“The Dark Side of the Moon” made a memorable album title for the rock group Pink Floyd in 1973. But the term is a common misnomer. The moon rotates like Earth, but so slowly it keeps one side facing Earth though synchronous rotation. When we have new moon on Earth, the lunar far side is fully illuminated; there is no “dark side.”
It may seem hard to believe, but a few decades ago we didn’t have clue as to what the far side of the moon looked like. One cartoon showed it to be the backside of a highway billboard.
At the birth of the Space Race in the late 1950s the Soviet Union sent the Luna 3 probe to take the first ever snapshot of an unknown landscape so close to Earth.
To everyone’s surprise the fuzzy photos showed few mare — circular frozen lava seas — on the far side, unlike the “face of the man in the moon” near side. In fact there were only two regions, which the Soviets dutifully named Mare Moscovrae (Sea of Moscow) and Mare Desiderii (Sea of Desire). The moon was found to be two-faced.
Now little more the 50 years later we have the lunar far side topographically mapped to an extraordinary level of precision with instruments aboard NASA’s Lunar Reconnaissance Orbiter, which reached the moon in the summer of 2009.
It took about one year for LRO’s Lunar Orbiter Laser Altimeter (LOLA) to build an elevation map of the entire moon. The altimeter can pinpoint the height of lunar landforms to within four inches! The instrument sends laser pulses to the lunar surface and measures the time that it takes for them to reflect back to the LRO.
The false color altimetry map of the moon reveals in a new way the tortured impact history of Earth’s only natural satellite. What stands out dramatically in the far side map (blue corresponds to lowlands, yellow and red to highlands) is the immense excavation at the lunar south pole.
Clues to the existence of a giant far side basin go back as early as 1962, as seen in crude images transmitted from the Luna 3 and Zond 3 Soviet probes. This was a shallow angle blow by as asteroid, and reminiscent of the giant south pole crater on the rocky dwarf planet Vesta. (Sorry IAU, Vesta is a dwarf planet in my book.)
Previous lunar maps assembled over the past decades have different resolutions, viewing angles, and lighting conditions, which made it hard to make a topographic map that is consistent across the lunar surface. the LRO map at last gives us a solid, global model of the moon’s undulating surface.
The cratering record on the moon yields clues to similar impacts by comets and asteroids on the primeval Earth. Our planet’s bombardment history has been largely eroded by billions of years of weathering and geological activity.
LRO’s data confirm earlier theories the there was one last assault wave of interplanetary debris 3.8 billion years ago. At that time one of the moon’s largest impact features, Mare Orientale — seen below — was blasted out of the lunar surface. (Regrettably the feature is on the western limb of the moon and barely visible from Earth. If the moon were reoriented 90 degrees to the east, Mare Oriental, with its concentric ring of mountains, would give the moon the uncanny appearance of an eyeball staring at Earth.)
One idea is that 3.8 billion years ago the outer giant planets were shifting their orbits. This disrupted the asteroid belt and, like apples shaken out of a tree, a swarm of asteroid debris pelted the inner planets. Earth would have had as many as a couple dozen impact basins at least 600 miles across, a few would have been the size of the continental United States. But only the moon and Mercury have retained the forensic evidence of this late mass bombardment event.
The digital elevation and terrain maps from LRO will guide future scientific missions, and eventually human expeditions, back to the moon. By contrast, the Apollo astronauts barnstormed the Moon with photomaps form the Lunar Orbiter probes, and judicious application of Newton’s laws of gravity.
LRO will continue mapping with so much detail that a navigational system with GPS-accuracy could be available — especially for spacecraft landing in the moon’s heavily mapped polar regions.
I can imagine a scene out the legendary film “2001: A Space Odyssey” where lunar shuttle pilots leisurely punch in landing coordinates and let the vehicle touchdown autonomously. The 3D, high-resolution cockpit displays in the next generation lunar lander would offer a photo-realistic view of the landscape rolling beneath the crew.
I think eventually we will have a manned Antarctic-style outpost at the permanently shadowed Shackleton crater -– it’s just a question today of what nation builds it first. From here astronauts would venture out to assemble and maintain an immense far side radio telescope shielded from radio interference from Earth. Such a device could potentially detect evidence of extraterrestrial civilizations, if they exist.
