He breaks down the orbital mechanics of Aten asteroids, a class of near-Earth objects with semi-major axes smaller than Earth's distance from the Sun. The semi-major axis, he explains, is measured from the asteroid's farthest point to the midpoint of its orbit. If that measurement comes in under one astronomical unit (the Earth-Sun distance), the asteroid spends most of its time inside Earth's orbit. These objects cross Earth's path, making them a category worth understanding. The geometry is straightforward once you visualize it: most of an Aten's elliptical orbit lies closer to the Sun than we do, which changes the dynamics of potential encounters and the frequency with which we might detect them.