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People tend to think of the solar system as a static environment, in which the orbits of the planets, asteroids, and comets have remained the same over its lifetime. But although its current architecture has existed for roughly the past 4.5 billion years, the solar system is far from the unvarying environment that we imagine.
The gravitational influence of the planets over small bodies, particularly those in the solar system’s inner regions, has modified many asteroid orbits in quite dramatic ways. For example, the interactions can push asteroids from nearly circular orbits in the main asteroid belt between Mars and Jupiter to highly elliptical orbits that cross those of all the terrestrial planets. Eventually, those perturbations can move the asteroids’ perihelia—their closest orbital distance from the Sun—to within the star’s radius. They are known as Sun-grazing orbits. The asteroids’ transformation from main-belt orbiters to Sun grazers can take place in the surprisingly short time scale of a million years. That’s less than 0.02% the age of the solar system.
In addition to decreasing the asteroids’ perihelia, gravitational interactions can also decrease their aphelia—their farthest orbital distance from the Sun—and push them to increasingly smaller orbits. That movement progressively nudges asteroids onto hard-to-reach orbits that are closer to the Sun than either Earth’s orbit or Venus’s orbit; the asteroids are known as Atiras and Vatiras, respectively. The orbital evolution of those rare Atira and Vatira asteroids is a reminder that their trajectories can change dramatically over their lifetimes and take them throughout the inner solar system. That evolution can tell us where the asteroids have likely been and where they will likely go as their orbits continue to evolve. More practically, it can tell us where to point our telescopes to find those elusive objects.

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