A long-term danger of President Reagan's proposed ''Star Wars'' antimissile system is that it could actually be successful, according to a leading arms control expert.
If the Soviets also perfect such a defense, it could neutralize United States nuclear weapons and free the Soviets to be more aggressive, warns retired Air Force Lt. Gen. Brent Scowcroft. An adviser to President Ford, General Scowcroft also led a major study of US strategic forces for the Reagan administration that has become a basic reference.
Speaking at an arms symposium during the annual meeting of the American Association for the Advancement of Science (AAAS) Tuesday, Scowcroft reminded the audience that ''we are the ones, in 1952, who decided to defend the United States on the cheap by going to nuclear weapons rather than producing the conventional strength necessary to protect our interests around the world. Now I would hope that, by the end of the century or whenever SDI (the ''Star Wars'' Strategic Defense Initiative) could come in, we will have rectified that. But if we don't and . . . both sides have . . . a virtually impregnable shield, it could simply make the world safe for conventional Soviet aggression.''
''I'm a strong supporter of research and development in that (SDI) area,'' Scowcroft said. But he warned that he thinks that the proposal has not been well thought out from the strategic point of view.
He also noted that some of the money for the SDI is being taken out of funding for existing systems for defending US missiles - so-called point defense systems. He called this a mistake. Such point defense may be urgently needed should the US and USSR some time decide to amend the Antiballistic Missile (ABM) Treaty to allow for more protection of missile assets, he said.
Also, Scowcroft expressed a grave doubts that the US will be able to sustain the Star Wars program politically and carry it through to fruition. But he said he has much less doubt that the Soviets would do this if the US stimulates an arms race in space-based laser weaponry.
What he considered more serious, he said, is the possibility that ''one of the first casualties, if we're not careful . . . is likely to be arms control.''
''The first refuge of the Soviets is likely to be to preserve and enhance their ability to penetrate (US missile defenses),'' he explained. ''And thus they're unlikely to look sympathetically on our efforts to reduce their warhead numbers.''
On Tuesday, Robert Jastrow, an Earth scientist at Dartmouth University, defended the President's ''Star Wars'' program. As he noted, there is wide opposition to the program amoung US scientists because some of them ''have made a pessimistic judgment'' that it is technically infeasible.
But he explained that such a position exaggerates the uncertainties and discounts the possibility that research may overcome perceived difficulties. He added that similar exaggeration of cost projections obscures the fact that the program is being funded within the traditional level of US strategic spending.
Richard Garwin of IBM, a leading arms expert and vocal critic of the ''Star Wars'' plan, said flatly that the President had made his proposal with no technical foundation for it. Someone has to declare that ''the Emperor has no clothes,'' he said. Dr. Garwin went into detail as to why he considers the program infeasible. He explained that he also is concerned about diverting attention from arms control and allowing the US nuclear deterrent to deteriorate.