On January 16, 1969, for the first time in the history of cosmonautics, two cosmonauts transferred from one spacecraft to another.

It is worth noting that the Soyuz spacecraft orbited the Earth at an altitude of about 200 kilometres above its surface. At such altitudes there is practically no atmosphere, which supports and protects life on Earth. The conditions of outer space are extremely harmful to man. The reason is not only the absence of oxygen.

Powerful solar radiation, the greater part of which comprises ultra-violet emission, has fatal effects on living organisms. Pressure outside the spacecraft is practically nil. As is known, the boiling point of liquids goes down with pressure. In such conditions decompression of the cosmonaut's protective suit would cause the boiling of the blood and instant death. And finally, there is radiation from space which is harmful for living organisms. The greater part of its energy does not reach the surface of the Earth, it is expended in splitting the molecules and ionising the upper layers of the atmosphere. But at the altitudes of space flights special protection against radiation hazards is necessary. In view of these plus psychological factors, performing operations in outer space is extremely complex.

Alexei Yeliseyev and Yevgeni Khrunov emerged into outer space wearing space suits equipped with an extravehicular life support system. The oxygen supply bottle and chemical agents for disposal of the impurities of "internal atmosphere" are located in the box carried by the cosmonaut. The box may be attached, by the cosmonaut, to any part of his body, where it is convenient to him.

Cosmonauts Khrunov and Yeliseyev, for example, chose to attach it to their legs.

Normal breathing in space is not the only problem solved with the aid of the space suit. Another problem is proper heat exchange. Man continuously emits heat in the course of his activity, and the harder the labour performed, the greater the amount of heat emitted. This presents no problems on Earth. But in space unless special measures are taken, the heat accumulated inside the space suit would make condition for the cosmonaut at first difficult, and then simply unbearable. So what is done about the excess heat?

It should be noted that when a cosmonaut is floating in space one side of him faces the sun while the other is in shadow. The side illuminated by the sun is heated to a greater extent. The other side gives off heat into space. The fabric of the space suit is selected so as to reflect the greatest possible fraction of solar radiation. Since there is no natural air circulation inside the space suit, forced ventilation is provided.

When outside the spacecraft the cosmonauts maintained contact with their commanders through the umbilical lines connecting the space suits with the spacecraft. Apart from communication wires the umbilical line incorporates power supply cords, as well as telemetric lines for monitoring the physiological functions of the cosmonauts on the basis of data supplied by sensors taped to their bodies.

The spacecraft commanders could follow the movement of their colleagues by means of external TV-cameras installed on the hull of the spacecraft.

The crews of Soyuz-4 and Soyuz-5 demonstrated that it was possible not only to build orbital space stations but also to replace crewmen and rescue people in an emergency.