By slamming into the planet at many times the speed of sound, triggering tidal waves, setting off earthquakes, vaporising anything in the impact radius, and causing centuries long dust storms, I hear you say.
But I’m not asking the question in the physical repercussions sense. I’m asking the question in the the sense of how asteroids in relatively stable, near-circular orbits out in the main belt between Mars and Jupiter, can be dislodged and get into the inner system where they could cause, in Busta Rymes infamous phrase, “Extiction Level Events”.
If we can understand that, then maybe we can save ourselves having to send Bruce Willis out to save the planet.
One method is obviously via collisions between asteroids within the belt, or collisions between asteroids and passing objects such as comets. Given the vast number of objects that make up the main belt, and the gravitational fluctuations that arise due to the relative positions of the Sun and Jupiter, I would anticipate that these kinds of collisions are not uncommon.
However, these types of interactions are by their nature fairly chaotic and unpredictable. This is why the skies are constantly being scanned by teams of astronomers dedicated to looking out for rogue asteroids. An unexpected collision could set a asteroid on a trajectory that would impact with Earth in a matter of years. Sounds a long time, but given the high cost and technical difficulty of easing one of these asteroids onto an alternative path, we might need every last day of warning to avert catastrophe.
Are there any other more predictable means by which asteroids might be subjected to forces that move them into dangerous inner system elliptical orbits?
Strangely enough, the combination of the fact that most asteroids are spinning to a greater or lesser extent, and that they’re bathing in a constant stream of solar radiation, means that all asteroids feel a steady radiation thrust in a direction perpendicular to the inward gravitational force. The effect comes about because solar radiation takes time to be absorbed and re-emitted by the surface of the asteroid. In this time, given the rotation of the asteroid and the differential temperatures between the sun-side and the dark-side, more heat radiation is emitted on one side than the other, corresponding to a small thrust. The effect is named after its discoverer Ivan Yarkovsky.
Of course the effect is tiny, but over the course of millions of years this process can steadily alter the orbit of an asteroid. In this way, the main belt can act as a steady reservoir for missiles that can be flung at the inner planets causing craters like the one found at Meteor Crater near Flagstaff, Arizona, or the tree-flattening devastation of the Tunguska Event in Siberia.