Nowadays, at any time and any place, numerous artificial satellites are bustling above our heads beyond the tranquil sky. Unmanned Spacecrafts bearing the most sophisticated equipment and cameras are sent to study other members of the Solar System. The problem is: if machines can explore the space for us, why we still take the risk to send human into space?

Besides as a demonstration of scientific and financial powers of a nation, there is a simple reason to send man into space. Human is superior to the best machine in many aspects. 

Firstly, astronauts can handle all kind of unpredictable problems and even actively solve them. Secondly, with a pair of skillful hands, they can perform tasks proved to be far beyond the ability of robotic arms. And more importantly, in the distant future, depletion of the Earth's resources, impacts of asteroids, abrupt changes of solar activities, etc. will force human to leave the Earth and find a new home in the endless universe. To prepare for the challenges ahead, we need to understand the effects of prolonged weightlessness and living in confined environment on human's psychology and physiology.

Sending man into space is note merely a problem of transportation. Outer space is too hostile to human. The spacecraft should provide a suitable environment, food, water and hygienic facilities, needless to say to bring the astronauts home safely. The human, financial and technological resources required are far exceeding any unmanned space project.

The spacecraft for astronauts to live and work in should be perfectly airtight. It should also:

• provide an environment with the right temperature, humidity and pressure;

• has facility to absorb all metabolic gases (e.g. carbon dioxide, water vapour and hydrogen sulphide);

• provide foods and water as well as collect human wastes (e.g. urine, excrement and vomit);

• provide lightings, dials, viewing windows and communication system;

• has emergency rescue system for different flight stages;

• allow the switching to manual control if necessary;

• has backup units for all key equipment;

• provide spare parts and tools to allow astronauts to do onboard repairs;

• be able to limit the deceleration during reentry below human's limit.