The first spacecraft built by the United States to observe solar flares yesterday flew into Earth orbit, where for the next two years it will study the sun during one of its most intense periods of flare activity.

At 10:57 a.m. a 5,200 pound satellite named the Solar Maximum Mission Observatory sped away from Cape Canaveral aboard a Delta rocket into a circular orbit 360 miles above the Equator.

Carrying seven instruments that study the gamma rays, X-rays and ultraviolet rays that pour off the sun, the satellite is to be used for the sole purpose of watching the flares that erupt from time to time on the surface of the sun and release as much energy as a 100 trillion kiloton nuclear blast.

That's enough energy to cause widespread radio blackouts across the Earth, disrupt communications satellite traffic, overheat the atmosphere for days at a time and trigger global magnetic storms.

"The flare is probably the most energetic manifestation of changes in the sun that we know about," said Dr. Harold Glazier, director of the Solar-Terestrial Division of the National Aeronautics and Space Administration. "It's very important for us to understand the physics of the solar flare and to understand how flares come about."

The 11-year cycle in which solar flares wax and wane is about to enter its most turbulent period, during which weak flares erupt on the sun once or twice every day and the huge flares that cause atmospheric storms on Earth occur once every month.

Solar flaring is expected to peak in April, May and June, when four or five big flares may break out on the sun's surface. Scientists have said that the current cycle is one of the most active since 17th Century astronomers began keeping records of solar observation.

When a flare erupts on the surface of the sun, scientists suspect that nuclear processes beneath the surface suddenly release a torrent of electrons that break through the solar atmosphere and generate huge amounts of superhot X-rays.

Scientists believe that the flood of electrons and X-rays produced by the flare cascades through the solar atmosphere with such force that it carries part of the sun's gaseous shell with it. Scientists have literally detected parts of the sun being blown past the Earth just after a major solar flare has erupted.

The Solar Maximum Mission put into orbit yesterday cost the United States $99 million, including $20 million for the Delta rocket that carried it from Earth.

The satellite is the first equipped with a grapple so it can be retrieved in Earth orbit by the nascent space shuttle's robot arms and brought back to Earth. If the space shuttle astronauts retrieve the satellite in late 1982, it can be used all over again at a saving of $20 million to $50 million.