# What does acceleration in deep space feel like?February 29, 2012 10:38 AM   Subscribe

Humans can supposedly sense acceleration. So what would piloting a spaceship in deep space feel like? General confusion about relativity inside.

Presumably a spacecraft would accelerate by throwing mass in the opposite direction. Without a large source of gravity nearby, would you be able to feel that acceleration inside the ship? I would assume so, but how does that play into the concept of relative motion? Wouldn't you only be accelerating relative to whatever you threw in the opposite direction? If so, couldn't anything else floating through space act as a frame of reference?
posted by parallellines to Science & Nature (29 answers total) 1 user marked this as a favorite

Wouldn't you only be accelerating relative to whatever you threw in the opposite direction?

Acceleration isn't relative; velocity is relative. You accelerate equally in any inertial frame of reference (not worrying about near-light speeds for now). Bob and Frank may measure Bob's velocity differently, but they'll measure the rate of change of Bob's velocity the same.

And acceleration feels just like gravity. In fact it's indistinguishable.
posted by dfan at 10:52 AM on February 29, 2012 [1 favorite]

I'm no scientist (though I play one on TV!), but a lot of space sci-fi that I've read seems to think that, given enough acceleration, a likeness of normal gravity can be produced. A character in the story may say "Hey pilot, how about giving us a few gees so we can sit down to eat?" or something of the sort.

I would guess that the key here is acceleration, not velocity, as the Shuttle crews working in established orbit experienced weightless or close to it, while moving at a good, though fixed, velocity.
posted by xedrik at 10:54 AM on February 29, 2012

Presumably a spacecraft would accelerate by throwing mass in the opposite direction. Without a large source of gravity nearby, would you be able to feel that acceleration inside the ship?

Not sure if I'm understanding you correctly but I've read a lot of books written by the Apollo astronauts. When they were in space and firing the Service Module engine, they had a sense of acceleration because they were pushed into their seats. Otherwise, they couldn't tell they were moving at 5 miles a second or whatever. Even when they reached a point where the moon's gravity stronger than Earth's and thus accelerating them, there wasn't a noticeable pull.
posted by Brandon Blatcher at 10:55 AM on February 29, 2012

You're maybe conflating motion and acceleration here. You would sense your acceleration, not your motion, so relative motion isn't important to this sense.

If it helps, you could think of your acceleration as your motion relative to yourself in the past.
posted by RobotHero at 10:55 AM on February 29, 2012

Haven't you ever taken a ride in an airplane? It would feel similar, only with weightlessness. Meaning, you would feel changes in acceleration and gross changes in vector.
posted by Cool Papa Bell at 10:57 AM on February 29, 2012 [1 favorite]

The short answer is that accelerating to light speed (as is common on spaceships) would feel roughly the same as riding in a dragster or rocket on earth, because the main force would be pushing you back in your seat with several g's of force. The difference, of course, would be that on the Space Shuttle, for example, the maximum force is about 3 g's and the whole launch takes about eight and a half minutes. Accelerating to even 1/10th the speed of light would take (1/30)*c/g = 11.8 days. And these are highly trained astronauts in peak physical condition. Once you got to that speed, assuming you weren't being accelerated by anything else, you would feel weightless.
posted by wnissen at 11:00 AM on February 29, 2012

Meaning, you would feel changes in acceleration and gross changes in vector.

It maybe terminology but I think you mean that you feel acceleration (which is a change in velocity) not just change in acceleration.

Velocity is a vector of course, so changes in velocity also means changing direction wthout changing speed.
posted by vacapinta at 11:04 AM on February 29, 2012

To illustrate the basics of acceleration some more ...

Rocket engines on spacecraft do indeed accelerate by throwing stuff out the back -- their own exhaust. Literally, the mass of the combustion products spewing out the nozzle of the engine rearward provides the reactive force that accelerates the spacecraft forward.

Airplane jet engines work similarly, except they are gulping the AIR coming into the front of the engine and accelerating it out the back of the engine. Accelerating the air in the rearwards direction provides a reactive force in the forwards direction. The engines do consume fuel and spew that out the back as well, but that's a secondary effect. The bulk of the work is done by the air being gulped in the front and thrown out the back.

For advanced information on this topic, look up "ion engines". Demonstrated on the Deep Space 1 probe, in use now on Dawn, and planned for ISS, these engines are far more efficient than chemical rockets. While they provide a weaker thrust, they can do so for hours and days (and weeks and ...) on end and achieve much of the same results. They just can't be used for the initial launch, since that's a high-thrust scenario.
posted by intermod at 11:06 AM on February 29, 2012

Yeah...I think you have some basic confusion over acceleration and velocity.

You are at this moment experiencing exactly one G of force the earth exerts on you via gravity. You're not accelerating because your legs provide equal and opposite force.
Which is exactly what you will feel if you're standing inside a rocket ship accelerating at one G.

If your rocket ship has no windows to look outside, you will not be able to tell the difference whatsoever whether you're in a rocket ship or on earth.
That's the gist of general theory of relativity.
posted by 7life at 11:30 AM on February 29, 2012 [1 favorite]

Without a large source of gravity nearby, would you be able to feel that acceleration inside the ship?

You don't need gravity to feel acceleration. General relativity (and most classical physics) holds that forces due to acceleration are indistinguishable from forces due to gravitation (the equivalence principle). Gravity, in fact, can be thought of as a kind of acceleration. If your ship started to accelerate at 32 ft/s/s, and you were standing on a floor of the ship perpendicular to the direction of acceleration, you would go from feeling weightless to feeling exactly the same weight that you feel on the surface of the earth.
posted by mr_roboto at 11:33 AM on February 29, 2012 [2 favorites]

You would feel weightless by default, because the craft is in free flight, with no forces acting on it.

When the main engines kick in, you are pushed back into your seat, like in a sports car. You stay pinned in your seat as long as those engines are burning. As soon as the engines cut out, you become weightless again.

When small thrusters for steering kick in, you would be slightly jostled in your seat. If the ship starts to spin fast, you would be pushed to the outer edge of the spacecraft as if you were on a merry-go-round.

All this would be without the 1g pushing you down into your seat that you are accustomed to on Earth.
posted by colinshark at 11:33 AM on February 29, 2012

If your ship started to accelerate at 32 ft/s/s, and you were standing on a floor of the ship perpendicular to the direction of acceleration, you would go from feeling weightless to feeling exactly the same weight that you feel on the surface of the earth

I think this may be what is confusing me. Constant velocity in space produces weightlessness, when acceleration is 0 ft/s/s. But on earth, to experience weightlessness the idea is to let yourself accelerate at 32 ft/s/s. In my head, something about that doesn't seem to add up.
posted by parallellines at 12:06 PM on February 29, 2012

You will never feel weightless if you are accelerating. Acceleration = weight.
posted by mr_roboto at 12:08 PM on February 29, 2012

Well, mass * acceleration = weight, but you know what I mean.
posted by mr_roboto at 12:10 PM on February 29, 2012

Many other people have answered a good part of the question, with regard to what humans (or for that matter any other instrumentation) are able to sense within a particular reference frame.

I'll just add that in most conceptions of deep-space flight, including the actual Apollo lunar missions, what typically occurs is that the spacecraft accelerates up to some 'cruising speed', then coasts for a significant part of the flight, and then turns itself around and accelerates in the opposite direction in order to bring itself to rest (or orbit) relative to the target. Depending on the relative velocities of your start and finish, the turnaround might not be in the middle of the flight.

The absolute fastest way to get from point A to B in space is to accelerate continuously, first in one direction, and then immediately turn around and begin accelerating in the other. But this consumes a lot of reaction mass if you are using current technology, as you seem to understand. However, it would have the nice side-effect of creating an artifical gravity-like effect on the occupants, except during the brief time when the ship was turning around to point its engines in the reverse direction. Some SF novels hint at this sort of propulsion, although they typically have to handwave away exactly what sort of propulsion mechanism you'd need in order to generate that sort of acceleration of a large ship continuously, without burning through an unrealistic amount of mass.
posted by Kadin2048 at 12:13 PM on February 29, 2012

So what is happening to produce weightlessness on zero-g flights?
posted by parallellines at 12:16 PM on February 29, 2012

I think the deal with zero-g flights is that the airplane around you is flying downward at the same velocity that you are falling at, so it has the appearance of zero gravity. Because the walls and floor and ceiling are falling at the same speed you are. The gravity's not actually gone, but you do appear to be weightless. I'm not sure how the vestibular system in your ear copes with that, but I would hazard a guess that people on those flights can still distinguish up from down. In space, there is no up and down.
posted by vytae at 12:22 PM on February 29, 2012

Yeah, in free fall, there are no stationary objects to push against you as you accelerate towards the surface of the earth. So you don't feel the "weight" sensation of the ground or your chair pushing up against you.
posted by mr_roboto at 12:27 PM on February 29, 2012

You feel "accelerated" (that is, normal) when the fluids in your inner ear are supported in their various canals. You have several ear canals looping in different directions so that you can tell the direction of the acceleration. When you're stationary on earth, this direction is down; when you're accelerating, it's down plus some contribution in another direction. (Have you ever seen an enclosed movie-theatre flight simulator at a museum or theme park, which tilts and twists in sync with the film so that the folks inside feel like they are moving? That's why those work.) It's all about the fluids in your ear: twist your head, and they fall onto a different part of the canal. Every time you make a little motion, the fluid has another opportunity to fall — a real, physical acceleration — to a different part of your ear.

You feel "weightless" when the fluids and the canal are accelerating at the same rate. Now the motion of the fluid relative to the canals is fundamentally different: imagine dropping a stack of books, and think of the way it would slowly come apart. Fill up a bucket of water and drop it off your balcony.

I guess you could say that your body is only sensitive to differential acceleration: when the acceleration of the free-moving fluid in your ear is different from the acceleration of your ear around it. But because there's no such thing as a completely rigid object, differential acceleration is all that there is.
posted by fantabulous timewaster at 1:02 PM on February 29, 2012

Okay, let's consider several scenarios here to separate out the different things that are going on:

1. Sitting on Earth, stationary (relative to your desk or whatever)
2. Sitting on an airplane, moving at Mach 0.7 (~cruising speed for a general aviation flight)
3. Being in a "zero-g" flight during the downward part of its parabolic arc (or the initial part of a skydive)
4. Sitting on the International Space Station
5. Sitting on the Apollo 11 flight vehicle, halfway between the Earth and the Moon

Now, in which of these are the following true:
Gravity is exerting a 1 g force on you: True for 1, 2, 3, and 4. Whenever you are on or near the Earth, this is true.
Your fall is accelerating at 9.8 m/s^2: True for 3 and 4. Note that in low Earth orbit, you are simply moving forward fast enough that the curvature of the Earth drops off at the same rate as your acceleration, keeping your altitude constant.
Your subjective experience is weightlessness: True for 3, 4, and 5. This happens when your actual acceleration and the acceleration produced by gravity match.

Note that there's no difference between #1 and #2, and there's no difference between #3 and #4. General relativity tells us that forces produced from acceleration are indistinguishable from those produced by gravity - the equivalence principle. Basically, being in a spaceship (with the ground pointed 'back') accelerating at 9.8m/s^2 cannot be differentiated from being on a spaceship (with the ground pointed 'down') on Earth.
posted by 0xFCAF at 1:31 PM on February 29, 2012

The usual description of a zero-g airplane flight is that the plane follows a parabolic trajectory: fast steep climb, slowing towards the top, gaining speed on the way down. Of course, every baseball that gets thrown follows a parabolic trajectory, because that's what you get from motion in a uniform gravitational field. On a horizontal straight-line trajectory, you continuously need the plane to correct you so that you don't accelerate downwards on a parabola towards the earth; but if the plane is also following that parabola, you and the plane will take the same path whether you are touching part of the plane or not. No differential acceleration — weightlessness.

A zero-g flight is different from orbital zero-g only in duration. The zero-g flights usually have many intervals of parabolic flight, each lasting maybe a minute: the plane enters the climb, levels off, and dives. At some point the pilot has to pull out of the dive, the crew cleans up from the previous experiment, and then you can do another parabola, until fuel runs low.

It's probably not fair to call those paths parabolas: a better approximation is that each loop is a segment of a very tall, very skinny ellipse, which has one focus way down at the center of the earth. If you made that ellipse less skinny, by having more horizontal speed initially, more and more of the ellipse would be above Earth's surface. The limit of this is where the entire ellipse is above Earth's surface, in which case you would call it an orbit, and your airplane could "fall" all the way around the Earth without every having to pull out of the dive. That's exactly what's happening in, say, the ISS. Local gravity isn't substantially weaker at that altitude than on the Earth's surface, but the whole space station is falling together, so there's no differential acceleration.

You don't even have to get to space or to a special NASA plane to do this yourself. Just go to a playground with a swingset, get going real fast, and leap off into the air. For about a second you'll be a freely-orbiting satellite of the Earth, weightless. Don't hurt your ankles.
posted by fantabulous timewaster at 1:33 PM on February 29, 2012 [1 favorite]

While it is true that you can detect relative acceleration in the inner ear, it is not at all the case that this provides an entirely accurate or useful frame of reference. Pilots in airplanes without visual reference are under the constant effect of gravity, and can further feel other changes in forces on their flight, but without reference to instruments pilots regularly fly out of the clouds upside down or worse.

Remember that gravity is a force imparting an acceleration on your body. So, on a zero-g flight, to feel "weightless" you match the rate of acceleration with the entire frame of reference. In this case the airplane dives at a rate such that you have no motion or force relative to the walls of the cylinder. If this could be left going long enough and there were no other markings you would quickly lose track of which way is "up". In deep space the effects of gravity become minimized quite quickly due to distance, so in that case acceleration is only effectively by engine action. In orbit you are essentially always falling "past" the source of gravity, so, just as in the airplane your feeling is weightlessness as the entire frame of reference you have shares the same acceleration force and a constant velocity (but constantly changing vector).
posted by meinvt at 1:35 PM on February 29, 2012

Let's say you're in a rocket-shaped spaceship which is accelerating at 1G - 9.8 m/s^2 (yes, metric).

If you're sitting inside this rocket in an orientation where the top of your head is pointing towards the nose of the rocket, you will feel the same gravity effect that you feel sitting vertically on Earth. If, on the other hand, you are sitting oriented such that your face is pointing towards the nose of the rocket, you will feel the same effect as if you were lying on the ground on Earth - your back will press into the seat.

If the pilot turns off the engines you will feel weightless, despite the fact that you might be travelling at 4234 km/second.

You cruise in weightlessness for a while.

To decelerate, the captain turns the rocket around and activates the engines back to 1G. You feel exactly the same effect as when you were accelerating. You are just accelerating in the opposite direction, and since the rocket is turned around, relative to your position it feels the same as when you were accelerating.
posted by Diag at 1:39 PM on February 29, 2012

Weightlessness is not 0g. It's freefall. Here's some of JPL's rulebook on the idea. The sensation of something pressing on us, or us pressing on something, and the degree of that pressure, is generally what we feel. The vestibular system is a whole 'nother beast that is really easy to screw up. It gives rough clues for comparison to what your eyes and other senses are feeling, and often is either barely noticeable or completely OMG.

As a completely hypothetical example, if you were in a car hurtling down the road at a decent clip and went into a deep dip, you'd "launch" off the other side. If the angles are right, your eyes can't see what happened but you'd get an instinct that something is different... a vestibular instinct.

Your movement in the car is different than the car's movement in the atmosphere though. The car is dealing with a lot of drag from air flow and no longer has propulsion (the wheels are off the ground). So it'd start to slow down, and since cars aren't lifting bodies, would begin to fall. Your first sensation of that would be the seatbelt (you were wearing one, weren't you?) cutting into your shoulder. There's literally thousands of pounds pulling you down, accelerating you with it.

Hopefully you caught it before then though. If you didn't, you won't have the wheels straight in time and everything goes to hell. Which would happen without a seatbelt since you'd come to rest on the steering wheel and get the air knocked out of you.

So, if you were in a spaceship... you'd get sick from space sickness, as your vestibular system "learns" the new environment. You'd also feel whatever wall/seat/belt/etc. is pressing against you due to acceleration in the opposite direction. That wall accelerates you, and when the acceleration stops, you and it are going to same speed. This brings up the main problem of space flight: It's basically spam in a can. Protect the spam and the can.

Lessons: Seatbelts are good, pay attention to instincts, and always keep the wheels straight when airborne in a land vehicle, regardless of the vehicle type (bicycles count too!).

One last thing: funny (as in weird) things happen with these issues. I learnt most of this on a bike and took the handlebars in a not very comfortable place as a result. Which is why I'll never be an astronaut. My vestibular system triggers a sensation in that same area when I go weightless without significant drag (like air from skydiving). Thankfully that's pretty rare at 1g, especially after establishing that I know how to fly and land my car. Gs aren't much fun after that.
posted by jwells at 3:57 PM on February 29, 2012

I think this may be what is confusing me. Constant velocity in space produces weightlessness, when acceleration is 0 ft/s/s. But on earth, to experience weightlessness the idea is to let yourself accelerate at 32 ft/s/s. In my head, something about that doesn't seem to add up.

Not so. A spacecraft in orbit changes velocity all the time-- it's constantly accelerating and decelerating, to say nothing of turning (which is another form of acceleration). A perfectly round orbit has constant velocity, but it's a special case. Most (closed) orbits are ellipse-shaped; slow at the highest parts, fast at the lowest parts. Kepler's Laws talk about this.

The weightlessness comes from the fact you're falling without any (noticeable) force deterring your fall. The fact that you fail to splat into something is related to your sideways motion-- you're still in freefall. Freefall = weightlessness.

I think you may be confusing "acceleration" with "stepping on the accelerator." It's true that when the spaceship is not forcing a change in things by firing up a thruster, but it is being accelerated by its movement within the gravitational field of the Earth or whatever heavenly bodies are around, the ship's contents are "weightless." When you turn on the thrusters to change your velocity, first the spaceship changes velocity, and then the spaceship runs into you (hopefully seat-first) and changes your velocity. That's why you strap in for (deliberate) acceleration.
posted by Sunburnt at 4:22 PM on February 29, 2012

But on earth, to experience weightlessness the idea is to let yourself accelerate at 32 ft/s/s. In my head, something about that doesn't seem to add up.

In terms of general relativity, you are accelerating when you are sitting in your chair on the surface of Earth, and you are not accelerating when you're in the vomit comit doing a parabolic trajectory. In other words, gravity has warped spacetime so that a straight path is the one the vomit comit is taking, and a curved path is sitting motionless in your chair.
posted by Rhomboid at 5:23 PM on February 29, 2012 [1 favorite]

So what is happening to produce weightlessness on zero-g flights?

The story here is all about frame of reference. The gravity between you and the Earth continually applies a 32 ft/s/s acceleration on you toward its center. So you and other objects (birds, trees, little squirrels) are all feeling acceleration relative to your frame of reference (you and the Earth). That's why you feel weight, essentially.

In a zero-g flight, your surroundings are accelerating along with you (accelerating downward at 32 ft/s/s, instead of pushing back with dirt and grass and whatnot). In your frame of reference (you, other flight participants, and the plane), you experience zero acceleration, and feel weightless.
posted by BevosAngryGhost at 8:26 PM on February 29, 2012

I had written up a long exposition about spaceships and vomit comets and weightlessness and net force and the equivalence of gravitational and inertial acceleration, but on reflection I will simply point you to some relevant Wikipedia articles and suggest that you should do your best to digest those before attempting to work this stuff out intuitively.

Once you're comfortable with the idea that the physical effects of standing on the Earth are locally indistinguishable from the effects of standing in a spaceship accelerating at 32 ft/s/s, a lot of the confusion will dissipate. You will need to understand fictitious forces and in particular gravity as a fictitious force, and for that you will want a solid grounding in Newton's laws of motion, and in order to gain that you will need to pay close attention to separating the concepts of velocity and acceleration.
posted by flabdablet at 10:47 PM on February 29, 2012

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