Two Soviet cosmonauts successfully docked their Soyuz spacecraft with a long-empty orbiting space station today and then entered the space lab for what may be an attempt at breaking the U.S. record of 84 days in earth orbit.

The Soyuz 26, launched early yesterday morning, succeeded in approaching and joining the space lab through a rear-entry docking collar, according to Tass, the official news agency. An earlier attempt by another spacecraft in October to dock with the lab ended in failure, dashing Kremlin hopes to a space spectacular to mark the 60th anniversary of the Russian revolution and the 20th anniversary of space flight.

Tass said that cosmonauts Yuri Romanenko, 33, and Georgi Grechko, 46, docked on the "instrument section" of the Salyut, which it said was "on the opposite side" from the space station's transfer section. The existence of two docking stations on the space lab said Tass, "is important for replacing crews, for carrying out rescue operations and delivering foodstuffs and equipment to an orbital scientific lab."

The Salyut lab is configured with a docking, or transfer unit at one end, through which the cosmonauts crawl to rach main section of the station, where orbital and space experiments are carried out and the two-man crew lives.

In the past, Soyuz spacecraft, which are substantially smaller than the U.S. Apollo craft used for similar purposes, have experienced difficulties and failure in trying to dock with the Salyut at the so-called "transfer" end of the craft. Tass made it clear that the second entry port and the cosmonauts' ability to use it, has perhaps simplified Soviet docking techniques, which are for the most part controlled from the ground, independent of cosmonaut judgment.

The docking occurred at 6:02 am Moscow time and was reported more than four hours later by Tass. This type of delay is common in Soviet handling of their space program. The Salyut was launched Sept. 29 and the Soyuz 25 a few days later. The lab remained unmanned after the failure of that spacecraft to link up, and Tass reported only that the Salyut was functioning in an "automatic" mode and returning information to Soviet earth stations.

Today, Tass reported, "A piloted scientific station began functioning in orbit." The agency later said that when flight engineer Grechko and spacecraft to link up, and Tass reported only that thcommander Romanenko entered the lab, "this was the beginning of a new space expedition with plans for important and complicated work." A camera aboard relayed pictures of the pair to Soviet television.

Tass said that the experiments will include study of physical processes and phomena in outer space, exploration fo the earth's surface and atmosphere for data in the interests of the national economy, and other experiments of testing cosmonaut's adaptation to space conditions and proving out the onboard spacelab systems."

The Soviets have lagged behind the United States in the ability to research from space such important resources as metal deposits, soil conditions and harvest prospects. Presumbly the spacelab crew will seek improvements in these reporting methods. At the same time, the Soviet space program has delved far more deeply into the biological effects of prolonged space travel on humans than has the U.S. program.

The U.S. space program has turned away from manned orbital flight while the National Aeronautics and Space Administration conducts tests of a re-usable space shuttle, which has undergone some preliminary flight tests. The shuttle is envisioned as the primary manned space vehicle of the next decade.

Recently, American space scientists and officials of NASA were here to begin preliminary discussions of a joint space shuttle-Soyuz-Salyut mission in the next decade. The joint Apollo-Soyuz mission of 1975, in which the two spacecraft flew together for two days, is still popular here. Reminders can be found in everything from a special brand of cigaretts to book matches.

Soviet cosmonauts succeeded in spending 63 days aboard a Salyut in 1975. That mission was recalled abruptly after the crew apparently began to become disoriented. The Salyut and Soyuz craft linked togehter comprise a single unit weighing more than 25 tons and about 75 feet in length, a reasonably comfortable spacehome for extended periods, according to U.S. scientists.