The Eighth Continent

The northern hemisphere of the moon as viewed from the Jupiter-bound Galileo spacecraft during an Earth gravity-assist flyby. Parts of the spectrum invisible to the human eye are rendered as green and bluish tints; these provide hints to the complex, barely understood mineralogy of lunar features. Image credit: NASA

Earth's moon is for most people just a light in the sky. Many of us who have followed the course of spaceflight know that it is in fact a place we have barely explored.

I like to think of our planet's moon as its eighth continent. In terms of extent, Asia is, at 44.6 million square kilometers, the largest continent. The moon is next in order, with 37.9 million square kilometers. Africa comes in third with 30.4 million square kilometers.

Another way to look at this is, the moon has more area than North America (24.7 million square kilometers) and Europe (10.2 million square kilometers) combined. Or one could say that the moon has about the same surface area as five Australias (7.7 million square kilometers each) or three Antarcticas (14 million square kilometers each).

South America, at 17.8 million square kilometers fifth in area after Asia, the moon, Africa, and North America, has a little less area than one lunar hemisphere. When we look up at the hemisphere the moon holds forever turned toward Earth, the Nearside, we view a surface area that could comfortably encompass Brazil, Argentina, Colombia, Venezuela, Peru, Chile, Ecuador, Bolivia, Uruguay, Suriname, French Guiana, Guyana, and Paraguay, plus the nations of Central America and the Caribbean. Half a billion people call those countries home.

If there's one hemisphere we can see from Earth, it follows that there's a hemisphere we cannot see. Called the Farside, we have observed it only from lunar-orbiting spacecraft or more distantly, from interplanetary spacecraft flying through the Earth-moon system (see image at top of post). No lander, rover, or astronaut has explored there. The Farside is home to the South Pole-Aitken Basin, the largest impact basin on the moon and one of the top five largest impact basins in the entire Solar System. Its full extent and depth were not confirmed until the last years of the 20th century.

Many people - including space enthusiasts, who really should know better - look at the moon and say, "been there, done that." The fact is, we have examined up close far less than 1% of the moon's surface. All the territory that the Surveyor and Luna robot landers, Apollo astronauts, and Lunokhod robot rovers of the 1960s and 1970s explored could fit within a smallish city.

All of this unexplored territory is close at hand - on average just 385,000 kilometers away, a distance approximately equal to 10 times Earth's circumference. There exist automobiles and many ships and aircraft that have racked up more kilometers than that. If we still possessed Saturn rockets and Apollo spacecraft, in three days you could climb a ladder down to the surface of the moon. That compares favorably with the travel time to remote places on Earth, such as Antarctica or the ocean abyss.

The eighth continent. The phrase makes the moon seem more real, more like a place, more like a part of Earth. That's how it should be. Earth and moon form a unique system. When we see the Earth as everything and its moon as separate, remote, and insignificant, we are only a step removed from ancient peoples who thought the Earth was flat. It's time we stopped that nonsense and gave our eighth continent its due consideration.