Photo courtesy NASA One astronaut lifts another with her
have often seen pictures of astronauts floating around inside
Space Station or Mir.
While weightlessness looks like fun, it places great demands
on your body. Initially, you feel nauseated, dizzy and
disoriented. Your head and sinuses swell and your legs shrink.
In the long term, your muscles weaken and your bones become
brittle. These effects on your body could do severe damage on
a long voyage, such as a trip to Mars.
In this edition of HowStuffWorks,
we'll take you on an extended journey aboard the International
Space Station, where we will examine what weightlessness is,
what happens to your body, how these changes come about and
what can be done to prevent or reverse these adverse effects.
Encountering Microgravity Imagine that you
are dressed in your spacesuit
and lying on your back in the flight deck of the space
shuttle. You have been on your back in the chair for
several hours as the pilots and mission control have gone
through the preflight launch preparations. Normally, when
you're standing upright, gravity pulls blood downward so it
pools in the veins of your legs. However, because you've been
lying on your back, the blood is distributed differently
through your body, shifting slightly toward your head because
your feet are elevated. Your head may feel a little stuffy,
much like it does at night when you sleep.
engines fire and you feel the acceleration. You get pushed
back into your seat as the shuttle ascends. You feel heavy as
the G-forces of the shuttle's acceleration increase to up to
three times normal gravity (some roller coaster rides can
achieve this level of acceleration). Your chest feels
compressed and you may have some difficulty breathing. In
about 8 and a half minutes, you are in outer space,
experiencing an entirely different sensation:
Weightlessness is more correctly termed
microgravity. You are not actually weightless, because
the Earth's gravity is holding you and everything in the
shuttle in orbit. You are actually in a state of
free-fall, much like jumping from an airplane except
that you are moving so fast horizontally (5 miles per second
or 8 kilometers per second) that, as you fall, you never touch
the ground because the Earth curves away from you. It's like
this: When you stand on a bathroom scale, it measures your
weight because gravity pulls down on you and the scale.
Because the scale is resting on the ground, it pushes up on
you with an equal force -- this equal force is your weight.
However, if you were to jump off a cliff while standing on a
bathroom scale, both you and the scale would be pulled down
equally by gravity. You would not push on the scale and it
would not push back against you. Therefore, your weight would
NASA Objects such as long
hair, water, M&Ms and toys behave strangely in
action, there is an opposite, but equal
reaction." To move in any direction in
microgravity, you must push against something so that it
can push against you in the opposite direction.
Furthermore, whatever you push on must be either
anchored or more massive than you, such as the wall of
Because the shuttle and all of the objects in it are
falling around the world at the same rate, everything in the
shuttle that is not secured floats. If you have long hair, it
floats around your face. If you pour a glass of water out, it
assumes a large, spherical drop that you can break up into
separate, smaller drops (see this
page for information on liquid surface tension).
Food and candy gently float to your mouth if you push them in
that direction. While sitting in your seat, you have no sense
that you are seated because your body does not press against
the seat. If you are not secured to something, you float.
Furthermore, if you cannot reach a wall or hand/foot-hold, you
cannot move from your position because you have nothing to
push against. For this reason, NASA has placed many
restraints, hand-holds and foot-holds throughout the cabin of
When you first
encounter microgravity, you have the following feelings:
Loss of appetite
The longer you stay in microgravity,
the more your muscles and bones weaken.
These sensations are caused by changes in various systems
of your body. Let's take a closer look at how your body
Spacesick The nausea and
disorientation that you feel are like that sinking feeling in
your stomach when your car hits a dip in the road or you
experience a drop on a roller coaster ride, only you have that
feeling constantly for several days. This is the feeling of
space sickness, or space motion sickness, which is
caused by conflicting information that your brain receives
from your eyes and the vestibular organs located in your inner
ear. Your eyes can see which way is up and down inside the
shuttle. However, because your vestibular system relies on the
downward pull of gravity to tell you which way is up versus
down and in which direction you are moving, it does not
function in microgravity. So your eyes may tell your brain
that you are upside-down, but your brain does not receive any
interpretable input from your vestibular organs. Your confused
brain produces the nausea and disorientation, which in turn
may lead to vomiting and loss of appetite. Fortunately, after
a few days, your brain adapts to the situation by relying
solely on the visual inputs, and you begin to feel better.
NASA has issued medication patches to help astronauts deal
with the nausea until their bodies adapt.
Puffy Face and Bird
Legs In microgravity, your face will feel full and
your sinuses will feel congested, which may contribute to
headaches as well as space motion sickness. You feel the same
way on Earth when you bend over or stand upside down, because
blood rushes to your head.
Shifts in your blood and bodily fluids upon
exposure to micro-gravity
On Earth, gravity pulls on your blood, causing significant
volumes to pool in the veins of your legs. Once you encounter
microgravity, the blood shifts from your legs into your chest
and head. Your face tends to get puffy and your sinuses swell,
as shown below. The fluid shift also shrinks the size of your
Photos courtesy NASA Astronaut Story Musgrave on Earth (left) and
in orbit (right). You can see the puffiness around his
eyes and cheeks caused by
When the blood shifts to the chest, your heart
increases in size and pumps more blood with each beat. Your kidneys
respond to this increased blood flow by producing more urine,
much like they do after you drink a large glass of water.
Also, the increase in blood and fluid decreases anti-diuretic
hormone (ADH) secretion by the pituitary gland, which
makes you less thirsty. Therefore, you do not drink as much
water as you might on Earth. Overall, these two factors
combine to help rid your chest and head of the excess fluid,
and in a few days, your body's fluid levels are less than what
they were on Earth. Although you still have a slightly puffy
head and stuffy sinuses, it is not as bad after the first
couple of days. Upon your return to Earth, gravity will pull
those fluids back down to your legs and away from your head,
which will cause you to feel faint when you stand up. But you
will also begin to drink more, and your fluid levels will
return to normal in a couple of days.
Space Anemia As your
kidneys eliminate the excess fluid, they also decrease their
secretion of erythropoietin,
a hormone that stimulates red blood-cell production by bone
marrow cells. The decrease in red blood-cell production
matches the decrease in plasma
volume so that the hematocrit (percentage of blood
volume occupied by red blood cells) is the same as on Earth.
Upon your return to Earth, your erythropoietin levels will
increase, as will your red blood-cell count.
Weak Muscles When you are
in microgravity, your body adopts a "fetal" posture -- you
crouch slightly, with your arms and legs half-bent in front of
you. In this position, you do not use many of your muscles,
particularly those muscles that help you stand and maintain
posture (anti-gravity muscles). As your stay aboard the
Space Station lengthens, your muscles change. The mass of
your muscles decreases, which contributes to the "bird leg"
appearance. The muscle
fiber types change from slow-twitch to fast-twitch. Your
body no longer needs slow-twitch endurance fibers, such as
those used in standing. Instead, more fast-twitch fibers are
needed as you push yourself quickly off of space station
surfaces. The longer you stay on the station, the less muscle
mass you will have. This loss of muscle mass makes you weaker,
presenting problems for long-duration space flights and upon
returning home to Earth's gravity.
Brittle Bones On Earth,
your bones support the weight of your body. The size and mass
of your bones are balanced by the rates at which certain bone
cells (osteoblasts) lay down new mineral layers and
other cells (osteoclasts) chew up those mineral layers.
In microgravity, your bones do not need to support your body,
so all of your bones, especially the weight-bearing bones in
your hips, thighs and lower back, are used much less than they
are on Earth. In these bones, the rate at which your
osteoblasts deposit new bone layers is reduced (no one knows
exactly why, though it is thought that changes in force and
stress are somehow involved), while the rate at which
osteoclasts chew up bone stays the same. The result is that
the size and mass of these bones continue to decrease as long
as you remain in microgravity, at a rate of approximately 1
percent per month. These changes in bone mass make your bones
weak and more likely to break upon your return to Earth's
gravity. It is not known how much of the bone loss is
recoverable upon return to Earth, although it is probably not
100 percent. These changes in bones may limit the duration of
space flights. More research is needed in this area.
In addition to weak bones, your blood's calcium
concentration increases slightly as your bones get chewed up
by osteoclasts. Your kidneys
must get rid of the excess calcium, which makes them
susceptible to forming painful kidney stones.
Countermeasures What can be done to help you
deal with the microgravity environment? With respect to
non-living things, every object in the shuttle or space
station must be stowed in lockers, strapped down or attached
to the wall with Velcro.
NASA Everything and everyone
in microgravity must be
For example, when you eat a meal in microgravity, you must
be held to the shuttle with footholds, and your food tray is
attached to you with a strap. Your food tends to be in forms
that are sticky or pasty, like rice or peanut butter, so that
it does not float away. If you are at a work station, you use
straps and footholds to restrain yourself. Portable equipment,
such as a laptop
computer, is strapped to either you (as shown above), an
equipment rack or the wall of the spacecraft.
As for all of these changes that occur in your body during
your stay aboard the International Space Station, what can you
do to remain healthy, especially upon your return to Earth?
Remember that we have to deal mainly with three changes:
Loss of muscle tissue
Loss of bone mass
Fluid Loss One
countermeasure to deal with fluid loss is a device called
lower body negative pressure (LBNP), which applies a
vacuum-cleaner-like suction below your waist to keep fluids
down in your legs. This device might be attached to an
exercise device, such as a treadmill. You might spend 30
minutes per day in the LBNP to keep your circulatory system in
Photo courtesy NASA Test of LBNP
Also, just prior to your return to Earth, you can drink
large volumes of water or electrolyte solutions to help
replace the fluids you've lost. This can prevent you from
fainting when you stand up and step out of the shuttle.
Deterioration of Muscles and
Bones NASA and the Russian Space Agency have found
that the best way to minimize loss of muscle and bone mass in
space is to exercise frequently. This trains your muscles,
prevents them from deteriorating and places stress on your
bones to produce a sensation similar to weight. You exercise
as much as two hours every day on various machines (treadmill,
rowing machine, bicycle). You have to be restrained during
your exercise, usually by tension-producing straps, such as
bungee cords, that hold you to the machine.
NASA Exercising in
Much more research needs to be done to develop
countermeasures to the body's changes in microgravity. This
research must be conducted both on the ground and in outer
space -- aboard the International
Space Station -- using both humans and animals. The
results of such research will help to improve the health of
astronauts and pave the way for long-term space exploration,
such as a trip to Mars.
How to Simulate
Micro-gravity on Earth
Here are some human and animal models
for simulating and studying microgravity on Earth:
Head-down tilt - A person lies down on a
bed tilted head down about five degrees from
horizontal. The tilt reproduces the headward shift of
body fluids encountered in microgravity. In addition,
weight-bearing bones and muscles will not be used and
will deteriorate or atrophy as seen in astronauts in
Swimming-pool immersion - Place a subject
in a warmed swimming pool of water for extended
periods of time. The buoyancy of the water will
produce the fluid shifts and relieve the
weight-bearing bones and muscles as in microgravity.
Tail-suspended rats - Rats are suspended
head down by their tails in cages for extended periods
of time. The tilt reproduces the headward shift of
fluids, and inactivity of the hind legs reproduces the
deterioration of muscles and bones.
KC-135 ''Vomit Comet" - Ride an airplane
through a series of up and down (parabolic) flight
paths that achieve brief periods (30 seconds each) of
microgravity at each peak. NASA uses this technique in
astronaut training and has even made it available for
student research projects.
How You Sense Position and Motion
Photo courtesy NASA Vestibular
and motion are sensed by using the vestibular system,
which is located in the upper portion of the inner ear.
Here is how the vestibular system senses orientation
with respect to gravity:
It has otolithic organs that contain crystals
of calcium carbonate (chalk).
The crystals are attached to hair-like sensory nerve
cells in different orientations (x-, y- and z-axes).
When you bend your head in different directions
(forward, backward, sideways), gravity pulls on the
crystals that are oriented in the direction of the pull.
The affected crystals stimulate the attached hair
cells to send nerve impulses to the brain.
The brain interprets these signals to find out which
way the head is oriented in space.
Here is how the vestibular system senses motion:
There are three semicircular canals for sensing
motion, specifically acceleration.
They are oriented at right angles to one another, and
each is in one of the three directions (x-, y- or z-axis).
They contain fluid called endolymph and
hair-like sensory nerve cells.
As your head accelerates in a given direction, the
endolymph lags behind because of its initial resistance to
change in motion (inertia).
The lagging endolymph stimulates the appropriate hair
cells to send nerve signals to the brain.
The brain interprets them to find out which way the
head has moved.