Lowest Radar Echo Detected from Moon

A team of scientists has detected the lowest frequency radar echo off the moon ever picked up by Earth-based receivers, it was announced today.

In the lunar echo experiment, high-power radio waves were sent toward the moon with a transmitter in Alaska. The reflected signals, weakened by the long journey to the moon and back, were detected 2.4 seconds later by receiving antennas in New Mexico.

The transmitter sent out two-second pulses every five seconds over a period of two hours each day of the two-day study period (Oct. 28 to 29, 2007), one hour for each frequency at which it operated.

The echo signals at 7.4075 megahertz, made by the pulse bouncing off the moon, are believed to be the lowest frequency (or longest wavelength) at which these radar measurements have been conducted.

This radar experiment, and others such as NASA's Clementine mission that looked for ice at the lunar poles, work similarly to sonar experiments over the ocean, telling scientists more about the moon's structure.

"Analysis of the echo gives information on the properties of the sub-surface topography, because the low-frequency radar waves propagate to varying depths below the visible surface of the moon," said Naval Research Laboratory consultant scientist Paul Rodriguez.