QUESTION:
Is there any chance that Mars, or any other planets, will crash into the sun
or each other?  And if not, why not?

ANSWER from Bruce Jakosky on November 13, 1997:
A single planet orbiting around a star at a safe distance will have a very
stable orbit.  In our solar system, each planet also exerts a gravitational
force on the others.  This force is capable of perturbing the orbits.  Once
perturbed in their orbits, objects run the risk of colliding with each
other.  During the earliest periods in the history of our solar system, in
fact, proto-planets were abundant, they did perturb each other in their
orbits, and collisions were common.  As collisions swept the solar system
clean of most of the debris, we were left with a few planets.  What is
special about the planets is only the fact that they are now far away from
each other and collisions are rare.  In essence, these are the objects that
survived the collisions.

Any time you start with a lot of objects, collisions will bring them
together into a smaller number of bigger objects (in other words, planets).
Once the number of objects drops to a small value, and the distance between
surviving objects gets big enough, no more collisions occur.

Of course, there still are a lot of asteroids in the asteroid belt between
Mars and Jupiter, and the gravitational pull from Jupiter changes their
orbits into ones that cross the Earth's orbit.  These objects do continue to
collide with the Earth--we see impact craters on Earth (such as at Meteor
Crater, Arizona) to remind us of this.