objects in the Solar System, including the planets and their moons,
move in circular orbits around the Sun. Their orbits are far apart
and change very little from eon to eon, so they will never crash
into each other.
are lots of small objects in the Solar System called asteroids
and comets that move in oval-shaped orbits
that sometimes cross the circular orbits of the planets. Most of
the time, the small objects cross a planet's orbit when the planet
is somewhere else in its orbit, so no collision occurs. It is like
crossing a railroad track when no train is present. But every now
and then, an asteroid or comet crosses a planet's orbit exactly
where the planet is, causing a huge crash!