By Ron Cowen
Nearly 400 years after Galileo first viewed the moon’s nearside through a telescope, scientists still know relatively little about the moon’s hidden half — the hemisphere that always faces away from Earth. Now, researchers have for the first time mapped the gravitational field of the moon’s farside.
Over the past few decades, lunar spacecraft have revealed that the moon’s two hemispheres show striking differences. The visible nearside is covered with smooth, dark “seas” of volcanic material, while the farside is more heavily cratered and consists of brighter, highland material. But because a lunar orbiter traversing the moon’s hidden half can’t be tracked directly from Earth, researchers had lacked a detailed map of the farside’s distribution of matter.
The Japanese SELENE (“KAGUYA”) mission, launched in 2007, has now remedied that problem, scientists report in the Feb. 13 Science. As SELENE’s main satellite orbits the moon, the craft slows down while passing over lower density regions and speeds up while passing over higher density regions, all the while broadcasting radio waves. Shifts in the frequency of these radio waves, due to the motion of the craft, are recorded by a small companion satellite in a higher-altitude orbit. The companion satellite is positioned to relay the signals to Earth, where they are transformed into a gravity map.