The Lunar Far Side

Why is the far side of the Moon important? Well, it's unique in that it is the only place in the solar system that is permanently shielded from the barrage of radio waves and other electromagnetic radiation streaming out from the Earth. This makes it valuable as a site for astronomical observation, not just at radio wavelengths, but right across the electromagnetic spectrum, including wavelengths that are normally inaccessible from the Earth's surface. An array of optical telescopes spanning the Moon's diameter could theoretically achieve a resolution close to a million times greater than the Hubble Space Telescope (HST), enough to image Earth-sized planets around nearby stars and resolve the innermost regions surrounding giant black holes in other galaxies. A similar array located on the Earth would be limited by seismic disturbance and atmospheric distortion, while a free-floating array placed in Low Earth Orbit (LEO) would be unable to achieve the same levels of precision as a stable lunar array.

Another advantage of the Moon is that it takes 28 days to rotate on its axis, meaning that it has 28 times longer than an Earthbound telescope to gather light from faint sources. The key to astronomical photography is exposure - the fainter the object, the longer you need to leave the aperture open. For example, the original Hubble Deep Field (the deepest exposure of the Universe to date) was created by combining 342 separate exposures, taken across 10 consecutive days. Using lunar telescope arrays, we could see even deeper into space, analysing the structure of the universe right back to the time when it first became transparent to electromagnetic radiation.

Before we get too carried away with the possibilities of lunar astronomical observatories, it's important to recognise that even though astronomy is a perfectly good reason to return to the Moon, it's going to cost money - a lot of it. Even a relatively modest array, involving just a handful of telescopes, would cost significantly more than the HST. And being located on the far side, it would require the support of at least one satellite in lunar orbit, in order to relay the data back to Earth. Also, the shuttle missions to repair and upgrade the HST have shown us the value of in-orbit servicing. A similar mission to service one or more lunar telescopes would be both problematic and extremely expensive. The truth is that investment in astronomy is rewarded with knowledge, not hard cash, so science alone won't be enough of an incentive to return to the Moon.

POSTSCRIPT: A note to Pink Floyd fans and readers of bad sci-fi: I avoid using the expression "dark side of the Moon". Just because we can't see it directly doesn't mean that it's dark. The far side receives just as much sunlight as the near side. In fact, if you take lunar eclipses into consideration, it actually receives more.

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