7. Where is the best site?

=> Detailed surface topology, magnetic fields, and electrical properties at candidate sites.

 

Precursor Studies

To help answer these questions quickly (and to better plan precursor missions), radio wave propagation around the Moon is being simulated. We solve the wave equation using the finite difference method on a uniform space-time grid.

  1. Shielding by the Moon: 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 will be simulated as a superposition of incoming plane waves in a frequency range 100-750kHz. The interference from various telecommunication transmitters will be simulated similarly. When a faster code is developed, we will find out at what longitude the noise will be attenuated to be comparable to the background level.
  2. Reflection and scattering of radio waves off the Lunar regolith are studied by introducing discontinuities in electrical properties under the surface. The Moon is simulated using data in the Lunar Sourcebook for relative dielectric permittivity and loss tangent. We are reminded of how helpful actual data from measurements at the specific site will be.
  3. Refractive and absorptive effects of the Lunar ionosphere: While a simulation can predict the effects, it's crucial to measure the density of Lunar ionosphere as soon as possible.

 

Precursor Missions to Plan Now

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 are suggested [24, 26, 28, 29].

  1. Lunar orbiter
  2. Two-element orbiting interferometer
  3. Near side lander
  4. Robotic array deployment.

 

To realize the dream of observing the universe from the Moon, it is time for an international team to begin seriously proposing these missions. A simple radio array on the Moon may be just another one small step for man... but it could also be the beginning of a giant leap in the mankind's view of the universe.

 

Acknowledgments

I would like to thank Dr Graham Woan for being a very approachable advisor. I would also like to thank the Fulbright Foreign Scholarship Board.

 

Chronological references

[1] Gorgolewski S. (1965) The Advantages of a Lunar Radio Astronomy Observatory. Astronautica Acta, New New Series 11, No 2 126, 130-131.

[2] Gorgolewski S. (1966) Lunar Radio Astronomy Observatory. Proceedings of the First Lunar International Laboratory (LIL) Symposium, 78-84.

[3] Douglas J. N. and Smith H. J. (1985) A very low frequency radio astronomy observatory on the moon. Lunar Bases and space Activities of the 21st Century, LPI, 301-306.

[4] Burns J. O. (1985) A moon-earth radio interferometer. Lunar bases and space activities of the 21st century, LPI, 293-300.

[5] Burns J. O., Asbell J. (1987) Radio astronomy on the moon. Radio Astronomy from Space, NRAO, 29-39.

[6] Douglas J. N., Smith H. J. (1988) A very low frequency radio astronomy observatory on the Moon. NASA, Future Astronomical Observatories on the Moon, 113-118.

[7] Burns J. O. (1988) MERI: An ultra-long-baseline Moon-Earth radio interferometer. NASA, Future Astronomical Observatories on the Moon, 97-104.

[8] Burns J. O. (1989) A Lunar far-side very low frequency array: proceedings of a workshop. NASA, Office of Management, Scientific and Technical Information Division.

[9] Burns J. O. (1990) The Lunar Observer Radio Astronomy Experiment (LORAE). Low frequency astrophysics from space, 19-28.

[10] Sith H. J. (1990) Very low frequency radio astronomy from the moon. Low frequency astrophysics from space, 29-33.

[11] Kuiper T. B. H. Jones D. L., Mahoney M. J., Preston R. A. (1990) A simple low-cost array on the lunar near-side for the early lunar expeditions. Low frequency astrophysics from space, 46-51.

[12] Basart J. P., Burns J. O. (1990) A very low frequency array for the lunar far-side. Low frequency astrophysics from space, 52-56.

[13] Kuiper T. B. H. Jones D. L., Mahoney M. J., Preston R. A. (1990) Lunar low-frequency radio array. Astrophysics from the moon, AIP, 522-527.

[14] Duric N. (1990) VLF radio astronomy from the moon - Probing astrophysical plasmas. Astrophysics from the moon, AIP, 515-521.

[15] Chua K. M., Johnson S. W., Yuan Z. (1990) Foundation design for a radio telescope on the moon. ECOS 1, 707-716.

[16] Akgul F., Gerstle W. H., Johnson S. W. (1990) Computer-aided structural design of a lunar radio telescope. EOCS 1, 697-706.

[17] Basart J. P., Burns J. O. (1990) Initial design of a lunar far-side very low frequency array. ECOS 1, 687-696.

[18] Burns J. O. (1991) Aperture synthesis imaging from the moon. Radio interferometry: Theory, techniques, and applications, 420-427.

[19] Landecker P. B., Choi D. U., Drean R. J., Edelsohn C. R., Gurley J. C., Hagen F. A., Su C. W., Tillman M. L., Wassgren C. R. (1991) Astronomical lunar low frequency array. IAF, IAC, 42nd, 5.

[20] Drean R. J., Caylor M. A., Choi D. U., Edelsohn C. R., Gurley J. C., Hagen F. A., Landecker P. B., Su C. W., Tillman M. L., Wassgren C. R. (1992) Engineering Design of an Unmanned Lunar Radio Observatory. Robotic telescopes in the 1990s, ASP Conf. Ser. 34, 347-365.

[21] Landecker P. B., Caylor M. A., Choi D. U., Drean R. J., Edelsohn C. R., Gurley J. C., Hagen F. A., Su C. W., Tillman M. L., Wassgren C. R. (1992) Telerobotically Deployed Lunar Farside VLF Observatory. Robotic telescopes in the 1990s, ASP Conf. Ser. 34, 335-346.

[22] Duric N. (1992) Very low frequency radio astronomy from lunar orbit. ECOS 2, 1925-1934.

[23] Marsh K. A., Mahoney M. J., Kuiper T. B. H., Jones D. L. (1992) Concept for a lunar array for very low frequency radio astronomy. ECOS 2, 1935-1940.

[24] International Space University (1993) International Lunar Farside Observatory and Science Station, 1993 ISU Design Project, W. W. Mendell.

[25] Lecacheux A. (1994) Solar system, low frequency radio astronomy from the Moon. AdSpR, 14, 6, 193-200.

[26] Bougeret J.-L. (1996) Very Low Frequency Radio Astronomy. AdSpR, 18, 11, 35-41.

[27] Woan G. (1996) Design considerations for a Moon-based radio telescope operating at frequencies below 16MHz. Large Antennas is Radio Astronomy, ESA workshop WPP-110, 28-29.

[28] Bely P. Y., Laurance R. J., Volonte S., Ambrosini R. R., Ardenne A., Barrow C. H., Bougeret J. L., Marcaide J. M., Woan G. (1997) Very Low Frequency Array on the Lunar Far Side. ESA report SCI(97)2, European Space Agency.

[29] Jones D. L., and Weiler K. W. (1997) Low Frequency Radio Astronomy from the Moon. Astronomy from the Moon, IAU, JD.

[30] Heidmann J. (1998) SETI from the Moon: Avoiding Radio Pollution for Future Radioastronomy. Astronomy from the Moon, IAU, JD.

[31] Woan G. (1999) A Very Low Frequency Radio Telescope on the Far Side of the Moon. Measuring the Size of Things in the Universe: HBT Interferometry and Heavy Ion Physics, CRIS '98, 347.

[32] Heidmann J. (2000) A Pristine Radio Window on the Universe: Just One Lunar Crater, but Opened at all Frequencies. AdSpR, 26, 2, 343-346.

[33] Maccone C. (2000) Laydown of A Tether from Earth Visible Location to Far Side for Lunar SETI. AdSpR, 26, 2, 359-370.

[34] Heidmann J. (2000) Sharing the Moon by Thirds: An Extended SAHA Crater Proposal. AdSpR, 26, 2, 371-375.

[35] Heidmann J. (2000) A proposal for a radio frequency interference-free dedicated lunar far side crater for high sensitivity radioastronomy: programmatic issues. Acta Astronautica 46, 10-12, 555-558.

[36] Heidmann J. (2000) Recent progress on the lunar farside crater Saha proposal. Acta Astronautica 46, 10-12, 661-665.

[37] Weiler K. W. (2000) The Promise of Long Wavelength Radio Astronomy. Radio Astronomy at Long Wavelengths, GMS, 119, 243-255.

[38] Kuiper T. B. H. and Jones D. (2000) Lunar Surface Arrays. Radio Astronomy at Long Wavelengths, GMS, 119, 351-357.

[39] Woan G. (2000) Capabilities and limitations of long wavelength observations from space. Radio Astronomy at Long Wavelengths, GMS, 119, 267-276.

[40] Gorgolewski S. (2002) Future space VLBI on lunar orbits and in the radio shadow on the far side of the Moon. In Heather D.J. (ed) New Views of the Moon, Europe: Future Lunar Exploration, Science Objectives, and Integration of Datasets. ESTEC RSSD, Noordwijk.

[41] Gorgolewski S. (2002) Highest sensitivity radio astronomy and SETI from the far side of the Moon (including Crater SAHA). In Heather D.J. (ed) New Views of the Moon, Europe: Future Lunar Exploration, Science Objectives, and Integration of Datasets. ESTEC RSSD, Noordwijk.

[42] Maccone C. (2002) RadioMoon: a space mission around the Moon to detect "RFI-free" and bioastronomical signals. In Heather D.J. (ed) New Views of the Moon, Europe: Future Lunar Exploration, Science Objectives, and Integration of Datasets. ESTEC RSSD, Noordwijk.