Book Review of Mission of Gravity

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Book Information

Title: Mission of Gravity
Author: Hal Clement
Copyright Date: 1954
Rating (out of 4 stars): ***
Reviewed on: Jan. 7, 2006


Approximately 100 years in the future, Earth will have interstellar flight. As humans explore the galaxy, they encounter the planet Mesklin and make contact with some of its intelligent inhabitants. Mesklin is a much more massive planet than Earth and is unusual because it rotates extremely quickly - one rotation every 20 minutes. Consequently, the planet is extremely oblate, and the gravitational acceleration felt at the surface varies from approximately 700 gees at the poles to about 3 gees at the equator. Mesklin also orbits far enough from its star that it is cold enough for methane to be liquid, solid, or vapor, depending on the season.

Because of the high surface gravity of the planet, the human explorers must usually remain in their orbiting ship, with only occasional forays to the planet in special suits or equipment. They have befriended a native ship captain named Barlennan. Mesklinites are described as being rather like centipedes with several sets of front pincers. Because of the high surface gravity, they are very low to the ground, and have extremely strong exoskeletons. Barlennan's civilization has approximately the same level of technology as 18th century on Earth: sailing ships, some weaponry, some ideas of cartography and science.

The humans have sent a probe to land at the south pole of the planet. However, because of a malfunction, the probe cannot return to the ship with its observations. The humans enlist Barlennan and his crew to travel to the pole and retrieve the crucial data storage components. The book then chronicles Barlennan's adventures as he travels with his ship and crew from his home hemisphere across the equator to the pole of the other hemisphere. Along the way, they have various encounters with other civilizations that have differences in technological capabilities.

When Barlennan and crew finally reach the probe at the pole, Barlennan refuses to return the crucial data to the humans until they agree to teach Barlennan and his crew basic science. Barlennan's intent is to use the knowledge in trade on his world. The humans agree.


This is a very pleasant book, which follows the principles of what I think of as "old style" science fiction. That is, it imagines an unusual situation, and by very carefully thinking out all of the science, determines what things would be like in that situation. The book then demonstrates this to the reader. The plot of the book is purely a device by which the author demonstrates all the strange physics that results from having a massive planet that rotates very rapidly. I have to say that it's very interesting.

For example, the author, via Barlennan, mentions repeatedly that Mesklinites are deathly afraid of being off the ground (for example, on a ladder), jumping, or falling. This is because Barlennan and his crew typically live some distance from the equator, where the surface gravity is hundreds of times the Earth's. Anything that falls experiences terrific acceleration - for a Mesklinite to fall even a foot would be deadly. The author plays with the effects resulting from high gravity throughout the book.

The second unusual property of the planet is its rapid rotation. Since the planet (which is much larger than Earth) rotates in just 20 minutes, the Coriolis Effect is correspondingly extreme. (The Coriolis Effect is what causes air at the Earth's surface to circulate in the counterclockwise direction in the northern hemisphere. In order for the Earth's equator to make one rotation in the same time as the Earth's higher latitudes, the surface at the equator has a higher linear velocity than the surface at higher latitudes. Air that travels northward from the equator has the same speed as the surface at the equator. As it travels northward, the air keeps the same speed, but the ground is moving slower and slower. Consequently, the air "races ahead" of the ground and travels to the east.) For this reason, the author reasons that the Mesklinites would have never developed projectile weapons, because the motion of the projectiles would be so complicated. Any bullet shot from a gun would follow a very curved path.

During the adventures of Barlennan and his crew, the author introduces numerous complications which are eventually overcome by cleverly analyzing the physics of the situation and coming up with solutions that are possible on this unusual planet but would be impossible on the Earth. I enjoyed this a lot - it was very logical in retrospect, but I hadn't thought of the solutions in advance.

My paperback version of the book has an Author's Afterword which contains an article that first appeared in Astounding Science Fiction in 1953. The article describes all the properties of the planet and how he developed it for use in the story. This is a wonderful bonus, and shows the level of detail and effort the author put into the book. As an astronomer, I found this article very interesting.

Astronomically speaking, I do have one problem with the planet the author developed. In the Afterword, the author states that the planet has a mass that is 16 times Jupiter's mass, and an average diameter of 34,000 miles (obviously since the planet is very oblate, the equatorial diameter is much larger than the polar diameter). This gives an average density of 22,000 kg/m^3 (about 4 times the average density of the Earth). This implies that the planet is composed mostly of very heavy materials, such as metals and rocks (which have been additionally compressed by the weight of material above them).

Most of the properties of the planet contradict our current ideas of how planetary systems form. First, an object 16 times the mass of Jupiter would typically be classified a brown dwarf, because objects that large would likely have had deuterium fusion, which is a criterion for being a brown dwarf. However, this boundary is not well-known, so that could be OK.

The second problem is that the planet should be a gas giant, and not have a solid surface. The planet is outside what is called the "frost line", where hydrogen compounds (like water, methane, and ammonia) can solidify. Indeed, the author has these solids in the surface of the planet. However, if an object formed with so much mass, its gravitational force would be high enough that it would attract a large atmosphere of hydrogen and helium gas and become a gas giant planet, just like Jupiter. In fact, we currently believe that Jupiter's core consists of approximately ten times the Earth's mass of rocks, metals, and hydrogen compounds, and its gravity was strong enough to attract its massive atmosphere. The "atmosphere" of Mesklin would extend for tens of thousands of kilometers, such that the pressure at the "surface" would be so extreme that nothing could survive - this is why we say gas giant planets have no surface.

I have no idea why the author thought he could "make" such a massive planet that was not a gas giant. I attribute this to the contemporary ideas of planetary formation in the 1950s - they certainly were aware that Jupiter was a gas giant planet (not solid), but perhaps they had not yet worked out why it was a gas giant. Regardless of this fact, there were no data to support the idea that such a massive planet could exist as a solid.

At any rate, this is my only real problem with the book. It didn't actually bother me as much as the above discussion might suggest - I am just trying to be thorough in my explanation. Once I just accepted the planet existed, the physics the author used was impeccable, as far as I could tell. The ideas in the book were quite entertaining, but it was very short on any sort of character development or any type of "deep meaning".

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