Simplify the following text for a Grade 11 native English-speaking person.
Original: A Kirkwood gap is a gap or dip in the distribution of main belt asteroids with semi-major axis ( or equivalently their orbital period ) , as seen in the histogram below .
Simplified: Kirkwood gaps are gaps or dips in the asteroid belt with semi-major axis ( or equivalently their orbital period ) , as seen in the histogram on the right .