Heavy Planet

The other month I read, in quick succession, two sci-fi classics involving heavier-than-Earth gravities: Robert Forward’s Dragon’s Egg (1980) and Hal Clement’s Heavy Planet (comprising four novellas, 1953–1971).

Dragon’s Egg is set on the surface of a neutron star, with a surface gravity equivalent to 67 billion Earth gravities, a rotational period of 0.2 seconds, and an “organic chemistry” based on the strong nuclear force rather than electromagnetic bonds. The native life-forms operate at such a rapid pace that multiple generations can rise and fall in an hour, the entire history of life is compressed into 5000 years, and the “modern history” of the cheela civilization — from the invention of agriculture to interstellar spaceflight — takes place over about a month (thanks to an extremely coincidental meeting with an expedition from Earth at just the right moment in their history).

Dragon’s Egg is often compared to Hal Clement’s 1953 Mission of Gravity (collected in Heavy Planet), in that both novels involve human contact with the inhabitants of a rapidly whirling, super-massive world. Heavy Planet, though, involves numbers on a much more human scale: its planet Mesklin is merely 5000 times the mass of Earth, with a rotational period of about 18 minutes. This gives Mesklin a surface gravity of… well, it depends on your latitude! Near the equator, 38600 km from the planet’s center, Mesklin’s rapid whirling (227 km/s angular velocity) produces a centrifugal acceleration of v²/r = 1330 m/s², canceling out most of the 1360 m/s² acceleration due to gravity at that distance. The result is that gravity at Mesklin’s equator is only three times that of Earth; but at the poles, where the centrifugal acceleration is nil and (because Mesklin is an extremely oblate spheroid with a polar radius of only 16000 km) we’re talking something like 275 Earth gravities. (My naïve arithmetic says 800 gravities, but my understanding is that it’s not terribly appropriate to approximate Mesklin as a point mass: at the pole, much of Mesklin’s equatorial mass is actually “above” you in its gravity well, producing a countervailing gravitational force. The 275-g number comes from a computer program written by the MIT Science Fiction Society back in 1953-something.) So, in Clement’s stories, the chemistry is ordinary organic chemistry, and human astronauts can even visit the surface of the planet and interact directly with the natives — as long as they stay near the equator!

Gregory Brannon has created a mod for Kerbal Space Program that simulates a “whirligig world” similar to Mesklin.

Hal Clement’s Mesklinites are technologically about where Earthlings were in the middle of the second millennium, and the narrative itself is a straightforwardly rollicking exploration-and-adventure travelogue, following the native ship’s captain Barlennan on a voyage similar to those of Captain Cook. One delightful bit of worldbuilding: Just like Earth, Mesklinite navigation has a longitude problem but no latitude problem, despite Mesklin’s cloudy skies. A Mesklinite sailor needs no sextant to measure latitude; all he needs is a wooden spring with a weight attached. The further you get from the pole, the higher the weight lifts.

But here’s the real reason I’m writing this post: Mesklinite gravity is so high that it’s hard for humans to comprehend. There’s a viral video going around right now that shows a simulated pallet of lumber falling on a car in Earth gravity, Jupiter gravity, and so on. Apparently it was made in the driving (and soft-body collision) simulator BeamNG.drive, which has a gravity slider; but according to the Reddit comments the slider maxes out at “Sun gravity” (274 m/s², or about 28 g), which is vastly lower than the gravity at Mesklin’s poles (which, remember, MITSFS estimated at 275 g = 2700 m/s²). From Mission of Gravity:

Seeing things fall free in triple gravity, Lackland found, was even worse than thinking about it. Maybe it would be better at the poles — then you couldn’t see them at all. Not where an object falls some two miles in the first second! But perhaps the abrupt vanishing would be just as hard on the nerves.

And again, when the crew encounters a three-hundred-foot cliff:

One of the crew […] rolled a bullet-sized pebble to the edge of the cliff and given it a final shove. The results had been interesting, to both Mesklinites and Earthmen. The latter could see nothing, since the only view-set at the foot of the cliff was still aboard the Bree and too distant from the point of impact to get a distinct view; but they heard as well as the natives. As a matter of fact, they saw almost as well; for even to Mesklinite vision the pebble simply vanished. There was a short note like a breaking violin string as it clove the air, followed a split second later by a sharp report as it struck the ground below.

Fortunately it landed on hard, slightly moist ground rather than on another stone[.] The impact, at a speed of approximately a mile a second, sent the ground splashing outward in a wave too fast for any eye to see while it was in motion, but which froze after a fraction of a second, leaving a rimmed crater surrounding the deeper hole the missile had drilled in the soil. Slowly the sailors gathered around, eyeing the gently steaming ground; then with one accord they moved a few yards away from the foot of the cliff. It took some time to shake off the mood that experiment engendered.

(To be fair, when I do this math, I think that either the cliff was 300 meters tall, or else the pebble was traveling only half a mile a second, or 1600 mph, at impact. Still.)

I’d love to see some animations of soft-body dynamics under Mesklin-like gravitational conditions; even better if the model takes into account the massive conversion of kinetic energy into heat that must follow such an impact. If you know of a way to simulate and render such an animation, please let me know!

Posted 2022-05-22