ALSE

The ALSE (Apollo Lunar Sounder Experiment) (also known as Scientific Experiment S-209, according to NASA designations) was a ground-penetrating radar (subsurface sounder) experiment that flew on the Apollo 17 mission.

Mission and science

This experiment used radar to study the Moon's surface and interior. Radar waves with wavelengths between 2 and 60 meters (frequencies of 5, 15, and 150 MHz) were transmitted through a series of antennas near the back of the Apollo Service Module. After the waves were reflected by the Moon, they were received using the same antennas and the data was recorded on film for analysis on Earth. The primary purpose of this experiment was to "see" into the upper 2 kilometers of the Moon's crust in a manner somewhat analogous to using seismic waves to study the internal structure of the Moon. This was possible because very long radar wavelengths were used and because the Moon is very dry, which allowed the radar waves to penetrate much deeper into the Moon than would have been possible if water were present in lunar rocks. (A radar experiment on the Space Shuttle has been similarly used to map ancient river valleys beneath the Sahara Desert.) This experiment also provided very precise information about the Moon's topography. In addition to studying the Moon, the experiment also measured radio emissions from the Milky Way galaxy.

This experiment revealed structures beneath the surface in Mare Crisium, Mare Serenitatis, Oceanus Procellarum, and many other areas. In the mare areas, layers were observed in several different parts of the basins and are therefore believed to be widespread features.