You know, this space tourism thing will just be a fad:

"It's not really like being weightless in water," she said. "Water has its own weight. You're still experiencing something like a pressure. But this is the feeling of no pressure." Going weightless made her realize "how rarely we experience an entirely new physical sensation over your whole body, and that was just so different. I couldn't have really anticipated what it would feel like."

In fact, she said, it wasn't until she got back on the ground that she "understood the magic" of the experience. "It was like I had gained this momentary super power that I couldn't access any more. I felt like I should be able to just launch off the ground and go flying across the hotel lobby."

I was going to respond to the first comment here, but Brian Swiderski beat me to it. I wish that more of his comments were helpful and civil, like most of his comments on space policy, and fewer (in fact, none) of them on other topics the product of obvious Bush Derangement. Unfortunately, the ratio is the other way around.

However, when commenter "kayawanee" writes that:

While it's true that there is no further acceleration due to gravity, it's really inaccurate to say that the skydiver is no longer in free fall once he achieves terminal velocity. Afterall, any object in orbit is considered to be in a perpetual state of freefall, even if (really, especially because) that object is at a constant velocity.

This is clearly (OK, well, not so clearly, or obviously) wrong, which gets back to the previous post on free fall.

Part of the confusion arises from the word "velocity," and the rest from the special case of a circular orbit.

[Sigh]

The whole reason that I stipulated that the orbit was circular in the previous post was because I didn't want to open up this new can of worms. I promise that I'll finish this post, but I have to go stir some chili, and I don't want people to be misled in the meantime.

[Update a few minutes later]

OK, back from chili stirring (and adding various ingredients to make it more chili-like).

The first issue is simple. Velocity is not speed. Velocity is a vector, and has a directional component. Speed is the scalar of that vector, that represents only the magnitude. Example: going fifty miles per hour east is a vector, going fifty miles per hour is a speed. When one runs in a circular race track at a constant speed (say, 120 mph) the speed is constant, but the vector is continuously changing (with a constant acceleration directed toward the center of the track, otherwise the car wouldn't be turning). So even if the speed doesn't change, there is similarly an acceleration in orbit as well.

Here's where it gets even more complicated. In a non-circular earth orbit, both speed and velocity are changing, because at apogee (the highest point of the orbit), speed is low, but altitude is high, whereas at perigee, it's the opposite. But in both cases, and all cases in between, the body is in free fall. And the energy of the orbit is constant throughout (thus maintaining Newton's laws). Free fall simply means that there are no forces acting on the body other than gravity.

In a parabolic aircraft, the only reason that the inhabitants of the airplane are in free fall is because the pilot is flying the trajectory that would apply if there were no atmosphere (that is, he is compensating for the air drag with the thrust of the engines). He is in fact flying an orbit that, if continued, would intersect the earth. In fact, it's useful to think of the airplane as "flying around" the free fall of the passengers, so that it doesn't cause an impact with them. He doesn't continue it, and pulls out after half a minute or so (for subsonic aircraft) for what I hope are obvious reasons.

Isn't the sensation of weightlessness identical to that of falling? I have never enjoyed the sensation of falling.

I think part of the sensation of falling is the mental knowledge that you ARE falling. Of course some people do get space sick.

Isn't the sensation of weightlessness identical to that of falling? I have never enjoyed the sensation of falling.

Posted by Kurt9 at April 30, 2007 01:43 PM

That's probably because you only ever experienced the first few seconds (maybe even milliseconds) of a fall. Additionally, your fall may have resulted in you hitting the ground, and possibly getting hurt. That mixture of shock, fear, and pain would certainly turn you off to the sensation of falling. But there is another side to it.

I've been a skydiver for about a dozen years, and I can tell you falling can be one of the greatest thrills a person can ever experience. Most skydivers refer to it as the 2nd best thrill you'll ever experience in your life.

After the first hundred milliseconds upon exit from a moving airplane (or about 10 seconds after you jump from a stationary platform like a hot air balloon) the "stomach-in-the-throat" sensation instantly disappears. It's replaced by one of the most freeing and intense sensations a person can ever experience. Every skydiver I've ever met all have the same complaint about our sport: "The time in freefall is just too damn short!!!"

Even for someone like me who has experienced this, I can barely imagine what a week long freefall would feel like. If this ever comes down to the price where a middle class guy like me could afford it, I'd be up in the ISS in a heartbeat.

The sensation of falling is in the transition, but it quickly fades if freefall persists, and you're left with a feeling that can only be described as transformative--the vestibular equivalent of opening an eye and seeing depth.

BTW, I've never skydived myself, but my understanding is that most of the trip down is not freefall--you attain terminal velocity and become buoyant, descending at constant speed.

I have never enjoyed the sensation of falling.

I have never enjoyed the sudden stop...
(I tried really hard not to post this)

TW, I've never skydived myself, but my understanding is that most of the trip down is not freefall--you attain terminal velocity and become buoyant, descending at constant speed.

Posted by Brian Swiderski at April 30, 2007 02:24 PM

That's true. Standard belly to earth position will permit a TV of about 120 mph. The latest craze, a head down position (a.k.a. lawn darts!!!) will yield a higher TV of about 160-180 mph.

While it's true that there is no further acceleration due to gravity, it's really inaccurate to say that the skydiver is no longer in free fall once he achieves terminal velocity. Afterall, any object in orbit is considered to be in a perpetual state of freefall, even if (really, especially because) that object is at a constant velocity.

I've never done anything so bold as to sky dive. In fact, I don't even like roller coaster rides. However, I do like the free fall type rides at the State Fair or Six Flags. They are the vertical contraptions that take you up to the top wait a undetermined amount of time and then suddenly let go for provide a moment or two of free fall. Also, when a friend of mine was current on his private pilots license there were a couple of times we'd nose the plane over to get a several seconds of near weightlessness. The fall itself didn't scare me it. It was knowing the plane we were in was 40 years old and mental visions of wings snapping off got me rather anxious so I tended to dissuade him from doing many acrobatics.

BTW, hooray for Mr. Hawking. He really looks like he's enjoying himself. We talk about how liberating the experience could be for all of us, just think how much so for someone held in the straight jack that is ALS.

Just think, Orbital Hospitals/Rehab centers.

kayawanee: "any object in orbit is considered to be in a perpetual state of freefall, even if (really, especially because) that object is at a constant velocity."

Actually, this isn't correct. Objects in orbit are traveling at constant speed, but there is delta-v because their direction of motion, which would otherwise be a straight line, is bent in toward Earth by gravitational force.

Josh Reiter: "However, I do like the free fall type rides at the State Fair or Six Flags."

I recommend the Supreme Scream at Knott's Berry Farm--it has the longest freefall of any drop tower ride that I've ridden, and is sufficiently long to pass through the "ack!" stage into the "freedom" zone. Unless you've done anything more extreme before, riding it three or four times will have you dreaming about it for days afterward. Even though you can't really move in the harness, that feeling of freedom is deep in your bones and nervous system.

Josh: "Just think, Orbital Hospitals/Rehab centers."

Unfortunately for Hawking and other patients with advanced ALS, they would still be unable to move under their own power--their muscles are destroyed, not merely weakened. It's a melancholy, but at the same time strangely beautiful image: Completely unable to move, the patient floats in his webbing in the darkened viewing bubble, staring out at the stars for hours on end, reaching out with his mind, and dreaming of far places.

Well i was thinking more about how it would spare the patient from compression (bed) sores. Particularly, wheel chair bound patients are prone to infections in their Sacral region. Bed ridden patients quickly loose the skin around their ankles, from lying on their side, and the underlying bone can be exposed. They say bone density losses are about the same between a astronaut and a bed ridden patient so that would be a wash.

Which brings up another possibility--cremation by reentry sans vehicle. The bereaved on the surface could look up at the sky and see their loved one streaking across as a meteorite, tell the predictable stories to their little children.

I think NASA recently did some research in that.

While cremation-by-re-entry sounds cool, I suspect that it would not be complete. Vaporization of water and fat would do a lot to protect the bulk of the body, and you'd end up with chunks of char-broiled grandma raining down.

<VOICE="Homer Simpson">Ummmmmm...char-broiled grandma...ggrrgrrlllrrggggeaarrrgglle</VOICE>

Seems like we ought to be able to simulate this and see.

What are the obvious reasons?

Forgive me....I just HAD to ask! ;-)

Let's think about this more deeply. Formerly living matter, while wrapped in some kind of flimsy envelope, would be exposed to vacuum and temperature extremes for anywhere from hours to days as it deorbits, desiccating it substantially if not completely. By the time it actually reached the upper atmosphere, it would be dry, charred, and shredded on a microscopic level by repeated temperature cycles--basically, dust crust in the shape of a body. Then, when it hits the upper atmosphere at near orbital velocity, the acceleration would tear it into tiny pieces, and the pieces would burn and fission further. At most, you might get fragments of a tooth or fingernail hitting the ground with very little kinetic energy, and if the idea freaks people out you could just make sure it all happens over ocean.

At the risk of being rather macabre(sp?), I suspect the Columbia accident may give us some data on this, though not the kind NASA would readily (and understandably) share. But I gather that *some* remains were recovered...

Of course, for intentional cremation purposes, you'd not wrap the deceased in anything as potentially durable as a pressure suit, nor would they be in a container that would protect them for part of re-entry. Indeed, you might saturate the body with oxygen (and add powdered aluminum, or other fireworks-related materials?) to insure a bright and complete cremation.

Thinking farther ahead, for those who find some appeal in the idea, one might want one's body deposited (energetically difficult as it would admittedly be) into the Sun...

This may be a tangential subject, but I also figure a potentially very profitable business would be depositing memorials on celestial bodies. A high-end customer might even have an engraving of the dearly departed's image and name carved in Moon or Mars rocks, or even (at huge expense) a bust.