Telescopes to be built by the United States in the next 10 years should tell astronomers whether planets as small as earth circle any of the 30 or 40 stars nearest our own sun.

"By the year 2000, we ought to be able to spot planets the size of Jupiter or Saturn out to a distance of 30 light years," Cornell University's Dr. Frank Drake said yesterday at a press conference here beginning the 144th annual meeting of the American Association for the Advancement of Science. "That takes in about 400 stars, the most distant being almost 180 trillion miles away."

Drake said the science of astronomy is entering an era of even greater promise than the last 20 years, when most of the astronomical discoveries in history were made. He cited an "army" bright young people now in astronomy and an enormous array of new tools being put at their disposal.

"Ninety percent of all the astronomers who ever lived are alive today," Drake said. "They're highly trained, some of the best mids we've ever had, and their medan age is 34."

Among the new tools Drake mentioned is a space telescope 71/2 feet in diameter that will be dropped off in earth orbit by astronauts flying the space shuttle in 1983. Drake said this telescope has the potential to identify for the first time planets no bigger than Earth "around the nearest 30 or 40 stars."

Some astronometers have already seen what they believe to be planets circling a star called Barnard's Star, which lies only 5.6 light years from Earth. Drake said these "controversial" observations would be cheared up one way or another in less than 10 years by the large space telescope.

A telescope being built in Arizona will be able to detect the faint "wobble" of nearby stars as neighboring planets pass them and exert a gravitational pull. The same Arizona telescope might even spot planets the size of the giant Jupiter if they exist around nearby stars, though it would be difficult to do through the haze of Earht's atmosphere.

Drake described the Arizona telescope an an array of six mirrors, each six feet across, arranged in hexagon on top of Mount Hopkins near Tucson. Drake said the six giant mirrors together will be the optical equal of a single telescope 175 inches across, which will make the Mount Hopkins telescope the world's third largest when it goes into service this spring.

The first mirrored array to be built in the United States, the Mount Hopkins six are already being called "rubber mirror" because of the ways they will have to "bounce" to compensate for slight wind and gravity chandes.

"Our goal is to have an array of hundreds of mirrors that would be the equivalent of an 82-foot optical telescope," Drake said." "An array that big would involve the use of lasers just to keep the mirrors in unison, but the result would be more than four times the light-gathering capability of the largest optical telescope in the world."

Other than optical telescopes, Drake said, the greatest new tool the nation's astronomers will use is the "Very Large Array" of radio telescopes, each 82 feet across, now built in remote region of New Mexico.

Eleven of these are already in operation. When all 27 are built they will describe a Y shape whose three legs will reach out 13 miles in each direction.