In 1970, Michael Vasin and Alexander Shcherbakov, of what was then the Soviet Academy of Sciences, advanced a hypothesis that the Moon is a spaceship created by unknown beings.^[2] The article was entitled "Is the Moon the Creation of Alien Intelligence?", and was published inSputnik,^[10] the Soviet equivalent of Reader's Digest.^[1][14]

Their hypothesis relies heavily on the suggestion that large lunar craters, generally assumed to be formed from meteor impact, are generally too shallow and have flat or even convex bottoms. They hypothesized that small meteors are making a cup-shaped depression in the rocky surface of the moon while the larger meteors are drilling through a rocky layer and hitting an armoured hull underneath.^[15]

The authors reference earlier speculation by astrophysicist Iosif Shklovsky, who suggested that the Martian moon Phobos was an artificial satellite and hollow; this has since been shown to not be the case. Sceptical author Jason Colavito points out that all of their evidence is circumstantial, and that in the 1960s the atheistic Soviet Union promoted the ancient astronaut concept in an attempt to undermine the West's faith in religion.^[2]

The Moon rang like a bell

Between 1972 and 1977, seismometers installed on the Moon by the Apollo missions recorded moonquakes. The Moon was described as "ringing like a bell" during some of those quakes, specifically the shallow ones.^[16] This phrase was brought to popular attention in March 1970,^[1] in an article in Popular Science.^[17] When Apollo 12 deliberately crashed the Ascent Stage of its Lunar Module onto the Moon's surface, it was claimed that the Moon rang like a bell for an hour, leading to arguments that it must be hollow like a bell.^[1] Lunar seismology experiments since then have shown that the lunar body has shallow moonquakes that act differently from quakes on Earth, due to differences in texture, type and density of the planetary strata, but there is no evidence of any large empty space inside the body.^[16]

Density

The fact that the Moon is less dense than the Earth is advanced as support for it to be hollow. The moon's mean density is 3.3 g/cm³ whereas the Earth's is 5.5 g/cm³.^[15] One explanation of this discrepancy is that the moon may have been formed by a giant impact which ejected some of the early Earth's upper crust into its orbit.^[4][18] The Earth's upper mantle and crust are less dense than its core.^[19]

Scientific perspective

Cornell University's Ask an Astronomer, run by volunteers in the Astronomy Department,^[20] answered the question "Can we prove that the Moon isn't hollow?". There, physicist Suniti Karunatillake suggests that there are at least two ways to determine the distribution of mass within a body. One involves moment of inertia parameters, the other involves seismic observations. In the case of the former, Karunatillake points out that the moment of inertia parameters indicate that the core of the moon is both dense and small, with the rest of the moon consisting of material with nearly-constant density. As for the latter, he notes that the moon is the only planetary body besides Earth on which extensive seismic observations have been made. These observations have constrained the thickness of the moon's crust, mantle and core, suggesting it could not be hollow.^[21]

Mainstream scientific opinion on the internal structure of the Moon overwhelmingly supports a solid internal structure with a thin crust, an extensive mantle and a small denser core.^[22][23] This is based on:

1. Seismic observations. Besides Earth, the Moon is the only planetary body with a seismic observation network in place. Analysis of lunar seismic data have helped constrain the thickness of the crust (~45 km)^[23][24] and mantle, as well as the core radius (~330 km).^[22]

2. Moment of inertia parameters. True (physical) libration of the Moon measured via Lunar laser ranging constrains the normalizedpolar moment of inertia to 0.394 ± 0.002.^[25][26] This is very close to the value for a solid object with radially constant density, which would be 2/5 = 0.4 (for comparison, Earth's value is 0.33).^[25] The normalized polar moment of inertia for a hollow Moon would be close to the value 2/3 for a thin sphere.

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