MOSCOW, OCT. 3 -- Sometime after sunset on an autumn evening 30 years ago, a solitary bugler strode onto the apron of a launch pad on the steppes of Kazakhstan, near the Aral Sea, and blew a series of prolonged notes.

The haunting send-off was for nothing more than a radio transmitter hooked up to a thermometer and powered by a pack of chemical batteries, as Soviet and American chroniclers describe it. Soviet scientists had put the 184-pound device into the nose of an A7 rocket, known as Semyorka -- "Old Number Seven" -- that was originally designed to carry thermonuclear bombs.

That night, Oct. 4, 1957, Semyorka fired its 20 engines and blasted its humble cargo into history, with stunning symbolic impact throughout the world.

Now, on the 30th anniversary of the dawn of the space age, the Soviets again are trumpeting the benefits of replacing, in a sense, warheads with alternative, peaceful payloads.

This week, in an atmosphere of reduced tension between the two superpowers and a new openness here, the Soviets are hosting a space forum that has attracted about 400 leading space scientists, astronauts and officials from the world's spacefaring nations, including 130 from the United States.

It is the first such gathering in the Soviet capital and the visitors are getting the red carpet treatment, from caviar to ballet to an unusual tour of the cosmonaut training facilities north of Moscow.

As expected, the Soviets are using the anniversary of Sputnik I to trumpet their steady record of milestones in space at a time when the U.S. space effort is essentially grounded and struggling for direction.

Today, they noted that one of the cosmonauts aboard Mir, the first permanently manned space station, has set a record for human endurance in space, 240 days. Soviet scientists also announced the first sighting of X-rays from an unusually close supernova, or exploding star, in the Southern Hemisphere by their astrophysics module Kvant, which is docked to Mir.

At the same time, they are using the celebration to intensify an already aggressive push for unprecedented levels of peaceful international cooperation in space and, according to some, to strengthen the hand of the nonmilitary side of the Soviet space program.

"Everything that is done in space should be done in the interest of all of us," Gury Marchuk, head of the Soviet Academy of Sciences, told the gathering. "Cooperation, coordination would create a synergetic effect that would make things possible that are now impossible."

The Soviets, he said, are "resolutely against turning space into a new arena of the arms race."

Cosmonaut Vladimir Shatilov, addressing the gathering of scientists, astronauts and scientists at the cosmonaut training center, shrugged when asked about a recent statement by James C. Fletcher, head of the National Aeronautics and Space Administration, that there is a "race" to Mars.

"The reason for Fletcher's statement was to whip up the American space program a little and bring about a little healthy competition," he said with a smile. "We have no clandestine, secret plan to get the upper hand over the United States and get to Mars first."

The slogan appearing on all the conference literature is "Let's make cooperation, not war, in space."

Most of the scientists expressed eagerness to do just that. Roger Bonnet, chief scientist of the European Space Agency, told the gathering, "As our missions are becoming more and more expensive, and our resources are limited, cooperation is very important."

Roald Sagdeyev, the charismatic head of the Soviet Academy of Sciences' Space Research Institute and host of the forum, has asked attending scientists in each discipline -- planetary specialists, astronomers, astrophysicists and so on -- to come up with a plan for coordinating their respective projects in order to avoid overlap and conserve money and other resources.

Many of the European scientists here, desperate for the access to space that the Soviets offer in abundance, already are cooperating with the Russians on major projects. For example, 14 nations participated in the Soviets' spectacular Vega mission to Halley's Comet, in which the United States played no major role.

American scientists here say they are as eager as any to cooperate on a larger scale, but the U.S. government remains cool to most such overtures.

U.S. officials say many of the Soviet proposals for joint activities would enable them to pilfer the secrets of more sophisticated U.S. technology. And in any case, they add, the Soviets themselves remain too secretive in many areas.

Although the Soviets have not mentioned the issue specifically at the conference, some believe that President Reagan's Strategic Defense Initiative, or "Star Wars," is their main target. The administration has made it clear it will not abandon research on the project, and U.S. officials say the Soviets, despite their denials and talk of peace, are developing a space defense system of their own.

Some U.S. officials say privately, however, that in light of the improving climate between the two countries and a new agreement with the Soviets signed earlier this year to continue cooperating on low-level science projects, there is some chance for a bigger breakthrough eventually. In case that happens, said one, "It's important to put forward our views."

Although the cream of American space scientists is here, the administration declined to send a top-level delegation to the Soviet event because they did not want to contribute to a Soviet propaganda show, according to some.

The highest-ranking official in attendance is Sam Keller, NASA's deputy associate administrator in the office of space science. Addressing the forum, he stressed the amount of work yet to be done -- in science as well as in building public support in the United States -- before any major joint undertaking, such as a mission to Mars, can be considered.

Still, many called the gathering unique. "This is the first time all the space-capable countries have come together in Moscow," said historian John Logsdon of George Washington University. "It gives you an idea of who is defining the future."

The gathering also is helping Sagdeyev by demonstrating that his peaceful space projects are good public relations for his country, according to some scientists. "This many Americans coming here is a recognition of that," said a former NASA official.

The Soviets bused about 300 visitors to Star City, an hour's drive north of Moscow, a privileged, closed city where cosmonauts are trained.

In a museum there, 41 astronauts and cosmonauts from Europe, Mexico, the Soviet Union and the United States sat on a stage before a huge picture of Lenin and took questions from the audience of scientists, occasionally taking a democratic vote on the answer.

Author Carl Sagan had summed up the mood here earlier when he told the gathering that the exploration of space is a "mythic endeavor" and cited the long list of achievements of both superpowers in the relatively short time since the first Sputnik began to beep.

"The sum of the two," he said, "is the stuff of legend."