The Soviet Union today launched an unmanned cargo-carrying spacecraft to dock with the manned, orbiting Salyut space station, thus taking one more hopeful step toward their goal of assembling large laboratories beyond the earth's atmosphere.

Tass, the Soviet news agency, said the unmanned Progress One cargo craft carries "fuel for the [Salyut] power units and different cargoes - equipment, apparatus, and materials for life support of the crew and for scientific exploration and experiments."

The launch follows the completion earlier this week of a successful roundtrip to the Salyut by two cosmonauts who spent five days in space with the longterm crew and then returned to earth. The Salyut crew, Lt. Col. Yuri Romanenko and flight engineer Georgi Grechko, have been aboard since Dec. 11, 42 days. They have become almost permanent features of the nation's nightly television news, which shows them in good spirits and trading homilies with the ground controllers.

The docking of the Progress could occur within about 26 hours of launch, according to observers here. Tass did not say, nor could it be found out, whether the fuel and "materials for life support" were crucial for continued habitation of the Salyut or not.

The Soviets have hailed this Salyut as the first step toward their goal of massive, permanent, manned stations orbiting the earth, assembled from smaller components terried up to it.

The Soviets said the unmanned Progress One was built "on the basis of a manned Soyuz," the standard two-man Soviet spacecraft in use this decade. The Soviets, with their smaller payloads, have banked their hopes for continued space exploration on assembling large space complexes from smaller components.

Cosmonaut Oleg Makarov, in a Tass interview said "1978 will be a turning point in the Soviet program of explorations in a near-earth orbit . . . It is quite clear that expeditions should be constantly replenished with fuel and scientific materials, air and water to enable the cosmonauts to work for long in orbital stations."

The new cargo craft was orbited "to test all possibilities for uninterrupted and economic supply of explorers in near earth orbits," he said.

The Soviets have long relied on docking procedures between spacecraft that are almost wholly automatic, in part because they have not had the capability for carrying into space the computers needed to solve orbital intercept problems. Direction has been from the ground instead.