Alec Nevala-Lee

Thoughts on art, creativity, and the writing life.

The moon is a harsh fortress

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The May 1947 issue of Air Trails and Science Frontiers

In the May 1947 issue of the nonfiction magazine Air Trails and Science Frontiers, which was edited at the time by John W. Campbell, the cover story was an article titled “Fortress in the Sky.” Its author, credited as “Capt. B. A. Northrop,” argues that the existence of the atomic bomb has rendered the notion of a defensible land or naval base obsolete. The only truly impregnable military position, he writes, is the moon, which will soon be “conquered” by man: “It will probably be reached in five years and completed in ten. Its possessor will be supreme over all nations and peoples of Earth.” Northrop discusses the possible technologies that could be used for a moon landing, and he goes on to make a very peculiar claim:

Here and there throughout the world many men have been thinking about rockets for some time. I recall that in 1930, L. Ron Hubbard, a writer and engineer, developed and tested—but without fanfare—a rocket motor considerably superior to the V-2 instrument of propulsion and rather less complicated.

In fact, “Northrop” was none other than Hubbard himself, and his pseudonym, which is spelled “Northorp” elsewhere in the issue, was evidently inspired by Northrop Aircraft, a frequent presence in the magazine’s pages. (At the time that he was allegedly conducting rocket research in 1930, incidentally, Hubbard was just nineteen years old.) “Fortress in the Sky” was Hubbard’s first major publication after the war. He had been suffering from depression and, unusually, writer’s block, and before Campbell give him the assignment, he had contributed just a few poems and short articles to the Catalina Islander. As such, the piece was a turning point in his career, but as far as I know, it has never been reprinted in its entirety. Recently, however, I got my hands on a copy of the original issue of Air Trails in which it appeared, and I was able to read the whole thing. Aside from Hubbard’s gratuitous reference to himself, which Campbell either believed or was willing to let slide, it’s a surprisingly plausible piece of futuristic speculation. Campbell appears to have provided much of the science, and a lot of the material relating to a future moon colony seems to have been drawn directly from the editor’s unpublished novel The Moon is Hell.

If you’re a science fiction fan, however, the most fascinating section comes about a third of the way through. While listing the moon’s strategic advantages as an atomic missile base, Hubbard notes:

The first [factor] might be termed the “gravity gauge” comparable to the weather gauge so desirable in the days of sailing ships of the line…The gravity gauge is important in the ratio of six to one, in that a missile would have to travel with an initial velocity of six miles per second to leave Earth, but would only have to travel with a velocity of one mile per second to leave the Moon. Such a missile, leaving Earth, would have to go nine-tenths of the way to the Moon on power before the latter body would begin to pull it in by its own gravity, whereas it would only have to travel one-tenth of the distance to escape the Moon and begin to ride down on Earth gravity.

On the next page, a diagram is provided to illustrate this point, with a caption that was presumably written by Campbell:

Skippers of old-time men-o’-war early learned the advantage of the “weather gauge,” which meant being up-wind from the enemy. The Moon affords a similar “gravity gauge” over the Earth by reason of its weaker pull. A missile shot from Earth to Moon must work against the stronger Earth gravity to the point where the pulls balance, nine-tenths of the way out. Missiles fired from the Moon work against one-sixth the pull through one-tenth the total distance, and ride free the rest of the way. Also, the moon has no retarding blanket of atmosphere to slow the take-off.

If I had to guess, I’d say the concept itself probably came from Campbell, while the term “gravity gauge” sounds more like Hubbard, a nautical enthusiast who casually uses the term “weather gauge” in one of his later novels, Masters of Sleep. But this is pure speculation.

The Moon is a Harsh Mistress

Yet if this all sounds a little familiar, it might be because you’ve read Robert A. Heinlein’s The Moon is a Harsh Mistress, which was first published in 1966. The novel recounts the revolt of a lunar colony much like the one that Hubbard describes, and in a crucial plot point, its narrator quickly figures out the advantage that gravity provides, with the help of a computer named Mike: “Luna…has energy of position; she sits at top of gravity well eleven kilometers per second deep and kept from falling in by curb only two and a half km/s high. Mike knew that curb; daily he tossed grain freighters over it, let them slide downhill to Terra.” And they don’t need to use missiles at all. A hundred tons of anything, falling to earth from the moon, would generate six trillion joules of kinetic energy, or the equivalent of two-kiloton atomic bomb. All they have to do is throw rocks: “That terrible speed results from gravity well shaped by Terra’s mass, eighty times that of Luna, and made no real difference whether Mike pushed a missile gently over well curb or flipped it briskly. Was not muscle that counted but great depth of that well.” Heinlein had also mentioned this concept before—although, revealingly, not in Rocket Ship Galileo, which is actually about a Nazi military base on the moon, but was written before Hubbard’s piece was published. In late 1947, a few months after the article appeared, Heinlein began work on the juvenile novel Hayworth Hall, later retitled Space Cadet, in which we find the exchange:

“The spaceship is the perfect answer in a military sense to the atom bomb, and to germ warfare and weather warfare. It can deliver an attack that can’t be stopped—and it is utterly impossible to attack that spaceship from the surface of a planet.”

Matt nodded. “The gravity gauge.”

“Yes, the gravity gauge. Men on the surface of a planet are as helpless against men in spaceships as a man would be trying to conduct a rock-throwing fight from the bottom of a well. The man at the top of the well has gravity working for him.”

Heinlein first publicly made the connection to the moon at a meeting of the County Librarians’ Association in Los Angeles on May 5, 1948. (One of the other writers in attendance, incidentally, was Theodor Geisel, who would later become famous as Dr. Seuss.) A writeup in the Los Angeles Times quoted Heinlein as saying that the moon would make an ideal military base: “A power on the moon would have the gravity gauge. The moon has one-sixth the normal earth gravity. It would be like throwing rocks downhill.”

Decades later, in 1965, Heinlein expanded on this point in a comment on his earlier essay “Pandora’s Box,” which was published in the collection Expanded Universe:

The disadvantage in being at the bottom of a deep “gravity well” is very great; gravity gauge will be as crucial in the coming years as wind gauge was in the days when sailing ships controlled empires. The nation that controls the Moon will control the Earth—but no one seems willing these days to speak that nasty fact out loud.

The italics are mine. Given the timing of its appearance in Space Cadet, Heinlein’s remarks on the subject in 1948, and his use of the term “wind gauge” in 1965, it seems clear that he read Hubbard’s “Fortress in the Sky” and was influenced by it while writing The Moon is a Harsh Mistress. We know for a fact that Heinlein read Air Trails. Campbell had approached him about writing an article for the magazine, and Heinlein wrote back to the editor on November 11, 1946: “Air Trails shows distinct improvement under your editing.” He was also friends with Hubbard, and there’s no question that he would have found this article intensely interesting. The fact that the three men had all spent time together over the previous couple of years means that we can’t rule out the possibility that Heinlein came up with the notion first and passed it along to one or both of the others. But I think that the simplest explanation—that Heinlein borrowed the concept from Hubbard’s article—is also the most likely one. It doesn’t seem to have occurred to Hubbard or Campbell that the gravity well could be used to deliver anything other than missiles, and utilizing it to “throw rocks” is, frankly, a much better idea. This implies that Heinlein read the article, mentioned it in passing in Space Cadet, thought up a distinct improvement, and then simply set it aside until he was ready to use it. (As far as I can tell, Hubbard was the first to use the term “gravity gauge.”) Hubbard’s official biographies refer to Heinlein as his “protégé,” which is a stretch even by their uncritical standards, but this is one case in which the direction of influence genuinely appears to have run the other way. As Heinlein wrote in Glory Road, using an image that he liked so much that he returned to it repeatedly: “That’s the way with writers; they’ll steal anything, file off the serial numbers, and claim it for their own.”

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  1. As I began to read this essay, I thought, “This may mean nothing, but Bob Cornog was working for Northrop Aircraft during the time that he and L. Ron Hubbard lived at Jack Parsons’s house beginning in late 1945.”

    Deeper in, I recalled that Heinlein, who had returned to Los Angeles, visited that house and engaged in bull sessions. In the 1930s Cornog and Heinlein, then both engineers, had bonded over a love of SF and spaceflight. Heinlein persuaded Cornog to join the American Interplanetary (later Rocket) Society, and he introduced Cornog to the Caltech rocket experimenters, possibly including Parsons.

    So what were Heinlein and Cornog talking about in the Parsons living room, in the first couple years of the Atomic Age, with Hubbard listening in?

    Imagining that scene, I am not at all convinced that “gravity gauge” was an idea of Hubbard’s that Heinlein picked up. While that’s possible, it seems more likely that “gravity gauge” flowed the other way, from Cornog or from Heinlein, the ex-Navy man, to the less technically-adept Hubbard. LRH was just the first into print with it. (Or Hubbard may just have gotten it from Campbell, along with other Moon base information.)

    Later in life, as you show, Heinlein preferred the “gravity well” analogy, which has been widely adopted in spaceflight discourse today. I first encountered it in Arthur C. Clarke’s 1951 book The Exploration of Space. But a bit of googling shows me that Robert S. Richardson (an astronomer and another buddy of Heinlein and other SF writers) first introduced “gravity wells” in “Space Fix,” in Astounding for April 1943.

  2. @Bill Higgins: Thanks for reminding me of the Cornog connection. Obviously, unless more documentation turns up, there’s no way of proving it definitively either way. But I still think that Campbell –> Hubbard –> Heinlein is the most plausible progression. It just seems very striking that Heinlein never mentions the concept, even in stories in which it would have fit very well, until after the article appears, at which point he brings it up gratuitously in his library talk and in Space Cadet. I’m hoping to write up a short piece about this for The Heinlein Journal, and I’ll credit you for the Cornog angle.


    March 7, 2017 at 9:09 pm

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