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.
The Hollow Moon concept is similar to the better-known Hollow Earth hypothesis, which was a recurring plot device in pre-spaceflight science fiction. The first discussion of a hollow Earth was by scientist Edmond Halley in 1692, with the first mention of a hollow Moon being in H. G. Wells' 1901 novel The First Men in the Moon.
The Hollow Moon hypothesis is the suggestion that the Moon is hollow, usually as a product of an alien civilization. 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 by Ludvig Holberg. 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.
It is now considered to be a fringe theory. It is often described in the media as a conspiracy theory, and the concept of the Moon as a spaceship is often mentioned as one of David Icke's beliefs.
Claims and rebuttals
In 1970, Michael Vasin and Alexander Shcherbakov, of what was then the Soviet Academy of Sciences, advanced a hypothesis that the Moon is an extension of the brain 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. 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.
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 g/cm3 whereas the Earth's is 5.5 g/cm3. 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 story set in the interior of a postulated hollow Moon which had an atmosphere and was inhabited.
- Nikolay Nosov, Dunno on the Moon (1965). A Russian fairytale novel with a hollow Moon.
- Isaac Asimov, Foundation and Earth (1986). Science fiction in which robot R. Daneel Olivaw is depicted living inside a partially hollow Moon.
- David Weber, Empire from the Ashes (2003). Science fiction in which the Moon is a giant spaceship which arrived 50,000 years ago.
- Christopher Knight & Alan Butler, Who Built the Moon? (2005). They suggest humans from the future travelled into the past to build the Moon in order to safeguard human evolution.
- David Icke, Human Race Get off Your Knees – The Lion Sleeps No More (2010). Icke suggests that the Moon is in fact a space station from which Reptilians manipulate human thought.
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- Although not described as such by the media, it is more accurately a fringe theory than a conspiracy theory.
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- Maria Nikolajeva (2000). From Mythic to Linear: Time in Children's Literature. The Scarecrow Press, Inc. p. 74. ISBN 9780810849525.
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- David G. Robertson (2016). UFOs, Conspiracy Theories and the New Age: Millennial Conspiracism. Bloomsbury Academic. p. 148. ISBN 9781474253222.