Manned Spacecraft

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 the earth's surface. In the earliest crewed spacecraft, this problem was solved simply by allowing the vehicle to travel along a ballistic path back to the 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 reentry 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 reentry is the heat that develops as the spacecraft returns to the earth's atmosphere. Friction between vehicle and air produces temperatures that exceed 3,000°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 are beginning to look beyond space shuttle flights and the International Space Station. While NASA's main emphasis for some time will be unmanned probes and robots—in terms of spacecraft variety, not funding (most of which goes to the manned spaceflight program)—the most tempting target for a manned spacecraft will surely 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.