Further Support For the Giant-impact Theory
Besides conducting computer simulations, researchers pursued other ways of testing the giant-impact theory. During 1998, Lunar Prospector, NASA's first moon mission since Apollo, was launched to gather a variety of data about the moon. As Prospector orbited just 100 kilometers (60 miles) above the moon's surface, researchers used Earth-based radio telescopes to monitor the moon's gravitational pull on the spacecraft. By mapping the results, the scientists were able to get a far more accurate picture of how the moon's mass is distributed than they had had before. The researchers calculated that the moon has a core, most likely composed of iron, with a radius of 220 kilometers to 450 kilometers (135 miles to 280 miles). This is well within the range expected if the moon had formed as the result of a devastating impact on Earth and thus is consistent with the giant-impact theory.
In December 1998, at an origin-of-the-moon conference in Monterey, California, Canup and planetary scientists Craig Aignor of the University of Colorado at Boulder and Harold Levison of the Southwest Research Institute announced further calculations in support of the giant-impact theory. They used a computer method called “symplectic integration,” which allowed them to simulate much greater lengths of time and thousands more sun-orbiting objects than ever before possible. Canup's group found that impacts on Earth, such as the one that may have formed the moon, were almost certainly common during the first 100 million years after the birth of the solar system. This conclusion was among the strongest indications available to astronomers that the giant-impact theory was not based on an unusual accident of nature, a finding that made the theory even more credible.