How Weightlessness Works

Imagine that you are dressed in your space suit 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.

The rocket 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.

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 read zero.

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. 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 the shuttle.

When you first encounter microgravity, you have the following feelings:

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 responds.

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

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 legs.

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 International 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.

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.

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:

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 near-Earth condition.

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.

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.

Orientation 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).

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.