TheHollow Moon and the closely relatedSpaceship Moon arepseudoscientific hypotheses that propose that Earth'sMoon is either wholly hollow or otherwise contains a substantial interior space. Noscientific 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 solid, differentiated interior, with a thincrust, extensivemantle, and a densecore which is significantly smaller (in relative terms) than Earth's.
While Hollow Moon hypotheses usually propose the hollow space as the result of natural processes, the related Spaceship Moon hypothesis[1][2] holds that the Moon is an artifact created by an alien civilization;[1][2] this belief usually coincides with beliefs inUFOs orancient astronauts.[2] This idea dates from 1970, when two Soviet authors published a short piece in the popular press speculating that the Moon might be "the creation of alien intelligence"; since then, it has occasionally been endorsed by conspiracy theorists likeJim Marrs andDavid Icke.[3][4]
An at least partially hollow Moon has made many appearances inscience fiction, the earliest beingH. G. Wells' 1901 novelThe First Men in the Moon,[1][5] which borrowed from earlier works set in aHollow Earth, such asLudvig Holberg's 1741 novelNiels Klim's Underground Travels.[6][7]
Both the Hollow Moon and Hollow Earth theories are now universally considered to befringe orconspiracy theories.[1]
The fact that the Moon is less dense than the Earth is advanced by conspiracy theorists as support for claims of a hollow Moon.[8] The Moon's meandensity is 3.3 g/cm3, whereas the Earth's is 5.5 g/cm3.[9] Mainstream science argues this difference is due to the fact that the Earth'supper mantle and crust are less dense than its heavy, iron core.[9][5][10]
Between 1969 and 1977,seismometers installed on the Moon by theApollo missions recordedmoonquakes. The Moon was described as "ringing like a bell" during some of those quakes, specifically the shallow ones.[11] This phrase was brought to popular attention in March 1970[1] in an article inPopular Science.[12]
On November 20, 1969,Apollo 12 deliberately crashed the Ascent Stage of itsLunar Module onto the Moon's surface; NASA reported that the Moon rang 'like a bell' for almost 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.[11]
In 1970, Michael Vasin and Alexander Shcherbakov, of theSoviet Academy of Sciences, advanced a hypothesis that the Moon is a spaceship created by unknown beings.[2] The article was titled "Is the Moon the Creation of Alien Intelligence?" and was published inSputnik, the Soviet equivalent ofReader's Digest.[1][13] The Vasin-Shcerbakov hypothesis was reported in the West that same year.[14]
The authors reference earlier speculation by astrophysicistIosif Shklovsky, who suggested that the Martian moonPhoboswas an artificial satellite and hollow; this has since been shown not to be the case.[15] Skeptical authorJason Colavito points out that all of their evidence is circumstantial, and that, in the 1960s, the atheistic Soviet Union promoted theancient astronaut concept in an attempt to undermine the West's faith in religion.[2]
In 1965, authorIsaac Asimov observed: "What makes a total eclipse so remarkable is the sheer astronomical accident that the Moon fits so snugly over the Sun. The Moon is just large enough to cover the Sun completely (at times) so that a temporary night falls and the stars spring out. […] The Sun's greater distance makes up for its greater size and the result is that the Moon and the Sun appear to be equal in size. […] There is no astronomical reason why Moon and Sun should fit so well. It is the sheerest of coincidence, and only the Earth among all the planets is blessed in this fashion."[16]
Since the 1970s, conspiracy theorists have cited Asimov's observations on solar eclipses as evidence of the Moon's artificiality.[17][18] Mainstream astronomers reject this interpretation. They note that the angular diameters of Sun and Moon vary by several percent over time and do not actually "perfectly" match during eclipses.[19] Nor is Earth the only planet with such a satellite: Saturn's moonPrometheus has roughly the same angular diameter as the Sun when viewed from Saturn.[19]
Some scholars have claimed that "the conditions required for perfect solar eclipses are the same conditions generally acknowledged to be necessary for intelligent life to emerge"; If so, the Moon's size and orbit might be best explained by theweak anthropic principle.[19][20][21]
Multiple lines of evidence demonstrate that the Moon is a solid body which formed from an impact between Earth and a planetoid.
Historically, it was theorized that the Moon originated when a rapidly-spinning Earth expelled a piece of its mass.[22] This was proposed byGeorge Darwin (son of the famous biologistCharles Darwin) in 1879[23] and retained some popularity until Apollo.[22] The Austrian geologistOtto Ampferer in 1925 also suggested the emerging of the Moon as cause forcontinental drift.[24] A second hypothesis argued the Earth and the Moon formed together as a double system from the primordialaccretion disk of theSolar System.[25] Finally, a third hypothesis suggested that the Moon may have been a planetoid captured by Earth's gravity.[22][26][27]
The modern explanation for theorigin of the Moon is usually thegiant-impact hypothesis, which argues aMars-sized body struck theEarth, making a debris ring that eventually collected into a singlenatural satellite, theMoon.[28][29] The giant-impact hypothesis is currently the favored scientifichypothesis for theformation of the Moon.[30]
Multiple lines of evidence disprove that the Moon is hollow.[1][31] One involvesmoment of inertia parameters; the other involvesseismic observations.[1][31] 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. Seismic observations have been made, constraining the thickness of the Moon's crust, mantle and core, demonstrating it could not be hollow.[32]
Mainstream scientific opinion on theinternal structure of the Moon overwhelmingly supports a solid internal structure with a thin crust, an extensive mantle and a small denser core.[33][34]
| Body | Moment of inertia factor |
|---|---|
| Hollow Sphere | 0.67 |
| Uniform Sphere | 0.4 |
| Denser at Core | < 0.4 |
| Moon | 0.39 |
| Earth | 0.33 |
| All Mass at Core | 0.0 |
Themoment of inertia factor is a number, ranging from 0 to .67, that represents the distribution of mass in a spherical body. A moment of inertia factor of 0 represents a body with all its mass concentrated at its central core, while a factor of .67 represents a perfectly hollow sphere.[35] A moment of inertia factor of 0.4 corresponds to a sphere of uniform density, while factors less than 0.4 represent bodies with cores that are more dense than their surfaces.[36] The Earth, with its dense inner core, has a moment of inertia factor of 0.3307[36]
In 1965, astronomerWallace John Eckert attempted to calculate the lunar moment of inertia factor using a novel analysis of the Moon'sperigee andnode.[37] His calculations suggested the Moon might be hollow, a result Eckert rejected as absurd.[38] By 1968, other methods had allowed the Moon's moment of inertia factor to be accurately calculated at its accepted value.[39]
From 1969 to 1973,fiveretroreflectors were installed on the Moon during theApollo program (11,14, and15) andLunokhod 1 and 2 missions.[40] These reflectors made it possible to measurethe distance between the surfaces of theEarth and theMoon using extremely preciselaser ranging. True (physical)libration of the Moon measured viaLunar laser ranging constrains the moment of inertia factor to 0.394 ± 0.002.[41] This is very close to the value for asolid object with radially constant density, which would be 0.4.[41]
From 1969 through 1972, Apollo astronauts installed severalseismographic measuring systems on the Moon and their data made available to scientists (such as those from theApollo Lunar Surface Experiments Package). The Apollo 11 instrument functioned through August of the landing year. The instruments placed by theApollo 12,14,15, and16 missions were functional until they were switched off in 1977.[42][43]
The existence of moonquakes was an unexpected discovery from seismometers. Analysis of lunar seismic data has helped constrain the thickness of the crust (~45 km)[34][44] and mantle, as well as the core radius (~330 km).[33]
In 1998, the United States launched theLunar Prospector, which hosted the Doppler Gravity Experiment (DGE) -- the first polar, low-altitude mapping of the lunar gravity field. TheProspector DGE obtained data constituted the "first truly operational gravity map of the Moon".[45] The purpose of theLunar Prospector DGE was to learn about the surface and internal mass distribution of the Moon.[45] This was accomplished by measuring theDoppler shift in theS-band tracking signal as it reaches Earth, which can be converted to spacecraft accelerations.[45] The accelerations can be processed to provide estimates of the lunar gravity field. Estimates of the surface and internal mass distribution give information on the crust, lithosphere, andinternal structure of the Moon.[45]
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