The Extremely Cool Physics of the Moon

Jeff Bezos is desperate to go to the Moon. Elon Musk is heading for Mars. Who’s right? What will realistic engineering look like on the Moon and how will the unfamiliar physics affect life there?

It’s clear that solar panels will be a vital lunar power source but getting enough daylight might be an issue for future colonists. The result of millions of years of gravitational interaction between the Earth and the Moon is something called “tidal locking” or synchronous rotation. On Earth, tidal locking means everyone always sees the same face of the Moon. On the Moon, the effect is more significant: it means a single one of their days lasts nearly 30 Earth days. As a lunar colonist, once the sun sets over the horizon, you aren’t going to see it rise again for a fortnight. I like a lie-in, but even I might find 2 weeks excessive.

The problem is, long nights could make farming and living difficult above ground for most of the surface. That’s one of the reasons the south pole is often proposed as a location for the first human lunar colony: unlike most of the Moon, bits of the poles are in almost constant sunlight.

Contrary to what you might read in your Christmas cracker, the Moon does have an atmosphere although it’s so thin it’s practically nonexistent. Most gas particles that do get captured by the lunar gravity get blown off again by the solar winds. Occasionally, water particles will stick and gradually migrate towards the poles where they form ice deposits. That’s another reason the poles are a good location for a base.

The downside for puny humans of an airless surface is fairly obvious — we have a pesky need to breathe. In that respect, your cracker is right: a party with no atmosphere is unlikely to be a success. I wouldn’t recommend it as your next theme.

For engineering, however, near-vacuum could be useful. There’s no air resistance on the Moon, nothing rusts, and there’s no weather to contend with. Storms are an increasing problem on Earth today and a major issue for the protagonists in my scifi books (which are set 30 years in the future). Fortunately, Moon colonists won’t have to worry about that.

However, there will be meteorite strikes. Any single area on the Moon is likely to be hit once every thousand years. That doesn’t sound much but if the Moon were densely covered with buildings that could be a problem.

The Moon is about a quarter the size of Earth and only about 60% as dense. Gravity depends on mass, so the gravity on the Moon is a sixth that here. If I was 9 stones on Earth, I would only weigh 1.5 stones on the Moon! That’s useful in some ways but bad in others. Humans aren’t designed to weigh so little and in a low gravity environment our skeleton and muscles will grow weak. Astronauts have to work hard to keep their bones strong. Without resistance exercises they couldn’t safely return to the tougher gravity of Earth.

A significant benefit of the Moon’s smaller size is it’s much easier to launch stuff into space from there than from here. If you fire things upwards fast enough, they’ll escape the pull of gravity and make it into space. For any given planet, that minimum firing speed is called the planet’s “escape velocity” and it applies to every projectile, independent of its own mass. The Moon’s escape velocity is 2.38 km/s, which is about twice the speed of a bullet. Our escape velocity is over 11 km/s. It’s much, much harder to fire stuff into orbit from the Earth than the Moon. (Just because I was interested and looked it up, Mars’ escape velocity is ~5km/s).

Another advantage is if you launch something into the “air” on the Moon it will travel a long way before the weak gravity pulls it back down (provided you kept the speed below escape velocity, or it will never hit the ground again). In the 1950’s, the science fiction writer Arthur C Clarke was keen to exploit the zero friction, low gravity conditions on the Moon by using electromagnetic cannons or mass drivers to cheaply and easily move payloads around on the surface and launch them into orbit. He even patented the concept. I’ve used the same idea in my books.

The Moon would be a cool place to visit, but it would be a pricey trip. Is there any stuff there that might be useful? Fortunately, there’s water (at the poles and also in moondust), oxygen in the form of minerals, and metals like iron and magnesium. For folk on Earth, it would be easier to get those things here than from the Moon. However, for people in space the Moon could be a useful source of materials that wouldn’t have to be lifted out of Earth’s gravity well.

If human society ever moves into space, the strategic location of the Moon and its resources will make it important. In my opinion, it’s a damn interesting place. Humanity should be there.

About the Author

As well as being an engineer for 25 years, Anne Currie is the author of a series of science fiction action and adventure novels that explore AI, surveillance, climate change and new forms of society.

Her new hard science fiction novel came out in December 2019. Denizen 43 (Amazon UK/US) is the third in the Panopticon trilogy.

Panopticon Series

“Utopia 5 asks the big questions about privacy, surveillance and free will in a networked society. All wrapped up in a page-turning thriller”

Hero image with thanks to NASA.

SciFi author interested in tech, engineering, science, art, SF, economics, psychology, startups. Chaotic evil.