Europe's unmanned Ariane rocket booster has begun to give the more expensive, manned U.S. space shuttle stiff competition for the $500 million-a-year international commercial space satellite business.

For the first time since they began competing, Ariane and the shuttle came out almost even in the number of satellite orders in 1984 for future flights. The shuttle signed five orders and Ariane four, a sharp change from the three-to-one edge the shuttle held over Ariane each year since 1980.

"I don't mind telling you I worry about Ariane," NASA Administrator James M. Beggs said in an interview. "This is the first time we've had to split the market with them, and it worries me."

The shuttle lost not only foreign business but a piece of its domestic market. Ariane last year signed up Satellite Business Systems of McLean for one of its future communications satellites. Ariane also received a letter of intent from the Direct Broadcast Satellite Corp. reserving cargo space for two satellites to carry direct television broadcasts to the U.S. pay television market.

Ariane also split two orders with the shuttle for Australian communications satellites and won the order for an Intelsat 6 in a toe-to-toe competition with the shuttle that one source described as fierce.

The shuttle still is ahead in the race for new satellite business, however. Indonesia chose the shuttle for its Palapa 3 satellite, and Great Britain's Ministry of Defense chose the shuttle for two Skynet communications satellites. The shuttle also signed three U.S. companies for two satellite launches apiece: U.S. Satellite Systems, Satellite Television Corp. and American Satellite Co.

Foreign and commercial satellite users still lean toward the shuttle. Through 1995, the shuttle has firm contracts for 34 and reservations for almost 100 satellites. Ariane has contracts for 30 satellites and reserved cargo space for 11 more.

The National Aeronautics and Space Administration long has assumed it will win as much as 75 percent of the foreign and commercial satellite market, a figure it has used to justify to Congress making 24 shuttle flights a year starting in 1989 at the latest. NASA said it hoped the foreign and commercial market would fill the cargo bays of as many as six shuttle flights per year. The other 18 would carry scientific and military satellites, the backbone of the shuttle's market.

But if it got only 60 percent of the foreign and commercial market, the shuttle's flight rate might not reach 24 a year until the mid-1990s, when a permanent space station will require six service and supply flights a year. A lower flight rate means higher shuttle operating costs, which could mean higher prices to customers.

Price is at the heart of the new competition. Ariane charges customers $23 million to $24 million to launch a satellite. NASA's base price in 1986 goes up to $71 million for a full cargo bay, which translates into as much as $35 million per satellite.

NASA says Ariane's prices are subsidized by the 11 member nations of the European Space Agency, which paid for the design and development of the Ariane rocket booster. Ariane officials say this is no different from NASA paying for development of the shuttle.

"Ariane Space gets no help for its operating expenses from ESA," said Douglas Heyden, executive vice president of Ariane Space Inc., the U.S. subsidiary of Ariane. "We have to collect the revenue from our customers to pay our bills."

Heyden said Ariane's advantage lies in its efficiency, its launch location near the equator and its no-frills unmanned rocket needing no expensive and heavy equipment to support astronauts.

"A launch from the ESA spaceport in French Guiana costs us about $40 million," Heyden said. "I believe NASA is still quoting a shuttle launch at somewhere near $150 million."

Launching Ariane only 7 degrees away from the equator, Heyden said, means a 10 percent performance advantage over the shuttle's launch from Florida's Cape Canaveral because the Earth's higher rotational speed near the equator gives Ariane an extra boost into orbit. This lowers Ariane's weight-carrying requirements.