Spoiler warning for Project Hail Mary
Part of the fun of sci-fi is getting an excuse to talk about real science, and Andy Weir exemplified this in Project Hail Mary. He goes into meticulous detail about the various problems that arise throughout the story, and while it’s not perfect, it does a good job of selling the the protagonist’s passion for science. And not to be that guy, but there’s one thing I need to correct, because it’s a misconception I’ve encountered frequently while teaching physics.
It happens when the protagonist, Ryland Grace, is piloting a spaceship named Hail Mary in an alien solar system. He needs to collect a sample from the upper atmosphere of a planet named “Adrian” without crashing his ship. Starting from orbit, he slows the Hail Mary to a crawl and uses the ship’s thrust to maintain altitude. He then drops a collection device attached to a 10-kilometer chain into Adrian’s atmosphere. The ship is tilted at 30 degrees so the engine won’t vaporize the chain. Here’s how Grace describes the scenario:
This is one of those things I frequently have to explain to my students. Gravity doesn’t just ‘go away’ when you’re in orbit. In fact, the gravity you experience in orbit is pretty much the same as you’d experience on the ground. The weightlessness that astronauts experience while in orbit comes from constantly falling. But the curvature of the Earth makes the ground go away at the same rate you fall. So you just fall forever.
So far, so good. Astronauts aboard the International Space Station are subjected to 88% of the gravity they would be subjected to on the ground. But since the station around them is falling at the same rate, they don’t notice.
I unhitch my restraints and climb out of the chair. Adrian’s 1.4 g’s of gravity pulls me down at a 30-degree angle. The whole room feels tilted because, actually, it is tilted. This isn’t engine thrust I’m feeling. It’s gravity.
Sorry, but no. It’s not gravity that Grace is feeling here. It’s engine thrust. In fact, not only is Grace not feeling gravity in this scene, no one has ever felt gravity. When you stand on the ground, you feel the ground pushing up against your feet. The force of contacting the ground compresses your skin, which compresses the tissues above it, and so on, all the way to the top of your head. This compression is what you perceive as “weight.” If you take away the ground, your perception of weight disappears. Being subjected to gravity alone feels like being subjected to no force at all1.
Normally, it doesn’t matter whether we’re careful about this distinction. If you’re standing still on the ground, the ground applies a force that is exactly opposite to the force of gravity, so whether you say you feel “pulled by gravity” or “pushed by the ground,” the result is the same. On a spaceship, it makes a difference. Imagine Grace drops a tennis ball from the middle of the ship: If he’s feeling Adrian’s gravity, as he claims, then the ball should fall at a 30-degree angle relative to the ship’s vertical axis. If he’s feeling engine thrust, then the ball should fall toward the ship’s engines. The following animation shows what that would look like to an external observer and to someone on board the Hail Mary. I included an animation without thrust for comparison.
With the engines off, the ship and the ball will fall at the same rate, so the ball will remain stationary next to Grace’s hand. With the engines on, the ship will move to the right, keeping the ball along a vertical path relative to the ship. The simple explanation for what we see on the ship is that both objects are subjected to Adrian’s gravity, but the ship is subjected to thrust and the ball isn’t.
To see where the book goes wrong, we need to do a first-year physics student’s favorite thing: draw a free body diagram. The two forces acting on the Hail Mary are gravity and thrust. The vertical component of the thrust needs to cancel the force of gravity to prevent the ship from falling. This leaves us with a net force to the right, with a magnitude of half the ship’s thrust2.
This contradicts the book’s description:
The Hail Mary isn’t falling anymore. The engines hold us up in the sky and our tilt makes us scooch forward at 127 meters per second—about 285 miles per hour. Fast for a car, but amazingly slow for a spaceship.
A horizontal net force doesn’t “scooch” an object at constant velocity. It causes horizontal acceleration. Maintaining a constant velocity would require the forces to be in equilibrium (i.e., zero net force). It seems Weir thought the forces were equivalent to resting the Hail Mary on a 100-kilometer-tall mountain3, but thrusting the engines at a 30-degree angle is not the same as making contact with the ground. Unfortunately, this spoils the whole “dangle a probe from a chain” plan. When Grace drops the chain, it’s going to fall the same direction as the tennis ball—straight into the engine exhaust. To avoid vaporizing the chain, Grace would need to have multiple engines firing in different directions.
This kind of mistake is why I have beef with the term “microgravity”. It fails to correct the misconception that gravity is the thing you’re feeling, and suggests that the gravity is just too weak for you to notice. (Weir avoided the second mistake, but fell victim to the first.) Imagine riding the Galileo probe on its final descent into Jupiter’s atmosphere: you would have experienced “microgravity” while being pulled 2.5 times harder than on Earth. Total nonsense. Taken literally, “zero gravity” would be even more wrong than “microgravity”. But if we clarify that we’re really talking about the perception of weight, then “zero gravity” correctly suggests that the forces causing that sensation (e.g., contact with the ground or engine thrust) are absent. Truly, the best term for the state of weightlessness is “free fall” (or in the case of unaccelerated coasting through empty space, “free float”). This makes clear that the perception of weight is caused by the forces preventing you from falling freely under gravity.
The realization that you can’t feel gravity is a powerful insight. It means that being held stationary in a gravitational field by some additional force feels indistinguishable from being accelerated by the same force through interstellar space. This idea makes a whole host of physics problems easier to reason about. One of the most common puzzles given to introductory physics students is, “What happens to a balloon floating inside a sealed box if the box is given a sudden push to the right?” Most people’s instinct is to say that the balloon will resist the sudden motion, appearing to be pulled to the left side of the box. However, once you understand that “acceleration to the right” is the same as “a gravitational field to the left” as far as the contents of the box are concerned, it’s easy to see that the balloon (which was already floating against a gravitational field) will float to the right side of the box4. This insight also led Albert Einstein to his General Theory of Relativity:5
The breakthrough came suddenly one day. I was sitting on a chair in my patent office in Bern. Suddenly a thought struck me: If a man falls freely, he would not feel his weight. I was taken aback. This simple thought experiment made a deep impression on me. This led me to the theory of gravity.6,7
So next time you’re tempted to say you feel the pull of gravity, remember: no, you don’t. And if you’re Ryland Grace, the fate of the world may depend on it.
- Tidal forces are negligible on human-sized scales, unless you’re falling into a black hole. Even in that case, you’d be feeling the difference in gravity at separate points on your body, not the value of the gravitational field at any point. Experiments performed within a box can’t determine the gravitational field without knowing how the box itself is moving. ↩︎
- sin(30°) = ½ ↩︎
- A 100-kilometer-tall mountain would be an incredibly unlikely feature to find on a super-Earth. Stronger gravity tends to make a planet less lumpy. ↩︎
- YouTube Short: Helium balloon moves forward when pushed within a container??? 🤔 #shorts #DrDawson ↩︎
- Equivalence Principle – History ↩︎
- Einstein, Albert, How I Created the Theory of Relativity ↩︎
- In Einstein’s theory, gravity isn’t even considered a force. A force is an influence that, if acting alone, would cause a change in an object’s motion. In Newtonian physics, “unchanged motion” means traveling in straight line through 3-dimensional space at constant speed. So gravity is a force because it causes objects to deviate from that motion. In general relativity, we need to consider an object’s path through curved, 4-dimensional spacetime. Gravitational time dilation near a massive object means that the straightest path you can draw through spacetime appears to accelerate toward the object. In general relativity, gravity doesn’t cause objects to deviate from their natural motion, it defines their natural motion. ↩︎