1959

Far Side of the Moon

The Moon orbits Earth with what is called synchronous rotation; that is, the Moon spins on its axis exactly once for every orbit about the planet. But from our perspective on Earth, it doesn’t look like the Moon is spinning at all, because in synchronous rotation the same hemisphere (or “face”) of a satellite is always pointed toward its primary planet. Our familiar view of the full Moon is thus a view of what astronomers call the near side, the side always facing us.

Until the space age, no one had ever seen the side of the Moon that always faces away from us—the far side. In fact, the only way to see it would be to send a spacecraft out beyond the Moon and turn it around to take a picture looking back. And that’s exactly what the Soviet Union did in 1959 with the Luna 3 mission—another first for the Soviet space program.

Luna 3 was launched toward the Moon on October 4, 1959 (just two years after Sputnik 1). Three days later, after passing by the Moon’s south pole, it became the first successful three-axis stabilized spacecraft when ground controllers directed it to take photos as it looped around the far side. Twenty-nine film pictures were taken on board and then later scanned, digitized, and radioed back to Earth.

The Luna 3 photos are fairly low quality compared to modern space photography, but they were good enough to enable Soviet space scientists to map and name features and to discover that the Moon’s far side is dramatically different from the near side. The far side is more uniformly bright, with fewer dark mare (lava-filled impact basins; pronounced MAH-ray) than the near side. Much better follow-up photos of the far side from the Soviet Zond 3 satellite in 1965 showed the bright areas to be heavily cratered, rugged highlands regions.

A modern digital astrophotograph of the more familiar near side of the Moon.

Synchronous rotation turns out to be the usual situation among large planetary moons—all of the major satellites are synchronous rotators. Astronomers believe that this is because of the effects over time of tidal forces between a planet and its moon(s). Tides dissipate energy, slowing down a moon’s spin until it reaches a stable, so-called tidally locked orbit, with a near side always facing the planet and a far side always facing away.

One of the first images of the far side of the Moon (the side always facing away from the Earth), taken by the Soviet Luna 3 satellite in October 1959.