For years planetary scientists have chalked up the moon's origin to an impact between Earth and a Mars- or larger-sized body called Theia. According to this theory about 4.5 billion years ago a newly-coalesced Earth was hit by another protoplanet. The impact pulverized a sizable part of our young planet, sending the debris into orbit where, under its own gravity, it accreted to make our moon. A study published today (Jan. 9) in Nature Geoscience is calling this into question, proposing several impacts of smaller bodies instead of a single huge one. "We have all been focused on the idea of finding a single impact," says Robin Canup, a planetary scientist at Boulder's Southwest Research Institute. "The out-of-the-box thinking here is to relax that assumption, asking, Can we form the moon in little bits?" Canup was not involved in the study, though she has herself proposed that the moon formed when a Earth collided with another protoplanet nearly the same size. The Earth-moon system is a