Alas public highdives are going the way of the dodo.[/OT]
Why is the G level in orbit not utterly zero? Are objects in satellites attracted to heavy parts of the satellite? Or is it decel from solar wind, or something?
Atmosphere (Earth’s) and tidal forces are the biggies. The atmosphere is not negligible at ISS altitudes, and tidal forces (i.e., the fact that there’s only one point on the station that’s at the center of gravity, and everywhere else will not have perfect cancellation between gravity and centrifugal forces) are also on the order of 10[sup]-6[/sup] gees.
As Dr Strangelove has pointed out, only the exact center of gravity is at the lowest state. The shuttle and the ISS could be oriented in different configurations to give a range of G levels for testing, called the “gravity gradient orientation”. Plus there are G zitters from equipment vibration and people moving astronauts moving about. Free floating tests hovering just off the station were planned for, I don’t know if they used them.
To answer one more question, the actual “Drop Tower” at Glenn is a different facility that gives 2.2 seconds of drop, it is indeed a tower to extends up from the Rocky River valley on the edge of the lab.
The ZARM at Bremen is a free standing tower:
https://www.zarm.uni-bremen.de/drop-tower.html
There used to be a 10 second facility at the JAXA in Hokkaido but it is no longer in operation.
Dennis
I respectfully disagree about jumping off a high dive. Being in freefall is not what makes the Vomit Comet an amazing experience. What makes it amazing is the illusion that you are stationary because the room which is around you is also in freefall and yet you do not drift toward the floor of the room. Jumping off a high dive does NOT give the same illusion because you don’t have a room around you which is also falling.
Even if you had a giant cardboard box with you when you jumped, the illusion would not be the same because wind resistance would slow down the box, making it appear that your feet are in fact drifting toward the floor of the box. This is a flaw with the weightless-in-a-falling-elevator illusion too. You would not feel totally weightless b/c the elevator itself would be falling slightly slower than 9.8 m/s2 due to wind resistance. Your feet would tend to stay in contact with the floor.
What makes an orbiting space ship an amazing experience is that you look at the walls, you feel like you aren’t moving, you don’t drift toward the floor, in fact there is no “floor”. This gives the illusion that gravity has let go of you. But this illusion can be shattered if you look out the window and see cities and continents whizzing by, revealing that you are actually moving very very very fast and neither you nor the room you are in is stationary. Then you feel like a person shot out of a cannon, moving so fast that you are going to miss Earth entirely (which is basically true, except for the part about the cannon). If you had total situational awareness, you’d realize that you actually are falling quite rapidly and that gravity has almost as strong a grip on you as it ever has during your mundane life up to that point.
A falling elevator would also lose the “no gravity” illusion if the elevator had glass walls and you could see the fact that you are actually falling.
You get ~3.8 seconds freefall on the Giant Drop at Dreamworld.
That’s long enough …
The Giant Drop does not give the illusion of weightlessness. On the contrary, you are very aware of the fact that you are falling; that is the whole point of the ride. “Weightlessness” is the illusion that gravity has let go of you, caused by the fact that #1 you are falling, and #2 the room you are in is also falling, and #3 the floor of the room is not pushing against your feet. ISS, the space shuttle, and the Vomit Comet all give the illusion (as long as you don’t look out the window or pay too much attention to what you see there). High dives, falling glass elevators, and theme park rides do NOT give the illusion.
FWIW, every time you go jogging, you experience free fall many many times over, for a fraction of a second each time (every time both your feet are off the ground simultaneously). This also does not give the illusion of weightlessness because you are aware of the fact that gravity still has a firm grip on you.
It’s not an illusion, there is a real difference. Jumping off a high board is not freefall. It may be a good approximation of freefall at first, but as speed builds up, air resistance increases, and slows you down. If you jump from high enough, you’ll reach terminal velocity, where you are falling at a constant speed, and you once again experience full 1G.
Parabolic flights (Vomit Comet etc) are different. It is a powered dive, using the engines to counteract air resistance and simulate a true parabolic trajectory. For the occupant, this means there is no air resistance, because the surrounding air is moving along at the same trajectory.
Those are exactly the same thing.
And looking out the window of the ISS won’t dispel any illusions, because you don’t see anything zipping past you. That’s like saying that the minute hand of a clock zips around the clock face: Yeah, you can see it moving if you’re paying attention, but it’s not very prominent.
I’d say we’re tripping over competing definitions of “illusion”.
A G-meter would be utterly “fooled” by falling off a high dive or using one of those space agency drop towers. It’d read pretty darn close to zero G as it fell, especially at first when it’s falling more slowly and air drag is negligible.
OTOH, the word “illusion” as applied to a human is the totality of one’s perception about the experience. IMO that’s what sbunny8 is talking about.
And IMO he’s got a point that watching the scenery rush by and feeling the wind building up after one jumps off a high dive or drop tower is a different total experience than riding in a vomit comet or the ISS. One that changes the sensation from floating to falling.
Even though your trusty pocket G meter would read the same during both experiences.
Consider the difference between jumping off the wrong side (deck side) of the high dive tower under magically zero G vs. under ordinary 1 G conditions. The former would be a serene floating sensation. The latter will be a (brief) zero G ride of rapidly increasing terror followed by a horrifying splat. Very different perception. Very different illusion. Still zero G. For most of it at least. 
What sorts of research/experiments were affected differently by the microgravity in orbit and the (I guess) nanogravity of a drop tower?
Falling off the high-dive board feels like falling. Being in the ISS also feels like falling, just for a really long time. Neither feels anything like floating.
EDIT:
The practical difference is mostly just that orbit lasts longer, but the drop tower is cheaper, with the Vomit Comet in between on both scores.
Sbunny8, if you really want to think about a tricky situation, consider the case when we launch upwards in the microgravity facilities. We only did this briefly at Glenn in Cleveland, but ZARM at Bremen has the capability. Here is how it goes:
The experiment is placed in a capsule at the bottom of the facility and hurled upwards by an accelerator. At Glenn we used a pneumatic piston with a 20’ stroke. As soon as the capsule leaves the piston, it travels upwards at zero G. It is going up, but accelerating downward from the pull of gravity. It crests at the top and drop backs down, and we have placed the decelerator cart in position by then. This give 10 seconds of microgravity.
Now.
All the way up, and all the way down, you are at zero G. There is no “up and over” feeling at the top. A delicate fluid experiment sees no disturbance what-so-ever during reversal. If you were aboard, you would float in the middle of the capsule during the entire event. You cannot tell which direction you are traveling at any moment.
Dennis
I think the disparity you are not addressing is the fact that, on Earth, with very few exceptions, when you “fall”, you feel air move past you. You also experience the visual of things like the ground moving closer, or the trees going by. I certainly consider this an integral part of “falling”, something I will admit that I am quite scared of (they call it fear of heights, but it’s not in my case, it’s fear I’ll fall OFF the high place).
I would think that the feeling of weightlessness, absent those aspects, would resemble falling, but not be quite exactly the same feeling. Am I wrong? I think that’s what LSLGuy is saying.
Personally, I think the farthest I’ve ever fallen is less than 10 feet. I imagine that’s true for most people, except for people who dive into pools from a high diving board (something I’ve never done). Falling from 10 feet, you’re in the air for about 3/4 of a second. Is that really long enough to become aware of air rushing past you; or trees going by, especially since you’re moving pretty slowly in the first part of the fall? I don’t know, perhaps it is. But I’m somewhat skeptical of the notion that air rushing past you is an integral part of the feeling of falling.
I guess a test of this theory would be to fall with your eyes closed, wearing a wetsuit. Would Sbunny8 and DSYoungEsq predict that this experience would be completely different from normal falling, more like floating?
I was going to post something similar myself. I don’t think the air rushing past is a significant factor- but I do think the visual input plays a role. I used to skydive a lot and there is no real sensation of falling once you reach terminal velocity, even though the air is rushing past very quickly. However, there is nothing visible rushing past.
You can feel the acceleration when you first exit the airplane, and again when you reduce your wind resistance by changing body position, e.g. going from a belly-to-earth position to vertical, either head or feet first. But once you reach terminal velocity again the feeling of acceleration stops.
I think a big part of the sensation of falling is the feeling of suddenly not having your weight supported by something, combined with the slight feeling of acceleration when gravity pulls you toward whatever object will be supporting your weight next- either the ground or whatever is between you and the ground. We spend so much time “on” something (the ground, the floor, a chair) that it is very disconcerting to suddenly not be supported by something.
It’s not a feeling of acceleration that you notice: It’s the lack of feeling of acceleration.
Our brains are accustomed to perceiving gravity in one of two ways. #1 When standing (or lying) on the floor, we feel the floor pushing back at us in an amount exactly equal to our weight. This push increases when we are carrying something (e.g. a backpack). #2 When we jump into the air or jump/fall from a height, our senses tell us that we are on a collision course with the ground and we know that it will probably hurt. In both those situations, we are aware that gravity is pulling us downward. These are the situations we experience for 99.999% of our lives.
When you step into an elevator that is going up, your senses tell you that you temporarily get heavier and then when you reach your floor you temporarily get lighter again. This is an illusion. Your weight does not actually change. But your perception of your weight is based on the 100% real push of the floor against your feet.
In some rare situations, our brains might temporarily be fooled into thinking that gravity has let go of us entirely and we have no weight at all. Being inside a room which is falling at the same speed which we are falling is the perfect example of that. However, it all depends on your state of mind. If you look at the walls and convince yourself that the room is stationary, you will think gravity has been switched off (although it has not). But if you convince yourself that you are actually falling (either by force of will or through visual cues) then maybe the mental illusion would shatter.
That would depend on your mental state. If you close your eyes and jump off the high dive thinking “oh my god oh my god oh my god I’m about to start falling downwards and I think it’s gonna hurt” then your brain will (correctly) infer that you are falling. But if you could relax your mind and focus on the sensations in a Zen Yoga sort of way, then, yes, maybe for that brief moment, you would feel as if gravity had let go of you. That’s my prediction.
But I think the illusion of having slipped out of gravity’s grasp would be much easier to achieve in a room with no windows.
I would imagine that skydiving would mess with your perception in a different way because the wind is there but it’s much stronger than what your brain associates with “falling”, and there’s the strange sense that you can see the ground and it does not appear to be getting any closer.
I am not an astronaut or a skydiver.
The feeling of acceleration (or lack thereof) is far stronger than the effect of visual cues. If you’re in that completely-sealed elevator, or on the ISS, or in deep intergalactic space, you don’t feel like gravity has been turned off. How could you? We’ve never experienced gravity being turned off, so we have nothing to compare it to. In any of those situations, even with all visual cues removed, you still feel like you’re falling.