Scientists are convinced that a double flash of light detected off the coast of South Africa by a U.S. satellite came from the test of a nuclear weapon, not a freak mix of lightning and meteor strikes or any other combination of natural events.

There had been speculation that the Vela satellite picked up a combination of natural events, such as an enormous streak of lightning followed immediately by a meteor burning in the Earth's atmosphere. Nuclear scientists discounted such a bizarre combination of circumstances, saying that Vela almost certainly spotted the signature of a nuclear explosion.

State Department and Pentagon spokesmen said the Vela satellite, launched in 1970 to look for violations of the Atmospheric Test Ban Treaty, detected a double flash of light, about one second apart, in the atmosphere south of South Africa in either the South Atlantic or the Indian Ocean.

Vela was built to detect the X-rays, gamma rays and neutrons released when an atomic explosion takes place in the atmosphere, but it also carries optical sensors that can detect the fireball of an atomic explosion. The X-ray, gamma ray and neutraon sensors of the Vela apparently had been damaged by nine years of exposure to the sun's untraviolet light in space, but the optical sensors still were working.

"What they saw was the double pulse of light that is only generated by a nuclear explosion," one scientist said. "A nuclear fireball has this tell-tale signature, unlike any other phenomenon."

The two light pulses triggered by an atomic explosion come about one second apart, scientists said. The second pulse, during which the fireball reaches its maximum brightness, lasts much longer than the first.

Scientists still are puzzled by the inability of surveillance aircraft to detect any radioactive debris in the atmosphere from the explosion, which reportedly took place five weeks ago in the middle of the night.

The State Department said the explosion appeared to have a force of only two kilotons, but this is enough to throw radioactive debris into the atmosphere that would be detectable for weeks.