The Hollow Moon hypothesis, or Spaceship Moon hypothesis, proposes that Earth's Moon is either wholly hollow or otherwise contains a substantial interior space. No scientific evidence exists to support the idea; seismic observations and other data collected since spacecraft began to orbit or land on the Moon indicate that it has a thin crust, extensive mantle and small, dense core, although overall it is much less dense than Earth and its inhabitants.
The Hollow Moon hypothesis is the suggestion that the Moon is hollow, usually as a product of an alien civilisation. It is often called the Spaceship Moon hypothesis, and often corresponds with beliefs in UFOs or ancient astronauts.
The suggestion of a hollow moon first appeared in science fiction, when H. G. Wells wrote about a hollow moon in his 1901 book The First Men in the Moon. The concept of hollow planets was not new; Wells borrowed from earlier fictional works that described a hollow Earth, such as the 1741 novel Niels Klim's Underground Travels. Academic proposals for a hollow Earth pre-dated that. Edmond Halley's hypotheses, advanced in 1692, was the first one to specify an actual void in the Earth.
Arguments advanced in support
In 1970, Michael Vasin and Alexander Shcherbakov, of what was then the Soviet Academy of Sciences, advanced a thesis that the Moon is a spaceship created by unknown beings. The article was entitled "Is the Moon the Creation of Alien Intelligence?", and was published in Sputnik, the Soviet equivalent of Reader's Digest.
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.
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. 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.
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. This phrase was brought to popular attention in March 1970, in an article in Popular Science. 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. 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.
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 gm/cm3 whereas the Earth's is 5.5. 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. The Earth's upper mantle and crust are less dense than its core.
Cornell University's Ask an Astronomer, run by volunteers in the Astronomy Department, 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.
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. This is based on:
- 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) and mantle, as well as the core radius (~330 km).
- Moment of inertia parameters. For the Moon, moment of inertia parameters have demonstrated that the core is ~1.4% of the total mass. One such parameter, the normalized polar moment of inertia, is 0.393 ± 0.001. This is very close to the value for a solid object with radially constant density, which would be 0.4 (for comparison, Earth's value is 0.33). The normalized polar moment of inertia for a hollow Moon would have a higher value, closer to 0.67.
- Fine-scale variation (e.g., variation along the orbit of the Lunar Prospector orbiter) of the lunar gravitational field, which is consistent with geologic processes involving a crust, mantle, and core.
- H.G. Wells, The First Men in The Moon (1901). Wells describes fictional insectoids who live inside a hollow Moon.
- Edgar Rice Burroughs, The Moon Maid (1926). A fantasy set in the interior of a postulated hollow Moon which had an atmosphere and was inhabited.
- C. S. Lewis, That Hideous Strength (1945). A hollow Moon is an important part of the science fiction novel's background.
- Nikolay Nosov, on the Moon (1965). A fairytale novel with a hollow Moon.
- Isaac Asimov, Foundation and Earth (1986). R. Daneel Olivaw is depicted living inside a hollow Moon in the novel's final chapters.
- David Weber, Empire from the Ashes (2003). Science fiction in which the Moon is a giant space ship which arrived 50,000 years ago.
- David Icke, Human Race Get off Your Knees – The Lion Sleeps No More (2010). Icke discusses the idea of a hollow moon.
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