The confusion over whether or not an atomic explosion occured near South Africa Sept. 22 arises in part because the vast U.S. network that once policed nuclear explosions in the atmosphere is no longer in place.

Not only is the Vela satellite network that watched for atomic explosions from Earth orbit down to three from eight, but the giant acoustic system that detected he sounds of nuclear explosions anywhere in he atmosphere is inoperative.

"This is a serious deficiency," a source in the Carter administration said. "We were literally quite fortunate to spot the event near South Africa that could have been an atmospheric test of an atomic device."

The United States scaled down its atmospheric police force when it signed the Atmospheric Testt Ban Treaty with the Soviet Union in 1963 and testing of nuclear weapons moved underground. France and China continued testing in the atmosphere, but India exploded its first atomic device in 1974 below the ground.

"There was a feeling that any new nation getting atomic weapons and testing them would do it underground," a weapons expert said. "People who watch this kind of thing were quite surprised by the event of Sept. 22, which still appears to a nuclear explosion in the atmosphere."

The detection was made by the newest Vela, which was put in orbit in 1970. I saw double pulse of light in a circle 3,000 miles wide over the South Atlantic and Indian oceans south and east of South Africa. Transmitted to Earth, the pulse registered as a strong double blip on a roll of paper.

The twin light pulse is characteristic of nuclear explosions. It is the fireball that briefly disappears from view when the explosion's shock wave makes it opaque from space, then reappears when the shock wave is dissipated.

"There is no natural phenomenon that gives us this double light pulse," a source said. "We know of nothing else that has given us this signal."

The Vela satellites that circle Earth looking for nuclear explosions have seen 42 double spikes of light in the last 15 years. The first 41 were all later confirmed as atmospheric nuclear tests, conducted either by China or France. The 42nd event took place Sept 22.

The explosion, if it indeed was an explosion, took place in the middle of a cloudy night, so that the fireball light filtered through the clouds in a way that allowed scientists to estimate the force of the explosion. The Pentagon has said that, if it was an atomic explosion, its force was less than four kilotons and probably no more than two kilotons.

That is quite small. Even the first two atomic explosions over Japan at the end of World WAR II in 1945 had a force equal to 20 kilotons, which is the explosive force of 20,000 tons of TNT.

Whatever Vela saw was discussed yesterday at the Old Executive Office Building, where a panel of seven outside experts in atmospheric science met for the second straight day with White House science advisers and officials from the Statt Department and Central Intelligence Agency.

The experts discussed whether the Sept. 22 event could have been a superbolt of lightening, a meteor striking the atmosphere or a freak combination of both. They also discussed the possibility that the Vela satellite itself malfunctioned.

"That was not likely," a source said. "The satellite's instruments were checked and calibrated a week before the event was witnessed and everything was working just fine."

Another source said that the expert panel convened by the White House did not rule out natural phenomena. Said the source: "No conclusions were reached and there is still the possibility that it was indeed a nuclear explosion."