As many as 10 unmanned spacecraft from the United States and the Soviet Union will visit the planet Venus late this year, according to plans disclosed yesterday by officials of the National Aeronautics and space Administration.

Six of the visiting spacecraft will be American, sent to the mysterious and cloud-covered planet by a pair of Pioneer-Venus spacecraft to be launched later this month and late in August from Cape Canaveral. The first Pioneer is to orbit Venus Dec. 4, and the second is to leave from a spaceborne "bus" one week later that will drop four probes into the atmosphere of Venus.

Soviet plans for Venus are still vague to U.S. planners, but NASA Planetary Programs Director A. Thomas Young said it is his understanding that the Soviets in August will launch two spacecraft that will separate into four spacecraft when they arrive at Venus late this year.

"We think that two of their spacecraft will fly by the planet and the other two will attempt to land on Venus," Young said after a news briefing at space agency headquarters yesterday. "We don't know yet when the Soviets will get to Venus, but we think it will be a little later than our own arrival."

The first U.S. launch to Venus is scheduled to take place May 20, when an Atlas-Centaur rocket is to lift off from Cape Canaveral with a Pioneer spacecraft aboard that's been built to fly into orbit around Venus in December. If successful, it would be the first U.S. spacecraft to orbit Venus.

The second Pioneer launch is set for Aug. 7, and is to carry to Venus a drum-shaped spacecraft "bus" that will piggy-back four proves to Venus. Three probes are small, weighting 200 pounds apiece. The fourth probe is more than 3 times that size.

The largest probe is to be dropped by parachute along the equator of Venus on the day side of the planet. Its parachute will drop off about 30 miles above the surface, allowing the bullet-shaped spacecraft to float down and take measurements of the thick Venusian atmosphere.

Two of the small probes will be aimed at the night side of the planet, near the north and south poles. The third small probe will be directed to the day side, even closer to the south pole than the other small probe. All three of the small probes will measure temperature and pressure of the atmosphere in different regions of Venus.

The Pioneer Orbiter that will arrive at Venus first is to stay in orbit around Venus for at least a year. It will map the cloud-covered surface with radar and take pictures of the entire planet in ultraviolet and infrared light.

U.S. space officials do not conflict in the arrivals of both U.S. and Soviet spacecraft at Venus in the same month. NASA's Young said that if talks between the two countries go well next month, the United States may be able to provide enough information about the planet so the Soviets can better target the two apacecraft they hope to land on the surface of Venus.