Criticism of the terms "Zero Gravity" and "Microgravity"
It is important to note, as stated at the beginning of this article,
that there is plenty of gravity pulling on a spacecraft in orbit around
the Earth. Gravity is the very reason why the spacecraft is orbiting.
Therefore it is totally inaccurate to say that astronauts are
experiencing "zero gravity" or "microgravity". What orbiting astronauts
experience is zero-g, a measure of acceleration relative to their
spacecraft, which results in weightlessness. But zero-g is not "zero
gravity". If there were "zero gravity" or "microgravity", their
spacecraft would not be pulled into an orbit around the Earth. It would
go in a straight line.
As a thought experiment, imagine a spacecraft that had the ability to
rise up to orbital altitude by going straight up like a helicopter and
hover over one spot on the Earth. The astronauts inside would not
experience weightlessness. Their ride inside this hovering spacecraft
would be similar to riding an elevator up an incredibly tall building
and stopping at the top floor. While hovering above Earth's atmosphere,
their weight would be very close to what they weigh on the surface of
the Earth, even as a space shuttle goes zinging by them. So astronauts
in a hovering spacecraft are being pulled by strong gravity just as
space shuttle astronauts are pulled by strong gravity. The difference
between them is that the orbiting shuttle is freely being pulled toward
the center of the Earth. The lack of relative acceleration between the
orbiting shuttle and its astronauts inside (who are also being freely
pulled toward the center of the Earth) result in them being weightless.
But the hovering spacecraft (as with an elevator at the top of an
incredibly tall building) is not freely falling. The pull of gravity it
is experiencing is being opposed by the hovering force. This force gets
transfered to the astronauts within (along with everything else within
the spacecraft) resulting in weight. This example illustrates the fact
that there is plenty of gravity out in space. If you were to take any
object that is orbiting the Earth and stop it dead in its track and
then release it, the Earth's gravity would pull it straight down back
toward the Earth's surface.
To use confused terms like "zero gravity" and "microgravity" is to
mistake the general concept of acceleration for the concept of gravity.
"Zero-g" and "micro-g" are perfectly accurate terms referring to the
lack of acceleration (in the frame of reference of the spacecraft) that
cause weightlessness, even while gravity is strongly pulling the
trajectory of that spacecraft into an orbit.
The specific point of confusion is that "g" does not mean "gravity".
The designator "g" is an arbitrary scale of acceleration not to be
confused with gravity itself. "Zero-g" means zero acceleration, not
zero gravity. "1-g" is the acceleration experienced on the surface of
the Earth due to gravity, but it is not gravity itself. This scale is
widely used because it is easy to relate to from common experience of
acceleration due to gravity. But any other scale of acceleration can be
used to describe the condition of weightlessness. It could be described
using a scale that has nothing to do with Earth's gravity. Similarly, a
distance can be measured in feet as well as meters, where a meter has
nothing to do with the length of a human foot. For a weightless
astronaut to say that they are in zero gravity is the same type of
error as saying that an object that has a length of 0.3048 meters is
identically one human foot in flesh and blood. "1 foot" is an arbitrary
scale for measuring length that was (at some point in history) based on
a person's foot, but not to be confused with an actual human foot.
"1-g" is an arbitrary scale for measuring acceleration that is based on
gravity, but not to be confused with actual gravity. A zero-g
environment is also a zero-meters/second^2 environment and a
zero-feet/second^2 environment. Any arbitrary scale of acceleration can
be used, and none of them have any exclusive relationship to gravity.
Another illustration of this type of mistake is when people erroneously
speak of a jet pilot blacking out as a result of "gravity-induced Loss
Of Consciousness". The proper term is g-induced Loss Of Consciousness.
It is the acceleration produced by their maneuvers that is the culprit
for g-LOC. It is clearly "g-induced" and not "gravity-induced", because
gravity obviously remained constant at 1-g the whole time for the
pilot. Likewise, the purpose of NASA's " Reduced Gravity Aircraft" is
not to reduce gravity, but rather to fly in a parabolic arc that brings
relative acceleration to zero. "g" is reduced while gravity stays
essentially the same. So clearly it is possible to experience zero-g
without going into space. Any aircraft can do this by pushing it over
into a parabolic arc. Even any car that hits a bump fast enough to
leave the ground will experience zero-g for the time that the wheels
are not in contact with the road. The easiest way to experience zero-g
is to bend your legs and jump off the ground. For the time that you are
in the air, you are experiencing weightlessness. The difference with
astronauts is that the experience is not momentary because their
spacecraft is continually getting pulled toward the Earth. It is
possible for non-astronauts to experience longer durations of
weightlessness by cliff diving, bungee jumping, freefall parachuting,
barrelling over a waterfalls or more safely by riding many types of
modern amusement park rides that put the occupant in a freefall. What
is common for the astronauts as well as these other examples is that it
is not gravity that is changing, but rather the acceleration in their
falling frame of reference goes to zero-g.
As it stands today, NASA itself is one of the biggest promoters of the
erroneous terms "zero gravity" and "microgravity" (along with the
similarly erroneous term "reduced gravity"). Astronauts themselves have
been quoted as having experienced "no gravity" while in space. Surely
they are aware that there was plenty of gravity throughout every orbit
they made, with gravity being the very thing that pulled them into an
Here is the version of the Wikipedia section as subsequently tweaked by
A well managed MEL1 can represent a near zero/micro gravity zone.
Well managed means exactly what it means, as being interactively managed
for the task of remaining as near to zero gravity as we're ever going to
get and survive to talk about it.
Venus L2 could be similar and even more station-keeping energy efficient
than within our moon's L1.
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG
Well, that section isn't right as a Wikipedia section, because of the
Neutral Point of View policy (
http://en.wikipedia.org/wiki/Wikipedia:NPOV ), and some others too
like verifiability. At least for me, it is easier to understand
weightlessness than it is to describe it. The Weightlessness article,
despite potential issues with terminology, does a pretty decent job,
for example saying "Weightlessness means a zero g-force or zero
The myth that satellites remain in orbit because they have "escaped Earth's
gravity" is perpetuated further (and falsely) by almost universal use of the
zingy but physically nonsensical phrase "zero gravity" (and its techweenie
cousin, "microgravity") to describe the free-falling conditions aboard
orbiting space vehicles. Of course, this isn't true; gravity still exists in
space. It keeps satellites from flying straight off into interstellar
emptiness. What's missing is "weight," the resistance of gravitational
attraction by an anchored structure or a counterforce. Satellites stay in
space because of their tremendous horizontal speed, which allows them --
while being unavoidably pulled toward Earth by gravity -- to fall "over the
horizon." The ground's curved withdrawal along the Earth's round surface
offsets the satellites' fall toward the ground. Speed, not position or lack
of gravity, keeps satellites up, and the failure to understand this
fundamental concept means that many other things people "know" just ain't
<tdadamemd-...@excite.com> wrote in message
Brad, I remember having discussed these types of points in that last
thread we engaged in regarding Lagrangian points. I'll resummarize for
anyone who may not be clear on it: the gravity of the primary bodies
do not cancel to zero at such points - the sum of forces is balanced
because of the inertia of the spacecraft in its circular orbit synched
with the two primaries. Or more succinctly...
There is plenty of gravity at all five of the Lagrangian points.
You've provided plenty more to get the Criticism re-added to the
article. And hopefully combined efforts will help get the tide turned
to squelch all such physics-deficient terminology. Hey, maybe even
NASA will start to adhere strictly to verbiage backed by sound physics!
>From Jim Oberg:
>Got my vote. I've been arguing the physics of this for years,
>welcome to the side of truth and accelerational justice!!
I've never understood what's wrong with simple, straightforward "free
fall," which workd from both Newtonian and Einsteinian POVs. Did some
primordial PAO decide that the word "fall," however accurate, was just
Here's my take on that...
The term freeFall applies to only half of your orbit at best! From
perigee to apogee, altitude is increasing. Freerise, if you will.
Another detractor is that the term freefall is *not* a synonym for
zero-g. Being well established in the realm of skydiving, freefall is
the falling part of the trajectory prior to parachute opening. It is
important to know that the term is decoupled from a definition of
acceleration. At terminal velocity, a skydiver in freefall is
experiencing 1-g, not 0-g. In contrast...
The term weightlessness is inherently inseparable from zero-g
acceleration. (The only exception would be a massless object, which is