THE BEST WAY to knock out an enemy satellite, the astrophysicist was telling a gathering of arms controllers, is to leave a trail of gravel in a path that will bisect the orbit of the satellite you want destroyed.

"This way you take advantage of the relative velocities of things in orbit," the physicst said matter-of-factly, explaining that the satellite would be smashed by the impact of colliding with the gravel at speeds as high as 10,000 miles an hour in frictionless space. "Besides, you're on the other side of the earth when it happens, nowhere near the gravel that was left behind and nowhere near the destruction it caused. There's no way you can be blame."

Time was when war in space was unthinkable. War in space meant unclear weapons, whose use in space was barred by the Outer Dpace Treaty of 1967. But in the 10 years since "weapons of mass destruciton" were banned, space technology has grown so fast that arms controllers believe a new treaty should be drawn up to keep war out of space.

"The Outer Space Treaty is obsolete," said one veteran arms control expert who attended a recent closed door seminar on the militarization of space. "There are so many military satellites in space and so many Buck Rogers methods of knocking them out today that the treaty is a joke."

Laying gravel in space is just one way of destroying a satelite in space. The Soviet Union for the eighth time recently tested a hunter-killer satelite that blows up alongside an enemy satellite and destroys it in a hail of shrapnel. The United States will soon test two killer satelites, one that fires canisters the size of tomato cans at an enemy and the other that colides head-on with its prey. NSC Study

AT A TIME when the satelite population is burgeoning, few experts know the outcome of an unprovoked attack in space. If one satellite were attraked there might be no response for lack of proof. But the "disappearance" of two or three satellites might provoke a hostile response.

"The loss of more than one satellite to an attack would be viewed in either Moscow or Washington with considerable alarm," one source close to the Central Intellignece Agency said. "It might take away one country's ability to police treaties like SALT and it could lead to a very cold resumption of the Cold War, replete wtih space gap theories and the like."

War in space is now one of the hottest topics in defense and intelligence circles. It's discussed behind closed doors in the White House, at CIA and the Pentagon just about every week. Hughes Aircragt, Lockheed, TRW and LTV all have big contracts to develop hunter-and'killer satellites.

The National Securtiy Council has undertaken a study to define what U.S. policy on space war should be in the years ahead. Among the questions being asked: What would happen if the White House proposed a ban on killer satellites at next round of SALT talks? What would happen if the White House went ahead with plans to develop a U.S. killer satellite?

In testimony before the House subcommittee on Europe and the Middle East, State Deparment special adviser Marshall D. Shulman suggested that the United States and the Soviet Union reach an agreement on killer satellites before the two superpowers perfect techniques to destroy satellites in space.

"There have been reports that the Soviet Union has been esperimenting with such capabilities," is the way Shulman put it to Congress the week before last," and if these were to continue the U.S. would clearly draw on its strong technologival base to develop capabilities at least as strong as those of the Soviet Union."

Two years ago, the "blinding" of a U.S. early warning satellite was attributed jprematurely to laser light beamed at the satellite from the Soviet Union. Blame was later traced to fires that broke out along a natural gas pipeline in Siberia, but the CIA is convinced the Soviets are two years away from testing laser light against spacecraft.

Lasers could be used against statellites in a variety of ways. One possible method would be for a laser beam to nudge an orbiting satellite, causing it to start tumbling in space so that its radio antennas lose contact with theirearthbound receivers.

The Pentagon worries about the laser threat because it is developing solar panels of an enemy satellite, destroying its power source. So seriously does the Pentagon take the laser threat that it has equipped three satellites with nuclear generators instead of the solar panels that present such tempting targets to lasers.

Space planners say that most if not all of the Pentagon's spy, navigation and communications satellites to be flown in the next 10 years will bet electricity from nuclear sources, which can be housed deep inside a satellite and surrounded by shielding against lasers.

A growing share of the money spent on space by the Defense Advanced Research Projects Agency will go to nuclear power sources and to methods of "hardening" electronics devices inside satellites to protect them from jamming by powerful radio beacons on land or at sea.

"There are those of us who worry more about false commands than any laser weapons," one intelligence source confides. "Lasers can't penetrate fog, cloud or rain. The right kind of radio signal can go through anything."

Imagine an American spy satellite orbited in a hurry to get pictures of Soviet ship deployments during a Middle East crisis. Imagine the Air Force about to signal the satellite to return its film package to earth. Then imagine a beaming a false signal to the satellite, causing the film package to land in the Soviet Union.

"There's talk of floating a cloud of aluminum particles around our military satellites to form protective screens against lasers and false commands," said one intelligence source."Schemes like that may sound far out, but let's face it . . . we're dealing with possibilities that are a lot farther out than aluminium smoke screens."

The United States is hard at work on other moves to counter attacks in space. The range of the space tracking network is being enlarged to pinpoint the whereabouts of the world's satellite traffic. A space attack warning system is under scrutiny. There is talk of disguising satellite missions, of using satellite decoys and of orbiting satellites that are "dark" - without radio beacons that can't be tracked.

"But it's much more difficult to protect a satellite than to destroy it," one intelligence source said. "There's almost no way to defend against the first use of a killer satellite. It will have all the advantage, plus the element of surprise." The End of Sanctuary

A GROWING NUMBER of space and arms control planners think the first use of a killer statellite is not too far away. It could come during a crisis, like a Mideast conflict. It could involve the use of a Soviet killer satellite to knock out a Chinese satellite. It could involve both Soviet and American killer satellites warring on each other.

There are now upward of 1,000 satellites from all nations in orbit, as many as 200 of them military. The Pentagon now spends almost $3 billion a year in space, and the CIA estimates that the Soviet Union spends four times the U.S. total in military space hardware and development.

More important, fresh arrivals will be drifting into space in the next five years. France and China have space programs, according to one expert, "that are overwhelmingly military." India has embarked on a program to develop satellites for spying, of navigation and communication, all within the military.

Australia, Great Britian and Canada also plan to orbit military satellites of some kind. Which brings to mind the prediction of Dr. Malcolm R. Currie when he left the Pentagon as director of Defense Research and Engineering two years ago: "Space is not going to remain the sanctuary it is today. It will not be unmolested territory."