The United States has quietly decided to build a new weapon that could destroy Soviet satellites in space.

The U.S. policy decision represents a giant step toward building weapons for deterring and, if that fails, fighting a war in space.

In the 1972 anti-ballistic missile treaty, the United States and Soviet Union agreed not to build weapons that would interfere with "national technical means of verification" - meaning, among other things, satellites. Pentagon spokesman Thomas B. Ross, when asked about the new system, said the treaty did not prohibittests.

He declined to confirm or deny that the Pentagon is now embarked on an anti-satellite weapon system.

The Air Force has awarded the Vought Corp. of Dallas a $58.7 million contract to build a satellite killer that is as simple as it is secret.

Basically, the satellite killer is a highly maneuverable, heat-sensing vehicle resembling a tomato can that would ram an enemy satellite as both vehicles fly thousands of miles an hour.

The satellite killer would carry no explosives but rely instead on the head-on collision to destroy the satellite.

Because the satellite killer will be so small, a cylinder about a foot long and eight inches in diameter, any number of rockets could carry it up to space where satellites fly or hover, depending on their mission.

Once hurled into space near the target, the killer would thrust itself on its rocket power into the enemy satellite. The rocket motors would make the tiny satellite killer highly maneuverable.

Although temperatures in space range hundreds of degrees above and below zero, the satellite killer would home in on the difference in temperature between the metal satellite and its surroundings.

Pentagon sources said that a primitive version of the satellite killer already has demonstrated in free-fall tests inside an airplane hangar that its heat seeker and guidance system can activate themselves in mid-air and hit the target.

Vought is expected to have a battle version of the satellite killer ready to test in space in about two years.

U.S. security has become increasingly dependent on satellites in the 20 years since the space age dawned with the Soviet launching of Sputnik I Oct. 4, 1957.

The United States spies from space on the Soviet Union and other nations with picture taking satellites.Other satellites hang in space to warn of Soviet rocket launches as sensors detect their heat. U.S. ships, submarines and planes navigate by satellites. Military leaders communicate around the world by satellite.

In a diplomatic crisis or war, U.S. military leaders long have feared that the Soviet Union would blind American spy satellites and destroy navigation and communications satellites.

These fears have intensified in recent years as military leaders concluded that a series of Soviet space shots were tests of hunter-killer satellites. The Soviet killer apparently is a satellite loaded with nonnuclear explosive that blows up when it gets near the target satellite.

Although years ago the United States put Thor rockets on Kwajalein Island designed to hit satellites that flew within their limited reach, the United States has not yet built and tested any anti-satellite weapons comparable to those the Soviet Union is believed to be developing.

William J. Perry, Pentagon research chief, said in a recent interview with The Washington Post taht the two basic options for responding to the Soviet satellite threat were to build anti-satellite weapons like the Russians have done or give added protection to American satellites.

He said the Pentagon was leaning toward the hardening of American satellites by such measures as making their wiring less vulnerable. But the Vought contract signifies that the Pentagon is opting for both offensive and defensive satellite programs.

This step into offensive measures was carefully camouflaged by the Air Force in this Sept. 8 contract announcement issued by the Pentagon. "Vought Corp., Dallas, TX, is beign awarded a $58,720,738 cost plus incentive fee contract, of which $1,000,000 is being obligated today, for the development and test of hardware in support of the Space Defense Technology Program."

The 1967 outer space treaty between the United States and Soviet Union prohibits both countries from putting in orbit any objects "carrying nuclear weapons or any other kinds of weapons of mass destruction . . ."

Given those restrictions, U.S. space weapon specialists in the 1960s studied anti-satellite weapons using nonnuclear explosives. That concept eventually gave way to the idea of arranging head-on collisions in space without explosives of any kind. Infrared heat seekers for anti-satellite vehicles were perfected to the point at which it appeared likely toat a single cylindrical vehicle could destroy a satellite. The cylinders are so light that one surplus Scout or Minuteman rocket could life several at once into space to hit satellites.

The Soviet Anti-satellite tests and the Pentagon's new contract with Vought suggest that both governments believe the treaty does not prohibit the two countries from shooting at their own target satellites in space as they test anti-satellite weapons.

Specialists working on the anti-satellite program said the rocket carrying the cylinder could be fired from the ground or an airplane. The rocket would be programmed to put the anti-satellite weapon close to the target.

Having the anti-satellite weapon in hand, these specialists argued, would deter Russia from waging space warfare. Critics countered that this new step by the Air Force will make it easier for the Army to increase its research on anti-ballistic missiles, possibly eroding the treaty in the process.