As discussed in the extraordinary claims box, when the giant impact idea was first proposed, it was generally thought to be so unlikely that it wasn't seriously considered until decades alter, after the apollo missions to the moon. what key piece of scientific understanding was missing that made it seem so unlikely when first proposed?

The correct answer is:

An understanding of the number and size of leftover planetesimals in the early solar system.

The giant-impact hypothesis proposes that the Moon was created out of what was left of a collision between Earth and an astronomical body the size of Mars, around 4.5 billion years ago, in the Hadean eon. The heavy bombardment was a period when leftover planetesimals hit planets during the solar system's first hundred million years.

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