Is the Moon Hollow?

When was the last time you ever stopped to have a good think about the giant natural satellite that orbits our planet? The Moon is something that has fascinated us for centuries, but by and large, we have now come to accept it as something that is commonplace within our night sky. Most of us will glance up at it now and again and maybe comment on how bright the Moon or what phase the Moon is currently in.  The real question is when was the last time you really thought about the Moon? What is going on under that familiar crater-scarred surface?


"That's no moon. It's a space station. " said Obi wan Kenobi about the Death Star.The lower left part of the image shows a portion of the moon visible from Earth. The dark area at the 8:00 position on the edge is Mare Crisium. To the right of that is Mare Smythii. The upper right area shows the heavily cratered lunar far side. The Moon is 3475 km in diameter and North is at 10:30 in this image. (Image credit: NASA, image AS16-3021)

“That’s no moon. It’s a space station. ” said Obi Wan Kenobi about the Death Star but this image shows a real natural moon. The lower left part of the image shows a portion of the Moon visible from Earth while the upper right area shows the heavily cratered lunar far side.  (Image credit: NASA, image AS16-3021)


Our wondrous satellite has been on our minds recently here at Armagh Planetarium. Did you know that there is a pretty major conspiracy theory out there about our Moon? We know there are conspiracies on almost everything nowadays, but the moon? Fear not, we are here to explain!

A remarkably popular “theory”  is the Hollow Moon Hypothesis. The big question that this theory asks is “Is the Moon Hollow?” At this point you are probably furrowing your brow and asking yourself that very question. Many people have asked this, and have been asking it since the first moon landings. However this concept of a hollow Moon isn’t a new one. In his 1901 novel The First Men in the Moon, the great H.G. Wells (1866-1946), known to many as the Father of Science Fiction, wrote about an epic journey to the Moon. His main characters soon find out however that the Moon isn’t all that it seems and that it is hollow and inhabited by a strange insect-like alien race. Quite fantastical for a novel that was published in 1901, but as soon as humans started landing on the Moon over 60 years later, the concept of a Hollow Moon seemed to come to the forefront a little more.


Buzz Aldrin beside the Apollo 11 seismometer. Use the next diagram to identify its components. (Image credit: Neil Armstrong/NASA)

Buzz Aldrin beside the Apollo 11 seismometer. You can use the next diagram to identify its components. (Image credit: Neil Armstrong/NASA)


What started this notion of a hollow Moon? During the lunar landings, the astronauts deployed several seismometers, instruments to record movements and vibrations, into the Moon’s surface and left them there so as to receive data regularly about activities on the moon. These seismometers, which were set up during the Apollo 11, 12, 14, 15 and 16 missions, radioed back information to the Earth until they were finally switched off (to save money!) in 1977.


Schematic diagram of an Apollo seismometer. (Image credit: NASA)

Schematic diagram of an Apollo seismometer. (Image credit: NASA)


These radio transmissions revealed a lot about the Moon and its internal structure, in particular the existence of moonquakes. These moonquakes were placed into four different categories:
1. Deep Moonquakes (700km below the surface)
2. Vibrations (caused my meteorite impacts)
3. Thermal Quakes
4. Shallow Quakes (20-30km below the surface)

It was the shallow quakes that caused this hare-brained notion of the hollow Moon to come about. It has been said that during these shallow quakes the Moon would “ring like a bell.” This meme of the Moon ringing like a bell first became established in an article written for Popular Science in March 1970. This phrase was used after Apollo 12 purposefully crashed the 2.5 ton Ascent Stage of its Lunar Module onto the Moon’s surface. It was claimed that the Moon rang like a bell for roughly an hour. With information like this, it is no wonder that some people outside of the science community started to think that the Moon might not be as solid as they thought.

A closely-related theory is the Spaceship Moon Theory, or for you science buffs out there, the Vasin-Shcherbakov Theory. This theory takes the concept of a hollow moon one step further into pure science fiction territory. In an article by Michael Vasin and Alexander Shcherbakov published in 1970 it was hypothesised that the Moon was actually an artificial, hollowed out Earth satellite that had been put into place by unknown beings. Vasin and Shcherbakov were members of the then Soviet Academy of Sciences, but the article was published not in a science journal but in Sputnik, a sort of Soviet Reader’s Digest. In the article they refused to speculate what type of unknown beings put the artificial satellite in the Earth’s orbit, but we do think that this is a discussion that should be had at another time. We won’t go too in depth into the finer details of this theory, as we could talk about it for a while, but the general feeling of this article that was produced is that the Moon is made of up everything that a spaceship could be made of, such as materials that can withstand both extreme heat and cold and are extremely tough so could withstand blow from meteorites. Some second hand reporting of Vasin and Shcherbakov’s article claims mica,  uranium, brass (a human-made alloy) and neptunium (a radioactive element which does not exist in nature) were discovered in samples returned by Apollo missions but this is not in the original article and is also untrue. Sadly not everything you read on the internet is to be believed.


Alan Shepard on the Moon with the MET, Apollo 14's unique pull along trolley. The landscape around him is completely natural. (Image credit: Edgar Mitchell/NASA)

Alan Shepard on the Moon with the MET, Apollo 14’s unique (but not very useful) pull along trolley. The landscape around him is completely natural. (Image credit: Edgar Mitchell/NASA)


This is a very unlikely theory surrounding the Moon. Now we need to look at some facts about the Moon in order to further our knowledge about it, and help us to decide what we think about these two theories.

Sadly the Moon is not made of cheese. We know this may come as a blow to some of you, but it’s time you know. The Moon is not a giant sphere of cheddar. It does however have a crust, a mantle and a core just like the Earth. In the past decade it has been discovered that the Moon does have an Earth-like core, meaning it would be solid. The core is rich in iron and is solid, with a radius of approximately 150 miles. Then there is a primarily liquid outer core and it has an estimated radius of 205 miles. The Moon’s core differs from the Earth’s by having a partially molten boundary layer. This layer is estimated to have a radius of 300 miles. How was this research conducted to gather this information? By using state of the art seismological techniques and applying this to the same data taken from the first lunar landings, NASA was able to make these discoveries. People who say that the Apollo seismometer results prove that the Moon is hollow have things exactly the wrong way round; the results show that the Moon is a solid body with a variegated internal structure!


The Moon's interior today as determined by actual science. (Image credit: Renee Weber/MSFC/NASA)

The Moon’s interior today as determined by actual science. (Image credit: Renee Weber/MSFC/NASA)


What about the mantle, we hear you ask? The mantle is directly under the crust of the Moon (the part that we can visibly see) and it divides the crust from the core. Now to get down to some hard core science (see what we did? Fine…no more bad jokes). It is believed that the mantle of the moon consists of the minerals olivine, orthopyroxene, and clinopyroxene. The actual composition of the Moon’s mantle is very similar to that of the Earth’s, however it is thought that the Moon’s mantle is richer in iron.

Then there is the Moon’s density which we have been able to measure for centuries. The average density of the Moon is about 3.3 g/cm3. This makes the Moon actually the second densest moon in the Solar System after Jupiter’s Io. This seems very inconsistent with it being a hollow spaceship. We also have fossil evidence of tides on Earth generated by the Moon going back hundred of millions of years. In Australia, there are known rhythmites, fossilised tidal deposits, dated to 620 million years agoSimilar structures have been found in South Africa, but are thought to be significantly older, being dated to have been laid down 3.225 billion years ago. The Moon is not a new arrival! Radiometric dating of samples returned from the Moon shows that parts of the Moon’s surface are at least 4.4 billion years old. All of these facts do seem to point towards the Moon as being rather solid and rather ancient.

To conclude this look into the hollow Moon theories, it is relevant to say from looking at the facts of how the moon is made up that it is probably most likely to be a solid celestial body. Hollow Moon theory can be safely join the Martian canals, planet Vulcan and the Black Knight satellite in the “silly space ideas bin”.

The Moon will always play a large role when it comes to Earth and it will still be the inspiration for many science fiction masterpieces. In fact another idea has recently cropped up in the SF world about the Moon. In the Doctor Who episode Kill the Moon (2014), it was stated that the Moon was actually a giant alien egg! The egg started to crack and a giant alien popped out of it and conveniently laid another Moon-sized egg. Think of that what you will!

(Article by Heather Taylor, Education Support Officer)