For centuries, people have dreamed of leaving the Earth and traveling through space to visit the moon and explore other planets and stars. Some of these dreams have become realities. Spacecraft have orbited the Earth and sent back data to ground-based scientists. They have traveled to other planets and transmitted images and information that have helped to expand our knowledge of the solar system. People have gone into space to orbit the Earth and even to visit the moon.

In 1992, a United States satellite, the Cosmic Background Explorer (COBE), detected slight variations or ripples in the background microwave radiation coming from far out in space. This information may be helpful in determining how the universe evolved.

Despite such achievements, space exploration is still in its infancy considering the vast scope of the universe and the many unanswered questions about it. For example, scientists estimate that there are 10 billion stars like our sun in the Milky Way galaxy, perhaps a million of which may have planets orbiting around them. Scientists want to know if these planets exist, and if they do, are any of them like those in our solar system, or do any harbor intelligent beings or other forms of life.

Someday, as a result of space exploration and travel, scientists may be able to solve the mysteries of the universe. Their discoveries may also change our view of life on Earth and of our planet's role in the universe.

Leaving the Earth

Before reaching space, scientists had to solve the problem of escaping from the Earth's gravity — the force that pulls objects toward Earth and prevents them from floating off into space. A spacecraft leaving Earth must travel fast enough to overcome this strong gravitational pull. The speed needed to overcome the Earth's gravity, called escape velocity, is about 7 miles (11 kilometers) per second, or 25,000 miles (40,000 kilometers) per hour. Reaching escape velocity does not mean that a spacecraft has freed itself completely from the Earth's gravitational pull, which extends far out into space. But it does mean that the spacecraft will not fall back to Earth even if no additional power is used. As the spacecraft continues to move away from the Earth, the gravitational force weakens until it no longer has a significant effect on the spacecraft.

For a spacecraft to enter orbit around the Earth, it must reach a speed called orbital velocity. The orbital velocity will depend upon how far above the Earth the craft is supposed to orbit. For example, a spacecraft must attain an orbital velocity of about 17,500 miles (28,000 kilometers) per hour to orbit the Earth at a distance of 100 miles (160 kilometers). A slower orbital velocity is needed to keep a spacecraft in orbit farther from Earth.

A spacecraft is sometimes put into a temporary, or parking, orbit before it is sent farther out into space. There are two reasons for doing this. A spacecraft launched directly into space would need more powerful, more expensive rockets. Scientists have also found that it is easier to aim a spacecraft toward its destination if it is put into a parking orbit first.

Navigation, Tracking, and Monitoring

In space there are no fixed landmarks to indicate position. Yet a spacecraft is expected to travel immense distances to its destination and perhaps land within a few hundred yards of a specific target. Navigating a spacecraft to achieve this goal requires the help of many engineers and technicians and the use of complex equipment and systems.

Monitoring Systems and Crew
Even during the quietest moments of a spaceflight, many things are happening on board the spacecraft. Doctors working with flight controllers must know such things as the breathing rate, pulse rate, blood pressure, and body temperature of each crew member. Ground-based engineers need information about temperatures and pressures within the spacecraft, the condition of its machinery and instruments, and whether any dangerous situations may be arising. Scientists need information about the characteristics of planets and their satellites (such as data on their gravitational and magnetic fields) and information on atoms and molecules in space.

Much of this data is gathered by devices called sensors, which can detect and measure pressures, temperatures, radiation, pulse rates, and so on. The measurements are changed into radio signals that are beamed back to scientists on Earth. The technology concerned with systems that gather and send this information is called telemetry (from Greek words meaning "measuring at a distance").

Survival in Space

On Earth, people move about comfortably under the influence of Earth's gravity, and they extract oxygen from the air to breathe. The Earth's atmosphere protects them from deadly radiation and falling meteorites. Earth also has abundant supplies of water, which is necessary for survival. When people venture into space, however, they leave the only known place where they can live naturally.

G-Forces and the Human Body
During the first few minutes after a spacecraft is launched, an enormous force pushes astronauts down in their seats. You may have felt this force, although much more gently, in a rising elevator. As the elevator accelerates, you can feel the pressure of the floor against your feet. The force that holds you to the Earth and pulls you down is gravity.

The normal gravitational force that holds you to the Earth has a strength of 1 g. (The g stands for "gravity.") The pull of 1 g on your body is what is commonly called your weight. Suppose you stood on a scale inside an elevator as it accelerated upward. You would find that you had suddenly gained a few pounds. This is because gravity pulls you down and causes your body to resist being moved upward. This resistance has the effect of making you slightly heavier. The g-force on your body would be a little greater than the normal 1 g.

On early spaceflights, g-forces built up to 10 g as the spacecraft accelerated during launch, and also reached very high levels when the spacecraft slowed down. Care had to be taken to prevent these high g-forces from injuring the astronauts. Scientists learned that astronauts can withstand up to 10 g for about two minutes without harm if they are lying down and facing in the same direction as the spacecraft is moving. Their bodies must also be trained to withstand the strain.

Aboard the U.S. , the g-forces are never greater than 3 g. Since this is well within the physical limitations of most people, it allows individuals who have not had special training to go into space. It also allows the space shuttle to carry equipment that would be damaged by high g-forces.

Weightlessness and Its Effects
By the time a spacecraft reaches escape velocity or orbital velocity, there are no more high g-forces. In fact, even Earth's 1 g-force is gone. As a result, people aboard the spacecraft experience weightlessness, or zero gravity (0 g), a state in which they feel absolutely no gravitational pull. In a state of weightlessness, people feel lighter than a feather and float because they weigh nothing at all.

Several things are usually done to help crew members deal with weightlessness. Lines are strung in the spacecraft cabin so that crew members can pull themselves along in the directions they want to go. The soles of shoes and the floor of the cabin are covered with a special burrlike fabric that will stick to another similar surface. It is very similar to the Velcro that is used today as a fastener for clothes. This fabric enables crew members to walk around inside the spacecraft cabin during weightlessness. Astronauts are also carefully trained to live in weightless conditions. They learn how to move, eat, drink, and handle tools. They also learn how to sleep in a floating position, held in place in a type of sleeping bag attached to the walls of the spacecraft.

During periods of weightlessness, it is very important for astronauts to do certain exercises or there may be damage to their hearts, blood vessels, bones, and muscles, which are all adapted to the gravity of Earth. Astronauts who spend a few months in weightless conditions do not seem harmed, but doctors are concerned about the possible effects of weightlessness during long voyages if muscles and other bodily systems are not utilized as they are on Earth.

Space Sickness
One problem that affects many astronauts is space sickness, a violent motion sickness characterized by nausea, headaches, sweating, and vomiting. Space sickness is apparently caused by the absence of gravity. In the weightless environment the brain does not seem to be able to cope with the conflicting messages it receives from the eyes and the balance organs in the inner ear. As a result, a person's equilibrium, or sense of balance, is disrupted. After a few days, the body adapts to the weightless environment, but during the first few days of spaceflight, space sickness can make astronauts very uncomfortable. In many instances, certain drugs and mental training have been able to control the symptoms of space sickness.

Life-Support Systems
On Earth, every time you take a breath, fresh air is pushed into your lungs. That could not happen in space because there is no air and no air pressure to make air move. Instead, space is an almost perfect vacuum — an empty area with no air or atmosphere. Because of this vacuum, astronauts need special life-support systems to survive.

One type of life-support system is the pressurized space suit. This suit maintains proper air pressure and temperature and also supplies oxygen for breathing. It is made out of many layers of strong synthetic materials that can protect an astronaut from the vacuum of space and other dangers, such as radiation. A pressurized space suit is bulky, however, and it is uncomfortable and tiring to be inside one for very long. Therefore, scientists have created ways to provide a "shirtsleeve environment" for astronauts when they are inside the spacecraft. In this environment, the cabin is pressurized and its air is conditioned to protect astronauts from the extreme cold of space, the heat of the sun, and the heat of re-entry into the Earth's atmosphere. The cabin's air conditioning system also purifies the air, removes moisture and carbon dioxide from the air, and adds fresh oxygen to it. Within the cabin, crew members wear light, comfortable space suits that allow great freedom of movement and that can be pressurized quickly in an emergency.

When astronauts leave the shirtsleeve environment of the cabin, they must wear pressurized space suits and carry portable life-support systems. The Apollo astronauts who walked on the moon, for example, wore backpacks with special equipment that could keep them alive for up to four hours. An astronaut's backpack contains a small air conditioning system and a small battery to supply power. Oxygen for breathing and for pressurizing the space suit is stored in a small tank within the backpack. A ventilating system forces oxygen through the space suit and the pack and removes carbon dioxide and other contaminants, as well as perspiration.

Several radio transmitters in the backpack make up a communications system. One transmitter permits crew members to talk to each other and to ground controllers on Earth. Other transmitters relay information about the physical condition of the person wearing the space suit and about the oxygen supply, the pressure and temperature within the suit, and the strength of the battery in the backpack.

Food, Water, and Wastes
The food that astronauts eat on board a spacecraft must be nutritious, easy to eat, and convenient to store. Most ordinary foods are too bulky and heavy to take on a spaceflight, and many spoil if they are not refrigerated. Some foods used in space are dehydrated and freeze-dried, which is a process that removes water, leaving only a dry powdery or pastelike substance. Freeze-dried, dehydrated foods weigh as little as one tenth of their original weight. They take up very little space and can be kept in plastic bags at room temperature without spoiling. Before eating the freeze-dried food, an astronaut adds some water to the dry food while it is in the plastic bag and mixes the contents until the food is soft. The food can then be squeezed out of the bag or sometimes eaten with a spoon. Astronauts sometimes warm up frozen and chilled foods as well.

Astronauts must have water for drinking, washing themselves, and preparing freeze-dried foods. On long trips, devices called fuel cells are used to produce pure, fresh water. They also produce electricity for the spacecraft. When oxygen and hydrogen are piped into the fuel cells, the two gases combine, forming water. Electricity is produced during this reaction.

A special problem during spaceflight is how to deal with bodily wastes, such as perspiration, urine, and solid waste. Liquid wastes, such as perspiration and urine, are processed in a special purifying system that separates the water from the other materials in the wastes and purifies the water so it can be used again. Solid waste materials are stored in plastic bags that are discarded after returning to Earth.

Space Food in the Future
A trip to Mars will take about two and one-half years. Although this is a long time, space travelers will probably carry processed food and use chemical purifying systems for atmosphere and water. For longer trips, on-board "space farms" will probably be developed to provide most of the food needed. The biological processes of growing plants could also be used to purify water and the spacecraft's atmosphere. A space station 40 feet (12 meters) long and 14 feet (4 meters) across could house a farm large enough to support a crew of four.

Dangers of Spaceflight

Astronauts in space face a number of dangers quite unlike the dangers on Earth. One of the most obvious dangers is the lack of atmosphere in space and the vacuum that exists there. Without life-support systems, astronauts would be quickly killed by the space environment. Space holds other dangers as well, including the physical and emotional stresses of spaceflight itself.

Physical and Emotional Stress
The environment aboard a spacecraft can be physically and emotionally stressful. Spacecraft are rather small, and there is little room for astronauts to move around or to have any real privacy. In a spacecraft environment, astronauts are also deprived of many of the emotional, physical, and sensory stimulations they experience in their lives on Earth. Astronauts must also cope with the constant worry of life-threatening dangers to themselves and the spacecraft. Such conditions can be very stressful and can lead to conflicts among crew members, impaired judgment, and actions and behaviors that could endanger lives. While short missions do not pose a problem, long missions might cause tremendous stress.

Early spaceflights lasted only a few hours or a day, and stress was not a serious problem. Scientists were not sure, however, how astronauts would react to longer flights. To find out, they developed special test chambers to imitate the conditions in a spacecraft. Volunteers lived and worked in these test chambers for long periods of time, while scientists carefully observed them and kept records of their reactions. These tests helped scientists improve the methods of choosing and training astronauts.

Several other things can be done to reduce the stresses of spaceflight. Spacecraft cabins can be designed to allow privacy and provide room for recreation and exercise. Private spaces can be personalized. Astronauts can be given plenty of opportunities to rest, to engage in enjoyable activities, and to communicate with loved ones back on Earth. Such measures can be quite effective in reducing levels of stress.

Dangers from Micrometeoroids
Space is not completely empty. Floating around in it are countless tiny particles of solid matter called micrometeoroids. Although most of these particles are much smaller than grains of sand, they move through space at tremendous speeds — from 70,000 to 160,000 miles (112,000 to 258,000 kilometers) per hour.

Space is so vast that micrometeoroids are widely scattered throughout it, even though there are billions of them. Usually micrometeoroids pose no great danger to a spacecraft. Sometimes, however, they occur in great swarms. A spacecraft traveling through a swarm may be hit by some particles, which could puncture the skin of the spacecraft. While tiny puncture holes may not be dangerous, a larger hole could allow air to escape from the spacecraft cabin and cause the air pressure to decrease quickly. If too much air escaped and the air pressure decreased enough, breathing would become impossible. While such an accident is unlikely, astronauts are trained to respond almost automatically. Helmets are snapped into place, space suits are pressurized, and the oxygen supply is turned on. Safe within their space suits, the astronauts can devote their attention to repairing the damage to the spacecraft.

Danger from Radiation
In addition to micrometeoroids, space contains tiny, invisible particles of matter, known as radiation, that are emitted by the sun and other stars. These radiation particles travel at speeds ranging from 1 million miles (1.6 million kilometers) per hour to many times faster. There are many types of radiation, including X rays, ultraviolet rays, gamma rays, radio waves, and infrared rays.

Radiation can be very dangerous to life. Exposure to some radiation can cause physical illness and other serious health problems. Exposure to large amounts of radiation can cause death. On Earth, the atmosphere surrounding the planet acts as a filter to prevent most harmful radiation from reaching us. The radiation that gets through the atmosphere is weakened enough to make it relatively harmless. In space, however, there is no atmosphere to filter out harmful radiation particles.

Surrounding the Earth are two regions of strong radiation, discovered in 1958 by the American scientist James A. Van Allen. Known as the Van Allen belts, they are doughnut-shaped and one lies inside the other. They begin about 400 miles (640 kilometers) above the Earth and extend outward for about 12,000 miles (19,000 kilometers).

The early spaceflights did not go far enough into space to reach the Van Allen belts, but scientists planning the Apollo moon missions had to consider the danger from these belts. There was less danger than had been feared. Apollo 8, which carried the first astronauts to orbit the moon, was launched directly through the Van Allen belts. However, the thickness of the spacecraft's skin and the metals used in its construction helped shield the astronauts from radiation. The spacecraft also moved very fast, so it was in the heavy radiation zone for only a short time.

Another radiation danger results from solar "storms" on the surface of the sun. During these storms, great eruptions of energy called solar flares sometimes burst out from the sun, causing unusually intense radiation to spread outward in space. Astronauts who are exposed to this radiation, especially those working outside a spacecraft, are in serious danger. Protecting them against intense radiation requires the development of some type of heavy shielding. But even with the best shielding, some radiation might still harm them. Some scientists propose using intense electromagnetic fields that could shield astronauts by "pushing" radiation away. Fortunately, the most severe solar flares usually occur during the active parts of an eleven-year solar cycle, and the worst radiation occupies only a portion of the space around the sun.

Space Shuttles and Stations

Many space missions in the future will depend on a permanent station orbiting high above the Earth. In 1998, the United States, in partnership with 15 other countries — Russia, Canada, Japan, eleven European nations, and Brazil — began assembling the International Space Station, or ISS. It was the largest science project ever undertaken, costing $60 billion and taking an expected five years to complete. Plans called for a station powered electrically by an array of solar panels as wide as a football field, with living and working quarters as large as the combined cabin size of two 747 jetliners — enough for a crew of up to seven scientists and astronauts. The first two pieces, or modules, of the space station were carried into orbit, one by the U.S. shuttle Endeavour and the other by a Russian rocket. Shuttle astronauts started assembling the modules in space in what would be a series of missions to build the station piece by piece. They and future space construction workers will rely on various tools developed for the shuttles, such as the Remote Manipulator System (a robotic arm that moves objects in and out of the shuttle's cargo bay) and the MMU (manned maneuvering unit) that allows shuttle astronauts to "fly" and work in space without using a tether connected to a spacecraft.

The construction of an orbiting station will provide experience that can then be used in building lunar landing bases or research stations on the moon or on another planet. A space station also will provide an opportunity for astronauts and scientists to live and work in space for long periods of time, testing the long-term effects of weightlessness and exposure to the environment of outer space.

This ambitious project is not the first space station to orbit the Earth. The first experimental space station, Salyut 1, was launched by the former Soviet Union in 1971. Several teams of cosmonauts visited the station and conducted experiments. The Soviet Union put several other small space stations in orbit as well, including other Salyut stations and the Mir space station. Cosmonauts aboard the Mir set the record for the longest time human beings have remained in space. The first U.S. space station to orbit the Earth was Skylab, launched in 1973. Several crews visited and worked in Skylab before it re-entered the Earth's atmosphere and disintegrated in 1979. Skylab provided information and experience that helped make many subsequent U.S. space shuttle flights successful.

Future Space Explorations

One of the central quests of space exploration is to discover whether life exists anywhere else in the solar system. Life as we know it has evolved on Earth because the planet lies at just the right distance from the sun to allow water to remain liquid and temperatures to be moderate. Without these two conditions, life-forms as we know them could not exist.

Search for Life on Other Planets

Among the planets, Mars appears to be the most hospitable to life, although it is farther from the sun and much colder than Earth. Mars has some water, frozen in polar ice caps and perhaps in its soil, much like the permafrost in Earth's arctic lands. Mars also has a thin atmosphere, composed mostly of carbon dioxide. Scientists think that long ago the planet may have had sufficient amounts of atmosphere and surface water to support life. Although studies of the planet's surface have not revealed any signs of life, future probes will no doubt look for remnants of life at the sites of Martian lakes that no longer exist.

Some scientists believe that remnants of Martian life may have already been found in several meteorites thought to have come from Mars and crashed on Earth. In the late 1990's, NASA scientists discovered tiny wormlike features resembling fossilized bacteria in meteorites found in Antarctica, Egypt, and India. The meteorites varied in age from 4 billion years to 165 million years, suggesting that life once existed — and perhaps still exists — on the red planet. Other researchers were skeptical, arguing that the microscopic features might not be signs of life at all. The debate will no doubt continue until more convincing evidence can be found in samples of Martian soil brought back to Earth by future space probes.

Another possible candidate for life in the solar system appears to be Europa, Jupiter's large ice-covered moon. Scientists think that some form of life could exist under Europa's ice, just as it does in frozen lakes of Antarctica. In the late 1990's, the Galileo space probe found signs of ice that had melted and shifted, indicating warm slush or even liquid water beneath Europa's cracked icy surface. Some scientists speculate that a gravitational "tug of war" exerted by Jupiter and its other moons could keep large parts of Europa's ocean liquid. If that is so, then Europa may harbor some form of marine life — perhaps similar to creatures found thriving around hot deep-sea vents on Earth. Future space probes, designed to peer below Europa's surface using ice-penetrating radar, may offer further clues.

Human Bases and Colonies in Space

Sometime in the future, it is possible that we will establish permanent bases on the moon or perhaps Mars. The moon is the most likely site because it is close to Earth and its weaker gravity would allow spacecraft to use less fuel when taking off from its surface. The moon's surface also contains various mineral resources that could be used in building a base. In 1998, the space probe Lunar Prospector, in orbit above the moon's poles, found signs of frozen water — the raw material from which hydrogen fuel and breathable oxygen could be extracted and used by future lunar colonists.

If we establish bases on the moon, these bases will be excellent sites for astronomical research. The far side of the moon, for example, would be ideal for telescopes because there would be no glare from the sun and no atmospheric distortion. With near-perfect viewing conditions, astronomers could search for planets around other stars in the galaxy, conduct long-term studies of stars and other distant objects, and look much farther into space than with any Earth-based telescopes. The moon would also be an ideal site to listen for radio signals that might come from intelligent life elsewhere in the universe.

Much farther in the future lies the possibility of voyages to other stars. With current methods of rocket propulsion, however, a trip to the nearest star would take many more years than exist in a person's lifetime. Space travel within our Milky Way galaxy, therefore, will probably require spaceships to be "colonies," with generations of inhabitants whose entire lives will be spent on board the space colonies as they travel on their journeys.

Given enough time and advances in technology, it might even be possible to colonize other parts of the galaxy. In the 1990's, scientists began discovering evidence of planets orbiting other stars beyond our own. It may also be possible to build floating space colonies around nearby stars. These colonies would be located in regions of space near enough to a star so that there would be enough light, heat, and solar energy for human beings to survive.

More practical than colonies far out in space would be an orbiting space colony near the moon with room for 10,000 people. The colony could be built of lunar material, which would be easy to transport from the moon's surface because the weak gravitational force would allow rockets to take off easier than they can on Earth. Solar energy would supply an unlimited amount of power to the colony.

The Days Ahead

While exciting to contemplate, centuries-long space voyages, bases on the moon, and human colonies in space will not come about for many years. However, space tourism has become a reality. In 2001, American businessman Dennis Tito became the first civilian to pay for a trip into space. He joined a Russian crew on an eight-day mission to the International Space Station. Others lined up immediately for their turn in space.

Achievements in space have been truly remarkable. Yet the future may hold even greater triumphs. The day may come when human beings explore and inhabit the distant reaches of space and unravel the mysteries of the universe and of life within it.

Peter W. Waller Ames Research Center, NASA
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