Two Soviet cosmonauts this morning broke the United States manned space record of 84 days in earth orbit, thus capturing for the Soviet Union one more in a string of records set by the crew of the space station.

Salyut commander Yuri Romanenko, 33, and flight engineer Georgi Grechko, 47, have been in space since Dec. 10. The American record of 84 days, 1 hour and 16 minutes fell at 5:36 a.m. this morning Moscow time (9:36 p.m. Friday EST) The U.S. record was set in 1973-74 by the three-man crew of the SSkylab-4, Edward Gibson, Gerald Carr and William Pogue.

On hand with Romanenko and Grechko when they set the new record were two other cosmonauts newly arrived at Salyut 6, Czechoslovak "cosmonaut researcher" Vladimir Remek, 29, and veteran Soviet cosmonaut Col. Alexei Gubarev, 47. Remek is trhe first person to be lauched into space who is not from the Soviet Union on the United States. He and Gubarev rocketed into space Thursday and docked with the Salyut space station at 8.10 p.m. Friday night Moscow time.

Informed Western sources here speculated that Romanenko and Grechko, refreshed by their new visitors from the monotony of their duties and responsibilities inside the pressurized living and working quarters of the space station, may try for a 100-day record or longer. One source said they could possibly stay aloft until May 1, a traditional holiday in this Communist country.

"Every day is new now, and it makes sense to keep them up there as long as they can," said a source. He suggested that it may be some time before another attempt at an endurance record is made "because the Soviets will have enormous amounts of medical data to digest from this flight."

The Soviet eclipse of the American space record is viewed as a concrete, identifiable measure of the depth and seriousness of the country's manned space effort.Despite numerous reversals and tragedies over the years since 1957 when the Soviets opened the Space Age with the lauch of Sputnik I, the world's first satellite, the Soviets have continued to press forward with their space efforts.

The nation carved for itself an early, proud history: first space satellite, the Sddoviets ahve continued to press forwafirst animal (a dog) to orbit the earth, first man and then, first woman to orbit earth, first payload to hit the moon, first artificial stellite in solar orbit.

Soviet officials have long spoken of a sustained manned orbital program, envisioning massive space stations built from smaller components, capable of orbiting for years. With those, if would be possible to conduct long term medical and industrial experiments, research in astronomy and biologyplus military surveillance.

The Soviets have now established a new series of norms for manned spaceflight at a time when the United States' manned space program is inactive until the massive new reusable space shuttle, now in a testing program, is proven safe.

In a peculiar twist of fate that seems to symbolize this changed set of circumstances, the Soviet achievement comes just as the United States is mapping a chancy effort to save Skylab 4 from plunging into the earth's atmosphere and possibly causingan international incident similar to the one triggered by the Soviet's cosmos nuclear-powered that recently fell out of orbit over Canada, dripping radioactive debris on the tundra.

The Soviet manned space program, which lagged in the mid-1960s just as the Americans were gathering momentum to shoot for the moon, has now scored a series of solid new firsts. The new records began when Grechko and Romanenko were launched Dec. 10 and docked the following day with the Salyut. The station had orbited empty since September, when it was launched and an earlier attempt to dock a crew with it failed. That setback had come at an awkward time, just as the Soviets were hoping for a space spectacular to mark the 60th anniversary of the Bolshevik Revolution.

Romanenko and Grechko activated the spacelab's systems and soon were conducting experiments in such widely diverse fields as mettalurgy and astronomy, earth resources photography and space biology. They became almost regular fixtures on this country's nightly television news.

In January, they were joined for five days by a two-man team from a Soyuz ferry craft, who achieved the world's first double docking with a space station. Last month, Grechko and Romanenko were joined by an unmanned spacecraft, called Progress 1, that brought fuel, supplies, mail, and new experiments. For the first time in manned space history, the two successfully transfered dangerous feul and oxidizers from the cargo craft to the Salyut.

The Salyut weighs about 19 tons and is more than 75 feet long. Electric power comes from movable, winglike solar cells. Its living and working spaces are about 30 feet long and 12 feet wide and are painted in green, yellow and blue pastels.

There is not much living room for the longterm occupants, and when the newest visitors arrived last night the four looked positively cramped - but happy.

The Soviets said recently that Grechko and Romanenko had been specially screened for psychological balance to help insure against adverse emotional effects of long isolation in space. It was noted that both U.S. and Soviet spacemen on long missions suffered psychological depression about the 15th day of flight. It has been reported in the West that the previous longest Soviet flight, of 63 days, ended abruptly when one of the two-member crew began exhibiting signs of emotional strain.