Far Side of the Moon

February 6, 2012

Our Moon is a mystery. Of the three terrestrial planets of our Solar System other than the Earth, Mercury and Venus have no moons, and Mars has two diminutive moons. Earth, however, has a huge moon that's sized and located precisely enough to give us spectacular solar eclipses. These eclipses may have jump-started our scientific revolution by getting people interested in astronomy. The Moon's tidal forces may have been a factor in the origin and development of life on Earth.

There were many theories of the origin of the Moon. The most generally accept one is the giant impact hypothesis, the major evidence for which comes from mineral specimens collected from the Moon. The Earth-Moon system is thought to have arisen from the collision of a proto-Earth and the hypothesized protoplanet, Theia. The remains of this collision, which appears to have happened about four and a half billion years ago, just a few million years after the these objects first formed, settled into the present two circling orbs.

As if the moon wasn't mysterious enough, one side has been hidden from view until a few decades ago. Although the moon is tidally-locked to Earth, so that one side always faces us, not an entire 50% of the Moon's surface is invisible from Earth. Another 9% can be observed because of libration. This is a consequence of the slightly elliptical orbit of the Moon, the slight inclination of the Moon's axis to its orbit, and the fact that observers can look from slightly different vantage points at both sides of the Earth.

No, it's not a coffee stain. It's the first image of the far side of the moon, taken on October 7, 1959, by the Soviet Luna 3 spacecraft. North is up in this image. (Via NASA Web Site).

The remaining 41% of the Moon's surface, colloquially known as the far side of the Moon, was hidden until 1959, when the Soviet spacecraft, Luna 3, transmitted a series of twenty-nine low resolution photographs that encompassed about 70% of the far side. The photographs did reveal some large craters. Subsequent photographic explorations in preparation for manned landing revealed much detail and major differences between the visible and invisible faces.

The side that we see, the near side, has large, smooth areas, called maria because of their resemblance to oceans. The maria are actually lava outflows from volcanic eruptions, and they cover a little more than a quarter of the near side. Because of their iron-rich composition, the maria are darker than their surroundings. The far side is quite different, showing very little maria surface and many more craters. The near side was shielded from many impacts by the presence of the Earth, whereas the far side is completely exposed.

The Luna 3 images, low resolution though they were, were a sensation when they were published. Likewise, the first manned landing on the moon more than four decades ago had my family glued to our television.[1] One family member, my maternal grandmother, was born before the Wright Brothers flight. After the Apollo program, interest in the Moon has waned, and it doesn't help that NASA has proposed, then canceled, manned lunar return missions.

In order to generate renewed interest in the Moon among schoolchildren, NASA has added a camera, called the MoonKAM, to their twin GRAIL spacecraft. The GRAIL mission, short for Gravity Recovery and Interior Laboratory, has two spacecraft orbiting the Moon to make extremely precise measurements of lunar gravity. Eventually the spacecraft will be brought to nearly circular orbits just 34 miles (55 km) above the lunar surface. This will help to elucidate the Moon's internal structure. NASA intends to use the MoonKAMs to snap photographs of lunar areas selected by middle school students for their further study.[2]

A still image of the Moon's South Pole from NASA's recently released video of the Moon's far side. The video was taken on January 19, 2012, by the MoonKAM aboard GRAIL's Ebb Spacecraft. (Still image from video at NASA GRAIL Mission Web Site).

The twin spacecraft, which were named "Ebb" and "Flow" in a NASA contest won by fourth grade students at Emily Dickinson Elementary School, Bozeman, Montana, entered their orbits on New Year's Eve and New Year's Day.[2] Sally Ride, who was America's first woman in space, leads the MoonKAM program (the first woman in space was Valentina Tereshkova). Ride has a Ph.D. in physics from Stanford University. There are presently more than 2,500 schools participating.[2]

References:

1. I built that color television from a kit. It had about twenty vacuum tubes. From my Internet searches, it must have been the Heathkit Model GR-295. It worked at power-up, lacking only the red drive, which was the fault of a new, just-out-of-the-box, but defective, vacuum tube.
2. NASA Mission Returns First Video From Moon's Far Side, NASA Press Release No. 2012-031, February 1, 2012.
3. Video:GRAIL Mission Returns First Video of Moon's Far Side.
4. NASA GRAIL Mission Web Site.
5. MoonKAM Web Site.