Since the first of the year, while the United States has been hit by three consecutive rocket disasters that have grounded its space program, the Soviet Union has launched 43 rockets carrying 57 satellites with no known failures, according to government figures.

This string of successes has reinforced the Soviets' edge in world rocketry. The Soviets launched 98 rockets in 1985 compared with 17 for the United States and three for the European Space Agency. The Soviets this year have also conducted a highly publicized probe of Halley's Comet, put up the new Mir space station and, since March 13, have had two cosmonauts in orbit on a mission that is expected to break the 237-day world record for a stay in space.

The Soviet advances drew new attention earlier this month when "Jane's Space Flight Directory" declared that the Soviets had taken an "almost frightening" 10-year lead over the United States in space exploration, an edge that some experts say has been compounded by an expected two-year delay in shuttle flights as a result of the recommendations of the presidential commission on the Challenger disaster.

"The Soviets are so far ahead of the U.S. in space experience that they are almost out of sight," wrote Reginald Turnill, editor of the 453-page Jane's publication.

The National Aeronautics and Space Administration officially disputed the Jane's account, saying that while the Soviets have made "impressive strides" in space, their overall program "tends to be conservative and evolutionary." The agency, in a statement, noted that the Soviets have yet to fly their version of a shuttle-like vehicle while the United States has demonstrated an ability to retrieve and repair satellites in space that is "well beyond anything the Soviets have done."

Soviet space analysts, aerospace experts and U.S. officials agreed in interviews that the Jane's assessment is probably overstated. But at the same time, they say, the Soviets have made breakthroughs in a range of space-related areas -- including man-hours in space, materials and biological processing experiments, orbiting space stations and development of an untested heavy-lift launch vehicle -- that threaten U.S. technological preeminence and have potential national security implications.

"This has been quite a stellar year for the Soviets," said Marcia S. Smith, president of the American Astronautical Society. "They've gone out on two spacewalks, they have two space stations in orbit and they've tested a new spacecraft that can be used to ferry crews up into orbit . . . . Clearly, the Soviets are ahead of us in space operational areas."

"What the Soviets have done, in their typical plodding fashion, is to keep plugging away," said Sen. John Glenn (D-Ohio), a former astronaut. "They recognize the importance of space for basic research and they have an appreciation of the importance of being in this for the long haul . . . . And here we are, coming under increasing competition around the world in basic research, and we're questioning whether we even want to go forward or not."

Many experts contend the Soviets are moving steadily toward a long-range goal of a manned mission to Mars -- and because of recent advances, they may be in a position to do so well before the United States, possibly by the turn of the century.

Most notably, they have repeatedly pushed back the frontiers of longevity in space with six- to eight-month manned missions that appear primarily designed to test biological responses to the prolonged weightlessness of interplanetary flight. A round trip to Mars would take about one year.

"They're certainly putting together the building blocks for doing it a manned mission to Mars ," said John Pike, a space analyst for the Federation of American Scientists. "But it's hard to know what they're going after other than proving the superiority of the socialist system."

Other Soviet space advances have more practical advantages. Soviet cosmonauts have used their space stations to conduct extensive scientific work manufacturing crystals for semiconductor computer chips, to develop a new influenza vaccine and experiment with other biological substances -- the kind of work that has been planned by U.S. companies for the shuttle.

"The Soviets have done thousands of experiments that we haven't had a chance to do," said Nicholas L. Johnson, advisory scientist with Teledyne Brown Engineering, a Colorado-based firm that monitors the Soviet space program.

The Soviets, like the Chinese, have also made fledgling efforts to break into the commercial launch business, a potentially lucrative endeavor now that the West's launch capacity has been crippled by the U.S. accidents and the May 30 explosion of the European Ariane rocket. The Soviets have already launched payloads for France, India and Sweden, and last year they bid to launch a satellite of Inmarsat, the London-based international maritime satellite organization.

Recently, the Soviets stepped up efforts, apparently to take advantage of a business opportunity and reap a public relations bonanza. In a letter to United Nations Secretary General Javier Perez de Cuellar that proposed creation of a World Space Organization, Soviet Prime Minister Nikolai Ryzkhov announced that the Kremlin stood ready "to launch peaceful space vehicles of other countries and international organizations with Soviet carrier rockets on mutually acceptable terms."

The Soviet ability to break into the commercial launch business is strengthened by a fleet of at least four reliable expendable rockets that in recent years have been launched five times as often as all U.S. rockets combined, including the shuttle. According to Pentagon figures, the Soviets surpassed the annual U.S. launch rate in the mid-1960s and, over the past six years, have launched 533 rockets compared with 98 for the United States.

Many U.S. experts discount these figures, noting that Soviet communications, early-warning and photo-reconnaissance satellites have much shorter lifespans than their U.S. counterparts, forcing the Soviets to launch more frequently.

"These launch rates, paradoxically, demonstrate the comparative strength of the American space program," Pike said. "In the photo-reconnaissance area, our satellites last a couple of years and their's last only a few months."

But experts also say the Soviets have made new strides in gradually extending the lives of their satellites. And, more importantly, they are not dependent on a single launch vehicle while the United States phased out expendable unmanned vehicles and has now found itself almost totally dependent on a grounded shuttle.

This points to the real difference between the two countries, said Thomas Paine, chairman of the presidential National Commission on Space. "The Soviets have a real national space policy while we're coasting on decisions made over a decade ago," he said.