Why not be hopeful about Geneva? After all, Ronald Reagan has been reelected by a margin that gives him a free hand to strike out in any direction he pleases. And he would please the public, according to the polls, if he reaches an agreement with the Soviets. Further, it is said, the First Lady is focusing on her husband's legacy and thinks "peace president" would look great carved in marble.

And is it not true that the Soviets have come crawling back to the table, having vowed a year ago they never would while the Euromissiles were in place?

The Earthly circumstances seem promising. But it is the celestial dimension that shadows the meetings. Our negotiators go in with "Star Wars," a new and ominous "defense system" tucked underneath their arms.

When Reagan first proclaimed his Strategic Defense Initiative in March 1983, there was much head-scratching among the military and derision among scientists. Its slogan was a variation on the wildly successful gun-lobby retort: "Guns don't kill people; people kill people." Star Wars was the benign technological breakthrough, a system of lasers and satellites to kill weapons, not people.

Since then, it has undergone changes. Even its friends admit that as a shield it would be as leaky as the domestic "safety net." But, they say, at least it will protect our missiles and give the Soviets second thoughts about launching an all-out attack.

In Washington, which is exceptionally hospitable to preposterous ideas that have the president's backing, a loony notion is acquiring respectability. One-time critics are keeping their lines to the administration open by claiming that whatever its shortcomings, the initiative merits a full-court research press.

Some people say the impetus comes from the two U.S. weapons-design laboratories. Hugh DeWitt, a non-conforming theoretical physicist at the Lawrence Livermore National Laboratory, wrote in the Bulletin of the Atomic Scientists that "the scientists in these laboratories are a major force in driving and perpetuating the nuclear arms race."

The record bears him out. The impetus for Star Wars comes out of Livermore, out of the mouth of Edward Teller, father of the H-bomb, a melodramatic Hungarian of deepest anticommunist dye. It was after a conversation with him that Reagan became possessed with the idea of transferring the arms race to the skies.

Another scientist, physics professor J.G. Dash of the University of Washington, points out in a letter to the Bulletin how off-the-wall ideas become cemented into the national agenda:

"By the time a new weapons system evolves from research to national decision, it will have accumulated a host of enthusiasts, involving not only the laboratories but also military, industrial and congressional advocates."

Watch how soon the hawks will be telling us that to declare a moratorium on space research will be a form of "unilateral disarmament." No other collection of letters in the English language has become more pejorative than "unilateral."

Now those desirous of presidential favor are saying that while the shield theory is horsefeathers -- the leakage of 10 bombs alone could wipe out the East Coast -- we should explore the concept as a defense for our missiles. This might abrogate the ABM treaty, but the pervasive philosophy of those running arms control for Reagan is that the Soviets violate treaties anyway.

The defects of Star Wars have become its virtues. The knockout argument is that it is obvious that Star Wars drives the Soviets up the wall and therefore is demonstrably a good thing, if only as a bargaining chip.

Its appeal to Reagan is palpable. The cost, estimated by former defense secretary James R. Schlesinger at $1 trillion, recommends it. That feeds Reagan's pet theory, that by forcing the Soviets into ruinous competition we can spend them into the ash heap of history.

Reagan enjoys speaking grandiosely of the initiative's ability to make nuclear weapons "impotent" and "obsolete," while continuing his buildup of them. He likes to appear as a visionary and an optimist.

The history of weapons with shifting rationales is discouraging. The MX missile was rescued from annihilation by a bipartisan commission, which admitted it was a hopeless case but said it deserved perpetuation as an expression of "national will."

So with Star Wars. The train is leaving the station. One who scrambled aboard is Henry A. Kissinger, a former opponent, who, in exile from high office, has seen the light. A bipartisan commission, he murmurs, would be just the thing. And who do you suppose could be prevailed upon to be chairman?

Reagan has cautioned against excessive expectations from Geneva. He hardly needed to. By sending Star Wars as a grenade in our negotiators' pocket, he has practically assured us that peace talks do not necessarily mean arms control.