President Reagan's secret decision last December to go ahead with the "Zenith Star" chemical laser program was motivated by his longstanding desire to have at least a prototype laser weapon in space before he leaves office.

But he won't make it. His faith in the Strategic Defense Initiative, popularly known as "Star Wars," led Reagan to order Lt. Gen. James Abrahamson, SDI director, to proceed with Zenith Star even though the president thought it would eventually violate the 1972 Antiballistic Missile Treaty.

The same sources in the White House, the Pentagon and the intelligence community who confirmed the story of Reagan's decision to Dale Van Atta predict that the hydrogen-fluoride laser under development will not be ready to test in space until long after Reagan leaves office.

The earliest Zenith Star can have a chemical laser weapon ready to send up is late 1990, barring technical problems, these sources said.

The news has been a keen disappointment to the president. As one top White House aide explained, Reagan "would like to demonstrate one of these exotic weapons tomorrow, if he could."

Shortly after his election in November 1980, Reagan confided to several key Republican senators that he intended to accelerate the development of space-based laser weapons as an antimissile defense. The laser of choice at that time, known as Alpha, was fueled by hydrogen fluoride. It had been under development since the late 1970s.

But the relatively simple chemical laser lost its charm after Reagan's Star Wars speech in March 1983. Attention was focused on free-electron lasers and other exotica. Abrahamson's new SDI Office all but spiked the modest Alpha program.

But by mid-1986, Abrahamson had changed his position. The White House wanted early results, and the general knew that every laser but Alpha was at least 10 years away. Thus the Zenith Star program was born, with the Alpha chemical laser as the centerpiece of a fully integrated weapon.

When the president was finally sold on Zenith Star at a meeting last Dec. 17 with Abrahamson and Defense Secretary Caspar W. Weinberger, he insisted on making the program supersecret. The reason was the GOP loss of Senate control and the fear of opposition if the Democrats learned about it.

Zenith Star was so secret in its first phase that few if any members of Congress were cleared to know about it. But then, as the second phase began, some of the secrecy was lifted. Parts of the program have been downgraded to a "secret" classification and removed from the special access codes that kept the earliest phase under wraps.

On Oct. 5, the Pentagon even issued a press release about Phase Two -- without using the still- classified Zenith Star code name. The contract is "to develop a comprehensive road map and a ground verification and integration testing plan for existing key elements of the space-based laser program," the Pentagon announced. It added solemnly: "The study and the resulting plan will be entirely compliant with the 1972 ABM Treaty."

The statement is technically true, but it attempts to hide the program's real goal, which is to design a fully integrated chemical laser weapon to be tested in space at the earliest possible moment.