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Modeling suggests that collisions between planetesimals initially occur at low velocities, allowing them to merge and grow (Box 1.3). At the Earth's distance from the Sun, it takes only about 1,000 years for 1‐km‐sized objects to grow into 100‐km objects. Another 10,000 years produces 1,000‐km diameter protoplanets, which double in diameter over the next 10,000 years. Such models indicate that Moon‐sized objects can form in a little over 20,000 years.

As planetesimals within the protosolar disk grow larger and more massive, their gravity increases, and once a few of the objects reach a size of 1,000 km, they begin to stir up the remaining smaller objects. Near encounters accelerate the smaller, asteroid‐sized chunks of rock to higher and higher speeds. Eventually, they are traveling so quickly that when they collide, they pulverize each other instead of merging.