If you want to do any kind of successful spaceflight operation on, or near, the Moon, you absolutely need to know some facts about it. If you need to orbit it, land on it, and/or take off from it, the primary data you need are:
Once you have those two, it's a simple matter to figure out how much lunar gravity is available to keep your spacecraft in orbit, hold down your lander and maybe your astronauts, and resist your attempt to escape from it to get back home. Well, NASA has done some pretty successful stunts up there over the years—not just Apollo but many photographic and scientific orbiters. The Russians have had their successes with the Luna and Lunakhod missions. The Chinese have Chang'e and the Jade Rabbit lander—the Indians have two Chandrayaan landers to their credit.
So I can tell you with 100% confidence that we know how big the Moon is and how heavy it is. Here's the data:
- Mass 7.35 x 1022 kg
- Diameter (mean) 3474 km
Since the volume of a sphere is given by 4πr3/3, it's a trivial matter to calculate:
- Volume 2.2 x 1019 cubic metres
...and from that...
- Density 3340 kg/m3 (since density = mass divided by volume)
More rubbish from George Noory
I mention this—and I repeat, this data is not guesswork but certainty—because Coast to Coast AM last night donated three hours of broadcast time to an ignoramus called Rob Shelsky. Shelsky is described as an investigative journalist. His proposition was that the Moon is hollow, and he wanted us to know that there were three pieces of evidence for this idea. The first of them was that the overall density of the Moon was much less than it could be expected to be if the whole thing was solid.
Well now, that's quite an easy factoid to check, now that we know with certainty what that density is. We happen to have, down here on Earth, 382 kg of moon rocks from Apollo, 326 grams from Luna, plus a few bits from meteorites. We can measure their density and see if the density of the whole Moon really is a surprise. People in white coats and face masks have indeed done that, and here's what they found:note 1
Mare basalts: 3270 - 3460 kg/m3
Feldspathic highlands rocks: 2510 - 2840 kg/m3
Breccias: 2360 - 2520 kg/m3
We can pretty much forget the breccias because by definition they are highly porous — full of air (actually vacuum, since this is the Moon.) Then once we realize that most of the Moon is basalt, the conclusion must be that there is no reason at all to think that the Moon is hollow on the basis of its density.
The investigative Mr. Shelsky was perhaps misled by the fact that the overall density of the Moon is only six-tenths that of Earth. But that's because the Moon lacks that really heavy iron core that is siting there under our feet, giving us weight in addition to our inherent mass.
Like a bell
The second part of Shelsky's "evidence" was that, when Apollo 13's SIVB upper rocket stage was deliberately crashed into the Moon, the seismometer left there by the Apollo 12 crew reacted and went on reacting for more than 10 minutes. Clive Neal, associate professor of civil engineering and geological sciences at the University of Notre Dame, entered the annals of sound-bite history by remarking that the Moon "rang like a bell." However, that has more to do with the Moon's perfect dryness than its inherent musicalitynote 2. It is not credible evidence for the hollow Moon.
Finally, the ill-informed Mr. Shelsky told us that even the Great Carl Sagan thought that maybe the Moon was hollow, and his co-author Ivan Shklovskii agreed. Shelsky cites Shklovskii—try saying that 12 times quickly. Wrongly, as it turns out. The booknote 3 he's thinking of speculated that Phobos, the moon of Mars, might be partly hollow. That was based on orbital data available at the time which has since been discredited. Now the error-prone Richard Hoagland is the only person who thinks Phobos is hollow, and even he would scoff at Shelsky's pseudo-data.
Would somebody please tell George Noory and Lisa Lyon, his Exec. Producer, to stop misinforming their audience? Thank you.
 Kiefer, W. S., Macke, R. J., Britt, D. T., Irving, A. J., and Consolmagno, G. J. (2012) The Density and Porosity of Lunar Rocks. Geophysical Research Letters, v. 39, L07201, doi: 10.1029/2012GL051319.
 Sagan, Carl, and Shklovskii, I.S. (1966) Intelligent Life In the Universe. Dell. ISBN-13: 978-0440540564