Would a blind astronaut be able to sense the gravity of a nearby star/planet/moon?

Would an astronaut lost somewhere inside the Sun be able to sense which way is the core and which way is the surface?


  • For question 1, the astronaut is in the vacuum of space, the pressure outside of his spacesuit is in the order of $10^{-10}$ Pa.
  • For question 2, the futuristic spacesuit of the astronaut can resist the high temperatures inside the Sun.
  • For both questions, the astronaut does not have any object at his disposal, he cannot communicate with anyone else and his spacesuit does not have any integrated devices.

3 Answers 3


For Question 1, if he's in orbit, and assuming he started at a fully rested position (not spinning or anything) he could likely detect it using Tidal Forces. If he went entirely limp, gravity would slowly pull him into a position that he could use to determine which way was down and which was up. This works because part of his body is technically in a larger orbit than the other, so they'll slowly stretch apart. He would either end up with his feet pointed down (if his center of gravity is low in his suit) or feet up if his center of gravity is high. Note that "slowly" probably means several orbits. I know you said he doesn't have any objects, but if he can improvise something like a line or string with a weight on the end, that would work better for this than just using his own body.

For Question 2: He won't be able to detect the gravity, as the turbulence of the plasma will prevent him from getting stable. However, he can form a Y shape (arms out and up, legs together) then theoretically he'll end up falling feet first into the Sun as that's the most aerodynamic shape. So if he does that and waits a while, he could be fairly confident that his head is "up" - or at the very least, that it's not down.

  • $\begingroup$ For question one, are you assuming the astronaut knows they are in orbit or can they determine that some other way first? Assuming that knowledge, how does the astronaut determine that their tidal-force-maneuver is complete or which of the two listed orientations was reached? $\endgroup$
    – Samuel
    Mar 9, 2015 at 22:27
  • $\begingroup$ It's entirely a gamble. He needs to assume he's in an orbit, and depending on his knowledge of physics he could then determine how long it would take him to end up in a stable state. But there's no way for him to verify it. $\endgroup$ Mar 9, 2015 at 23:26
  • $\begingroup$ Fair enough. I had assumed the point of the astronaut being blind was they had no other way of knowing there was a planet nearby and this gravity sense was a way to detect it in the first place. $\endgroup$
    – Samuel
    Mar 9, 2015 at 23:27
  • $\begingroup$ @Samuel: If he had a fairly long string/line and a weight, he could probably use that to detect tidal forces. But I don't think the length of a human body would be sufficient. $\endgroup$ Mar 9, 2015 at 23:30
  • 1
    $\begingroup$ The difference between gravity ("tidal force") between the top and bottom of a 2m person orbiting at 6800 km (400 km altitude) is around 10 to -14th power. I'm thinking that even over many orbits, these will be far overshadowed by the irregularities in the Earth gravitational field. In any event, 10 to the -14th is probably beyond the vestibular system of any human, blind or otherwise. $\endgroup$ Mar 1, 2016 at 16:43

Humans can't tell the difference between gravity and acceleration, they're indiscernible. Without vision, the astronaut would need to rely entirely on their vestibular system. The vestibular system on its own doesn't do a really great job. It needs the feedback from the visual system and proprioceptors. The astronaut will have proprioception, but without anything to push against, it's nearly pointless.

Question 1:

Maybe, but probably not. Humans can detect acceleration, sort of, but can not detect velocity. Even then we can't detect low levels of acceleration, especially accelerations aligned with our bodies. The astronauts first clue they were near a planet would likely be when the were entering the atmosphere. I was a licensed skydiver for several years, do know what freefall feels like? Windy, it does not feel like falling (except during the first couple seconds).

Question 2:

Not very likely at all. There are a lot more things to worry about inside the Sun than which way is up. Even if that was the astronauts primary concern and focus, the problem is the same as for a planet, but further confused by being crushed and immersed in the plasma currents. Have you ever been inside a crashing wave in the ocean? It's nigh impossible to determine which way is up. If the suit was perfectly protecting the astronaut from the temperature and pressure they would presumably only know something was going on because of the sound being transmitted through the suit changing as they reached different layers of the Sun. Even then it might be hard to tell if they were in the atmosphere of a planet or in the Sun.


Only when he stops falling (i.e., when he hits the floor). A little earlier, if the planet has atmosphera.

An astronaut as you describe it is in free fall. All of the particles in his body feel the same acceleration, so he has no way of sensing which it has acceleration (when you stand on the ground, your feet feel the force on the ground needed to annulate acceleration due to gravity, which is what you are actually feeling).

Even if the speed becomes so big to begin having relativistic effects, for him everybody will be normal (if he had a watch that he could test by touch, he would not feel any time dilation/contraction).

Once he enters an atmosphera, he will begin sensing friction. Of course, that will not mean that he will know where down is, because friction will just mean a deceleration is applied with the direction opposite of his velocity. If he is facing in the direction of his movement, he will feel some force pushing his front body back.

If the direction of movement becomes down, he will feel that such forces increases progresively, as the atmosphera becomes denser and denser. He may feel the heat, too.

Of course, once he reaches the ground, what remains of him cannot continue advancing. The impact at terminal speed should give a good indication of what the direction of gravity is; and after all, each reamining chunk of astronaut (if any) feels the force the ground applies on it to compensate to gravity (since he is subject to gravity and does not move, the ground is exerting the force needed to compensate gravity).

If the planet has no atmosphera, the first hint of the action of gravity would be act of landing (and cratering) such planet.


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