After a seven-month journey from Earth, the first of five Pioneer spacecraft built to explore Venus flew into orbit yesterday around the planet whose clouds are deeper than any of Earth's oceans.

Following a path that will take it over the notth and south poles of Venus once a day, the drum-shaped Pioneer became the first American space craft to orbit Venus, just after 11 a.m. EST. The 810-pound Orbiter was in an elliptical orbit -- as cole to the planet as 250 miles and as far away as 42,000 miles.

"Everything went really good today," Pioneer Venus Project manager Charles F. Hall said at Ames Research Center in Montainview, Calif., where the mission to Venus is being directed. "Obviously, we're all very happy."

The four other Pioneers will be flown this Saturday into the thick clouds that cover the entire planet. A 700-pound space craft will be parachuted through the atmosphere while three smaller craft (200 pounds each) will free-fall to the surface. They will measure the Venusian atmosphere at four different spots on the planet, two of them on the night side.

The orbiter will serve as radio relay for the four probes that will descend to the surface on Saturday, and will also conduct scientific observations of its own for most of the next year. The orbiter carries 11 scientific instruments, including three cameras and a radar device to pierce the thick clouds for a look at the planet's surface.

The radar was out of sight of Earth last night for the first scan of a part of Venus never observed by radars on Earth -- two strips of the surface near the equator.

Today, one of the three cameras is to take a picture of the planet, when the orbiter nears its farthest point from Venus, about 41,600 miles away.

Scientists cautioned reporters not to expect too much of the first picture, because no more than a crescent of the planet would be lit by the sun at the time of the photograph. Venus should look like a twoday-old moon, no more.

The orbiter's two other cameras will "photograph" the tops of the clouds in ultraviolet and infrared light. This will produce maps of cloudtop temperatures rather than pictures.

Four instruments on the orbiter will measure the makeup and temperature of the upper atmosphere of Venus, the only planet in the solar system whose cloudtops are bombarded directly by the solar wind that streams off the sun.Three other instruments will measure the solar wind itself, right at the tops of the clouds where it strikes the planet day after day.

"Unlike other planets with thick atmospheres, like Earth and Jupiter, the solar wind acts directly with Venus," said project scientist Lawrence Colin. "This direct interaction should change the makeup of the atmosphere of Venus, and with our seven instruments we should be able to tell how and why."