Geomagnetic researchers think that it takes thousands of years to reverse the field. Kent says his simple test argues against the likelihood that these two craters formed hours apart, leaving unexplained the straight line with the Saint Martin crater. Spray contends that the magnetic test results may not be as clear-cut as they initially seem. Because the Manicouagan crater is so much larger than Rochechouart, the molten rocks would have cooled much more slowly at the Canadian site, taking thousands or hundreds of thousands of years longer to lock in a record of Earth's magnetic field. I f Spray and others can strengthen their claim of an impact chain, it would raise some problems.
Standard theories say there is almost no chance of finding such a string of craters on Earth.
Earth's gravity is not considered strong enough to capture a comet except in very unlikely situations. In 1994, planetary scientists received a vivid lesson on how crater chains can form on a larger planet. The comet had started out as a single body that strayed too close to Jupiter sometime this century and was trapped in an orbit around the planet. On a close pass by Jupiter in 1992, the planet's gravity tugged the weakly constructed comet apart, severing it into at least 20 large pieces aligned like a string of pearls. On the comet's next pass, in 1994, the fragments plunged into the atmosphere one by one over a period of 6 days.