The first American spacecraft built to orbit the planet Venus raced away from Cape Canaveral yesterday on the first leg of a journey that will cover 300 million miles.

The 1,280-pound Pioneer Venus left the Kennedy Space Center atop an Atlas-Centaur rocket at 9:13 a.m. EDT and flew into a parking orbit 115 miles above the earth, where its Centaur engine was fired a second time to send it outward toward Venus. The spacecraft will swing outside the earth's orbit and then inside it on a path that will take seven months to reach Venus.

When it arrives at the planet Dec. 4, Pioneer Venus will be placed into an egg-shaped orbit that will take it as close as 90 miles to the surface of Venus. At this low point, the spacecraft will fly through the thin clouds in the planet's upper atmosphere.

It is the dense carbon dioxide atmosphere of Venus that most intrigues scientists and does the most to make Venus different from the earth. Even though Venus and the earth are "sister" planets in size and mass, the earth is water-rich and supports life while Venus is hot-dry and desolate.

Scientists want know why Venus (900 degree at its surface) is more than 10 times hotter than the earth, why the temperature at the poles of Venus apparently is only 18 degrees cooler than at the equator and why its day and night-time temperatures seem to better same.

These are some of the questions that Pioneer Venus will attempt to answer during eight months in orbit around the planet. The spacecraft is equipped with 12 instruments, including cameras to photograph the clouds of Venus in ultra-violet and infrared light and the first planetary radar to get a contour map of the surface of Venus.

One of the biggest questions about Venus concerns its weather - why does the climate on Venus never appear to vary? Why is the climate apparently the same day in and day out, month after month?

"Venus is one large machine where the weather and the climate are one and the same thing," Pioneer Project Scientist Lawrence Colin explains. "It may be that the easiest way to study meteorology is at the planet Venus."

The Pioneer that left yesterday will be followed by a second to be launched Aug. 7 from Cape Canaveral. The second craft, on a more direct trajectory, is to arrive at the planet a week after the first, but instead of going into orbit it will drop four probes into the atmosphere of Venus.

Three identical probes will fall into the planet's atmosphere, two on the night side at high-northern and mid-southern latitudes and the third on the day side at a mid-southern latitude. The fourth probe will be parachuted through the atmosphere on the day side at the equator.

The two Pioneer spacecraft will be joined at Venus by two Soviet spacecraft, the first of which is expected to leave the Soviet Union in August. Both Soviet spacecraft are built to land on the surface of Venus. None of the American craft is designed to survive a landing.

Cost of the two Pioneers is $175 million, including operations and tracking.