Spacecraft, Manned | Research & Encyclopedia Articles

Manned spacecraft are vehicles with the capability of maintaining life outside of Earth's atmosphere. Partially in recognition of the fact that women as well as men are active participants in space travel programs, manned spacecraft are now frequently referred to as crewed spacecraft.

In its earliest stages, crewed space flight was largely an exercise in basic research. Scientists were interested in collecting fundamental information about the Moon, the other planets in our solar system, and outer space. Today, crewed space flight is also designed to study a number of practical problems, such as the behavior of living organisms and inorganic materials in zero gravity conditions.

A very large number of complex technical problems must be solved in the construction of spacecraft that can carry humans into space. Most of these problems can be classified in one of three major categories: communication, environmental and support, and re-entry.

Communication refers to the necessity of maintaining contact with members of a space mission as well as monitoring their health and biological functions and the condition of the spacecraft in which they are traveling. Direct communication between astronauts and cosmonauts can be accomplished by means of radio and television messages transmitted between a spacecraft and ground stations. To facilitate these communications, receiving stations at various locations around Earth have been established. Messages are received and transmitted to and from a space vehicle by means of large antennas located at these stations.

Many different kinds of instruments are needed within the spacecraft to monitor cabin temperature, pressure, humidity, and other conditions as well as biological functions such as heart rate, body temperature, blood pressure, and other vital functions. Constant monitoring of spacecraft hardware is also necessary. Data obtained from these monitoring functions is converted to radio signals that are transmitted to Earth stations, allowing ground-based observers to maintain a constant check on the status of both the spacecraft and its human passengers.

The fundamental requirement of a crewed spacecraft is, of course, to provide an atmosphere in which humans can survive and carry out the jobs required of them. This means, foremost, providing the spacecraft with an Earth-like atmosphere in which humans can breathe. Traditionally, the Soviet Union has used a mixture of nitrogen and oxygen gases somewhat like that found in the earth's atmosphere. American spacecraft, however, have employed pure oxygen atmospheres at pressures of about 5 lb per square inch, roughly one-third that of normal air pressure on the earth's surface.

The level of carbon dioxide within a spacecraft must also be maintained at a healthy level. The most direct way of dealing with this problem is to provide the craft with a base, usually lithium hydroxide, which will absorb carbon dioxide exhaled by astronauts and cosmonauts. Humidity, temperature, odors, toxic gases, and sound levels are other factors that must be controlled at a level congenial to human existence.

Food and water provisions present additional problems. The space needed for the storage of conventional foodstuffs is prohibitive for spacecraft. Thus, one of the early challenges for space scientists was the development of dehydrated foods or foods prepared in other ways so that they would occupy as little space as possible. Space scientists have long recognized that food and water supplies present one of the most challenging problems of long-term space travel, as would be the case in a space station. Suggestions have been made, for example, for the purification and recycling of urine as drinking water and for the use of exhaled carbon dioxide in the growth of plants for foods in spacecraft that remain in orbit for long periods of time.

An important aspect of spacecraft design is the provision for power sources needed to operate communication, environmental, and other instruments and devices within the vehicle. The earliest crewed spacecrafts had simple power systems. The Mercury series of vehicles, for example, were powered by six conventional batteries. As spacecraft increased in size and complexity, however, so did their power needs. The Gemini spacecrafts required an additional conventional battery and two fuel cells, while the Apollo vehicles were provided with five batteries and three fuel cells.

One of the most serious on-going concerns of space scientists about crewed flights has been their potential effects on the human body. An important goal of nearly every space flight has been to determine how the human body reacts to a zero-gravity environment.

At this point, scientists have some answers to that question. For example, we know that one of the most serious dangers posed by extended space travel is the loss of calcium from bones. Also, the absence of gravitational forces results in a space traveler's blood collecting in the upper part of his or her body, especially in the left atrium. This knowledge has led to the development of special devices that modify the loss of gravitational effects during space travel.

One of the challenges posed by crewed space flight is the need for redundancy in systems. Redundancy means that there must be two or three of every instrument, device, or spacecraft part that is needed for human survival. This level of redundancy is not necessary with uncrewed spacecraft where failure of a system may result in the loss of a space probe, but not the loss of a human life. It is crucial, however, when humans travel aboard a spacecraft.

An example of the role of redundancy was provided during the Apollo 13 mission. That mission's plan of landing on the Moon had to be aborted when one of the fuel cells in the service module exploded, eliminating a large part of the spacecraft's power supply. A back-up fuel cell in the lunar module was brought on line, however, allowing the spacecraft to return to Earth without loss of life.

Space suits are designed to be worn by astronauts and cosmonauts during take-off and landing and during extravehicular activities (EVA). They are, in a sense, a space passenger's own private space vehicle and present, in miniature, most of the same environmental problems as does the construction of the spacecraft itself. For example, a space suit must be able to protect the space traveler from marked changes in temperature, pressure, and humidity, and from exposure to radiation, unacceptable solar glare, and micrometeorites. In addition, the space suit must allow the space traveler to move about with relative ease and to provide a means of communicating with fellow travelers in a spacecraft or with controllers on the earth's surface. The removal and storage of human wastes is also a problem that must be solved for humans wearing a space suit.

Ensuring that astronauts and cosmonauts are able to survive in space is only one of the problems facing space scientists. A spacecraft must also be able to return its human passengers safely to Earth's surface. In the earliest crewed spacecrafts, this problem was solved simply by allowing the vehicle to travel along a ballistic path back to Earth's atmosphere and then to settle on land or sea by means of one or more large parachutes. Later spacecraft were modified to allow pilots some control over their re-entry path. The space shuttles, for example, can be piloted back to Earth in the last stages of reentry in much the same way that a normal airplane is flown.

Perhaps the most serious single problem encountered during re-entry is the heat that develops as the spacecraft returns to Earth's atmosphere. Friction between vehicle and air produces temperatures that approach 3,092°F (1,700°C). Most metals and alloys would melt or fail at these temperatures. To deal with this problem, spacecraft designers have developed a class of materials known as ablators that absorb and then radiate large amounts of heat in brief periods of time. Ablators have been made out of a variety of materials, including phenolic resins, epoxy compounds, and silicone rubbers.

Some scientists are beginning to plan beyond space shuttle flights and the International Space Station. While NASA's main emphasis for some time will be unmanned probes and robots, the most likely target for a manned spacecraft will be Mars. Besides issues of long-term life support, any such mission will have to deal with long-term exposure to space radiation. Without sufficient protection, galactic cosmic rays would penetrate spacecraft and astronaut's bodies, damaging their DNA and perhaps disrupting nerve cells in their brains over the long-term. (Manned flights to the Moon were protected from cosmic rays by the earth's magnetosphere.) Shielding would be necessary, but it is always a trade-off between human protection and spacecraft weight. Moreover, estimates show it could add billions of dollars to the cost of any such flight.

Space and Planetary Geology; Space Physiology

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