MOSCOW -- The three Soviet cosmonauts who set an endurance record for space flights died mysteriously today on their return to earth, the Soviet news agency Tass reported.
Soyuz 11 cosmonauts Georgy Dobrvolsky, 43, Vladislav Volkov, 35, and Viktor Patsayev, 38, were found strapped in their seats "without any signs of life" when their spacecraft softlanded as planned in Soviet Central Asia.
Tass said the causes of the crew's death are being investigated. The men spent 23 days, 17 hours and 40 minutes in orbit, and space scientists have speculated in the past that prolonged weightlessness and the sudden shock of re-entering earth's gravity could cause heart failure.
It may take weeks and months before the exact cause of the disaster is ascertained and then even more time to devise methods of overcoming the dangers.
Nation In Mourning
The Soviet Union plunged into national mourning for the three heroes who were in space six days and 41 minutes longer than any other men. Moscow radio played funeral music and Muscovites expressed shock and horror at the latest of the trouble and tragedy that has plagued the Soyuz manned space program.
The cosmonauts' ashes will be buried in the Kremlin wall with other heroes of the Soviet Union following a state funeral in Red Square.
Messages of condolence poured in from around the world -- Pope Paul VI, President Tito of Yugoslavia, western scientists, including the U.S. National Aeronautics and Space Administration Administration which said "the loss of the cosmonauts is a terrible tragedy."
To Americans it recalled the loss of three American astronauts astronauts who died on Jan. 27,1967, when an electrical spark ignited the pure oxygen in their cabin on the ground at Cape Kennedy -- Virgil I. Grissom, Edward H. White and Roger B. Chaffee.
That tragedy postponed the American Apollo program for eight months. The tragedy of Soyuz 11 is expected also to postpone the Soviet space program and Russian plans to build "cosmograds" or orbiting cities in the sky.
Col. Francois Violette, head of the French space medical service, said in Paris today the death of the three men could come from blood circulation problems -- that prolonged weightlessness could make the heart and circulatory system lazy and that it could fail with shock of re-entry.
Aloft 23 Days
The cosmonauts were last heard from only minutes before, when they fired their retrorockets slowing the Soyuz 11 ship to re-enter Earth's atmosphere after 23 days, 17 hours and 40 minutes in orbit.
Western space experts said the shock of gravitational forces after so long in a weightless could have caused their death. Without gravity, man's body grows "lazy" and the heart does not have to pump so hard.
The experts speculated that hearts of the astronauts may have been unable to cope with such a sudden and enormous burden. The previous record-holders for space flight -- the two-man Soyuz 9 crew -- were dizzy and suffering from weight loss and heart irregularities when they landed after their 11-day odyssey in 1970. It took them a month to readapt to gravity.
Soviet space doctor Ivan Vorobyov said June 24 when Soyuz 11 cosmonauts passed the Soyuz 9 record that "every new day of the flight is a step into unknown." He said: "Do the negative bodily processes stop at some time in weightlessness? Is there a limit to the body falling out of Earth habits? Is there a fringe which should not be out-stepped. So far, we cannot give a final answer to these questions."
Soviet experts had taken major steps to protect the three cosmonauts' bodies from this laziness by providing their Salyut orbiting space lab with equipment to exercise their muscles.
Tass, in announcing the tragedy, said "the causes of death are being investigated."
The Soviet Union went into mourning. Moscow radio, after announcing the deaths, played funeral music. Condolences began pouring in from around the world.
In Houston, the official spokesman for NASA, John E. McLeaish, said "we are very sorry to hear this. I'm sure this is the reaction of all U.S. space personnel." He would not say what affect this would have on the planned U.S. Skylab space station.
In Washington, NASA's deputy administrator, George Low, called the cosmonauts' deaths a "terrible tragedy." He extended NASA's "deepest sympathy" to their colleagues and their families.
Dobrovolsky, 43, Volkov, 45, and Patsayev, 38, were launched June 6 aboard Soyuz 11, docked with Salyut the next day and boarded the station to establish the first manned orbital space station.
On June 23 at 5:54 p.m. EDT, they surpassed the endurance record of 17 days, 16 hours and 59 minutes set by Soyuz 9 cosmonauts Andrian Nikolayev and Vitaly Sevastianov June 19, 1970. "Every day of the flight is now a step into the unknown," space doctors said at the time.
A few hours before the fatal descent, a Soviet space doctor said on Moscow television the cosmonauts seemed slightly fatigued. Dr. Arkady V. Yeryomin added, however, that "all the information so far indicates that they will complete their task and.' fulfill the entire program. We will greet them with pleasure on Earth."
Tass said the word to return to Earth came Tuesday night and the entire procedure went off without a hitch. "The crew of the Soyuz 11 spaceship reported to Earth the unlinking operation passed without a hitch and systems were functioning normally," it said.
Tass said' that at 1:35 a.m., Moscow time (6:35 p.m. EDT Tuesday) Soyuz 11 "braking engine was fired and functioned throughout the estimated time.
"At the end of the operation the braking engine, communication with the crew ceased. According to the program, after aerodynamic braking in the atmosphere, the parachute system was put into action and before landing the soft-landing engines were fired.
"The flight of the descending apparatus ended in a smooth landing in the pre-set area," Tass added.
"Landing simultaneously with the ship, a helicopter-borne recovery group, upon opening the hatch, found the crew ... in their seats without any signs of life."