The shuttle Atlantis' five-member crew rocketed into orbit Friday on a mission to launch a $617 million space observatory that will peer into the hellish cores of exploding suns, brilliant quasars and other cosmic enigmas.

Two of Atlantis' astronauts also plan to take the first U.S. spacewalk in more than five years on Monday - an action-packed payload bay excursion to test space station construction techniques.With the 35,000-pound Gamma Ray Observatory nestled in its payload bay, Atlantis blasted off with a ground-shaking roar at 9:23 a.m. EST - five minutes late because of low clouds - kicking off the first of six shuttle flights planned for 1991.

"Two, one, zero and liftoff of the space shuttle Atlantis and of Gamma Ray Observa-tory seeking out the explosive forces of the universe," said launch commentator George Diller as Atlantis majestically climbed through a partly cloudy sky.

With commander Steven Nagel, 44, and co-pilot Kenneth Cameron, 41, at the controls, Atlantis rolled smoothly about its vertical axis and arced east over the Atlantic Ocean, thrilling thousands of spectators as the four-man, one-woman crew thundered away on a five-day mission.

Joining Nagel and Cameron aboard Atlantis for the 39th shuttle flight were Linda Godwin, 38, an expert operator of the shuttle's robot arm, and spacewalkers Jerry Ross, 43, and Jay Apt, 41.

Eight and a half minutes later, Atlantis' three main engines shut down, putting the ship in its planned preliminary orbit. A rocket firing 42 minutes into the flight was scheduled to circularize the shuttle's orbit at an altitude of 280 miles.

The primary goals of the 39th shuttle mission are the deployment of the Gamma Ray Observatory - GRO - satellite Sunday and a spacewalk the day after by Ross and Apt, the first such excursion by American astronauts since 1985.

If all goes well, Nagel and Cameron will guide Atlantis to a touchdown Wednesday morning on a concrete runway at Edwards Air Force Base in the Mojave Desert northeast of Los Angeles.

Atlantis' mission marks the first of three shuttle launches planned over the next 50 days, a record flurry of flights triggered by cracks in critical fuel line doors that forced NASA to delay a March launch by the shuttle Discovery.

While Atlantis had similar cracks, they were deemed too small to pose a safety threat and the ship was cleared for takeoff as is. The now-repaired Discovery, already mounted on nearby pad 39-A, is scheduled for launch April 25 with the shuttle Columbia set to follow suit in late May.

Nagel and company faced a light first day in orbit. Along with activating on-board experiments, Godwin planned to use television cameras on Atlantis' mechanical arm to inspect the 17.5-ton Gamma Ray Observatory, the heaviest civilian cargo ever carried aloft by a space shuttle.

The $617 million GRO satellite is the second in a planned series of four "great observatories" built to study the cosmos with electronic eyes sensitive to low-energy infrared, visible light and ultraviolet emissions as well as X-rays and even more powerful gamma rays.

With the $1.5 billion Hubble Space Telescope already in orbit snapping pictures in visible and ultraviolet light, four massive instruments aboard GRO will record extremely high-energy gamma rays from the most violent objects in the universe.