A Revised Capture Scenario

In 1972, Ernst J. Opik, an Estonian-born astrophysicist who worked in Northern Ireland, proposed a totally different theory. Opik suggested that a primordial object may have streaked past the Earth in the early solar system and come within Roche's limit, a boundary located about 18,500 kilometers (11,500 miles) from the center of the Earth. Inside Roche's limit, the Earth's gravity can pull a weak body apart. (Every planet has its own Roche's limit, which varies according to the planet's mass.) Some debris from the disintegrating body may have gone into orbit around the Earth and then coalesced to form the moon.

Scientists agreed that Opik's “disintegrative capture” theory convincingly explained how the moon could have been made from different material than the Earth—material from a distant part of the solar system. And the theory provided an answer to the question of how a large object—or at least part of it—could have been pulled into orbit around the Earth, a test that the previous capture theory had failed. Nonetheless, by 1985, researchers had made detailed calculations showing that a fast-moving object would not have spent enough time within Roche's limit to be pulled apart and leave debris around the Earth before it flew on. Thus, until better calculations might somehow yield contrary results, the disintegrative capture theory was shelved.