|WikiProject Physics||(Rated Start-class, Mid-importance)|
Several major problems
I see several major problems in this article. The most serious is with the term "microgravity" itself. I've added a link to weightlessness where a subarticle there details the issue. I recommend scratching all use of the term "microgravity" and replacing it with "micro-g" (starting with the title).
- I did just that as I was annoyed with wikipedia spreading misleading knowledge, and slightly changed the first sentence and added a reference. TobiasBengtsson (talk) 15:16, 28 August 2008 (UTC)
The next problem is with the statement "only three methods". There is a fourth method. Theoretically, the gravity field within a hollow sphere of uniform thickness (uniform mass throughout the shell) cancels out completely, leaving zero net gravity at every point within the shell. It is often wise to avoid words like "only" and "always" because you can paint yourself into a tight corner.
Another problem is with the second method of "falling". I see this as an overly restrictive characterization. Micro-g is also experienced in that part of the trajectory that is increasing in distance from the Earth, whether that be a basketball as it is "rising" to its apogee above the rim or a satellite in an eliptical orbit as it is "rising" from perigee. Half of the parabola flown by the Vomit Comet is a "freerise", so to speak, before it reaches the apex and continues into "freefall".
...which leads into the next problem in the statement that "falling...approaches microgravity [sic] only when the fall is in a vacuum...". I agree that air resistance will spoil the zero-g effect, but a simple approach to countering this is to counter the force of drag with the force of thrust. This is exactly what the KC-135 did. No vacuum required at all.
The next problem I see is with the third given method of "orbiting a planet". This is overly restrictive on two counts:
- The massive body being orbited need not be a planet. It could just as well be a sun, or a moon, or a black hole.
- The trajectory need not be an orbit. Zero-g is experienced within spacecraft that are on parabolic and hyperbolic trajectories (non-orbiting) as well.
Finally, it would be more conceptually sound to combine the given second and third methods since they are exactly the same effect experienced on different scale. The micro-g environment results out of an unforced (no net external force) trajectory within a gravity field.
This article has potential to be very useful, but as it stands now it needs lots of work.
Moving candle flame
That is a very good question. I would guess that it would look more similar to a candle in a strong wind, but at low speeds it would probibly lack the rolling flicker that we associate with a flame because there is no convection. The airflow from movement would be less turbulent and more even than inside a flame. — Preceding unsigned comment added by Cam Forman (talk • contribs) 10:10, 7 April 2013 (UTC)
A Ridiculous Misnomer - delete and redirect
This term is a ridiculous misnomer wrongly given credibility by NASA. NASA's use of the term is not reliable, as in "references used by Wikipedia must be reliable". Sure, NASA's use is not cited (and that is good), but the promulgation of the term is a result of NASA's usage, and the presence of the article implies (if you will) a citation of NASA. For the record, that (implied) NASA reference is unreliable. You will see the term used more by NASA public affairs people who know less about the matter than actual NASA experts. So, a muddling occurs with this word. NASA is a center of expertise, but the "speaking part" of NASA is not. Wikipedia too-often muddles the too together, taking non-expert NASA statements and publications to be expert.
In addition to correction of the technical meaning of the term, the article is also in need of fixing of the many logical errors cited above in "Several major problems". I don't want to fix it -- because I hate this awful term and I want it to go out of use! :-). The best bet (in seriousness) is to delete the whole article and just redirect it to Weightlessness.
Okay, here is the technical reason why the term is so awful. My understanding is that the term was created to address the incorrectness of the term "zero gravity". In orbit, because of a number of factors, the acceleration difference between two free-floating objects is not always exactly zero. "Zero" was changed to "Micro" and the term was flaunted(!) as a proper and correct term compared to "Zero-gravity". The trouble is that it was the "gravity" part of "Zero-gravity" that was way more wrong than the "Zero" part. So the term was flaunted as being a corrected, prim and proper, new term, but it was quite the opposite! It is this contrast between what it is so flaunted to be, and the fact that it isn't, that makes the term so awful. If it was just another misnomer, that would be no big deal, but people hold it up as an example of such perfect correctness. It's nauseating! :-)
And worse, people (this article) try to "retcon" the definition to work around the fact that it is just the wrong word. Look at the first sentence of this very article:
- "[A micro-g environment (microgravity)] is one where the acceleration induced by gravity has little or no measurable effect - gravity itself does not change."
This is all a muddled walk around the fact that gravity is not zero, nor even small, nor "not measurable". In a low orbit "microgravity environment", gravity is about 90% what it is on the surface, and it is very measurable, it pulls the vehicle around the earth for crying out loud. The vehicle would otherwise fly in a straight line. The attempt to fix this way-off definition with " - gravity itself does not change" is again a muddlement because gravity does usually change in such a so-called "microgravity environment". As an orbiting object changes altitude between apogee and perigee, gravity changes because height changes.
I can fix all this, but I don't do that anymore. Fixes just get screwed up down the line by 1) rabid citation demanders demanding citations regardless of reliability and 2) by the less-than-well-enough-informed who think they're fixing it, but are actually breaking it.
So, like I said, I recommend just deletion and redirection. Do it! Go! :-)
- Re: "Several major problems": A hollow sphere does not shield from gravity due to external masses.--Patrick (talk) 20:06, 8 June 2010 (UTC)
Can we not just reform this into a mistaken name that is endorsed by NASA and the media? It is about as correct as calling Saccharin a micro-sweetener instead of a non-nutritive sweetener. Weight is the sensation of gravity, therefore weightlessness is how one would describe the absence of such sensation, even while possibly still under it's influence. The term has relevance described in other Wikipedia articles where the force of gravity is less than on millionth of 9.8m/s/s which would occur at a distance of about 1000 earth radii. Few, if any, man made objects have ever returned from such distant locations. The term microgravity is incorrect or at least misleading because most references to it occur where the subject is still under the influence of at least 80% of the strength of gravity we experience on the surface of this thing we call home. The reason for the weightlessness seen in orbit is the same as that seen by the vomit commit, rollercoasters and skydivers; they are in freefall. A great example of the confusion caused by the them "microgravity" is NASA's own attempt to explain it: http://www.nasa.gov/centers/glenn/shuttlestation/station/microgex.html
"G" vs. "Gravity"
Another source of muddlement on this issue is the difference between "G" and "Gravity". Strangely, "Micro-G" is as neatly correct as "Micro-Gravity" is awful and incorrect. The reason is that "G" is a measure of acceleration, and "Gravity" is a natural phenomenon that causes a force between two masses. They are two different things.
"G" is a measure of acceleration in this context, and is simply how fast two objects accelerate in relation to each other compared to how fast an object accelerates downward on Earth's surface. In a slightly different context, "G" is a similar measure of weight when the two objects are pressing against each other. In that context, a pilot pulling "2.5 G's" is pushed into his seat 2.5 times harder than sitting on the ground at 1 G. Even then, if the pilot dropped something in the cockpit, it would accelerate towards the cockpit floor at a rate 2.5 times faster than if he/she dropped it while standing on the ground.
So "G" is a measure of acceleration (or weight) as compared to the acceleration or weight caused by gravity on Earth's surface, hence the "G". "Gravity" is, well, gravity itself. Two different things that are frequently muddled, even by NASA!
In the context of this article, "G" is the acceleration of two objects relative to each other divided by the acceleration due to gravity on Earth's surface.
"One gravity" is a spelled-out form of "One G". It is still the same unit of acceleration. "One gravity" or "4.5 gravities" or "9 g" is differentiated by context from "the force of gravity". That is, "a gravity" is a unit of acceleration and is another name for "a g", as in "When the astronaut dropped the pen during launch, it fell at 2.5 gravities". Where, at the same time, "During second-stage firing, the rocket, being some 65 miles high, was pulled downward by only 96% of the gravity as on the surface.". In the first context, "gravity" is a unit of acceleration, in the second context, "gravity" is the familiar old "force of gravity".
The term "Microgravity" unfortunately has a context (in itself) that implies "the force of gravity". If the term was "Microgravities", the context would imply "a gravity" as a unit of acceleration, which is better. But, "a microgravities environment" would be as wrong as "the car had a lot of miles-per-hour". You wouldn't say that, you would say "the car had a lot of speed". You could say "a microacceleration environment" and that would be okay, as long as it was clear that the acceleration was between two objects in the same environment, not the acceleration of both objects compared to a third (like the earth).
If the article is to remain, it should make all the muddled stuff clear. If it doesn't, it will only perpetuate the confusion and the misnomer.
A hollow sphere does not shield from gravity due to external masses.--Patrick (talk) 20:06, 8 June 2010 (UTC)
- I'd be happier if g were defined somewhere in the first paragraph. —Tamfang (talk) 20:48, 16 June 2015 (UTC)
"stationary" micro-g environment
The assumed frame named so will be in fact not stationary (relative to Earth), but Earth’s center of momentum frame considered within special relativity or Galilean relativity. The true geostationary frame would add the centrifugal force which overwhelms a gravity in most directions on much shorter distance than 200,000 km or so, mentioned in article. Virtually, no spacecraft or planetary engineering structure is supposed to be in rest in Earth’s CoM frame. Therefore, this line of reasoning is nothing more than a theoretical speculation. Incnis Mrsi (talk) 14:10, 23 June 2011 (UTC)
"Attenuation" of gravitational field?
The article mentions that stationary microgravity must be attained by large distances from the earth; the decrease in the gravitational field strength is attributed to *attenuation*. What is meant here is the inverse-square behavior of the gravitational field, which can be interpreted as the reduction of "flux density" due to geometric spreading of the gravitational "flux".
However, the article on attenuation states: "Attenuation does not include the decrease in intensity due to inverse-square law geometric spreading." (This statement is made in the context of electric fields, but should equally apply to gravitational fields.) Rather, this article reserves the term "attenuation" for decreases in intensity due to a medium that actually absorbs (diminishes) the flux rather than merely spreading it.
These two uses of the word are contradictory. Either this article should replace "attenuation" by a different term ("inverse-square behavior", "geometric spreading"), or the article on attenuation should be changed.
- You address a minor problem, but actually entire Micro-g environment#Absence of gravity section has to be thrown away as a crap and original research. Compare to Weightlessness article, where invalidity of such terms as "zero gravity" and related conceptions is explained in details. Incnis Mrsi (talk) 11:58, 3 February 2012 (UTC)
- http://dictionary.reference.com/browse/attenuation says attenuation includes spreading.--Patrick (talk) 12:16, 3 February 2012 (UTC)
Effects of radiation pressure on orbiting bodies
The article mentions altidude of the orbitibg body not varying the radiation pressure from the solar wind. It seems that if the body being orbited had a magnetic field that altitude would have a significant effect on attenuating solar wind effects while outside the orbited bodies' atmosphere. Compare a body orbiting inside earth's field to a body orbiting Venus, which lacks a magnetic field. Both planets are close in mass and solar radiation pressure (as opposed to using the outer planets as an example). — Preceding unsigned comment added by 126.96.36.199 (talk) 01:04, 15 October 2014 (UTC)