Low Frequency Astronomy from Lunar Orbit

Basart, John P.; Burns, Jack O.; ,

Low Frequency Astronomy from Lunar Orbit

by John P. Basart, Iowa State Univ, Ames, United States,
Jack O. Burns, Iowa State Univ, Ames, United States,

Document Type: Proceeding Paper

Part of: Engineering, Construction, and Operations in Space III

Abstract:

The Radio Astronomy Explorer satellite verified that the far-side of the Moon is well shielded from manmade and natural Earth-based interference at frequencies below 30 MHz. Above a few MHz, most of the interference is produced from ionospheric breakthrough of manmade communication signals and over-the-horizon radar. Below about 1 MHz, the Earth's magnetosphere is a powerful source of Auroral Kilometric Radiation. Thus, the lunar far-side is the only location in the Earth-Moon environment where the noise level reaches the expected Galactic Background at low frequencies. The recent Presidential Space Exploration Initiative advocates the establishment of a permanently manned lunar outpost in the first part of the next century. In preparation for, and in support of this outpost, a number of satellites will be placed into orbit about the Moon. One such program for the late 1990's is the Lunar Observer which is designed to image the Moon at high resolution and to map its gravitational field. We are proposing to place relatively simple phased arrays of dipoles on individual lunar satellites and perform interferometry between the spacecraft. The dipole arrays will be deployed as inflatable structures. The experiment will function at 13 and 26 MHz. For a scenario in which two spacecraft are in perpendicular lunar polar orbits at altitudes of 100 km, the useful physical baselines will range from about 1 km to about 1500 km. The maximum useful baseline is limited by phase incoherency produced by interplanetary scintillation. With a mission lifetime of one year, the two element interferometer will produce synthetic aperture imaging with a resolution of about 2 arcseconds. This resolution is comparable to the Very Large Array radio telescope in New Mexico operating at a 20-cm wavelength, and is more than 100 times better than the best previous ground-based mapping at 38 MHz. The rms sensitivity for a 10⁶ second integration is about 2 Jy. Our proposed lunar interferometer has the advantages of being relatively simple, inexpensive, and opportunistic relative to current national space initiatives. It is also ground-breaking in terms of resolution, and astronomical productivity.

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