Over the Desert, A Nuclear Dawn

`MINUS 30 seconds,'' intoned physicist Samuel Allison over the network. ''Twenty-five ... 20 ... 15 ....'' At 10 he began calling off each second: ''Nine ... eight ... seven ... six ... five....''

At five seconds, the shortwave radio in the Carrizozo, N.M., tourist court failed momentarily. Los Alamos scientist Diz Graves continued her own countdown: ''Four ... three ... two ... one ... zero. It didn't go,'' she said to her scientist husband, Alvin. But she had counted too fast. At exactly 5:29:45 a.m., the sky ignited with a light brighter than the noonday sun.

In Albuquerque, Capt. Thomas Jones's hotel room lighted up as if a photo-flood lamp had been turned on. ''Wonder if anyone is still alive at Trinity, Tom?'' Phil Belcher said to his roommate. Then, realizing the significance of his remark, Belcher tried to contact Trinity base camp. After a suspenseful minute, a voice on the phone answered calmly, ''All is well.''

Inside the control shack, his back to the tower, Robert Oppenheimer had been looking south through the open door toward Mockingbird Gap when the flash illuminated the sky with a brilliance that for an instant bleached the desert to a ghastly white. Across Oppenheimer's mind flashed a passage from the sacred Hindu text, the Bhagavad-Gita: ''I am become death, the destroyer of worlds.'' For a second there was stunned silence in the shelter. Then the men rushed outside.

A great reddish-yellow ball of fire a quarter-mile wide was boiling and churning over ground zero. The suddenly unleashed nuclear force expanded, its fireball reaching back to the ground as if the sun had suddenly come down to sear the earth. Then the fireball shot upward, sucking up the desert floor beneath it into a column that within 20 seconds reached a mile into the sky.

Thirty seconds after the flash of light, the explosion's thunderous boom reached South-10,000, accompanied by a shock wave that buffeted the men standing in the open and sent desert debris flying before it.

At base camp, physicist Enrico Fermi counted off the seconds after the flash of light. Just before he estimated the blast wave would arrive, he held his right hand high in the air and released some scraps of paper he had been clutching. As the scraps fluttered toward the ground the blast wave hit and blew them several feet away. Fermi measured the distance, made some hasty calculations, and predicted to his colleagues that the bomb's yield would prove to be on the order of 20,000 tons of TNT.

All over the desert amphitheater, the observers shook hands or clapped each other on the back in elation. At South-10,000, Oppenheimer climbed to the roof of the control shelter and joined George Kistiakowsky. The day before, with Oppenheimer's spirits at their lowest, Kistiakowsky had bet his next month's salary against Oppenheimer's $10 that the detonators would fire. Now, as they watched the massive, mile-wide cloud mushrooming and climbing to 40,000 feet, Kistiakowsky exuberantly demanded payment. Oppenheimer dug out his wallet and looked inside. There was nothing in it except the four-leaf clover his wife, Kitty, had given him. Oppenheimer grinned and showed his friend the empty wallet. ''Guess you'll have to wait for your money,'' he said.

As the initial elation subsided and the mushroom cloud dispersed, the scientists began collecting the data needed to measure the bomb's explosive yield. By 10 a.m., enough evidence had accumulated to determine that the gadget was indeed a potential superweapon.

Physicist Herbert Anderson had made a brief expedition to ground zero in a sealed, lead-lined Army tank. Through the tank's periscope he had seen a huge, shallow crater 1,200 feet in diameter. The ground was covered with a green, glassy substance, pieces of which Anderson gathered with a clawlike device attached to the tank. The heat from the explosion had been so intense that the sand and gravel from the desert near the tower had fused into this ceramiclike material, which the scientists named ''Trinitite.'' In some of the samples of Trinitite there were streaks of brown, the only remains of the steel tower, vaporized by the fireball.

From the force of the shock wave, the size of the fireball, and the level of radiation in the samples taken from the bomb crater, it was determined that the yield was, as Fermi's simple experiment had predicted, equivalent to 20,000 tons of TNT. It had exceeded by 2,000 tons the highest guess in the betting pool. Nobel Prize-winner I. I. Rabi's guess of 18,000 tons won the $102 pot.

Two major problems remained: the danger from fallout and the maintenance of secrecy. From Santa Fe to El Paso, Texas, queries were being received at newspaper offices and sheriffs' stations from people who had seen the flash or heard the noise. Bomb-project head Maj. Gen. Leslie Groves decided to put out the cover story, and the commandant of the Alamogordo Air Base released an announcement that a remotely located ammunition magazine containing a considerable amount of high explosive and pyrotechnics had exploded.

The fallout pattern was still unknown. It appeared that most of the mushroom cloud was drifting over the sparsely settled area northeast of the site, but high-altitude winds were carrying off some possibly dangerous radioactive particles in other directions. General Groves and his chief medical adviser, Col. Stafford Warren, were concerned that excessive radiation might fall in populated areas. Monitors with Geiger counters were dispatched along roads leading in all directions from Trinity.

While awaiting word from the radiation monitors, Oppenheimer went for a walk alone on the desert. The first flush of satisfaction had passed, and the full import of what he had witnessed was beginning to register: The awful potential of a nuclear bomb dropped, not on a barren desert, but over a throbbing city. As he walked along, his thoughts were interrupted by the sight of a large land turtle.

''It was helpless, probably knocked over by the blast wave,'' Oppenheimer told me during a 1960 interview in his Princeton, N.J., office. ''So I knelt down and turned him right side up. It just seemed we had done enough harm for one night.''

Then he walked back to camp to help Groves and his deputy, Brig. Gen. Thomas Farrell, prepare a coded message to Washington so that the news of the superweapon's success could be relayed to President Truman, about to meet with Allied leaders in Potsdam, Germany.

The Potsdam declaration to Japan to surrender or face prompt and utter destruction was rejected by Japan's government: The untested Little Boy uranium bomb leveled Hiroshima on August 6, 1945. Three days later, a duplicate of the Trinity plutonium bomb was dropped on Nagasaki. Another Trinity-type bomb, which had been shipped from Los Alamos in early August, was never needed. Japan surrendered August 14.

When the Hiroshima announcement shattered the Manhattan Project's wall of secrecy, rumors circulated that the Trinity bomb's radioactivity might prove harmful to the residents of south-central New Mexico. The only ill effects appeared on a few animals, however. Some cattle grazing near Trinity lost patches of hair due to skin burns from wind-carried radioactive particles. The cattle were purchased by the federal government and shipped to Los Alamos and Oak Ridge, Tenn., for medical studies.

A black cat owned by a family 30 miles northeast of ground zero also received skin burns. When gray spots later appeared on her black coat, she was renamed ''Atomic'' and became Highway 380's foremost tourist attraction.

How much did Trinity change the world? Fifty years later, arguments persist over whether the United States should have dropped the bomb on a vacant site rather than on a city. History has shown that bombing Hiroshima and Nagasaki ended the war without the need for invading Japan.

And the testing of a hydrogen-fusion bomb at Bikini atoll in 1954 showed the world that bombs a thousand times more powerful than the first atomic weapons were a reality. Most observers now concede that the development of nuclear bombs and the threat of their use have been the major deterrent to a third world war.

Thirty-five years ago, Robert Oppenheimer told me: ''If I had had then the view of the state of the Japanese government that I have now - and that may be wrong, too - I would have been at the very least in favor of a much more explicit warning. I think I would have tried to get a negotiated peace without the use of the bomb.''

''Trinity was the beginning of a new age for man,'' Oppenheimer said. ''Yet the problems that bedeviled him in the past were not going to stop bedeviling him. That this was a new [problem] which would alter the light in which man's problems were looked on was the true sense of what we breathed that morning on the New Mexico desert.''

* Parts I and II ran Monday and Tuesday. Robert Cahn won a Pulitzer Prize for a Christian Science Monitor series on America's national parks. This article is adapted from one he wrote for The Saturday Evening Post in 1960.

Across Oppenheimer's mind flashed a passage from the Bhagavad-Gita: 'I am become death, the destroyer of worlds.'