The outlook for manned space flights looks bleak as an explanation is awaited for the Russian cosmonaut tragedy. After a brief early morning announcement of the death of the three men returning after 24 days in space, Moscow remained silent yesterday.

As the official silence lengthened, speculation among experts in the West centred increasingly on the possibility of human failure after prolonged weightlessness in space.

"If this was a failure of men, not of engineering, then the whole future of space flight not just Russian space flight is in jeopardy," one expert from Farnborough's Institute of Space Medicine, said. This gloomy view was echoed in France and the United States, although experts at the Manned Space Flight Centre at Houston said they believed the failure was technical.

News of the tragedy was broken to the world in a brief message from Tass. After completing the long mission in Russia's orbiting space station Salyut, the three cosmonauts, Lieutenant Colonel Georgy Dobrovolsky, Vladislav Volkov and Viktor Patsayev, returned to earth in their shuttlecraft Soyuz 11.

The Soyuz made a normal re-entry and soft landing in the expected recovery area. "Landing simultaneously with the spaceship, a helicopter-borne recover crew, upon opening the hatch found the cosmonauts in their seats without any signs of life," Tass said.

Russia was stunned. For over three weeks, the record-breaking flight of the cosmonauts has been followed closely on radio and television, and since it was a flight which put Russia clearly in the lead in the development of permanent orbital laboratories, the Russian public was jubilant.

The announcement of the disaster was followed by the continuous broadcasting of solemn music and the showing of portraits of the astronauts on Moscow television. The Tass account described re-entry as normal. "In order to carry out the descent to earth on June 30 after orienting the Souyur 11 space ship, retro-firing took place at 01.35 (23:35 BST Tuesday) and functioned for the planned length of time.

"At the end of the retro-firing, communication with the crew ceased. According to plan, the parachute system was put into action after aerodynamic braking and retro-engines were fired just before soft landing."

The message then went on to tell of the cosmonauts' death. The simplicity of this statement gives it an added eeriness. Everything was apparently normal. Yet there is no mention of the re-establishment of radio contact with the crew after the normal "black out" of communication systems caused by the initial fiery part of re-entry.

The Russian crew, at the end of the long and arduous activities aboard Salyut, appeared to be normal and cheerful during the approach to earth. They died suddenly in radio silence, in a blanket of fire, and within only a few minutes of touchdown. All were married and had children.

There is nothing in the message from Moscow to indicate a cause. If something had gone wrong with the re-entry flight path, then the spacecraft would not have been on target. Had the parachute system opened early - with the spacecraft travelling too fast - it would certainly have been observed from the ground and would have resulted in damage to the parachute system.

The automatic landing devices of the Soyuz craft have been well tested in operational use, and, as happened in the case of the early Soyuz failure which resulted in the death of cosmonaut Vladmir Komarov in April 1967, the Russians would have made an early announcement if the cause of the tragedy had been a simple engineering failure.