President Reagan's decision to demonstrate decisive American superiority in antisatellite weapons is a gamble that could alter the course of the superpowers' arms race, specialists said yesterday. Critics said the move could unravel arms-control efforts and jeopardize numerous American military operations, while supporters defended the decision as prudent in the face of Soviet programs.

The administration announced Tuesday that it plans to test an antisatellite (ASAT) weapon this fall. If the test succeeds, it will mean the United States has a capability far superior to the Soviet Union's, according to official assessments by the Central Intelligence Agency and others.

John Steinbrunner, director of foreign policy studies at the Brookings Institution, yesterday assailed Reagan's decision to go ahead with an ASAT test shortly before the November summit meeting with Mikhail Gorbachev as a "gun to the head approach" that will drive both superpowers into a military space race that "will leave both sides worse off."

"The whole strategic relationship of the United States and Soviet Union depends on getting some kind of antisatellite agreement," Steinbrunner said, referring to the superpowers' dependence on space satellites to observe what each other is doing with its strategic missiles and for many more mundane military missions.

Controlling weaponry in space "is the linch pin for all arms-control agreements," Steinbrunner said, because the Soviet Union is determined to try to head off an arms race in space.

Reagan's rebuttal to such attacks was contained in his report to Congress on Tuesday. He said the United States must develop the capability to combat "a growing threat posed by present and prospective Soviet satellites which, while not weapons themselves, are designed to support directly the USSR's terrestial forces in the event of conflict." The president added that the Soviets have at least one operational ASAT weapon.

The Soviets' operational antisatellite weapon threatens only U.S. military satellites that orbit at relatively low altitude over their launching sites.

Military satellites now at relatively low altitudes include several that watch Soviet facilities, collect intelligence data or help U.S. Navy ships navigate. Far in space, out of Soviet ASAT range, are others that control defense satellite communications and positioning of U.S. forces, including submarines.

Even in such small operations as the Grenada invasion and the bombing of Lebanon in 1983, satellites were used to provide reconnaissance and communications. The Pentagon is working on many ways to protect them, including parking them in deep space until needed closer to Earth, increasing their maneuverability and shielding them from attack.

U.S. ASAT critics, such as Steinbrunner, say the Soviets would be compelled to match a perfected U.S. weapon, ensuring a costly space arms race. If arms-control agreements halt competition, the strategic balance will be more easily stabilized, they say.

ASAT proponents say Soviet ability to destroy some U.S. satellites means America must develop a deterrent capacity.

According to a recent CIA report, "While the Soviets seek to be able to deny enemy use of space in wartime, current Soviet antisatellite capabilities are limited and fall short of meeting this apparent requirement."

Paul B. Stares wrote in "The Militarization of Space" that the Soviets conducted 20 tests with their antisatellite vehicle between 1968 and 1982, of which 11 failed. The Soviets have not tested the weapon since June 1982, according to military officials.

The Soviet weapon is launched by a relatively primitive liquid-fuel rocket and cannot be used on short notice. It follows its target through space in Earth orbit, blowing up when it approaches the target satellite.

The U.S. antisatellite weapon, by contrast, is a sophisticated hunter and killer that is highly mobile, because it can be launched from an F15 fighter plane that carries the two-stage boosting rocket to the edge of space. This puts Soviet satellites on different orbital routes at risk, a tremendous advance over the Soviet weapon.

The U.S. weapon, designated MV for miniature vehicle and built by the Vought Corp., appears to be a wonder of miniaturization and computerization, with a Gatling gun arrangement of infrared sensing devices to home in on the heat of the enemy satellite. The MV is designed to collide with the satellite going the orbital speed of 17,500 miles an hour, a crash that needs no explosive charge for sure destruction.

Concerned about the impact on arms-control talks of a successful test of the MV, Congress has repeatedly restricted its testing. The compromise fiscal 1986 military authorization bill pending in Congress allows three tests in the coming fiscal year.

The administration apparently concluded that a successful test this fall would strengthen rather than complicate Reagan's negotiating hand at the summit.