The first commercially built orbital rocket to be launched from a National Aeronautics and Space Administration facility on the Virginia shore exploded shortly after liftoff last night, likely destroying a payload of 14 scientific experiments and dealing another blow to the nation's fledgling commercial space industry.

The $75 million, five-story Conestoga rocket, designed by a Prince George's County firm, was blown up by NASA controllers 45 seconds into its flight when it began veering off course, officials said. NASA officials and representatives for the firm, EER Systems Inc., said last night they had not yet determined what caused the 100-ton rocket to change its angle of ascent.

"There are dozens of possibilities as to what happened," said EER spokesman Mike Bryant.

The rocket, launched from the NASA Wallops Island Flight Facility on the Delmarva Peninsula, turned into smoke and flaming debris over the Atlantic Ocean about 12 miles off the coast at an altitude of 25,000 feet, said NASA spokesman David Steitz. No one was injured.

"It was a beautiful liftoff. It looked great and then there was this major malfunction," said Steitz, who witnessed the explosion. Steitz said he saw three of the rocket's four strap-on booster engines ignite and detach from the rocket shortly before the explosion.

Coast Guard boats trolled the Atlantic last night for debris that might yield clues to the cause of the malfunction. Although the rocket's payload module was not located, Steitz said it "normally would not survive this type of explosion."

The explosion is a setback for scientists, business leaders and state officials who are trying to transform the Wallops Island facility, a frequent target of budget-cutters in recent years, into a commercial spaceport that backers say would eventually attract dozens of private launches each year and pump as much as $50 million into the economy of the rural lower peninsula.

The Conestoga launch took place from a $5 million complex EER built on NASA property. The company's launch pad and tower are the first such commercially built facilities in the United States.

Last night's failure was also the latest in a string of mishaps to plague private companies, several of which are in the Washington area, trying to enter the commercial launch business.

In August, a 65-foot rocket built by Bethesda-based Lockheed Martin Corp. was blown up shortly after its launch from Vandenberg Air Force Base in California when it began tilting from side to side. In 1994 and again this year, Pegasus rockets built by Orbital Sciences Corp., of Sterling, were destroyed because of flight problems.

"It's a setback and a disappointment at a time when the commercial launch industry is having a heck of a time getting stuff into space," Steitz said.

EER officials said last night they will investigate the explosion with NASA and the Department of Transportation, which regulates commercial space activities.

The explosion occurred before a crowd estimated by NASA officials at 5,000 that had gathered in marshland on the edge of the launch area.

The launch of the Conestoga rocket, developed by EER at a cost of $100 million, had been delayed six times. In August, a planned launch was scrubbed 98 seconds before liftoff because of a loss of pressure in two of the rocket's engine steering nozzles.

The rocket is designed to carry small to medium-size payloads into orbit, eventually to a space station.

Eight of the 14 experiments on board belonged to NASA. Among the space agency's experiments were tests of crystal growth in zero gravity and a study of how weightlessness affects plant life.

Some of the experiments were supposed to stay in orbit aboard a small spacecraft called METEOR-1 for as long as two years, relaying data on a daily basis to an EER control center in McLean.